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 Spintronics Topological insulator Bi 2 Te 3 Y.L.Chen et al., Science 325 (2009) 178. Spin-orbit interaction High-resolution spin-resolved ARPES Electronic-field induced spin-current Rashba 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, E z ) Surface Rashba effect Spin-orbit interaction surface potential Effective magnetic field spin-resolved ARPES Time reversal symmetry E(k,↑) = E(-k,↓) 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 (retarding-type) (high-energy type)  E = 30 meV  E = 70 meV (Fe(001)p(1x1)-O) Sb(111) Bi 1-x Sb x (x=0.13) [9] A. Nishide et al., PRB 81 (2010) (R). [8] T. Okuda et al., RSI 79 (2008) [1] K. Iori et al., RSI 77 (2006) [2] S. Qiao et al., RSI 68 (1997) [3] T. Kadono et al., APL 93 (2008) [1,2] [3] Au(111) Mott scattering E K = 25 keV Mott scattering E K = 60 keV Electron diffraction E K = 6 eV [6] M. Hoesch et al., PRB 69 (2004) (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) [4,5] [6] [9] [7,8]

High-resolution spin-resolved photoemission spectrometer 7

A B C D Spin-resolved ARPES system 8 PzPz PyPy (A,B) (C,D) Spin polarization x y z Spin-integrate ARPES Energy Angle Spin-resolved ARPES spin up spin down

Energy resolution at MCP Au metal T = 3.5 K Nb superconductor simulation BCS function Tc = 9.2 K Gap size  = 1.5 meV Broadening  = 200  eV 900  eV T = 3.5 K simulation FD function Energy resolution at MCP Xe I eV 9

High-resolution spin-resolved photoemission spectrometer 10 S. Souma et al., RSI 78 (2007) Xe I photons8-11 eV Intensity 2 x photons/sec Operation pass energy Ep = 1,2,5 eV Energy Mott = 8-40 meV Ep: pass energy Energy Mott ~ 0.008Ep eV

Side view High-resolution spin-resolved photoemission spectrometer 11

Discharge problem Au4f ch1 ch2 12 ch2 ch1

Solving for discharge of Mott detector Channeltron Scattering chamber Feed through Safety cover To HV supply Au target Focus cup Channeltron Scattering chamber Focus cup V 2200 V 1300 V Spark － Solutions － 1. Re-polishing of high voltage electrodes 3. Washing all parts 4. Baking 5. 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 , Noise at channeltron

Test measurement with gold sample ch A ch B ch C ch D Au He I  T=300K Ep 10eV ch A ch B ch C ch D Au T=10K Ep 1eV Xe I eV 14 Energy 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) semimetal Surface peculiar metal Bi, Sb bulk 15 Crystal structure of Bi

16 Previous spin-resolved ARPES studies Bi(111) film H. Hirahara et al., PRB 76 (2007)

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

Binding Energy (eV) EFEF Wave vector k x (Å -1 ) Electronic structure near E F of Bi(111) surface Wave vector k x (Å -1 ) Wave vector k y (Å -1 ) hole pocket electron pocket hole pocket electron pocket 21

Spin-resolved ARPES of Bi(111) surface Binding Energy (eV) EFEF B  Wave Vector k x (Å -1 ) y z up spin down spin z direction Intensity (arb. units) Binding Energy (eV) EFEF up spin down spin Intensity (arb. units) y direction

Binding Energy (eV) EFEF Wave Vector k x (Å - 1 ) Problem in Bi(111) surface state Time reversal symmetry E(k,↑) = E(-k,↓) Degeneracy of surface band at  (k=0) point Sb(111) Bi(111) Bi(111): surface band is unclear at  due to bulk band projection ARPES on Sb(111) same crystal structure no bulk projection at  near E F 23

Band structure near E F of Sb(111) surface 24 K. Sugawara et al., PRL 96 (2006)

Band structure near E F of Sb(111) surface 25 K. Sugawara et al., PRL 96 (2006)

Surface band of Sb(111) at  point 2nd derivative 26 K. Sugawara et al., PRL 96 (2006)

Spin-resolved ARPES spectra of Sb(111) spin up spin down Bulk band Surface band 27 K. Sugawara et al., PRL 96 (2006)

SUMMARY Spin-resolved ultrahigh-resolution ARPES study of Rashba effect on semi-metal surface Energy resolution  E= 8 meV Observation of Spin-splitting of surface band on Bi and Sb (111) Time reversal symmetry holds at  Surface Rashba effect on group-V semimetal surface