Post on 01-Jun-2020
Orenstein Group Research Overview
Overview and history of “quantum materials” group at UCB
What is the “quantum materials” problem?
Spin-charge separated excitations
Spin transport experiments in GaAs quantum wells
Future experiments
Outline
Theory
SynthesisCharacterization
DH Lee
J. OrensteinD.S. ChemlaJ.C. Davis
Materials research triangle (ca. 2001)
Theory
SynthesisCharacterization
DH Lee
J. OrensteinD.S. Chemla
Materials research triangle (ca. 2002)
Theory
SynthesisCharacterization
D.H. LeeJ.E. Moore (2002)A. Vishwanath (2004)
J. OrensteinD.S. ChemlaA. Lanzara (2002)F. Hellman (2004)R.C. Dynes (2003)R.J. Birgeneau (2004)
Y. Suzuki (2003)R. Ramesh (2004)
Materials research triangle (present)
Quantum materials: highly correlated electron systems
Non-interacting electrons
+-
charge fluctuates: non-magnetic metal
electrons that repel each other
charge fluctuation suppressed: antiferromagnetic spin order
Other types of order in correlated systems
orbital ordercharge order: stripes
Hallmark of QM’s: rich, complex phase diagrams
Ruthenates
Manganites
Cobaltates
Cuprates
Examples from transition-metal oxides
Applied science and technology
Multifunctionality (multiferroics) Improved ferroelectrics, piezoelectrics, ferromagnets Integrated technology based on functional interfaces
Unifying concept: quantum criticality
g
T
order “A” order “B”
quantum critical regime:physics determined by
fluctuations between A and B
new phase
Real world examples
High-Tc cupratesCePdSi2
Spin chargeseparatedexcitations
(Vishwanath)T Quantum critical
ggc
Applications to:
Understanding exotic phases in TM oxides, heavy fermions Confinement of quarks (see Laughlin)
Quantum criticality and spin charge separated quasiparticles
Fractional quantum number cartoon
ground state:array of singlets
Two particles eachhave q=0, s=1/2
Removing an electronyields a particle with
have q=e, s=0
excited state:triplet s=1
Spin chargeseparatedexcitations
(Vishwanath)T Quantum critical
ggc
Quantum criticality and spin charge separated quasiparticles
How to detect spin-charge separation?
argchespinσσ≠
Jspin
v
v
How to create Jspin?()sFFBJzz↑↓∂−∂∝∝∂∂
or()snnJz↑↓∂−∝∂
In the presence of spin-orbit coupling, photonscan inject spin polarization in a metal
Transient spin gratings
Interference of two orthogonallypolarized beams…. Creates a photon helicity wave…
which generates a spin density wave.
Cameron et al., Phys. Rev. Lett. 76, 4793 (1996)
Ideal for measurement spin diffusion coefficient
Spin grating dynamics
λ=2π/q
Sz
1/τs
q2
γq
Slope=Ds
(,)(0)exp()zzqSqtStγ=−
Sz decays due to thecombined effect of diffusionand relaxation
Slope and intercept ofγq vs. q2 yield Ds and 1/τs.
21/qssDqγτ=+where
Probing diffusion and relaxation:the transient grating technique
θ θ
transmitteddif
fracte
d
Coherent heterodyne detection of transient gratings
Technical innovations
Phase mask array forrapid variation of q
Phase-modulated heterodyne detection
of diffracted wave
N.Gedik and J. Orenstein, OpticsLetters, 29, 2109 (2004).
Demonstration of coherentheterodyne detection
0 20 40 60 800.1
1
Spi
n po
lariz
atio
n
Time [ps]
14 µm4.8 µm3.5 µm2.5 µm
Decay rate of a fluctuation with wavevector q:
γτ=+21/qssDq
Direct measurement of spindiffusion coefficient, Ds, in 2DEG
0 1 2 3 4 5 6 70.00
0.02
0.04
0.06
0.08
0.10
0.12
γ (ps-1
)
q2 (x 108 cm-2)
Ds=120 cm2/s
0 50 100 150 200 250 3000
1
2
3
4
5
6
D (1
000
cm2
/s)
T (K)
Comparison of spin and chargediffusion coefficients
Ds
Comparison of spin and chargediffusion coefficients
0 50 100 150 200 250 3000
1
2
3
4
5
6
0 100 2000.0
0.2
0.4
D (1
000
cm2
/s)
T (K)
T (K)
Ds/D
cDs
Dc
0 50 100 150 200 250 3000
1
2
3
4
5
6
D (1
000
cm2
/s)
T (K)
cJ
spinJ
spinJ
e-e collisions conserve total momentum, butexchange momentum between spin up and spin
down populations creating spin drag resistance ρ↑↓
Spin Coulomb drag (D’Amico &Vignale)
0 1 2 3 4 50
2
4
6
8
Dc
0/Ds
Direct comparison with theory
Charge and spindiffuse at same rate
Spin Coulomb drag
ρ↑↓/ρ
0 1 2 3 4 50
2
4
6
8
Dc
0/Ds
Direct comparison with theory
7.8 E11 cm-2
4.3 E111.9 E11
0 1 2 3 4 50
2
4
6
8
Dc
0/Ds
Charge and spindiffuse at same rate
Spin Coulomb drag
ρ↑↓/ρ
Future directions
Spin dynamics in correlated electron materials
Quasiparticle recombinationIsotope effects, Zn substitution (Lanzara, Ando)
Time-domain terahertz spectroscopyTM oxide interfaces