Giti A. KhodaparastDepartment of Physics, Virginia Tech

Supported by NSF and AFOSR

Graduate Students : Kanokwan Nontapot, Aliya Gifford, Matt Frazier

Post doc.: Rajeev Kini

Spin/Carrier Dynamics in Narrow Gap SemiconductorsSpin/Carrier Dynamics in Narrow Gap Semiconductors

InSbInSb Based Structures:Based Structures:

Prof. J. K. Furdyna, InMnSbDepartment of Physics, University of Notre Dame

Prof. M. B. Santos , InSb QWsDepartment of Physics & Astronomy, University of Oklahoma

Intel and Qinetiq researchers have recently demonstrated prototype InSb quantum well transistors

InSb QW has the lowest energy dissipation and gate delay which is an important metric for logic microprocessors

IIIIII--V SemiconductorsV Semiconductors

InSbInSb Based Based HeterostructuresHeterostructures

Small effective mass,large g-factor

Large spin-orbit coupling

Small e-e interaction

An ideal model of a narrow-gap semiconductor

Narrow Gap : Revisited Narrow Gap : Revisited Spin Orbit Interaction

Rashba Effect

Electronic systems with finite spin splitting in the absence of external magnetic field!!!

Z

Ez

X

Y

V

Beff =E×V/c

Symmetric quantum well

InSb

AlxIn1-xSb AlxIn1-xSb

Asymmetric quantum well

InSb

AlxIn1-xSb

AlxIn1-xSb

InSbInSb Quantum Well StructuresQuantum Well StructuresProf. M. B. Santos Prof. M. B. Santos

Department of Physics & Astronomy, Department of Physics & Astronomy, University of OklahomaUniversity of Oklahoma

Density: 1-4x1011 cm-2

Mobility: 100,000-200,000 cm2/VsAlloy concentration: 9%, 15%

RashbaRashba EffectEffectAsymmetry of the confining potential in a QW remove the degeneracy of band structure Rashba effect:

ttsub

z kmkEE α±+= *

22

2h

E

K

Electronic systems with finite spin splitting in the absence of external magnetic field!!!

Y.A. Bychkov and E.I. Rashba, J. Phys. C 17, 6039 (1984);Y.A. Bychkov and E.E. Rashba, [JETP Lett. 39, 78 (1984)]

RashbaRashba Splitting at B>0Splitting at B>0

E(k)=ħ 2k2/2m ±αk

In addition to:J. Luo, et al. PRB, 41,7685 (1990)

Change in g* at low magnetic fieldG. A. Khodaparast, et al. Phys. Rev. B 70, 155322 (2004)

Spin RelaxationSpin Relaxation

Equilibrium Excitation Recombination Non-equilibrium

Why Why SpintronicSpintronic??

ISD

VG

Normal Field Effective Transistor (FET) B

s

Spin Polarized FET

<Sx>∝cosωtω ∝E - E

Efficiency!!Efficiency!!

Spin Field Effective Transistor: Datta and Das, 1990, APL. Phys. Lett., 56, 665

Our MotivationOur Motivation

Understand charge/spin dynamics in narrow gap structures

Study phenomena such as interband and intraband photo-galvanic effects, in order to generate spin polarized current

We will need FEL!!

Probe the effect of magnetic impurities on the spin/charge dynamics: Poster by Matt Frazier today

Carrier dynamics:Carrier dynamics:PumpPump--Probe SpectroscopyProbe Spectroscopy

TiTi--Sapphire laser Sapphire laser –– 800nm 800nm

Carrier density ~10Carrier density ~101717 cmcm--33

Optical Parametric Optical Parametric Amplifier Amplifier –– 1.11.1μμm m –– 2.4 2.4 μμm, Carrier density ~10m, Carrier density ~101919

cmcm--33

Optical Induced Magneto Optical Optical Induced Magneto Optical Kerr Effect (MOKE)Kerr Effect (MOKE)

•• Selection rules for inter-band transitions, Spin-polarized carriers using circularly polarized beams

Spin Dynamics:Spin Dynamics:Magneto-Optical Kerr (reflectivity)Magneto-Optical Faraday (Transmission)

• Optical MOKE signal arises from the difference between the optical coefficients of a material for left and right circularly polarized light.

The rotation of the polarization plane is proportional to the magnetization

Carrier RelaxationCarrier Relaxation

• Pump – 800 nm

• Carrier density ~1017 cm-3

• Sharp decrease, followed by an exponential recovery

• Carrier lifetime ~ 40psΔR

/R (2

% P

er D

iv.)

-200 -150 -100 -50 0 50 100 150 200Time Delay (ps)

77K

b)Asymmetric QWSymmetric QW

Spin relaxationSpin relaxationPh

oto

Indu

ced

MO

KE

(1m

V pe

r div

.)

-200 -100 0 100 200Time Delay (ps)

77 K

Fluence50 μJ/cm2

σ+

σ−

• Spin relaxation for left and right circularly polarized pump.

• Relaxation time ~ 40ps

Carrier relaxation at 2Carrier relaxation at 2μμmm

Asymmetric Quantum Well

• Pump – 2 μm, Probe 800 nm

• Carrier density ~1019 cm-3

• Sharp decrease, followed by an exponential recovery.

• Carrier lifetime ~ 8ps

RT

-1.0

-0.5

0

0.5

1.0

-10 0 10 20

Delay (ps)

Δ R/R

(%)

RT

Spin relaxation at 2Spin relaxation at 2μμmmAsymmetric Quantum WellAsymmetric Quantum Well

• Spin polarized carriers excited using circular polarized light

• Spin lifetime ~ 4 ps

-1.5

0

1.5

3.0

0 5 10

Delay (ps)

MO

KE( μ

V) σ-

RT

Carrier relaxation at 2Carrier relaxation at 2μμmmSymmetric Quantum WellSymmetric Quantum Well

• Pump – 2 μm, Probe 800 nm

• Carrier density - ~1019 cm-3

• Sharp decrease, followed by an exponential recovery.

• Carrier lifetime ~ 4 ps

-3

-2

-1

0

1

2

-2.5 0 2.5 5.0 7.5 10.0

Delay (ps)

ΔR/R

(%)

RT

Spin relaxation at 2Spin relaxation at 2μμmmSymmetric Quantum Well

• Spin polarized carriers excited using circular polarized light

• Spin lifetime ~ 2 ps

-10

0

10

-3 0 3 6

Delay (ps)

MO

KE

(μV

)

σ+

RT

Time Resolved Cyclotron Resonance Measurements, Time Resolved Cyclotron Resonance Measurements, Stanford FELStanford FEL

delay

Pelliclebeamcombiner

FELprobe

Ti:Spump

synchronization

Ga:Ge detectorBSample

0-8 T

λ = 800 nmpulse ~ 200 fsmax 1 mJ/pulse

λ = 3-60 μmpulse 600 fs – 2 psmax 1 μJ/pulse

Prof. Kono’s Group at Rice University

Time Resolved Cyclotron MeasurementsTime Resolved Cyclotron Measurements

(T0 -T)/T

0

Delay (

ps)

B (T)

Interband pump + Intraband probe

Monitor dynamics of relaxing carriers in conduction band directly in time:

Effective mass m*(t)Density n(t)Scattering time τ(t)

G. A. Khodaparast et al, PRB 67 035307 (2003)

Carrier Dynamics in Carrier Dynamics in InSbInSb QWsQWs

0

0.1

0.2

0.3

0.4

0.5

0 100 200 300 400

ΔT/

T

Time Delay (ps)

Differential Transmission

Interband Pumping (800 nm)

Probe Beam (42 μm)

Undoped InSb MQW containing 25 periods of 35 nm InSb wells

1.5K

Spin Polarized CurrentSpin Polarized Current

S. D. Ganichev and W. Prettl, J. Phys: Condens. Matter 15 (2003) R935-R983

Spin Polarized CurrentSpin Polarized CurrentUsing FELUsing FEL

•Spin Current as a result of spin flip scattering

•Both inter-band and inter-subbandhas been observed in GaAs based QWs

S. D. Ganichev and W. Prettl, J. Phys: Condens. Matter 15 (2003) R935-R983 C. L. Yang et al. , PRL, 96, 186605 (2006)

Summary/ Future WorkSummary/ Future Work

- InSb based structures important: fundamental physics and applications

High carrier mobility, large Rashba effect,….

-We have some understanding of carrier/spin relaxations in this system

- We can take advantage of FEL to probe spin polarized current in this structure

Thank you for your attention