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Book of Abstract
CEE2D | PISA IT | 2015 3
Collective Electronic Excitations in 2D
FP7 INDEX Conference
Scuola Normale Superiore | Pisa IT
September 20-24 | 2015
Organizing Scientific Committee
François Dubin, ICFO (ES), Vittorio Pellegrini, Scuola Normale Superiore & IIT
(IT), Aron Pinczuk, Columbia University (USA), David Ritchie, University of
Cambridge (UK), Massimo Rontani, Cnr Nano (IT), Thomas Satzoukidis, Scuola
Normale Superiore (IT), Masha Vladimirova, CNRS (FR).
Organizing Committee
Thomas Satzoukidis, Elisa Guidi, Gisella Chiné, Scuola Normale Superiore (IT),
Luisa Neri, CNR NANO (IT).
Websites
http://web.nano.cnr.it/CEE2D/
http://indexitn.coulomb.univ-montp2.fr/
INDEX
PROGRAM 5
POSTER SESSION 9
ABSTRACTS OF TALKS 11
ABSTRACTS OF POSTERS 40
CEE2D | PISA IT | 2015 5
INDEX
PROGRAM
POSTER SESSION
ABSTRACTS OF TALKS
ABSTRACTS OF POSTERS
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PROGRAM
Sunday September 20th
18:30 Welcome cocktail, Sala Gran Priore in Scuola Normale Superiore.
Monday September 21st
08:30-09:15 Registration
09:15-09:30 Conference opening: V. Pellegrini, M. Rontani
09:30-10:15
Aleksey K. Fedorov, Russian Quantum Center, Moscow (RU) & LPTMS, CNRS
(FR)* Roton Phenomena of weakly interacting dipolar excitons in a semiconductor
layer
10:15-10:45 Anton Nalitov, Clermont Ferrand University (FR) Topological polariton states and Kibble-Zurek mechanism in zigzag chains of
pillar microcavities
10:45-11:15 Coffee Break
11:15-12:00 Cory Dean, Columbia University (US)*
Hofstadter’s Butterfly in the strongly interacting regime
12:00-12:45 Massimo Rontani, CNR NANO S3, Modena (IT)* Giant orbital magnetic moment and spin-orbit coupling of a carbon nanotube as
an excitonic insulator
12:45-14:30 Lunch Break
14:30-15:00 Serguei Andreev, CNRS, Orsay (FR)
Resonantly paired gas of dipolar excitons
15:00-15:30 Fedor Fedickhin, Laboratoire Charles Coulomb, University of Montpellier (FR)*
Transport of indirect excitons in polar GaN/AlGaN quantum well structures
grown on sapphire and GaN substrates
15:30-16:00 Dario Ballarini, NNL Istituto di Nanotecnologie CNR, Lecce (IT)
Non linear interactions in high-speed organic polariton flow at room temperature
16:00-16:30 Coffee Break
16:30-17:15 Andrea Gamucci, Laboratorio Nest Scuola Normale Superiore, Pisa (IT)* Evidence for electron-hole pairing in graphene-GaAs double layers
17:15-17:45 Guido Pupillo, CNRS Strasbourg (FR)
Cavity enhanced transport of excitons and charge
CEE2D | PISA IT | 2015 7
Tuesday September 22nd
09:00-09:45 Yuliya Kuznetsova, Columbia University, New York, NY (US)* Artificial Graphene: Lattices in Nano-patterned Semiconductors
09:45-10:30 François Dubin, ICFO (ES)*
Looking for the dark exciton condensate
10:30-11:00 Coffee Break
11:00-11:45 Daniele Sanvitto, NNL Istituto di Nanotecnologie CNR (IT)* Quantum fluid dynamics of polariton condensates
11:45-12:30 Ursula Wurstbauer, Technische Universität München TUM & Nanosystems
Initiative Munich NIM (DE)* Confocal shift interferometry of coherent emission from trapped dipolar excitons
Free afternoon, social event, dinner
Wednesday September 23rd
09:00-09:45 Jeremy Baumberg, University of Cambridge (UK)* Ultralow Energy Switching of Ferromagnetic Condensates in Semiconductor
Microcavities
09:45-10:30 Masha Vladimirova, CNRS & Université de Montpellier (FR)* Spin coherence in coupled quantum wells amended by in-plane magnetic field
10:30-11:00 Coffee break
11:00-11:45 Alexey Kavokin, University of Southampton (UK)*
11:45-12:30 Jelena Klinovaja, University of Basel (CH)* Engineering Topological Quantum States: From 1D to 2D
12:30-14:30 Lunch break
14:30-15:15
Peristera Andreakou, Laboratoire de Photonique et Nanostructures, CNRS,
Marcoussis (FR)* Transport of indirect excitons in coupled quantum wells:Basic physics and
devices
15:15-15:45 Karsten Leding Vendelbjerg, Cnr Nano S3 & University of Modena & Reggio
Emilia, Modena (IT) Manipulation of spin transfer torque using light
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15:45-16:15 Coffee break
16:15-18:30 Poster session
18:30-20:00 SB meeting Index Project
20:00-22:00 INDEX dinner
Thursday September 24th
09:00-09:45 Rainer Mahrt, IBM Research Zurich (CH)* Bose-Einstein Condensation in a Polymer: Towards Quantum Simulation
09:45-10:15 Ugo Siciliani de Cumis, University of Cambridge (UK) Electronic transport and optical properties of electrically-generated electron-
hole bilayers with minimum inter-layer separation
10:15-10:45 Vanik Shahnazaryan, University of Iceland, Reykjavik (IS)
Adiabatic preparation of a cold exciton condensate
10:45-11:15 Coffee break
11:15-12:00 Marco Polini, IIT Graphene Labs & Nest Scuola Normale Superiore, Pisa (IT)*
Collective non-local transport in graphene heterostructures
12:00-12:30 Federico Grasselli, University of Modena & Reggio Emilia & Cnr Nano S3,
Modena (IT)
Time-Dependent Quantum Dynamics of 2D Spatially Indirect Excitons
12:30-14:30 Lunch break
14:30-15:15 Carlo Andrea Rozzi, CNR NANO S3, Modena (IT)*
Ultrafast dynamics in light-harvesting and photovoltaics: a theoretical and
experimental investigation
15:15-15:45
Gabriel Gil, Cnr Nano S3 & University of Modena & Reggio Emilia, Modena
(IT)
Excitation energy transfer in hybrid nano-systems by a multi-scale beyond-
Förster approach
15:45 Conference closing, discussion remarks
* Invited Speaker
CEE2D | PISA IT | 2015 9
INDEX
PROGRAM
POSTER SESSION
ABSTRACTS OF TALKS
ABSTRACTS OF POSTERS
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POSTER SESSION
Room-Temperature Rectification In Graphene/LAO/STO Heterostructures
I. Aliaj, E. di Gennaro, V. Miseikis, I. Torre, A. Gamucci, C. Coletti, M. Polini, F. M.
Granozio, V. Pellegrini, F. Beltram, and S. Roddaro
40
Macroscopic Occupation Of Dark Excitonic States In A Trap
Mussie Beian, Mathieu Alloing, Romain Anankine, Edmond Cambril, Carmen Gomez
Carbonell, Aristide Lemaître, and François Dubin
41
Lateral Transport Of Indirect Exciton Spins In Double Quantum Well Structures
S. Büyükköse, C. Hubert, A. Violante, and P. V. Santos
42
Oxide Heterostructures As A Possible Innovative Road For Studying Indirect Excitons
L. Maritato, A. Galdi, C. Sacco, and D.G. Schlom
43
Cavity Polaritons Under The Influence Of The Landau Quantization, Rashba Spin-Orbit
Coupling, Zeeman Splitting And Gyrotropy Effects
S. A. Moskalenko, I.V. Podlesny, E.V. Dumanov, and M.A. Liberman
44
Magnetoexciton In Nanoring Of Non-Uniform Thickness
L. C. Porras and I. D. Mikhailov
45
Biased Controlled Dipole Oriented Polariton Bistability
Pramod K Sharma, Simeon I. Tsintzos, Gabriel Christmann, Zacharias Hatzopoulos,
Jeremy J Baumberg, and Pavlos G Savvidis
46
Exciton Dynamics In Disk-Like Quantum Dots With A Magnetic Impurity
V. Moldoveanu, I. V. Dinu, R. Dragomir, and B. Tanatar
47
Quantum Rings And Quantum Dots: Optical Properties And Annealing Process
M. Triki, D. Elmaghraoui, and S. Jaziri
48
CEE2D | PISA IT | 2015 11
INDEX
PROGRAM
POSTER SESSION
ABSTRACTS OF TALKS
ABSTRACTS OF POSTERS
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ROTON PHENOMENA OF WEAKLY INTERACTING DIPOLAR
EXCITONS IN A SEMICONDUCTOR LAYER
A.K. Fedorov
1,2, I.L. Kurbakov
3, and. Yu.E. Lozovik
3
1Russian Quantum Center, Skolkovo, Moscow 143025, Russia
2LPTMS, CNRS, Univ Paris Sud, Universite Paris-Saclay, Orsay 91405, France
3Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow 142190, Russia
Many-body systems with dipole-dipole interaction provide an interface between physics of strongly
and weakly correlated quantum matter. The anisotropy and the region of attraction of the dipole-
dipole interaction potential provide a set of interesting many-body phenomena. In the limit of
strongly correlated system of in-plane dipoles, the ground state of the system has the chain
structure; the 3D system of parallel dipoles has the chain structure as well.
Being typical for strongly correlated systems, local minimum in a non-monotonic excitation
spectrum – roton-maxon character – originally observed in liquid helium appears in a weakly
interacting gas [1]. This facts itself opens fascinating a prospective for revealing of non-
conventional structural properties of dipolar condensates close to the threshold of instability.
In this contribution, we predict the formation of the roton-maxon excitation spectrum and the roton
instability effect for a weakly correlated Bose gas of dipolar excitons [2]. The formation of the
roton minimum and the roton instability is the result of important features of the dipole- dipole
interaction – anisotropy and attraction region – as well as the layer geometry, which passes through
unstable 3D and stable 2D regimes. According to numerical estimations, the threshold of the roton
instability for Bose-Einstein condensed exciton gas with roton-maxon spectrum is achievable
experimentally in GaAs semiconductor layers.
References
[1] L. Santos, G.V. Shlyapnikov, and M. Lewenstein, Phys. Rev. Lett. 90, 250403 (2003).
[2] A.K. Fedorov, I.L. Kurbakov, and Yu.E. Lozovik, Phys. Rev. B 90, 165430 (2014).
CEE2D | PISA IT | 2015 13
TOPOLOGICAL POLARITON STATES AND KIBBLE-ZUREK
MECHANISM IN ZIGZAG CHAINS OF PILLAR MICROCAVITIES
D. D. Solnyshkov, A. V. Nalitov, and G. Malpuech
Institut Pascal, PHOTON-N2, Université Clermont Auvergne, CNRS, 4 Avenue Blaise Pascal,
63178 Aubière Cedex, France.
We theoretically consider a zigzag chain of coupled pillar microcavities, taking into account
the polarization. We show that the TE-TM splitting leads to the appearance of topological
polariton states at the edges of the chain. Their polarization depends on the parity of the
number of pillars. This follows from a tight-binding model and is confirmed by full numerical
simulations. A random choice of polarization of the bulk states during the condensate
formation is equivalent to dimerization of polymer chains. We show that topological defects
appear as domain walls between polarization domains, analogous to Su-Shriffer-Heeger
solitons in polymers. Their density in a polariton condensate is governed by the condensation
quench speed via Kibble-Zurek mechanism.
Reference
[1] D. D. Solnyshkov, A. V. Nalitov and G. Malpuech, arXiv:1506.04626 (2015).
14 www.web.nano.cnr.it/CEE2D
HOFSTADTER’S BUTTERFLY IN THE STRONGLY INTERACTING
REGIME
Cory Dean
Columbia University, New York City, NY, USA
In 1976, Douglas Hofstadter predicted that in the presence of both a strong magnetic field, and a
spatially varying periodic potential, Bloch electrons confined to a 2D quantum well exhibit a self-
similar fractal energy spectrum known as the "Hofstadter's Butterfly". In subsequent years,
experimental discovery of the quantum Hall effect gave birth to an expansive field of research into
2D electronic systems in the presence of a magnetic field, however, direct confirmation of the
fractal spectrum remained elusive. Recently we demonstrated that graphene, in which Bloch
electrons can be described by Dirac fermions, provides a new opportunity to investigate this nearly
40 year old problem. In this talk I will discuss the experimental realization of Hofstader's butterfly
by exploiting nano-scale interfacial effects between graphene and hexagonal boron nitride
substrates, together with application of extremely high magnetic fields. Utilizing newly developed
techniques to fabricate ultra-clean graphene devices, I will additionally discus the capability to
probe for the first time the effect of strong electron interactions , within the fractal Hofstadter
spectrum.
CEE2D | PISA IT | 2015 15
GIANT ORBITAL MAGNETIC MOMENT AND SPIN-ORBIT COUPLING
OF A CARBON NANOTUBE AS AN EXCITONIC INSULATOR
M. Rontani
CNR-NANO, Research Center S3, Via Campi 213a 41125 Modena Italy,
We suggest that an undoped carbon nanotube might be an excitonic insulator---the long-sought
phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We show theoretically that
the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously
occurs at equilibrium if the tube radius is sufficiently small [1]. Our claim contradicts previous
studies that neglected the coupling between K and K’ valleys [2]. We predict that the signatures of
exciton condensation are its sizable contributions to (i) the energy gap, (ii) the spin-orbit interaction,
and (iii) the magnetic moment per electron. The increase of the gap might have already been
measured, albeit attributed to the Mott-Hubbard insulating state [3], whereas giant values of spin-
orbit energy splittings were recently reported with no explanation [4]. The enhancement of the
quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of
the ground-state condensate of excitons. This property could rationalize the anomalous magnitude
of magnetic moments recently observed in different ultraclean devices close to charge neutrality [5].
Figure 1. Orbital magnetic moment (left) and spin-orbit energy gap (right) vs radius of a nominally metallic
carbon nanotube. The inter-valley exchange interaction w2 drives the excitonic insulator phase enhancing
both observables with respect to their non-interacting values (respectively the straight line in the left panel
and the dashed line in the right panel).
This work was supported by EU-FP7 Marie Curie ITN INDEX and MIUR PRIN MEMO.
References
[1] M. Rontani, Phys. Rev. B 90, 195415 (2014).
[2] T. Ando, J. Phys. Soc. Jpn. 66, 1066 (1997); R. R. Hartmann et al., Phys. Rev. B 84, 0354437
(2011).
[3] V. V. Deshpande et al., Science 323, 106 (2009).
[4] G. A. Steele et al., Nat. Commun. 4, 1573 (2013).
[5] E. A. Laird et al., Rev. Mod. Phys. (2015), in press, available at arXiv:1403.6113.
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RESONANTLY PAIRED GAS OF DIPOLAR EXCITONS
Sergey Andreev
Laboratoire de la Physique Théorique et des Modèles Statistiques (LPTMS), CNRS, Orsay, France
ITMO University, St. Petersburg, Russia
The phenomenon of resonant pairing lies at the heart of superconductivity in metals. Here
Cooper pairs of fermionic particles – electrons, can Bose-Einstein condense to form a
superfluid state. Advances in the field of ultracold atomic gases have allowed a thorough
study of resonantly paired fermionic superfluids [1] and have also inspired theorists to
envisage their bosonic counterpart [2]. However, experimental realization of a bosonic analog
of the Bardeen-Cooper-Schrieffer (BCS) superconductor have so far been precluded by a
short lifetime of the atomic molecules.
In this talk I suggest that the quantum molecular phase of bosons may already have been
observed in the experiments on long-living dipolar excitons in semiconductor quantum wells.
A striking feature discovered in these experiments is the spontaneous fragmentation of the
photoluminescent ring of cold excitons into an array of bright spots, dubbed the
Macroscopically Ordered Exciton State (MOES) [3]. I develop my earlier theoretical idea
[4,5] to show, that the MOES appears as a result of an interplay between the dipolar repulsion
and resonant pairing of excitons. The molecular phase occupies the central part of each spot
and can be distinguished by shift-interferometry measurements of the exciton
photoluminescence.
References
[1] See V. Gurarie and L. Radzihovsky, Ann. Phys. 322, 2 (2007) and references therein.
[2] L. Radzihovsky, J. Park, and P. B. Weichman Phys. Rev. Lett. 92, 160402 (2004).
[3] L. V. Butov, A. C. Gossard, and D. S. Chemla, Nature (London) 418, 751 (2002).
[4] S. V. Andreev, Phys. Rev. Lett. 110, 146401 (2013).
[5] To be published in Rapid Communication section of PRB in 2015.
CEE2D | PISA IT | 2015 17
TRANSPORT OF INDIRECT EXCITONS IN POLAR GAN/ALGAN
QUANTUM WELL STRUCTURES GROWN ON SAPPHIRE AND GAN
SUBSTRATES
F. Fedichkin
1, B. Jouault
1, M. Vladimirova
1, T. Guillet
1, C. Brimont
1, P. Valvin
1, T. Bretagnon
1,
N. Grandjean2, and P. Lefebvre
1
1Laboratoire Charles Coulomb, CNRS-INP-UMR 5221, Université Montpellier 2, F-34095
Montpellier, France 2Institute of Condensed Matter Physics, EPFL, CH-1015 Lausanne, Switzerland
Spatially resolved PL patterns measured at 10 K for 7 m-wide GaN/Al0.19Ga0.81N QWs grown on
GaN (a) and sapphire (b) substrates, respectively. The PL blue-shift is proportional to the exciton
density, the spatial dependence of which can therefore be determined. In (c) and (d), we plotted the
full widths at half maximum of this dependence, versus temperature, for different excitation powers.
Clearly, for the GaN substrate, we observe pure excitonic propagation, free of the secondary,
parasitic PL. Moreover, contrary to the case of sapphire substrate, suppression of nonradiative
recombination, with GaN substrate, allows for an excitonic propagation above 30-35 μm, that is not
quenched by increasing temperature.
An indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in
two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character.
Current research on transport properties of IXs is opening a pathway to the development of novel
optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1-
3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly
promising because of much larger exciton binding energies and natural dipoles induced by strong
built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on
sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (μ-
PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long
distances from the excitation spot (30 < r < 100μm) turned out to be a secondary PL, excited by the
light emitted at the central spot, guided along the plane, due to the refractive index contrast between
the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance
reached by the measurable PL is limited by recombination of excitons at non-radiative defects.
Using GaN substrates instead of sapphire should both suppress the secondary emission and the
nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this
work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures,
except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed
observe the mere propagation of excitons up to 35 μm away from the excitation spot and up to 250
K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole-
dipole repulsion in high-density regions and for disorder along the sample plane.
Spatially resolved PL patterns measured at 10 K for 7 m-wide GaN/Al0.19Ga0.81N QWs grown on
GaN (a) and sapphire (b) substrates, respectively. The PL blue-shift is proportional to the exciton
density, the spatial dependence of which can therefore be determined. In (c) and (d), we plotted the
full widths at half maximum of this dependence, versus temperature, for different excitation powers.
Clearly, for the GaN substrate, we observe pure excitonic propagation, free of the secondary,
parasitic PL. Moreover, contrary to the case of sapphire substrate, suppression of nonradiative
recombination, with GaN substrate, allows for an excitonic propagation above 30-35 μm, that is not
quenched by increasing temperature.
18 www.web.nano.cnr.it/CEE2D
References
[1] Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A.
C. Gossard, Optics Letters 35 (10), 1587 (2010).
[2] A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard, Science 321, 229 (2008).
[3] A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard, Optics Letters 32, 2466 (2007).
[4] F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T.
Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre, Phys. Rev. B 91, 205424 (2015).
CEE2D | PISA IT | 2015 19
NON LINEAR INTERACTIONS IN HIGH-SPEED ORGANIC POLARITON
FLOW AT ROOM TEMPERATURE
G. Lerario1, D. Ballarini
1, A. Fieramosca
1, A. Cannavale
1, A. Genco
1, F. Mangione
1, S. Gambino
1,2,
L. Dominici1,2
, M. De Giorgi1, G. Gigli
1,3, and D. Sanvitto
1
1NNL, Istituto di Nanotecnologie - CNR, Via Arnesano, 73100, Lecce, Italy
2CBN-IIT, Istituto Italiano di Tecnologia, Via Barsanti, 73100, Lecce, Italy
3Università del Salento, Via Arnesano, 73100 Lecce, Italy
One efficient way to dress photons with interactions is through their strong-coupling with the
electronic dipole of excitons. The physics of strong-coupling regime between the electromagnetic
and the excitonic fields in semiconductors is described by new eigenmodes of the coupled system -
exciton-polaritons - which allowed the observation of condensation and superfluidity in microcavity
structures.[1] Once polaritons have been put in motion in the plane of the cavity, the combination of
small mass and non-linear interactions provides a fertile ground also for investigations on all-optical
devices and circuits.[2-4] However, polariton propagation has been observed so far only in GaAs-
based microcavities at cryogenic temperature. Here we demonstrate exciton-polaritons propagating
at room temperature for long distances (≈ 150 micron) and high group velocity (50% the speed of
light) in an organic layer coupled to a Bloch Surface Wave (BSW). Crucially, this is the first time
that, in organic materials, the energy resonance of the propagating mode is controlled by increasing
or decreasing the polariton density through resonant excitation. This is of potential relevance for the
implementation of fast switches and polariton devices, but also for the effect of nonlinear
interactions in a wide range of accelerating optical wave-packets.[5]
The BSW is a non-radiative surface optical mode located within the band gap of the periodic mirror
and which, coupled to a thin layer of small organic molecules, assures long lifetime and fast group
velocities for the Bloch Surface Wave-Polaritons (BSWP).[6] The optical measurements have been
performed through leakage microscopy of the evanescent waves from the top of the structure. The
typical anticrossing behaviour is observed from the photoluminescence dispersion under non-
resonant continuous-wave excitation as shown in Fig. 1a. The blue-shift of the BSWP energy is
reversible, red-shifting for lower excitation power. In Fig. 1b the low-power (blue line) and high-
power (red line) reflectance spectra are shown. In Fig. 1c, an experimental image of propagating
BSWP is shown, with the polariton flow evidenced by the presence of few defects on the sample
surface.
References
[1] T. Byrnes, N. Y. Kim, and Y. Yamamoto, Nat. Phys. 10, 803 (2014).
[2] I. Carusotto and C. Ciuti, Rev. Mod. Phys. 85, 299 (2013).
[3] D. Ballarini, M. De Giorgi, E. Cancellieri, R. Houdré, E. Giacobino, R. Cingolani, A. Bramati,
G. Gigli, and D. Sanvitto, Nat. Comm. 4, 1778 (2013).
[4] T. C. H. Liew, A. V. Kavokin, and I. A. Shelykh, Phys. Rev. Lett. 101, 016402 (2008).
[5] P. Zhang, Y. Hu, T. Li, D. Cannan, X. Yin, R. Morandotti, Z. Chen, and X. Zhang, Phys. Rev.
Lett. 109, 193901 (2012).
[6] G. Lerario, A. Cannavale, D. Ballarini, L. Dominici, M. De Giorgi, M. Liscidini, D. Gerace, D.
Sanvitto, and G. Gigli, Optics Letters 39, 2068 (2014).
20 www.web.nano.cnr.it/CEE2D
Figure 1: a) Dispersion relation (energy vs in-plane momentum) showing the coupled BSWP (dashed white
line superimposed to the experimental data as obtained from PL under non-resonant excitation), the light line
(red line), the exciton (yellow line) and the bare BSW (green line). b) Reflectance measurements showing the
BSWP resonance at low power (blue line) and high power (red line), showing a blueshift of about 1.8 meV.
c) Real space two-dimensional experimental image of BSWP propagation. The polariton flow, propagating
downwards, is highlighted by the presence of few defects on the surface of the sample structure.
CEE2D | PISA IT | 2015 21
EVIDENCE FOR ELECTRON-HOLE PAIRING IN GRAPHENE-GAAS
DOUBLE LAYERS
A. Gamucci
1,2, D. Spirito
1,2, M. Carrega
1, B. Karmakar
1, A. Lombardo
3,
M. Bruna3, L. N. Pfeiffer
4, K. W. West
4, A. C. Ferrari
3, M. Polini
1,2,
V. Pellegrini1,2
1National Enterprise for Nanoscience and Nanotechnology (NEST), Istituto Nanoscienze-Consiglio
Nazionale delle Ricerche and Scuola Normale Superiore, I-56126 Pisa, Italy 2Istituto Italiano di Tecnologia, Graphene labs, Via Morego 30, I-16163 Genova, Italy
3Cambridge Graphene Centre, University of Cambridge, 9 JJ Thomson Avenue,
Cambridge CB3 0FA, UK 4Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08540, USA
Spatially-separated two-dimensional systems of electrons and holes are predicted to condense
below a critical temperature into a neutral superfluid state of electron-hole pairs, called “exciton
condensate”. Evidence of this scenario will be presented in systems of massless Dirac holes in a
graphene flake in close proximity of electrons hosted in a gallium arsenide quantum well. A
logarithmic enhancement of Coulomb drag at zero magnetic field and temperatures below 5 K,
which we attribute to pairing fluctuations extending above the temperature for exciton
condensation, has been found experimentally. Our Dirac-Schrödinger hybrid system offers a new
benchmark to study superfluidity in reduced spatial dimensions.
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CAVITY ENHANCED TRANSPORT OF EXCITONS AND CHARGE
J. Schachenmayer
1, C. Genes
2, E. Tignone
3, G. Pupillo
3
1JILA, NIST, Department of Physics, University of Colorado, USA 2Institut für Theoretische Physik, Universitaet Innsbruck, Austria
3University of Strasbourg and CNRS, Strasbourg, France
We show that exciton-type transport in certain materials can be dramatically modified by their
inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure
introduced by the cavity leads to transport via delocalized polariton modes rather than through
tunneling processes in the material itself. This can help overcome exponential suppression of
transmission properties as a function of the system size in the case of disorder and other
imperfections. We exemplify massive improvement of transmission for excitonic wave-packets
through a cavity, as well as enhancement of steady-state exciton currents under incoherent
pumping. These results may have implications for experiments of exciton transport in disordered
organic materials [1].
We further report on recent experiments and theory [2] on charge transport in disordered molecular
semiconductors strongly coupled to the vacuum field on plasmonic structures: the coupling to the
electromagnetic field enhances charge transport by an order of magnitude at resonance in the
coupled state. A theoretical model is presented that confirms the delocalization of the wave-
functions of the light-hybridized states and the consequences on conductivity. While this is a proof-
of-principle study, conductivity mediated by light-matter hybridized states is easy to implement and
we expect that it will be used to improve organic devices.
References
[1] J. Schachenmayer, C. Genes, E. Tignone, and G. Pupillo, Phys. Rev. Lett. 114, 196403 (2015).
[2] E. Orgiu, J. George, J. A. Hutchison, E. Devaux, J. F. Dayen, B. Doudin, F. Stellacci, C. Genet,
J. Schachenmayer, C. Genes, G. Pupillo, P. Samori, and T. W. Ebbesen, arXiv:1409.1900, Nature
Materials (in press, 2015).
CEE2D | PISA IT | 2015 23
ARTIFICIAL GRAPHENE: LATTICES IN NANO-PATTERNED
SEMICONDUCTORS*
Yuliya Kuznetsova
Dept. of Physics, Columbia University, New York, NY, USA
Charge carriers in graphene behave as massless Dirac fermions (MDFs) with a linear energy-
momentum dispersion. MDFs provide a platform for the studies of quasiparticles with relativistic-
like dispersion and of topological protection in the presence of a large spin-orbit coupling.
Engineered lattices with a honeycomb topology, called artificial graphene (AG), form a condensed-
matter platform for exploration of the new physics that emerges due to the tunable interplay
between topology and quasiparticle interactions. In the research reported here, artificial graphene is
realized by superimposing a small lattice constant (in this work, as small as 40 nm) honeycomb
potential on a two-dimensional electron gas hosted by a GaAs quantum well.
Optical methods form an effective probe of the band structure and other properties of electrons in
the engineered potential. Resonant inelastic light scattering spectra reveal transitions in artificial
graphene that are interpreted in terms of joint density for transitions between the AG bands. The
agreement of observed transitions with calculated joint density of states of the AG band structure
offers evidence of the occurrence of Dirac bands and MDFs in the artificial lattice. The ability to
create small-period artificial graphene lattices in semiconductors opens new, highly tunable
pathways for the exploration of fundamental condensed matter science, including the possibility of
access to novel topological phases in semiconductors with strong spin-orbit coupling.
This work is done in collaboration with: Sheng Wang, Diego Scarabelli, Loren N. Pfeiffer, Ken
West, Geoff C. Gardner, Michael J. Manfra, Vittorio Pellegrini, Shalom J. Wind, and Aron Pinczuk.
(*) Supported by the US Department of Energy, Basic Energy Sciences award DE-SC0010695.
24 www.web.nano.cnr.it/CEE2D
LOOKING FOR THE DARK EXCITON CONDENSATE
François Dubin
ICFO-The Institute of Photonic Sciences, Av. Carl Friedrich Gauss, num. 3,
08860 Castelldefels (Barcelona), Spain
Predicted in the 1960’s, Bose-Einstein condensation (BEC) of semiconductor excitons remains an
open question both theoretically and experimentally. This situation was not foreseen since excitons
are light mass quasi-particles made by a Coulomb bound electron-hole pair. BEC is thus expected to
occur below a few Kelvins, i.e. directly accessible by standard cryogeny. Nevertheless, it has so far
not be signalled by the long expected strong photoluminescence emitted by a condensed population
of low energy bright excitons. Indeed, recent theoretical works have pointed out that the excitons
ground-state being optically inactive, i.e. dark, BEC has then to be signalled by a macroscopic
population of dark excitons [1]. These can naturally not be directly observed optically, nevertheless
here we report experiments that provide strong evidence for the dark state condensation of excitons.
These experiments are performed in a microscopic trap where the overall exciton density is
controlled deterministically while the bath temperature is varied. Thus, quantum statistical
signatures are resolved and the quantum phase transition is mapped [2].
References
[1] M. Combescot et al., Phys. Rev. Lett. 99, 176403 (2007).
[2] M. Beian et al., arXiv:1506.08020 (2015).
CEE2D | PISA IT | 2015 25
QUANTUM FLUID DYNAMICS OF POLARITON CONDENSATES
Daniele Sanvitto
NNL, Istituto di Nanotecnologie, via per Arnesano, Lecce (Italy)
Polaritons are one of the most striking quasi-particles, made of light and matter, which have
recently demonstrated phase transition to a Bose-Einstein condensate state in semi-conductors [1].
The physics of these objects has been investigated with several impressive achievements, from
superfluidity [2], lasing at room temperature [3], solitons [4], optical logic [5], black holes [6], and
many others. Here we will discuss many aspects related to polariton condensation with the
peculiarity to exhibit out of thermal equilibrium. In particular thanks to the relative ease for
generation and detection of polariton states we will get insight on the intrinsic dual nature of its
photon and exciton components and their oscillating behaviour under specific excitation conditions
[7,8]. Using ultrafast techniques we can observe the dynamics of a quantum fluid under direct
injection, which will span from vortices of different spin nature to the appearance of a surprising
diffraction-limited self-localisation of the polariton condensate–independent of the pump
dimension–similar to the backjet of a water droplet splashing into a pond [9,10]. Interestingly, a
similar self-focusing effect was also observed under non-resonant excitation on the top of a
potential hill, followed by a ballistic expansion of a multimode condensate outside of the excitation
region.
These results show that much is left to explore in the high-density and ultrafast dynamics of
polaritons, with a striking and unique phenomenology that could open new areas of research and
applications not accessible to their atomic counterparts.
References
[1] J. Kasprzak et al., Nature 443, 409 (2006).
[2] A. Amo et al., Nature 457, 291 (2009).
[3] P. Bhattacharya et al., Phys. Rev. Lett. 112, (2014).
[4] M. Sich et al., Nat. Photon. 6, 50 (2011).
[5] D. Ballarini et al., Nat. Commun. 4, 1778 (2013).
[6] H. S. Nguyen et al., Phys. Rev. Lett. 114, 036402 (2015).
[7] L. Dominici et al., Phys. Rev. Lett. 113, 226401 (2014).
[8] D. Colas et al., Light: Science & Applications (2015).
[9] L. Dominici et al., ArXiv13093083.
[10] L. Dominici et al., ArXiv14030487.
26 www.web.nano.cnr.it/CEE2D
CONFOCAL SHIFT INTERFEROMETRY OF COHERENT EMISSION
FROM TRAPPED DIPOLAR EXCITONS
U. Wurstbauer
1,2, J. Repp
1,2,3, G. J. Schinner
2,3, E. Schubert
2,3, A. K. Rai
4, D. Reuter
4,5, A. D.
Wieck4, J. P. Kotthaus
3, and A. W. Holleitner
1,2
1Walter Schottky Institut and Physik-Department, Am Coulombwall 4a, Technische Universität
München, D-85748 Garching, Germany 2Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München, Germany
3Center for NanoScience and Fakultät für Physik, Ludwig-Maximilians-Universität, Geschwister-
Scholl-Platz 1, 80539 München, Germany 4Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße 150,
44780 Bochum, Germany 5Department Physik, Universität Paderborn, 33098 Paderborn, Germany
In coupled double quantum wells (CDQW) photogenerated and spatially indirect excitons can be
efficiently manipulated via gate-voltage-induced control of the quantum confined Stark Effect
(QCSE). On an InGaAs-GaAs CDQW heterostructure we have realized electrostatically widely
tunable trapping devices for dipolar indirect excitons. Utilizing the QCSE resonantly excited direct
excitons transform into indirect excitons and are collected via electrostatically shaped energy
landscapes. With their electron and hole confined to two different quantum wells, these indirect
excitons exhibit a large dipole moment and long lifetimes of about 100 ns. We introduce a confocal
shift-interferometer based on optical fibers [1]. The presented spectroscopy allows measuring
coherence maps of luminescent samples with a high spatial resolution even at cryogenic
temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a
semiconductor double quantum well. We find that the measured spatial coherence length of the
excitonic emission coincides with the point spread function of the confocal setup. The results are
consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.
Reference
[1] J. Repp et al., Applied Physics Letters 105, 241101 (2014).
CEE2D | PISA IT | 2015 27
ULTRALOW ENERGY SWITCHING OF FERROMAGNETIC
CONDENSATES IN SEMICONDUCTOR MICROCAVITIES
A. Dreismann
1*, H. Ohadi
1, Y. G. Rubo
2, Y. del Valle-Inclan Redondo
1, S. I. Tsintzos
3,
Z Hatzopoulos3, P. G. Savvidis
1,3, and Jeremy J. Baumberg
1*
1Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE,
United Kingdom, [email protected], [email protected] 2Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos,
62580, Mexico 3Foundation for Research and Technology–Hellas, Institute of Electronic Structure and Laser,
71110 Heraklion, Crete, Greece
A key requirement for the implementation of polariton-based spin-optotronic devices is the
electrical control of polariton spins. While the flow of polariton condensates can be guided using
all-optical methods, electrical fields or semiconductor post-processing,[1] the manipulation of
polariton spins is still restricted to optical means. Here we demonstrate for the first time the ability
to control the spin of a polariton condensate with external electric fields [2].
Fig. 1: (a) Illustration of a trapped polariton condensate: four linearly polarised laser beams induce a
potential landscape, giving rise to a trapped condensate at the centre of the excitation pattern. (b) In the
absence of an external electric field, the condensate stochastically adopts left- or right-circular polarisation
for each realisation of the experiment. If a field is applied, the condensate polarisation changes depending on
the magnitude and the polarity of the field. (c) Bias-pulses are employed to realise an ultra-low energy
optoelectronic spin-switch.
We create trapped condensates that are spatially separated from the excitonic reservoir by
patterning the optical excitation into four laser spots, giving rise to a confining potential (Fig. 1a).
As shown recently [3], this geometry allows the observation of a variety of different polarisation
states under linear non-resonant excitation, ranging from the pinned linear polarisation reported in
the literature to strong stochastic circular polarisation. By applying an electrical field perpendicular
to the quantum-well plane we are able precisely tune the polarisation of the condensate emission
(Fig. 1b), demonstrating the direct electrical control of the condensate spin. We utilise this
phenomenon to realise an electrical spin-switch, operating at record ultra-low switching energies of
order attojoules (Fig. 1c) and switching speeds that are only limited by the condensate dynamics.
References
[1] P. Tsosis et. al., Phys. Rev. Applied 2, 014002 (2014).
[2] A. Dreismann et al., Nat.Phys. Science (2015).
[3] H. Ohadi et. al., Phys. Rev. X 5, 031002 (2015).
28 www.web.nano.cnr.it/CEE2D
SPIN COHERENCE IN COUPLED QUANTUM WELLS AMENDED BY
IN-PLANE MAGNETIC FIELD
P. Andreakou1, A. Mikhailov, S. Cronenberger
1, D. Scalbert
1, A. Nalitov
2, N. A. Gippius
3, A. V.
Kavokin4, M. Nawrocki
5, L. V. Butov
6, K. L. Campman
7,
A. C. Gossard7, and M. Vladimirova
1*
1Laboratoire Charles Coulomb, UMR 5221 CNRS/ Université de Montpellier, France
2Institut Pascal, PHOTON-N2, Université Blaise Pascal, CNRS, Aubière Cedex, France
3Skolkovo Institute of Science and Technology, Skolkovo, Russia
4School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom
5Institute of Experimental Physics, Warsaw University, Poland
6Department of Physics, University of California at San Diego, USA
7Materials Department, University of California at Santa Barbara, USA
Semiconductor coupled quantum wells (CQWs) offer an excellent laboratory for studying
both intra-well direct excitons (DX) and inter-well indirect excitons (IX), as well as their
interactions and spin dynamics. The presentation will focus on the pump-probe spectroscopy,
a powerful tool of nonlinear optics, that we have applied to biased CQWs. We will show how
DX and IX spin and population dynamics, as well as the spin polarization of residual
electrons may be detected via the modulation of reflectivity and Kerr rotation spectra [1, 2].
Spin relaxation and decoherence mechanisms for DX, IX and electron gas will be discussed.
An unusual enhancement of the exciton spin coherence time with in-plane magnetic field in
unbiased CQWs will be presented.
References
[1] A. V. Nalitov, M. Vladimirova, A. V. Kavokin, L. V. Butov, and N. A. Gippius, Phys. Rev. B
89, 155309 (2013).
[2] P. Andreakou, S. Cronenberger, D. Scalbert, A. Nalitov, N. A. Gippius, A. V. Kavokin, J. R.
Leonard, L. V. Butov, K. L. Campman, A. C. Gossard, and M. Vladimirova, Phys. Rev. B 91,
125437 (2015).
CEE2D | PISA IT | 2015 29
ENGINEERING TOPOLOGICAL QUANTUM STATES: FROM 1D TO 2D
Jelena Klinovaja
Department of Physics, University of Basel, Klingelbergstrasse 82, Basel, CH-4056 Basel,
Switzerland
I will present results on exotic bound states in one-dimensional (Majorana fermions and
parafermions) and two-dimensional (edge states in topological insulators) condensed matter systems
that have attracted wide attention due to their promise of non-Abelian statistics believed to be useful
for topological quantum computing. I discuss systems in which topological properties could be
engineered per demand. For example, Majorana fermions can emerge in hybrid systems with
proximity pairing in which the usually weak Rashba spin-orbit interaction is replaced by magnetic
textures. Here, I will discuss candidate materials such as semiconducting nanowires [1-2], graphene
nanoribbons [3], atomic magnetic chains or magnetic semiconductors [4]. One further goal is to go
beyond Majorana fermions and to identify systems that can host quasiparticles with more powerful
non-Abelian statistics such as parafermions in double wires coupled by crossed Andreev reflections
[5,6]. In the second part of my talk, I will focus on 'strip of stripes model' consisting of weakly
coupled one-dimensional wires [6-8], where interaction effects in the wires can be treated non-
perturbatively via bosonization. I will demonstrate that such systems can exhibit the integer or
fractional quantum Hall effect [6], spin Hall effect [7], and anomalous Hall effect [8]. In the
fractional regimes, the quasiparticles have fractional charges and non-trivial Abelian braid statistics.
References
[1] J. Klinovaja and D. Loss, Phys. Rev. B 86, 085408 (2012).
[2] D. Rainis, L. Trifunovic, J. Klinovaja, and D. Loss, Phys. Rev. B 87, 024515 (2013).
[3] J. Klinovaja and D. Loss, Phys. Rev. X 3, 011008 (2013); J. Klinovaja and D. Loss, Phys. Rev.
B 88, 075404 (2013).
[4] J. Klinovaja, P. Stano, A. Yazdani, and D. Loss, Phys. Rev. Lett. 111, 186805 (2013).
[5] J. Klinovaja and D. Loss, Phys. Rev. B 90, 045118 (2014).
[6] J. Klinovaja, A. Yacoby, and D. Loss, Phys. Rev. B 90, 155447 (2014).
[7] J. Klinovaja and D. Loss, Phys. Rev. Lett. 111, 196401 (2013); J. Klinovaja and D. Loss, Eur.
Phys. J. B 87, 171 (2014).
[8] J. Klinovaja and Y. Tserkovnyak, Phys. Rev. B 90, 115426 (2014).
[9] J. Klinovaja, Y. Tserkovnyak, and D. Loss, Phys. Rev. B 91, 085426 (2015).
30 www.web.nano.cnr.it/CEE2D
TRANSPORT OF INDIRECT EXCITONS IN COUPLED QUANTUM
WELLS:BASIC PHYSICS AND DEVICES
Peristera Andreakou
Laboratoire de Photonique et Nanostructures, CNRS, Route de Nozay, 91460 Marcoussis, France
An indirect exciton (IXs) is a bound pair of an electron and a hole, which are located in separated
layers of a heterostructure. The spatial overlap of electron and hole wavefunctions is controlled by
the voltage applied to the top design and ground plane electrode. Therefore, the lifetimes of indirect
excitons can exceed those of direct excitons by orders of magnitude. Long lifetimes allow IXs to
travel over sufficiently large distances to accommodate devices as well as to cool down to lattice
temperature for the observation of condensed matter phenomena.
My talk focuses on recent advances in a variety of excitonic devices based on IXs. The first part of
my talk explores devices based on a coupled quantum well system. These devices take advantage of
the controllable transport of IXs through modulated electrostatic landscapes. Excitonic diodes,
transistors and routers are presented [1]. Such devices promise to effectively eliminate the time
delay between signal processing and optical communication, a significant advantage over electronic
transistors. My presentation also includes the development of devices such as electrostatic traps and
stirring potentials that allow for studies of exciton condensates analogous of atomic condensates but
much simpler to achieve in a lab [2, 3].
A different system for the study of IXs is CdSe/CdS colloidal heterostructures. These quantum rods
are utilized as gain medium to realize a single mode, single exciton whispering gallery mode
microlaser [4].
References
[1] P. Andreakou, S. V. Poltavtsey, J. R. Leonard, E. V. Calman, M. Remeika, Y. Y. Kuznetsova, L.
V. Butov, J. Wilkes, M. Hanson, A. C. Gossard, Appl. Phys. Lett. 104, 091101 (2014).
[2] Y. Y. Kuznetsova, P. Andreakou, M. W. Hasling, J. R. Leonard, E. V. Calman, C. J. Dorow, L.
V. Butov, M. Hanson, A. C. Gossard, Optics Letters 40, 589 (2015).
[3] M. W. Hasling, Y. Y. Kuznetsova, P. Andreakou, J. R. Leonard, E. V. Calman, C. Dorow, L. V.
Butov, M. Hanson, A. C. Gossard, Journal of Appl. Phys. 117, 023108 (2015).
[4] C. Grivas, C. Li, P. Andreakou, P. Wang, M. Ding, G. Brambilla, L. Manna, P. G. Lagoudakis,
Nature Communication 4, 2376 (2013).
CEE2D | PISA IT | 2015 31
MANIPULATION OF SPIN TRANSFER TORQUE USING LIGHT
K. L. Vendelbjerg
1, L. J. Sham
2, and M. Rontani
1
1CNR-NANO, Modena, Italy
2Dept of Physics, University of California San Diego, California
We show that the spin transfer torque induced by a spin-polarized current on a nanomagnet as the
current flows through a semiconductor-nanomagnet-semiconductor junction is externally controlled
by shining the junction off-resonantly with a strong laser beam. The excitonic coherence driven by
the laser dresses the virtual electron-hole pairs coupling conduction and valence bands and inducing
an evanescent state in the proximity of the nanomagnet. The Fano-like quantum interference
between this localized state and the continuum spectrum is different in the two spin channels and
hence it dramatically alters the spin transport, leading to the coherent control of the spin transfer
torque.
This work is supported by EU-FP7 Marie Curie Initial Training Network INDEX.
32 www.web.nano.cnr.it/CEE2D
BOSE-EINSTEIN CONDENSATION IN A POLYMER: TOWARDS
QUANTUM SIMULATION
Johannes Plumhof
1, Lijian Mai
1, Thilo Stöferle
1, Ullrich Scherf
2, and Rainer F. Mahrt
1
1IBM Research Zurich, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland
2Bergische Universität Wuppertal,Fachgebiet Makromolekulare Chemie, Gaußstraße 20,
42119 Wuppertal, Germany
During recent years polaritonics has emerged as a new field of solid-state physics based on the
unique quantum properties of mixed light-matter quasiparticles, so called exciton-polaritons. Recent
discoveries of Bose-Einstein condensation (BEC) and superfluidity provide opportunities to harness
these coherent quantum effects in a new generation of opto-electronic devices. Until now, BECs
have been realized either with laser-cooled gases at nano-Kelvin temperatures or with high-quality
semiconductor crystals produced by only a few laboratories worldwide. By utilizing the extremely
large oscillator strength, exciton binding energy and saturation density of organic semiconductors
we demonstrate BEC at room temperature with an amorphous spin-coated polymer film embedded
in a Fabry-Pérot microcavity. Since no crystal growth is involved, our approach radically reduces
the complexity of experiments to investigate BEC physics and paves the way for a new generation
of opto-electronic devices, taking advantage of the processibility and flexibility of polymers.
Finally, experiments on sub-micron sized defect cavities and possible ways
CEE2D | PISA IT | 2015 33
ELECTRONIC TRANSPORT AND OPTICAL PROPERTIES OF
ELECTRICALLY-GENERATED ELECTRON-HOLE BILAYERS WITH
MINIMUM INTER-LAYER SEPARATION
U. Siciliani de Cumis1, J. Waldie
1, A. F. Croxall
1, J. Llandro, H. E. Beere
1, I. Farrer
1, D. A.
Ritchie1, A. Perali, D. Neilson
2, R. Anankin
3, M. Beian
3, F. Dubin
3, I. Aliaj
4,
T. Satzoukidis4, and V. Pellegrini
4
1Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE,
United Kingdom 2University of Camerino, Madonna delle Carceri, 9, 62032, Camerino (MC), Italy
3INP, Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France
4NEST, Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy
The sustained improvement in semiconductor growth techniques over the past 30 years has allowed
electrons and holes to be confined in layers so thin that they can be considered as quasi two-
dimensional systems. Further interesting and more complex possibilities arise when electrons and
holes are confined in two adjacent 2D layers kept apart by distance of few nanometres so that
particle recombination is hindered by the presence of a potential barrier. The behaviour of the
particles in such electron-hole bilayers is the product of the relative contributions of inter-layer and
intra-layer Coulomb interactions. In particular, when the distance between the layers is comparable
to the average separation between particles in the same layer, novel phases are predicted to emerge
like a supersolid Wigner crystal [1] or an excitonic superfluid phase of indirect excitons [2] with a
BEC to BCS crossover [3]. Indeed, a BEC of electron-hole pairs has been recently observed in an
optically-generated electron-hole bilayer system [4].
Electrically-generated and independently-contacted electron-hole bilayers have been intensively
studied in the recent past, with a particular focus on transport measurements, such as the Coulomb
drag technique, which is used to probe directly the inter-layer interactions [5], [6]. We present
Coulomb drag measurements of electron-hole bilayers generated in a GaAs/AlGaAs double
quantum well structure with an interlayer barrier thickness of just 5 nm, the narrowest ever realised
in such a system. Recent theoretical calculations suggest that our systems is tantalisingly close to
the transition to an electron-hole superfluid state [7]. We discuss the evidence, in both the Coulomb
drag and the sheet resistivities, for electron-hole pairing correlations in these bilayers and present
some recent studies focusing on the optical properties of these structures.
References
[1] Y. Joglekar, A. Balatsky, and S. Das Sarma, Phys. Rev. B 74, 233302 (2006).
[2] Y. E. Lozovik, and V. I. Yudson, J.E.T.P. 44 (1976).
[3] P. Pieri, D. Neilson, and G. Strinati. Phys. Rev. B 75, 113301 (2007).
[4] A. A. High, J. R. Leonard, M. Remeika, L.V. Butov, M. Hanson, and A. C. Gossard, Nano Lett.
12, 2605 (2012).
[5] A. F Croxall, K. Das Gupta, C. Nicoll, M. Thangaraj, H. E. Beere, I. Farrer, D. A. Ritchie, and
M. Pepper, Phys. Rev. Lett. 101, 246801 (2008).
[6] J. A. Seamons, C. P. Morath, J. L. Reno, and M. P. Lilly. Phys. Rev. Lett. 102, 026804 (2009).
[7] D. Neilson, A. Perali, A.F. Croxall, to be published.
34 www.web.nano.cnr.it/CEE2D
ADIABATIC PREPARATION OF A COLD EXCITON CONDENSATE
V. Shahnazaryan
1,2, O. Kyriienko
3, and I. Shelykh
1,4,5
1Science Institute, University of Iceland, Dunhagi-3, IS-107, Reykjavik, Iceland
2Institute of Mathematics and High Technologies, Russian-Armenian (Slavonic) University,
Hovsep Emin 123, 0051, Yerevan, Armenia 3Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, DK-2100 Copenhagen, Denmark 4Division of Physics and Applied Physics, Nanyang Technological University 637371, Singapore
5ITMO University, St. Petersburg 197101, Russia
We propose a scheme for the controllable preparation of a cold indirect exciton condensate by
means of optical pump based on Landau-Zener bosonic transfer in a dipolariton system.
Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C),
direct exciton (DX), and indirect exciton (IX) modes, and appear in a double quantum well
embedded in a semiconductor microcavity (see Fig.1). Controlling the detuning between modes of
the system, the limiting cases of exciton-polaritons and indirect excitons can be realized.
The protocol is based on several steps. First stage corresponds to initial preparation of polariton
condensate with high cavity photon and direct exciton fractions, while indirect exciton mode is
located high in energy at zero external voltage (Fig. 2a). Next, applying electric field the IX energy
is lowered to far red-detuned value, where adiabatic following of the lower dipolariton mode
converts particles to indirect excitons with inherited coherence properties (Fig. 2b). The following
allows for generation of a spatially localized cold exciton gas, on the contrary to currently used
methods, where IX cloud appears due to diffusion of carriers from spatially separated electron- and
hole-rich areas.
Finally, to reduce residual effects of cavity an optical incoherent pump of polaritonic reservoir
states shall be switched off during the transfer event. We analyzed the population transfer for
various sets of parameters and switching conditions, and demonstrated that adiabatic cold exciton
preparation is experimentally feasible in currently existing setups.
The main results of investigation were published in the paper [1].
Figure 1. (a): Sketch of the dipolaritonic system, representing double quantum well (DQW) structure
embedded in a microcavity formed by distributed Bragg reflectors (DBRs). (b): Schematic representation of
dispersion of lower (LP), middle (MP), and upper (UP) dipolaritons. Incoherent optical pump creates carriers
at high energy, which relax to LP reservoir, and consequently scatter to a macroscopically coherent ground
state.
CEE2D | PISA IT | 2015 35
Figure 2. (a): Time dependence of energies of the modes. The bias applied to the system causes linear
decrease of IX energy in t = 1 ns to t = 1.5 ns window, up to far red-detuned value. The dashed red line
corresponds to time dependence of the pump intensity (in arbitrary units). (b): The evolution of occupations
of the modes, where NC, NDX, NIX denote the occupations of cavity, direct exciton, and indirect exciton
modes, correspondingly. At the first stage (t < 1 ns) the formation of polariton condensate takes place. Next,
continuous change of an applied bias drives the system through an avoided crossing, leading to the transfer
of polariton occupation to an indirect exciton mode.
Reference
[1] V. Shahnazaryan, O. Kyriienko, and I. Shelykh, Physical Review B 91, 085302 (2015).
36 www.web.nano.cnr.it/CEE2D
COLLECTIVE NON-LOCAL TRANSPORT IN GRAPHENE
HETEROSTRUCTURES
Marco Polini
Istituto Italiano di Tecnologia, Graphene Labs, Via Morego 30, I-16163, Genova (Italy)
NEST, Scuola Normale Superiore, I-56126, Pisa (Italy)
Graphene sheets encapsulated in thin slabs of hexagonal boron nitride display micrometer-scale
ballistic transport [1] and ultra-high room-temperature mobilities [2,3], which are solely limited by
graphene acoustic phonons [4]. In this talk I will first try and convince you that transport at
sufficiently high temperatures in such samples can be described by the theory of hydrodynamics
[5,6], which is non-local and non-linear. I will then present results of recent combined experimental
and theoretical work in which the hydrodynamic shear viscosity of the massless Dirac fermion
liquid hosted by the encapsulated graphene sheet is accessed through non-local dc transport
experiments in a multi-terminal Hall bar [7,8]. I will conclude by highlighting the implications of
hydrodynamic flow for all-electrical detection of graphene Terahertz plasmons [9].
References [1] A.S. Mayorov et al., Nano Lett. 11, 2396 (2011).
[2] L. Wang et al., Science 342, 614 (2013).
[3] A. Woessner et al., Nature Mater. 14, 421 (2015).
[4] A. Principi, M. Carrega, M.B. Lundeberg, A. Woessner, F.H.L. Koppens, G. Vignale, and M.
Polini, Phys. Rev. B 90, 165408 (2014).
[5] L.D. Landau and E.M. Lifshitz, Course of Theoretical Physics: Fluid Mechanics (Pergamon,
New York, 1987).
[6] A. Tomadin, G. Vignale, and M. Polini, Phys. Rev. Lett. 113, 235901 (2014).
[7] D. Bandurin, I. Torre, R.K. Kumar, M. Ben Shalom, A. Tomadin, A. Principi, G.H. Auton, E.
Khestanova, K.S. NovoseIov, I.V. Grigorieva, L.A. Ponomarenko, A.K. Geim, and M. Polini,
submitted.
[8] I. Torre, A. Tomadin, A.K. Geim, and M. Polini, arXiv:1508.00363.
[9] I. Torre, A. Tomadin, R. Krahne, V. Pellegrini, and M. Polini, Phys. Rev. B 91, 081402(R)
(2015).
CEE2D | PISA IT | 2015 37
TIME-DEPENDENT QUANTUM DYNAMICS OF 2D SPATIALLY
INDIRECT EXCITONS
Federico Grasselli1,2
, Andrea Bertoni2, and Guido Goldoni
1,2
1Dept. of Physics, Informatics and Mathematics, Univ. of Modena and Reggio Emilia
2CNR-NANO S3, Via Campi 213/a, Modena, Italy
We study the quantum dynamics of a single spatially indirect exciton (IX) in prototypical potential
landscapes. The internal relative quantum dynamics of the correlated electron-hole pair is fully
taken into account. The two particles, localized in different layers, can be independently gated. The
quantum evolution of the electron-hole pair is then realized by means of a numerical solver of the
time dependent Schroedinger equation, based on the split-step Fourier method, which relies on a
Suzuki-Trotter factorization of the evolution operator [1]. Such method is numerically exact,
unitary, and can be also extended to possibly time dependent external potentials, no matter their
strength or range.
On the basis of the results obtained from a minimal 1D model [2], for which we observed a wide
variety of typical two-particle scattering processes (dissociation of the IX, excitation to higher
internal levels, dwelling around the potential edge, periodic transmission of the IX wave packet),
we developed a more quantitative and realistic 2D-, i.e. 4-degree-of-freedom- (4DoF), model. A
massive parallelization of the code has been required in order to cope with the huge amount of data
and operation that a 4DoF quantum problem require. We performed simulations of scattering
events, considering different potential energy configurations and incidence angles, in order to have
a deeper insight into such a composite-quantum-particle- collision event, and to emphasizing the
main differences between the 4DoF model and the ‘’rigid IX’’ one, often employed in the literature,
for which the IX relative motion is frozen in its ground state [Figure].
Figure: Scattering on electron potential well. Comparison between the rigid IX model (left) and the fully
4DoF one (right) at the same time, t=12ps. Centre of mass probability density (green-scale), and external
potential are shown.
References
[1] A. Bertoni, J. Comput. Electron. 2, 291 (2003).
[2] F. Grasselli, A. Bertoni, and G. Goldoni, J. Chem. Phys. 142, 034701 (2015).
38 www.web.nano.cnr.it/CEE2D
ULTRAFAST DYNAMICS IN LIGHT-HARVESTING AND
PHOTOVOLTAICS: A THEORETICAL AND EXPERIMENTAL
INVESTIGATION
Carlo Andrea Rozzi
CNR NANO S3, via G. Campi 213A, 41125 Modena, Italy
It is known that the primary steps of photoinduced energy and charge transfer may occur on
extremely fast time scales in many natural and man-made compounds that perform conversion of
sunlight into chemical or electrical energy. These processes have traditionally been interpreted in
terms of the incoherent kinetics of optical excitations and of charge hopping, but recently signatures
of quantum coherence were observed in energy transfer in photosynthetic bacteria and algae [1,2].
We have studied the early steps of photoinduced charge separation in reference systems for artificial
photosynthesis and photovoltaics by combining Time-dependent Density Functional Theory
simulations of the quantum dynamics and high time resolution femtosecond spectroscopy. Our
results show that the coherent coupling between electronic and nuclear degrees of freedom is of key
importance for charge delocalization and transfer in both of covalently and non-covalently bonded
systems [3,4]. We have exploited the results of our research to design, synthesize and characterize a
novel molecular scaffold for photovoltaic applications [5].
References
[1] G. S. Engel et al., Nature 446, 782-786 (2007).
[2] E. Collini et al., Nature 463, 644-647 (2010).
[3] C. A. Rozzi, S. Falke, et al., Nat. Comm. 4, 1602 (2013).
[4] S. Falke, C. A. Rozzi, et al., Science, 344, 1001 (2014).
[5] S. Pittalis et al., Adv. Func. Mat. (2014).
CEE2D | PISA IT | 2015 39
EXCITATION ENERGY TRANSFER IN HYBRID NANO-SYSTEMS BY A
MULTI-SCALE BEYOND-FORSTER APPROACH
G. Gil1,2
, A. Delgado1, A. Bertoni
1, G. Goldoni
1, and S. Corni
1
1CNR Institute for Nanosciences, Center S3, Modena, Italy
2Department of Physics, Informatics and Mathematics,
University of Modena and Reggio Emilia, Modena, Italy
We developed a new multi-scale approach to treat excitation energy transfer in hybrid nano-systems
composed by an organic molecule nearby a semiconductor nanoparticle, a class of nano-materials of
growing relevance in diverse fields, from artificial photosynthesis to nanomedicine. Since such
systems may be composed of millions of atoms, their optical excitations are often described by ab-
initio methods selecting a small active optical center, comprising the molecule and a cluster of
tens/hundreds of semiconductor atoms. However, this may not be appropriate in many situations,
e.g., when the optical excitations of the molecule are resonant with those of the nanoparticle, which
is extended over the entire nano-system.
In our method, optical excitations of each subsystem is accurately described within a quantum-
mechanical approach at the appropriate level of description, that is a state-of-the-art Time-
Dependent Density Functional Theory description for the molecule and an accurate -yet semi-
empirical- envelope-function based Configuration Interaction description of excited electron-hole
pairs (excitons) for the nanoparticle. Energy transfer from one subsystem to the other is described
here by a beyond-Förster [1] approach which considers the interaction between all the transition
multipole moments of the nanoparticle and the transition dipole moment of the molecule, an
approximation which is accurate for molecule-nanoparticle distances a few times the molecular
size. This method allows for an accurate description of the dynamical correlations inside each
segment.
This novel approach is applied to a case study relevant to photodynamic therapy, where the
molecule is a free-base porphyrin and the nanoparticle is a core/shell CdSe/ZnS semiconductor
quantum dot [2].
References [1] D.L. Andrews, C. Curutchet, G.D. Scholes, Laser Photonics Rev. 5 1, 114-123 (2011).
[2] J.M. Tsay, M. Trzoss, L. Shi, X. Kong, M. Selke, M.E. Jung, and S. Weiss, J. Am. Chem. Soc.
129, 6865-6871 (2007).
40 www.web.nano.cnr.it/CEE2D
INDEX
PROGRAM
POSTER SESSION
ABSTRACTS OF TALKS
ABSTRACTS OF POSTERS
CEE2D | PISA IT | 2015 41
ROOM-TEMPERATURE RECTIFICATION IN GRAPHENE/LAO/STO
HETEROSTRUCTURES
I. Aliaj
1, E. di Gennaro
2, V. Miseikis
3, I. Torre
1, A. Gamucci
4,3, C. Coletti
3,4, M. Polini
1,
F. M. Granozio2, V. Pellegrini
4, F. Beltram
1, and S. Roddaro
1
1NEST, Istituto Nanoscienze - CNR and Scuola Normale Superiore, I-56126 Pisa, Italy
2CNR-SPIN and Dipartimento di Fisica, Complesso Universitario di Monte S.Angelo, Via Cintia,
80126 Naples, Italy 3Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San
Silvestro 12, 56127 Pisa, Italy 4Istituto Italiano di Tecnologia, Graphene labs, Via Morego 30, I-16163 Genova, Italy
Two-dimensional electron systems (2DES) embedded in solid state materials are central to modern
electronics and to fundamental as well as applied research in condensed matter physics. Two recent
prominent examples are the chiral Dirac fermions in graphene [1] and the emerging 2DES at the
interface between the bulk insulating oxides LaAlO3/SrTiO3 (LAO/STO) [2], exhibiting multiple
electric field-tunable properties, such as superconductivity [3], magnetism [4] and spin-orbit
coupling [5].
Vertical heterostructures combining the two materials, in analogy to graphene/GaAs
heterostructures [6], are expected to display a strong interlayer Coulomb coupling that can drive
novel collective phases, and promise interesting device applications [7].
In this poster, I will illustrate our results on the room-temperature electrical characterization of
high-quality CVD graphene monocrystals transferred on a LAO/STO substrate. We demonstrate the
capability of the interfacial 2DES in LAO/STO to tune the graphene chemical potential across the
Dirac point, and show that the effect is dominated by graphene’s quantum capacitance.
Furthermore, in contrast with Metal/LAO/STO systems [8], we find a pronounced rectifying
behaviour in the room temperature electrical transport between graphene and the LAO/STO 2DES.
Our results can shed light on the fundamental physics of these novel material systems and open up
possible future applications.
References
[1] A. H. Castro Neto et al., Rev. Mod. Phys. 81, 109 (2009).
[2] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004).
[3] N. Reyren et al., Science 317, 1196 (2007).
[4] A. Brinkman et al., Nat. Mater. 6, 493 (2007).
[5] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010); M. Ben Shalom et al., Phys. Rev. Lett.
104 (12), 126802 (2010).
[6] A. Gamucci et. al., Nature Commun. 5, 5824 (2014).
[7] M. Huang et al., APL Materials 3, 062502 (2015).
[8] G. Singh-Bhalla et al., Nature Phys. 7, 80 (2010).
42 www.web.nano.cnr.it/CEE2D
MACROSCOPIC OCCUPATION OF DARK EXCITONIC STATES
IN A TRAP
Mussie Beian1, Mathieu Alloing
2, Romain Anankine
2, Edmond Cambril
3,
Carmen Gomez Carbonell3, Aristide Lemaître
3, and François Dubin
1,2
1ICFO- The Institute of Photonic Sciences, Av. Carl Friedrich Gauss, num. 3, 08860 Castelldefels
(Barcelona), Spain 2Institut des Nanosciences de Paris, UPMC Paris 06, CNRS UMR 7588, 4 pl. Jussieu, 75005 Paris,
France 3Laboratoire de Photonique et Nanostructures, LPN/CNRS, Route de Nozay, 91460 Marcoussis,
France
We present recent studies of Bose-Einstein condensation of spatially indirect excitons confined in a
microscopic 10 µm wide electrostatic trap [1]. We show that trapped excitons have a
photoluminescence which drops suddenly below 2 K as the indirect excitons temperature is
lowered. Our observations realized at a fixed density are then interpreted as a manifestation for the
condensation of excitons in the lowest energy optically inactive (dark) states.
We create spatially indirect excitons in a double quantum well where valence holes and conduction
electrons are spatially separated using an external electric field tilting the energy bands. Indirect
excitons are then characterized by their giant electric dipole moment while electrons and holes have
their wavefunctions which overlap weakly. The recombination between them is then damped
compared to direct excitons such that indirect excitons exhibit a radiative life-time increased up to
70 ns.
While cooling indirect excitons from 3.5 to 0.33 K, we show that the photoluminescence integrated
intensity decreases long after the laser pulse, i.e. when the trapped gas is well thermalized. At the
same time, we observe that the decay time of the photoluminescence decreases thus showing that
the excitons temperature is lowered [2]. Our experiments being realized at a fixed density in the
trap, these combined observations lead us to conclude that bright indirect excitons are strongly
depleted below a critical temperature ~2 K [3], inducing the theoretically predicted macroscopic
occupation of the lowest energy dark states [4].
References
[1] M. Beian et al., Europhys. Lett. 110, 270001 (2015).
[2] A. Ivanov et al., Phys. Rev. B 59, 5032 (1999).
[3] M. Beian et al., arXiv:1506.08020 (2015).
[4] M. Combescot, O. Betbedet-Matibet, and R. Combescot, Phys. Rev. Lett. 99, 176403 (2007).
CEE2D | PISA IT | 2015 43
LATERAL TRANSPORT OF INDIRECT EXCITON SPINS IN DOUBLE
QUANTUM WELL STRUCTURES
S. Büyükköse
1, C. Hubert
1, A. Violante
1, and P. V. Santos
1
1Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
Indirect excitons (IXs) in double quantum well (DQW) structures are promising particles for
information storage and processes due to their long, electrically controlled lifetimes together with
the strong interaction with photons [1]. Furthermore, their spin dynamics can be controlled by
applying external fields [2]. Here, we report on transport of IX spins in GaAs DQWs using spatially
and polarization resolved photoluminescence (PL). A voltage-tunable in-plane potential gradient
was used to transport excitons by exploiting the quantum confined Stark effect in combination with
a lithographically defined resistive top gate. Transport direction was chosen as [-110] on (001)
GaAs sample surface to minimize the symmetry effects on the spin-orbit interaction [3]. Sample
was excited with a focused and circularly polarized laser beam and resulting photoluminescence
was investigated by using spatially and polarization resolved photoluminescence spectroscopy.
Effect of different bias voltage and magnetic field conditions were investigated.
References
[1] K. Sivalertporn, L. Mouchliadis, A. L. Ivanov, R. Philp, and E. A. Muljarov, Phys. Rev. B 85,
045207 (2012).
[2] A. Violante, R. Hey, and P. V. Santos, Phys. Rev. B 91, 125302 (2014).
[3] A. Hernández-Mínguez, K. Biermann, R. Hey and P. V. Santos, Phys. Status Solidi B 251, 1736
(2014).
44 www.web.nano.cnr.it/CEE2D
OXIDE HETEROSTRUCTURES AS A POSSIBLE INNOVATIVE ROAD
FOR STUDYING INDIRECT EXCITONS
L. Maritato
1, A. Galdi
1, C. Sacco
2, and D.G. Schlom
3
1Dipartimento di Ingegneria dell'Informazione, Ingegneria Elettrica e Matematica Applicata-
DIEM, Università di Salerno e CNR-SPIN, UOS Salerno, Via Giovanni Paolo II 132, 84084,
Fisciano, Salerno 2Dipartimento di Ingegneria Industriale-DIIN, Università di Salerno e CNR-SPIN, UOS Salerno,
Via Giovanni Paolo II 132, 84084, Fisciano, Salerno 3Cornell University, Kavli Institute for Nanoscale Science, Ithaca, NY 14853 USA
Recent advances in the molecular beam epitaxy growth of oxide-based heterostructures have
allowed atomic-scale thickness control and abrupt interfaces with the possibility to change the
chemical composition over a distance of a single unit cell [1]. In view of their exploitation to the
studies on indirect excitons, oxide-heterostructures have the potential advantage of allowing higher
n and p carrier densities and shorter separation distances when compared to semiconductor-based
double quantum wells [2]. We present preliminary studies on manganite and cuprate based
heterostructures of interest for the study of indirect excitons.
References [1] See for example, J. Mannhart and D.G. Schlom, Science 327, 1607-1611 (2012).
[2] A. J. Millis and D. G. Schlom, Physical Review B 82, 073101 (2010).
CEE2D | PISA IT | 2015 45
CAVITY POLARITONS UNDER THE INFLUENCE OF THE LANDAU
QUANTIZATION, RASHBA SPIN-ORBIT COUPLING, ZEEMAN
SPLITTING AND GYROTROPY EFFECTS
S.A. Moskalenko
1, I.V. Podlesny
1, E.V. Dumanov
1, and M.A. Liberman
2
1Institute of Applied Physics of the Academy of Sciences of Moldova, Academiei str. 5, Chisinau
MD–2028, Republic of Moldova 2Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23,
10691 Stockholm, Sweden
The energy spectrum of the two-dimensional (2D) cavity polaritons under the influence of a strong
magnetic and electric fields perpendicular to the surface of the GaAs-type quantum wells (QWs)
with p-type valence band embedded into the resonators is considered. As the first step in this
direction the Landau quantization of the electrons and heavy-holes (hh) was investigated taking into
account the Rashba spin-orbit coupling with third-order chirality terms for hh and with non-
parabolicity terms in their dispersion law including as well the Zeeman splitting effects. The non-
parabolicity term proportional to the strength of the electric field was introduced to avoid the
collapse of the semiconductor energy gap under the influence of the third order chirality terms. The
exact solutions for the eigenfunctions and eigenenergies were obtained using the Rashba method
[1]. On the second step we derive in the second quantization representation the Hamiltonians
describing the Coulomb electron-electron and the electron-radiation interactions. This allows us to
determine the magnetoexciton energy branches and to deduce the Hamiltonian of the
magnetoexciton-photon interaction. On the third step the fifth order dispersion equation describing
the energy spectrum of the cavity magnetoexciton-polariton is investigated. It takes into account the
interaction of the cavity photons with two dipole-active and with two quadrupole-active 2D
magnetoexciton energy branches.
It was shown that the Rabi frequency ΩR of the polariton branches and the magnetoexciton
oscillator strength fosc increase in dependence on the magnetic field strength B as ΩR~ ,B and
fosc~B. The optical gyrotropy effects may be revealed if changing the sign of the photon circular
polarization at a given sign of the wave vector longitudinal projection kz or equivalently changing
the sign of the longitudinal projection kz at the same selected light circular polarization.
I.V.P. and E.V.D. thanks the Foundation for Young Scientists of the Academy of Sciences of
Moldova for financial support (14.819.02.18F).
Reference
[1] E.I. Rashba, Fiz. Tverd. Tela (Leningrad) 2, 1224 (1960) [Sov. Phys. Solid State 2, 1109
(1960)].
46 www.web.nano.cnr.it/CEE2D
MAGNETOEXCITON IN NANORING OF NON-UNIFORM THICKNESS
L. C. Porras and I. D. Mikhailov
Universidad Industrial de Santander - UIS, cl 9 cr 27, Bucaramanga, Colombia, A.A. 678
To study the magneto-optical properties of a neutral exciton confined in a quantum ring of non-
uniform thickness in the presence of magnetic field applied along the ring’s symmetry axis, we
consider a model of a nanostructure in a form of a volcano with a circular ridge of variable height.
We show that in the structural adiabatic limit, when the width of the pattern of the particles
pathways within the ring is much smaller than the ring’s radius, the wave function of the exciton
confined in such structure can be found in a form of a double Fourier series expansion [1]. We
calculate the density of the states, oscillator strength and the photoluminescence spectrum for
different ring radii and scales of the non-uniformity. The dependencies of the magneto-optical
properties of the exciton on the magnetic field strength are discussed. Our results show a
substantial change of the amplitudes of the Aharonov–Bohm oscillations of energy levels, density
of states and photoluminescence spectrum of neutral excitons induced by the non-uniformity.
Reference [1] L. C. Porras and I. D. Mikhailov, Physica E 53, 41 (2013); L. F. Garcia, S. Yu. Revinova, and I.
D. Mikhailov, Physica E 71, 101 (2015).
CEE2D | PISA IT | 2015 47
BIASED CONTROLLED DIPOLE ORIENTED POLARITON BISTABILITY
Pramod K Sharma
1,2, Simeon I. Tsintzos
2, Gabriel Christmann
2, Zacharias Hatzopoulos
2,
Jeremy J Baumberg3, and Pavlos G Savvidis
1,2
1Department of Materials Science and Technology, University of Crete, 71300 Heraklion, Greece
2Microelectronics Research Group, IESL-FORTH, PO Box 1385, 71110 Heraklion, Greece
3Dept. of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK
Strong light-matter coupling regime in asymmetric double quantum well (QW) based microcavities
has been shown to give rise to dipole oriented polariton states [1]. These structures are expected to
exhibit remarkable nonlinearities owing to unique possibility these systems offer to engineer
polariton-polariton interactions. Strong nonlinearities refer to the optical bistability which is the
primary element for the optical computing where the information stored as energy and switching a
device from logic 0 level to logic 1 level required a definite switching energy [2]. Optical bistability
has a great impact of intense research in quantum electronics because it has potential in field of, all
optical logic and quantum computing system [3]. Our recent experimental demonstration is mainly
emphasized on biased controlled Dipole-oriented Polariton Bistability (DPB) which shows the
bistable behaviour in a system where the relationship of output to input characterized by hysteresis
loop. This bistable characteristic is crucial for the basis of binary switching element it will be
responsible for the optical logic gate circuits. The p-i-n microcavity diode structure consists of two
DBRs forming a cavity and 4 sets of asymmetric double InGaAs Quantum wells (QWs) positioned
inside the cavity. To map out polariton dispersions transmission measurements using white light are
performed at 160K. Figure (a) shows clear anticrossing behaviour between DX, IX and cavity
modes when tuning the bias. When in resonance the spatially direct and indirect excitons become
coupled sharing oscillator strength and provide strong dipole moment in growth direction.
Bistability measurements are performed when pumping at 1.3923eV with near resonant laser under
application of electric bias.
250
200
150
100
50
0
Out
put
Inte
nsit
y (µ
W)
108642
Input Intensity (mW)
'0.2mW' '0.4mW' '0.6mW' '0.8mW' '1.0mW'
Applied Bias ~ 0.2V
Applied Bias ~ - 0.2V
Fig: (a) Polariton branches under application of bias close to the resonance in transmission configuration for
a mesa with 4nm barrier layer showing clear anticrossing at 160K. (b) Bias controlled bistability different
power shown in figure respectively.
References
[1] Cristofolini et al., Science 336, 704 (2012).
[2] SD Smith et al., Nature 316 (1985).
[3] Y R Shen et al., Nature 299 (1982).
48 www.web.nano.cnr.it/CEE2D
EXCITON DYNAMICS IN DISK-LIKE QUANTUM DOTS WITH A
MAGNETIC IMPURITY
V. Moldoveanu1, I. V. Dinu
1, R. Dragomir
1, and B. Tanatar
2
1National Institute of Materials Physics, PO Box MG-7, Bucharest-Magurele, Romania
2Department of Physics, Bilkent University, Bilkent, 06800 Ankara, Turkey
In the recent years semiconductor quantum dots doped with a single magnetic impurity turned into
promising candidates for optical spin manipulation [1,2]. The coherent precession of a Mn atom
embedded in a CdTe quantum dot has been recently probed [3] . Trojnar et al. [4] also investigated
the formation of biexcitons in self-assembled QDs in the presence of a Mn atom.
In this work we study theoretically the exciton dynamics under optical pulses in the presence of
both electron-Mn and hole-Mn exchange interactions. The latter coupling does not affect heavy-
hole (HH) spin but flips the spin of the light-holes (LH). We use the Master equation approach [5]
to investigate the optical properties of Mn-doped CdTe quantum dots by taking into account the
specific disk-like geometry of the system along with the corresponding optical selection rules. We
find that the systems exhibiting heavy-hole light-hole mixing present a more complicated dynamics
of both excitons and Mn spin. The numerical calculations include charge relaxation processes as
they are crucial for the dynamics of the p-shell excitons.
References [1] Paul M. Koenraad and Michael E. Flatte, Nat. Mater. 10, 91-100 (2011).
[2] J. Kobak, T. Smolenski, M. Goryca, M. Papaj, K. Gietka, A. Bogucki, M. Koperski, J.-G.
Rousset, J. Suffczynski, E. Janik, M. Nawrocki, A. Golnik, P. Kossacki, and W. Pacuski, Nat.
Commun. 5, 1-8 (2014).
[3] M. Goryca, M. Koperski, P. Wojnar, T. Smoleski, T. Kazimierczuk, A. Golnik, and P. Kossacki,
Phys. Rev. Lett. 113, 227202 (2014).
[4] A. H. Trojnar, M. Korkusinski, U. C. Mendes, M. Goryca, M. Koperski, T. Smolenski, P.
Kossacki, P. Wojnar, and P. Hawrylak, Phys. Rev. B 87, 205311 (2013).
[5] V. Moldoveanu, I. V. Dinu, and R. Dragomir, Phys. Rev. B 89, 245415 (2014).
CEE2D | PISA IT | 2015 49
QUANTUM RINGS AND QUANTUM DOTS: OPTICAL PROPERTIES AND
ANNEALING PROCESS
M. Triki
1, D. Elmaghraoui
1, and S. Jaziri
1,2
1Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Université
Tunis El Manar, Tunisia 2Laboratoire de Physique et des Matériaux, Département de Physique, Bizerte, Université de
Carthage, Tunisia
Semiconductor nanostructures are of major interest for technological applications as well as being a
very interesting arena for basic physics. Thus, a detailed computation and understanding of their
properties is much needed. Among various topics in which are interested many experimental and
theoretical researches, this works, has focused on two subjects reported in two separated parts.
In first part, we discuss the effects of interdiffusion on self-assembled InAs/GaAs quantum dot
(QD) under thermal process. In fact, although better device performance had been expected from
self-assembled QDs, it has been found that such growth method usually resulted in ensembles of
dots that vary in size and shape, resulting in inhomogeneous broadening of the photoluminescence
emission of the dots which negates their advantages and constitutes a technical barrier for the
development of optoelectronic devices. Therefore, the need to use a narrowing technique of the
photoluminescence emission is presently a major goal among the researchers. In other hand, several
published works [1–3] have evidenced that blue shifts and narrowing of optical emission of QDs
can be induced by a rapid thermal annealing (RTA) procedure.
In this context, we study the modification of the well depths and the effective masses profiles
induced by the interdiffusion of the QD’s constituent atoms across its interfaces under high
temperatures. Then, we calculate the electron and the hole spectrums in these intermixed QDs. A
good agreement is shown when calculated energy transitions are compared with some previously
published experimental data. We found also, that such QDs retain their zero-dimensional density of
states even after the diffusion of the potential[4].
The second part of the work is devoted to optical properties of InAs/GaAs quantum ring. Such
nanostructures are obtained when a pause is introduced after cupping the dots with a GaAs layer
thinner than the InAs island height [5] . Nanostructure with a ring like shape is of particularly
interest due to its peculiar optical emission. Otherwise, external electric fields applied on Quantum
rings can provides two different behaviors for the electronic structure which depend on the direction
of the electric field; this is due to the hole in the middle of the QR. While an axial electric field
changes the positions of the energy levels under the quantum confined Stark effect, a lateral electric
field decreases the symmetry of problem and mixes the energy levels and wave functions of the QR.
The main idea of this work is to investigate theoretically the optical emission and the electronic
structure of InGaAs/GaAs QRs subjected to electric field. By using a numerical method and a
model of QR very close to real one, our results clearly show that the one-electron ground state
presents an absolute minimum when studied as a function of the ring radius. For large sizes, the
electron levels become close to each other: quasibidimensional limit. Comparing our results to
experimental ones, we found that the calculated spectra well reproduce the experiment [6]. A lateral
electric-field applied to the QR gives rise to anticrossings between the energy levels.
References
[1] A. Babinski and J. Jasinski, Thin Solid Films 412, 84 (2002).
[2] E. A. Zibik, W. H. Ng, L. R. Wilson, M. S. Skolnick, and J. W. Cockburn, Appl. Phys. Lett. 90,
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This booklet was edited by Luisa Neri, Cnr Nano S3, Italy.
September 20. 2015