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