Time - domain THz spectroscopy on the topological insulator Bi2Se3 (and its superconducting bilayers)

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1 Time - domain THz spectroscopy on the topological insulator Bi2Se3 (and its superconducting bilayers) N. Peter Armitage The Institute of Quantum Matter The Johns Hopkins University

2 Acknowledgements Liang Wu (JHU) Luke Bilbro (JHU) John Cerne (UB) Rolando Valdes Aguilar (JHU) Deepu George (UB) Andreas SAer (JHU/UB*) Seongshik Oh (Rutgers) Namrata Bansal (Rutgers) Wei Liu (JHU) Andrea Markelz (UB)

3 States of matter traditionally characterized by the symmetries they break - translation, rotation, gauge, etc. Quantum Hall effect taught us that states can also be distinguished by topological properties of their wavefunctions. --> Plateau quantization exact to 1/10 9. Plateaus are different states with the same symmetry, but different wavefunction topology. Gauss Bonnet theorem κ is curvature; g is integer integral is quantized! Looks like a curvature

4 2D/3D topological insulators Spin - orbit coupling looks like a momentum dependent magnetic field - can drive a band inversion - like in HgTe Topological band theory based on Z2 topological band invariant of single particle states. (Fu, Kane and Mele, Moore and Balents, Roy)

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6 (a) z y (b) x y ()&*+),-"$ -./"0$ (c) C x A t 1 t 2 t 3 %&$!"'$!"#$ B C A B C

7 Transport in single crystals Xiong et al d ~ 0.1mm 2.6 x /cm 3 R ~ 400 Ω/ Gsurface/Gbulk ~ 1.35 Hor et al. 2010, d ~ 1mm 8 x /cm 3 Gsurface/Gbulk < 0.01 If made sufficiently insulating, thin films would be an idea platform for optical investigation

8 Experiments

9 High quality MBE grown Bi2Se3 films Grown on surface treated 0001 sapphire substrates Much lower defect density than other substrates (Si etc.) AFM shows large terraces (300 QL sample) From two carrier model: For less than 200 QL surface transport dominates.

10 High quality MBE grown Bi2Se3 films For aged sample surface <-- to bulk figure of merit only improves

11 Time-domain THz spectroscopy Auston switch (emitter/ receiver) Femtosecond laser THz radiation Femtosecond laser pulse excites photoconductive emitter and receiver Coherent detection of field allows both components of optical response to be measured. 100 GHz - 3 THz, 2K - 300K. "(# ) = 4n n +1 e i$ s n +1+% # ( )dz 0

12 THz Conductance Measurements phonon α mode Drude free e - like

13 THz Conductance Measurements

14 Thickness independent Low Energy <-- Conductivity Sum Rule

15 Aging effects in TI thin films 64 QL T < 10 K

16 Aging effects in TI thin films Can describe low energy response with two Drude + phonon One small and narrow and one broader and fatter. Small one (which we associate with bulk) spectral weight decreases after a few days. Consistent with bulk mobility measrements Decreasing due to Se out diffusion and O compensation Aged films are better for transport investigation

17 General theory of EM response of TI: from Kane 2010 Exotic mangetoelectric effects (axion electrodynamic response, induced monopoles) Qi, et al. '08; Essin, et al.'09; Tse et al. '10,'11; Maciejko '10 Can be described by and E*B term to EM Lagrangian Predict universal rotations the size of which set by fine structure constant α

18 Finite field

19 Longitudinal Conductivity Spectral weight supression # Surf. state gapping

20 Faraday and Kerr Measurements In TDTS, E field is measured in time. Multiple reflections in substrate can be time-separated. Faraday Kerr Ame Ame This is unique to TDTS, tradiaonal spectroscopies measure in frequency directly.

21 Experiment P1 P2 Θ B Without any rotation, amplitude of E field is: A cos 2 (Θ) If E field rotates by angle α, then: A sin(θ)cos(θ α) We sought to measure the 2 nd reflection, ie. proportional to the Kerr rotation.

22 Kerr Rotation I 1 st and 2 nd pulse in Sapphire 10 T ZERO ROTATION.

23 Kerr Rotation II 2 nd pulse in Bi 2 Se 10 T; 64 QL films # 65 o ROTATION!!!

24 Kerr Rotation III 2 nd pulse in Bi 2 Se 3 in cross 10 T; 64 QL film # 65 o ROTATION!!! similar results obtained by Hancock et al. on strained lower field

25 Kerr Rotation IV Frequency (THz) Magnetic Field (Tesla) QL film

26 Kerr rotation from Dirac fermion cyclotron resonance n is substrate index Z0 is impedance of free space αh/2e 2

27 A surface band bending cannot explain Kerr rotation ARPES sees evidence for surface band bending of width 40 nm with ARPES densities and observed mass ~ 0.13 me cannot explain data

28 What is needed for the axion regime? complete gapping of surface quantum Hall regime --> EF closer to Dirac point and higher mobilities required

29 TI/Nb bilayers

30 Superconductivity - 20 nm Nb on 32 QL Bi2Se3 Nb sputtered in situ; THz transmission through the bilayer Superfluid density suppressed for Nb on TI A consequence of spin-momentum locking on TI surface? inverse-proximity effect

31 Superconductivity - 20 nm Nb on 32 QL Bi2Se3 Gap edge conductivity suppressed in usual way à la Mattis-Bardeen Type II coherence factors

32 In conclusion we find... - Thickness independent transport - Sample aging --> Good thing - Colossal Kerr rotation - Suppressed superfluid density in SC/TI bilayer

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