Photoelectron Spectroscopy for Functional Oxides
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Transcript of Photoelectron Spectroscopy for Functional Oxides
International Summer School on Surfaces and Interfaces in Correlated Oxiides, Vancouver, 29 Aug – 01 Sep 2011
FOR
1346
Ralph Claessen (U Würzburg, Germany)
Photoelectron spectroscopy of functional oxides: Heterostructures and buried interfaces
• Photoelectron spectroscopy (PES)
• PES theory in a nutshell
• PES with hard x-rays (HAXPES)
• HAXPES of oxide heterostructures
Heterostructures of functional oxides
strong coupling between charge/orbital/spin/latticedegrees of freedom lead to:
3d transition metal oxides
- metal-insulator transitions- charge and orbital ordering- local magnetism (ferro, antiferro,…)- high-temperature superconductivity- collossal magnetoresistance- …
Epitaxial heterostructures by MBE, PLD
controlled interfaces, additional functionalities:- strain engineering- interfacial 2dim electron gas (2DEG)- electrostatic doping (by polarity or field effect) - artificial multiferroics- spin injection- …
"The interface is the device"(H. Kroemer, Nobel lecture 2000)
Want information on:
• chemical composition
• electronic structure
• vertical depth profile
photoelectron spectroscopy (PES) with soft and hard x-rays
Oxide heterostructures
Photoelectron spectroscopy (PES)
Photoelectron spectroscopy (PES)
Ekin = hν – EB - Φ0
sample
spectrum
hν
Ekin
measure kinetic energy distribution of photoelectrons
Photoelectron spectroscopy (PES)
Chemistry (core levels):→ composition→ chemical bonding→ valencies
Electronic structure (valence band):→ density of states → band structure → Fermi surface
→ spectral function A<(k,E)
sample
spectrum
Core level spectroscopy: ESCA
Inte
nsity
[a.u
.]
140012001000800600400Kinetic Energy [eV]
Bi2Sr2CaCu2O8+δ
•Cu 2p•CuO
C 1s
Ca 2p
O 1s
Bi 4f
hν = 1486.6 eV [Al - Kα]
Sr 3d
Inte
nsity
[a.u
.]1500149014801470
Kinetic Energy [eV]
Fermi level
Bi 4d
Inte
nsity
[a.u
.]
1340133013201310Kinetic Energy [eV]
Bi 4f7/2Bi 4f5/2
Bi 5d
Electron Spectroscopy for Chemical Analysis
courtesy of A.F. Santander-Syro
Core level spectroscopy: Chemical shift and valency
1080 1075 1070 1065
binding energy (eV)
Na1s
37%32%23%
10%4%
15%
dopi
ng x
(%
)
462 460 458 456 454
binding energy (eV)
Ti2p3/2(+ Na)
Ti3+Ti2+
Na
Example: alkali metal doping of TiOCl
valence change:Ti3+(3d1) Ti2+(3d2)
PRL 106, 056403 (2011)
TiOCl
O 2p / Cl 3p
Ti 3d
Valence band spectroscopy
k-integrated spectrum
PRB 72, 125127 (2005)
band structure and Fermi surface
Valence band spectroscopy: ARPES
Angle-Resolved PhotoElectron Spectroscopy
courtesy T. Deveraux/A. Damascelli
emis
sion
ang
le (i
.e. m
omen
tum
)
energy
PES instrumentation
• rare gas discharge lamp (<40.2 eV)• x-ray tube (1.256 and 1.486 keV)• synchrotron radiation (10 eV … 10 keV)
• hemispherical anylzer• time of flight (TOF) analyzer)
typically 10-10 mbar
Wikipedia
PES theory in a nutshell:
1) Independent electron approximation
Unperturbed electron system: one-electron states with energy E
Perturbation: classical radiation field with vector potential
Fermi´s Golden Rule for the photoinduced transition rate from initial to final states:
Hence, the total photoelectron current is:
)(2
0 νδψψ hEEpeAw ifirki
ffi −−⋅∝ ⋅→
ψ
PES theory: Independent electrons
)2(0),( trkieAtrA πν−⋅=
)()(,
ffi
fiPES EwI −∝∑ → εδε
Time-dependent perturbation theory
)(2
0 νδψψ hEEpeAw ifirki
ffi −−⋅∝ ⋅→
final state:inverted LEED state(eigenstate of semi-infinite crystal)
energy conservation
PES theory: Independent electrons
initial state:Bloch wave or core level
)(2
0 νδψψ hEEpeAw ifirki
ffi −−⋅∝ ⋅→
final state:inverted LEED state(eigenstate of semi-infinite crystal)
PES theory: Independent electrons
One-step model Three-step model
final state: high-energy Bloch state of infinite crystal,steps 2 and 3 incoherently decoupled
courtesy A. Damascelli
)(2
0 νδψψ hEEpeAw ifirki
ffi −−⋅∝ ⋅→
transition matrix element
PES theory: Independent electrons
If the radiadion field is only weakly modulated on atomic length scales, (i.e. >> few Å), the photon momentum can be neglected in the transition matrix element:
Examples:
hν = 20 eV λ ≈ 600 Å
hν = 2000 eV λ ≈ 6 Å
k
πλ 2= k
irefAipAfipeAf rki
⋅∝⋅≈⋅⋅000
Dipole approximation
PES theory: Independent electrons
Dipole approximation and k-selection rule for Bloch states
momentum conservation:
photonif kGkk
++=only"vertical" transitions
ARPES
oxides of the 3d transition metals: M = Ti, V, … ,Ni, Cu
basic building blocks: MO6 octahedra (or other ligand shells)
electronic configuration: O 2s2p6 = [Ne]
TM 3dn
cubic perovskites perovskite-like anatas rutile spinel
O2-
quasi-atomic,strongly localized strong intraatomic Coulomb interaction
and breakdown of independent electron approx.
Transition metal oxides: electronic correlations
TMX+
PES theory in a nutshell:
2) Many-body picture
Ekin
hν
N interacting electrons:
"loss" of kinetic energy due to interaction-related excitation energy stored in the remaining N-1 electron system !
Many-body effects in photoemission
Photoemission process:
sudden removal of an electron from N-particle system
Fermi´s Golden Rule for N-particle states:
with
N-electron ground state of energy EN, 0 ("initial state")
N-electron excited state of energy EN, s, ("final state")
consisting of N-1 electrons in the solid and a free photoelectron of momentum and energy ε
in second quantization with suitable one-electron basis
)(ˆ)( 0,,2
0,, νδε hEEI NsNs
isf −−Ψ∆Ψ∝∑
0,0, Ni =Ψ
sNksf ,1,, −=Ψ
k
ifif
N
iii ccMprA +
==⋅=∆ ∑
1)(ˆ
one-particle matrix element
Reinterpretation of Fermi´s Golden Rule
Electron removal spectrum
Fermi´s Golden Rule for N-particle states:
The ARPES signal is directly proportional to the
single-particle spectral function )()(Im1)( ωωπ
ω fGA ×−=<
)(εI
a little bit of mathand a few plausible assumptions (sudden approximation)
single-particle Green´s function
probability of removing an electron at energy ω from the system
)(ˆ)( 0,,2
0,, νδε hEEI NsNs
isf −−Ψ∆Ψ∝∑
ωµ
U
TiOCl
O 2p / Cl 3p
Ti 3d1
d1 → d0
LHBd1 → d2
UHB
Example: PES of the Mott insulator TiOCl
spectral function A<(ω) (DMFT)
Photoemission probing depth:
soft and hard x-ray PES
Inelastic scattering of the photoelectron
Three-step model
courtesy A. Damascelli
Step 2: photoelectron transport to the surface
inelastic scattering with other electrons (excitation of e-h-pairs, plasmons)
• generation of secondary electrons("inelastic background")
Ekin
intensity intrinsic spectrum
incl. background
Inelastic scattering of the photoelectron
Three-step model
Step 2: photoelectron transport to the surface
inelastic scattering with other electrons (excitation of e-h-pairs, plasmons)
• generation of secondary electrons("inelastic background")
• loss of unscattered photoelectron current⇒ inelastic mean free path λ
courtesy A. Damascelli
"conventional" VUV/XUV-PES:surface sensitive on atomic length scale !
Photoemission probing depth
λ(Ekin)
hνEkin
probing depth (3λ) up to >10 nm access to bulk, buried nanostructures, and
interfaces
depth profiling of thin films
λ(Ekin) "universal curve"
hard x-ray PES = HAXPESsoft x-ray PES (SX-PES)
0≠µ0=µ0=µ
O2-
TMX+
Transition metal (TM) oxides form lattice of ionic charges
Classification of surfaces (Tasker): - surface charge Q- electrical dipole moment in repeat unit
P. W. Tasker, J. Phys. C 12, 4977 (1979)
Transition metal oxides: Instability of polar surfaces
O2-TMX+
µ
0=Q 0≠Q 0≠Q
O2-
TMX+
type 3 surfaces are energetically unfavorable:
Transition metal oxides: Instability of polar surfaces
charge field potential
"polarization catastrophe"
will be avoided by atomic/ionic/electronic surface reconstruction
⇒ surface ≠ bulk
-σ+σ-σ+σ
8.2 Å
PRB 76, 075412 (2007)
Transition metal oxides: Instability of polar surfaces
Example: Fe3O4 (magnetite) different reconstructions of the (111) surface (STM)
VUV-PESsurface-sensitive
Soft X-ray PESprobing depth 2x larger
Transition metal oxides: Instability of polar surfaces
Example: Fe3O4 (magnetite)
EPL 70, 789 (2005)
∑∑ ↑↓+ +−=
iii
jiji nnUcctH
σσσ
,,
ˆ
kinetic energy,itinerancy
local Coulomb energy,localization
Surface effects in Mott-Hubbard-type oxides
t
Uspectral function (DMFT for n=1)
U/t
Surface effects in Mott-Hubbard-type oxides
spectral function (DMFT for n=1)
U/t
Example: CaVO3
A. Sekiyama et al., PRL 2004
surface
"bulk"
lower Hubbard band
quasiparticle peak
Surface effects in Mott-Hubbard-type oxides
Example: CaVO3
A. Sekiyama et al., PRL 2004
surface
"bulk"
lower Hubbard band
quasiparticle peak
reduced atomic coordination @ surface:
stronger electron localization
smaller effective bandwidthWsurf < Wbulk
surface stronger correlated:U / Wsurf >U / Wbulk
"conventional" VUV/XUV-PES:surface sensitive on atomic length scale !
Photoemission probing depth
λ(Ekin)
hνEkin
probing depth (3λ) up to >10 nm access to bulk, buried nanostructures, and
interfaces
depth profiling of thin films
λ(Ekin) "universal curve"
hard x-ray PES = HAXPESsoft x-ray PES (SX-PES)
hν = 6 keV λ ≈ 2 Å, kphot ≈ 3 Å-1
HAXPES: drawbacks and caveats
Non-negligible photon momentum
hν = 6 keV λ ≈ 2 Å, kphot ≈ 3 Å-1
• suppression of direct (k-conserving) transitions
Debye-Waller factor for direct transitions
HAXPES: drawbacks and caveats
ARPES of W(110) @ hν = 870 eV Plucinski et al., PRB 78, 035108 (2008)
( )atomphotdir MTkW 2exp α−=
Non-negligible photon momentum
hν = 6 keV λ ≈ 2 Å, kphot ≈ 3 Å-1
• suppression of direct (k-conserving) transitions
• atomic recoil effect
photon-absorbing atom takes up recoil energyat the expense of
photoelectron energy,
depending on atom mass and lattice stiffness
HAXPES: drawbacks and caveats
Non-negligible photon momentum
MkE photkin 222=
Y. Takata et al., PRB 75, 233404 (2007)
hν = 6 keV λ ≈ 2 Å, kphot ≈ 3 Å-1
• suppression of direct (k-conserving) transitions
• atomic recoil effect
• quadrupolar contribution to transition matrix element
HAXPES: drawbacks and caveats
Non-negligible photon momentum
( ) iprkiAfipeAf rki
⋅⋅+≈⋅⋅ 100
hν = 6 keV λ ≈ 2 Å, kphot ≈ 3 Å-1
• suppression of direct (k-conserving) transitions
• atomic recoil effect
• quadrupolar contribution to transition matrix element
• cross section for photoemission
• electron analyzer transmission
need bright x-ray source…
HAXPES: drawbacks and caveats
Non-negligible photon momentum
Low photoemission signal
( ) 3−∝ νσ h1−∝ kinEt
HAXPES set-up @ PETRA III (DESY, Hamburg)
X-rays fromPETRA III
"High-resolution hard x-ray photoemission for materials science" (BMBF)
• joint project with C. Felser (U Mainz) and W. Drube (DESY)
• photon energy: 2.5…15 keV
• energy resolution: 30 meV
• linearly/circularly polarized x-ray radiation
• commissioned in 2010
• user operation since 2011
other HAXPES instruments worldwide:- Spring-8, Japan (>4)- BESSY, Germany (HIKE)- ESRF, France (ID-9)- Soleil, France (under construction)- Diamond, UK (under construction)
HAXPES of oxide heterostructures:
(1) Fe3O4/GaAs
Fe3O4
GaAs
Epitaxial growth of Fe3O4/GaAsPRB 79, 233101 (2009)
surface Datta-Das spin transistor
semiconductor with large spin diffusion length
semimetallic ferromagnet(100% spin polarization @ EF)
resistively matched to semiconductor
Fe3O4 (magnetite), (RE,Sr)MnO3, CrO2, Heusler compounds, …
Fe3O4
GaAs
Epitaxial growth of Fe3O4/GaAsPRB 79, 233101 (2009)
surface
MBE growth of thin magnetite film:
• epitaxial Fe deposition @ RT
• postoxidation @ 600 - 800K / p(O2) = 10-5 mbar (10-30 min)
Fe valency?
mixed-valent Fe3O4 vs. (Fe2+ )O and (Fe 3+)2O3 ?
chemical depth profile ?
700 705 710 715 720 725 730 735 740 745 750
Fe2O3
Fe3O4
FeO
Fe
charge transfer satellites
binding energy (eV)
Valence signatures in Fe 2p spectrum
Fe3+
Fe2+
Fe0
Fe2+/Fe3+
2p1/2 2p3/2
Fe3O4
GaAs
Depth profiling of Fe3O4/GaAs
Fe 2p spectra
PRB 79, 233101 (2009)
surface
interface
surface
Depth profiling of Fe3O4/GaAs
Tuning the information depth by variation of
(1) photon energy, or (2) photoelectron escape angle
energy
mea
n fr
ee p
ath
θ
λeff
λeff = λIMFP cos θ
Fe3O4
GaAs
Depth profiling of Fe3O4/GaAs
Fe 2p spectra
PRB 79, 233101 (2009)
surface
film: mixed-valent Fe2+/3+
interface: divalent and metallic Fe (O-deficient)
interface
surface
Fe3O4
GaAs
interface (Fe, FeOx, GaOx, AsOx)
Depth profiling of Fe3O4/GaAs
Fe 2p spectra As 2p3/2 spectra
PRB 79, 233101 (2009)
surface
film: mixed-valent Fe2+/3+
interface: divalent and metallic Fe (O-deficient)oxidized Ga,As
Fe3O4
GaAs
interface (Fe, FeOx, GaOx, AsOx)
Validation by electron microscopy
surface
TEM
STEM-EELS
J. Verbeeck, H. Tian, and G. van Tendeloo, U Antwerp
Fe3O4
ZnO
Fe3O4/ZnO: An all-oxide structure
also PLD-grown contacts: R. Gross et al.
APL 98, 012512 2011
HAXPES TEM
film grown by reactive deposition in O2-atmosphere (∼10-6 mbar)
HAXPES of oxide heterostructures:
(2) Interface 2DEG in LaAlO3/SrTiO3
•epitaxial growth by PLD
A. Ohtomo et al., Nature 419, 378 (2004) S. Thiel et al., Science 313, 1942 (2006)N. Reyren et al., Science 317, 1196 (2007)
LAO/STO heterostructures in a nutshell
LaAlO3∆=5.6eV
SrTiO3∆=3.2eV
•epitaxial growth by PLD
•both oxides: wide gap insulators
• if LaAlO3 film thicker than 3 unit cells (uc) : → formation of a high-mobility 2DEG
at the interface
LAO/STO heterostructures in a nutshell
2DEGconductivity
sheet carrier density (Hall)
A. Ohtomo et al., Nature 419, 378 (2004) S. Thiel et al., Science 313, 1942 (2006)N. Reyren et al., Science 317, 1196 (2007)
LaAlO3∆=5.6eV
SrTiO3∆=3.2eV
LAO/STO heterostructures in a nutshell
2DEG
A. Ohtomo et al., Nature 419, 378 (2004) S. Thiel et al., Science 313, 1942 (2006)N. Reyren et al., Science 317, 1196 (2007)
properties of the 2DEG:
• tunable conductivity by electric gate field
• superconducting below 200 mK
•magnetoresistance
•coexistence of s.c and magnetism / electronic phase separation
origin of 2DEG, threshold behavior ?
LaAlO3∆=5.6eV
SrTiO3∆=3.2eV
Polar catastrophe
polar catastrophe
Nakagawa et al., Nature Mat. 5, 204 (2006)
AlO2
LaO
LaO
LaO
AlO2
AlO2
TiO2
SrOTiO2
SrO
-1+1-1+1-1+1
0000
charge:
electrostatic energy increases linearly with thickness of polar film
electronic or ionic
0.5e- per layer unit cell n2D = 3.5×1014 cm-2
partial Ti 3d occupation Ti3.5 (d0.5) = Ti3+/Ti4+
charge reconstruction
AlO2
LaO
LaO
LaO
AlO2
AlO2
TiO2
SrOTiO2
SrO
-1/2+1-1+1-1+1
-1/2000
∆q = -1/2
and how to avoid it
HAXPES of LAO/STO heterostructures
PRL 102, 176805 (2009)
undoped SrTiO3: |3d0> Ti4+
doped LAO/STO interface: |3d0> + |3d1> Ti3+/Ti4+
Ti 2p spectrum
Ti4+
2p1/2 2p3/2
2DEG
SrTiO3
LaAlO3
Ti3+
Dependence on LAO overlayer thickness
interface charge density increases with LAO overlayer thickness
non-zero Ti d1 signal already for 2uc sample (?)
PRL 102, 176805 (2009)
Ti4+
Ti3+
Ti3+
Depth profiling by angle-resolved HAXPES
θ
d
e-
e-
PRL 102, 176805 (2009)
2DEG thickness
sheet carrier density
Quantitative analysis: 2DEG thickness
*lattice constant of STO unit cell (uc) = 3.8 Åθ
d
e-
e-
PRL 102, 176805 (2009)
Sample 2 uc 4 uc 5 uc 6 uc
d (uc*) 3 ± 1 1 ± 0.5 6 ± 2 8 ± 2
interface thickness < 3 nm
consistent with
- CT-AFM Basletic et al. (2008)
- TEM-EELS Nakagawa et al. (2006)
- density functional theory Pentcheva et al. (2009)
- 2D superconductivity Reyren et al. (2007)
- ellipsometry Dubroka et al. (2010)
Quantitative analysis: sheet carrier density
Sample 2 uc 4 uc 5 uc 6 uc
n2D (1013 cm-2) 2.1 3.9 8.1 11.1
n2D << electronic reconstruction value
n2D >> Hall effect data
el. reconstruction
35
PRL 102, 176805 (2009)
RIXS on LAO/STO
RIXS eg-excitation as fct. of # LAO-overlayers
PRB 82, 241405(R) (2010)
t2g
Ti 2p
Ti 3deg
Ti3+ (3d1)
photon in photon
out
Sheet carrier density: HAXPES, RIXS & Hall effect
• n2D much smaller than expected for purely electronic reconstruction (35 x 1013 cm-2)
• n2D higher than Hall effect data
• photo-generated carriers cannot fully account for observed excess
• remaining excess due to additional localized Ti 3d electrons? (cf. DFT - Popovic et al., PRL 2008)
PRB 82, 241405(R) (2010)
~3 eV
LAO/STO: Valence band spectroscopy with HAXPES
2DEG
SrTiO3
LaAlO3
2DEG
SrTiO3
LaAlO3
Ti 3d electrons should be here, but HAXPES cross-section too small !(theor. estimate: 10-4 of O2p emission)
O2p-derived vb states
Band situation from density-functional theory
surf
ace2D
EGSTO LAO
CBM
VBM
E
core levels
EF
Yu Lin et al., arXiv 0904.1636 (2009)Pentcheva and Pickett, PRL 102, 107602 (2009)
holes@ LAO VBM
electrons@ STO CBM
Yu Lin et al., arXiv 0904.1636 (2009)Pentcheva and Pickett, PRL 102, 107602 (2009)
e-e- surf
ace2D
EGSTO LAO
CBM
VBM
E
core levels
EF
Band situation from density-functional theory
interface
holes@ LAO VBM
electrons@ STO CBM
Yu Lin et al., arXiv 0904.1636 (2009)Pentcheva and Pickett, PRL 102, 107602 (2009)
e-
E
EF
e- surf
ace2D
EGSTO LAO
CBM
VBM
E
core levels
Band situation from density-functional theory
Results from HAXPES
Al 1s core levelvalence band
~3 eV
VBM: ~ 3 eV below EF same width for all samples!
band theory versus experiment
EF
e- surf
ace2D
EGSTO LAO
CBM
VBM
E
core levels
also observed by Segal et al., PRB 80, 241107(R) (2009)
STO LAO
• VBMLAO above VBMSTO
• type II interface(valence band offset: 0.35 ± 0.1eV)
• confirmed by core level analysis
Valence band offsets
valence band analysisCB
VB
type I type II
STO LAO STO LAO
band alignment
0.35eV
Band alignment: A possible scenario
DFT band theory:
Photoemission:
STO LAOlocalized hole states induced by surface O-vacancies
interface states (itinerant and localized)
HAXPES of oxide heterostructures:
(3) LaVO3/SrTiO3 – electrostatic doping of a Mott a insulator
Electrostatic doping of a Mott insulator
LaAlO3band ins.∆=5.6eV
SrTiO3band ins.∆=3.2eV
q2DEG
LAO/STO
(AlO2)-
(LaO)+
(TiO2)0
(SrO)0
……
polar
non-polar
Idea:replace Al3+ by trivalent transition metal
LaVO3
LVO/STO
LaVO3Mott ins.∆≈1 eV
SrTiO3band ins.∆=3.2eV
???
Ohtomo/Hwang, Nature 427, 423 (2004) Hotta et al., PRL 99, 236805 (2007)
LaVO3: - valence configuration V3+ (d2)
- polar oxide
- Mott insulator (∆LVO << ∆STO)
electronic reconstruction and formation of interface 2DEG ?
extra carriers on which side of interface(LVO or STO) ?
Electrostatic doping of a Mott insulator
LaVO3Mott ins.∆≈1 eV
SrTiO3band ins.∆=3.2eV
???
LVO/STO
band-filling controlled Mott transition without chemical doping ?
LVO/STO: Sample growth and characterization
pulsed laser deposition
RHEED oscillations
RHEED pattern AFM image
STEM image
interface
LVO/STO: metal-insulator transition in transport
metal-insulator transition for n-type interface
p-type interface insulating
critical thickness: ∼ 9 uc LVO (Hotta et al.: 5 uc)
high carrier mobility
HAXPES of LVO/STO: V 2p depth profiles
6 uc LVO
STO
homogeneous "V3+" profile
extra electronic charge on V near interface
10 uc LVO
STO
insulating conducting
HAXPES of LVO/STO: Ti 2p
10 uc LVO
STO
no Ti3+ (d1) signal
possibly some bandbendingon STO side of interface
extra electronic charge on V near interface
LVO/STO: electronic reconstruction picture
Electrostatic doping of a Mott insulator
LaVO3/SrTiO3:
• creation of 2D metal states in a correlated electron system by interface engeering
• purely electrostatic doping
• no disorder by chemical dopants
LaVO3Mott ins.∆≈1 eV
SrTiO3band ins.∆=3.2eV
"q2DEG"
Summary
Photoelectron spectroscopy of functional oxides:Heterostructures and buried interfaces
• Photoelectron spectroscopy (PES)yields (destruction-free) information on- chemical composition, valencies, local chemistry - electronic structure (band structure, spectral function)
• PES with hard x-rays (HAXPES)- enhanced probing depth giving access to bulk and buried interfaces- needs high x-ray intensity ( synchrotron radiation)- caveat: high photon momentum (ARPES difficult, recoil effects)
• Future directions:- magnetic information with polarized x-rays (XMCD, XMLD) and/or spin detection- soft x-ray ARPES: band mapping of buried interfaces
Photoemission:
• S. Hüfner, Photoelectron Spectroscopy – Principles and Applications, 3rd ed. (Berlin, Springer, 2003)
• A. Damascelli, Angle-resolved photoemission studies of the cuprate superconductors,Rev. Mod. Phys. 75, 473 (2003)
HAXPES:
• K. Kobayashi: Hard x-ray photoemission spectroscopy, Nucl. Instr. Meth. Phys. Res. A 601, 32 (2009)
• László Kövér: X-ray photoelectron spectroscopy using hard X-rays,J. Electron Spectrosc. Rel. Phen. 178-179, 241 (2010)
HAXPES of oxide heterostructures
• R. Claessen et al.: Hard x-ray photoelectron specroscopy of oxide hybrid and heterostructures: a new method for the study of buried interfaces,New J. Phys. 11, 125007 (2009)
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