Ultrahigh-resolution spin-resolved ARPES of novel low-dimensional systems

Seigo Souma

Tohoku University

May 31, 2010

A. Takayama, K. Sugawara, T. Sato, and T. Takahashi

Collaborators:

1

WS10-ETLODs, Valencia-Spain

Anomalous electron spin phenomena

Spin dependence of electronic structure

Rashba effect SpintronicsTopological insulator

Bi2Te3

Y.L.Chen et al., Science 325 (2009) 178.

€

Beff =α

gμB

ky

−kx

⎛ ⎝ ⎜

⎞ ⎠ ⎟Spin-orbit

interaction

High-resolution spin-resolved ARPES

Electronic-field induced spin-currentRashba term

Spin switch via S.O. interaction

Edge state (surface state)Time reversal invariant

E(k,↑) = E(-k,↓)

2

Spin-splitting of surface Rashba effect

∇V= (0, 0, Ez)

Surface Rashba effect

Spin-orbit interaction

surface potential

Effective magnetic field

€

Beff ~ (∇V × p)

spin-resolved ARPES

Time reversal symmetry

E(k,↑) = E(-k,↓)

€

ˆ H SO = α Rσ ⋅ (∇V × p)

3

Space inversion symmetry

E(k,↑) = E(-k,↑)

Angle-resolved PES (ARPES)

e- freedom •Energy•Momentum

4

Detection of electron spin is

difficult !!Efficiency of instrument goes down by 3-4 order

Energy Resolution 100 meV

Spin-resolved ARPES

e- freedom •Energy•Momentum•Spin

5

Mott scattering

Mini Mott Detector

25 keV

c

6

Recent spin-resolved ARPES studies

VLEEDMott detector Mott detector(retarding-type) (high-energy type)

DE = 30 meVDE = 70 meVDE = 70 meV

(Fe(001)p(1x1)-O)

Sb(111) Bi1-xSbx(x=0.13)

[9] A. Nishide et al., PRB 81 (2010) 041309(R).[8] T. Okuda et al., RSI 79 (2008) 123117.

[1] K. Iori et al., RSI 77 (2006) 013101.[2] S. Qiao et al., RSI 68 (1997) 4390.[3] T. Kadono et al., APL 93 (2008) 252107.

[1,2]

[3] Au(111)

Mott scattering EK = 25 keV Mott scattering EK = 60 keV Electron diffraction EK = 6 eV

[6] M. Hoesch et al., PRB 69 (2004) 241401(R).[5] M. Hoesch et al., JESRP 124 (2002) 263.

[7] R. Bertacco et al., RSI 73 (2002) 3867.[4] V. N. Petrov et al., RSI 68 (1997) 4385.

[4,5]

[6] [9]

[7,8]

High-resolution spin-resolved photoemission spectrometer

7

A

B

C

D

Spin-resolved ARPES system

8

Pz Py

(A,B)

(C,D)

Spin polarization

x

yz

Spin-integrate ARPES

Energy

Ang

le

Spin-resolved ARPES

spin up

spin down

Energy resolution at MCP

Aumetal

T = 3.5 K

Nbsuperconductor

simulationBCS function

Tc = 9.2 K

Gap size D = 1.5 meVBroadening G = 200 meV

900 meV

T = 3.5 K

simulationFD function

Energy resolution at MCP

Xe I 8.437 eVXe I 8.437 eV

9

High-resolution spin-resolved photoemission spectrometer

10S. Souma et al., RSI 78 (2007) 123104.

Xe I photons 8-11 eVIntensity 2 x 1013 photons/sec

Operation pass energy Ep = 1,2,5 eV Energy resolution @ Mott

= 8-40 meV

Ep: pass energy

Energy resolution @ Mott~ 0.008Ep eV

Side view

High-resolution spin-resolved photoemission spectrometer

11

Discharge problemAu4f

ch1

ch2

12

ch2

ch1

Solving for discharge of Mott detector

ChanneltronScattering chamber

Feed throughSafety cover

To HV supply

Au target

Focus cup

Channeltron

Scattering chamberFocus cup

25000 V2200 V1300 V

•Spark

－ Solutions －

1. Re-polishing of high voltage electrodes

3. Washing all parts4. Baking5. Conditioning of electrode’s

surface by applying HV

•Field emission BG noise depends on voltage difference between the electrodes

Roughness of surface

2. Coating of electrodes with TiC

13

100,000 cps @18kV

0.1 cps @25kV

Noise at channeltron

Test measurement with gold sample

ch A

ch B

ch C

ch D

AuHe Ia

T=300KEp 10eV

ch A

ch B

ch C

ch D

AuT=10K

Ep 1eV

Xe I 8.437 eV

14

Energy resolution @ Mott= 8 meV

Peculiar surface states of group-V semimetals

Surface Rashba effect

with S.O.

without S.O.

Yu. M. Koroteev et al., PRL 93 (2004) 046403.

semimetalSurface

peculiar metalBi, Sbbulk

15

Crystal structure of Bi

16

Previous spin-resolved ARPES studiesBi(111) film

H. Hirahara et al., PRB 76 (2007) 153305.

In-situ preparation of Bi thin film on Si(111)Si(111) 7×7

Bi(111) 1×1 LEED

substrate

Flash annealing

Bi thin film (80ML) epitaxially grown on Si(111) surface

17

ARPES spectra of Bi(111) surface

surface BZ

bulk BZ

(111)

Xe I (8.436 eV)T = 30 K

Experiment

18

Band structure of Bi(111) surface

19

Spin-integrate band structure of Bi(111) surface

20

Bin

ding

Ene

rgy

(eV

)

0.10

0.15

0.20

0.05

EF

Wave vector kx (Å-1)0.0-0.2-0.8 -0.6 -0.4 0.2

Electronic structure near EF of Bi(111) surface

Wave vector kx (Å-1)0.0-0.2-0.8 -0.6 -0.4 0.2

Wav

e ve

ctor

ky

(Å-1)

0.0

0.1

0.05

-0.05

hole pocket electron pocket

hole pocket

electron pocket

21

Spin-resolved ARPES of Bi(111) surfaceB

indi

ng E

nerg

y (e

V)

0.1

0.2

EF

-0.2-0.4 0 0.2

BG

Wave Vector kx (Å-1)

y

z

up spin

down spin

z directionInte

nsity

(arb

. uni

ts)

Binding Energy (eV)0.10.2 EF

up spin

down spin

Inte

nsity

(arb

. uni

ts)

y direction

Bin

ding

Ene

rgy

(eV

)

0.1

0.2

EF

Wave Vector kx (Å-1)-0.2

-0.4

0 0.2

Problem in Bi(111) surface state

Time reversal symmetry E(k,↑) = E(-k,↓) Degeneracy of surface band at G (k=0) point

Sb(111)Bi(111)

Bi(111): surface band is unclear at G due to bulk band projection

ARPES on Sb(111) same crystal structureno bulk projection at G near EF 23

Band structure near EF of Sb(111) surface

24K. Sugawara et al., 　 PRL 96 (2006) 046411.

Band structure near EF of Sb(111) surface

25K. Sugawara et al., 　 PRL 96 (2006) 046411.

Surface band of Sb(111) at G point

2nd derivative

26K. Sugawara et al., 　 PRL 96 (2006) 046411.

Spin-resolved ARPES spectra of Sb(111)

spin up

spin down

Bulk band

Surface band

27K. Sugawara et al., 　 PRL 96 (2006) 046411.

SUMMARYSpin-resolved ultrahigh-resolution ARPES study of

Rashba effect on semi-metal surface

•Energy resolution DE= 8 meV• Observation of Spin-splitting of surface band on Bi and Sb (111)•Time reversal symmetry holds at G

Surface Rashba effect on group-V semimetal surface