New Quantum Transport Results in Type-II InAs/GaSb Quantum Wells

Transcription

1 New Quantum Transport Results in Type-II InAs/GaSb Quantum Wells Wei Pan Sandia National Laboratories Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy s National Nuclear Security Administration under contract DE-AC04-94AL SAND NO XXXXP

2 Work done in collaboration with Sandia National Labs: X. Shi M. Thalakulam M.J. Cich J.K. Kim J.F. Klem Georgia Tech: W. Yu Z. Jiang UC Irvine: S.K. Lyo

3 Part I - Quantum transport near the charge neutrality point in inverted type-ii InAs/GaSb field-effect transistors Part II - Transport studies of a superconductor- InAs/GaSb bilayer junction

4 Part I Quantum transport near the charge neutrality point in inverted type-ii InAs/GaSb field-effect transistors

5 Part I Outline The sample The experiment and results Electron transport at zero magnetic field Electron transport at low magnetic fields Electron transport at very high magnetic fields Summary

6 The sample: D E g0

7 Growth structure: air InAs 20A (or GaSb 20A) AlSb 500A GaSb QW 50A InAs QW 150A AlSb 1um GaSb 1um GaSb substrate (p-doped) Field-effect transistor: Yang et al, Appl. Phys. Lett. 69, 85 (1996). AlSb GaSb buffer on p-type GaSb E g0 D

8 R xy R xx V g I

9 R xx (k ) Electron transport at zero magnetic field: 6 B = 0 T T ~ 25 mk D 4 E g V g (V)

10 G xx (e 2 /h) 15 T ~ 25 mk B = 0 T e 2 /h V g (V)

11 s th xx e 2 /h E g0 /D Y. Naveh and B. Laikhtman, Europhys. Lett. 55, 545 (2001). E g0 ~ 15 mev E g0 D D~ 1 mev s th xx 15 e 2 /h G th xx = 5 e 2 /h ~ 4 e 2 /h

12 G xx (e 2 /h) 4.3 G xx = log (T) T (mk)

13 R xx (arb. units) Electron transport at low magnetic fields: 0.04 B = 2 T = V g (T)

14 R xy (h/e 2 ) 0.1 B = 2 T V g (V)

15 n, p (10 11 cm -2 ) n= xv g CNP p= xv g V g (V) At CNP (n + p =0) n = p ~ cm -2

16 s xx, s xy (e 2 /h) s xy (e 2 /h) s xx B = 5T h =-2 s B = 5T xy e = V g (V) V (V) g 8 6 s xx (e 2 /h)

17 Electron transport at high magnetic fields: 4 e =3 s xx & s xy (e 2 /h) h V g (V) 2 B = 20 T T ~ 30 mk

18 8 7 6 s xx (e 2 /h) s xy (e 2 /h) (s xx N) 2 + s xy 2 = N 2

19 (s xx N) 2 + s xy 2 = N 2 r xx = h/e 2 /(2N)

20 e =1 2 3 r xx & r xy (h/e 2 ) h =-1 B = 20 T T ~ 30 mk V g (V)

21 (s xx N) 2 + s xy 2 = N 2 r xx = h/e 2 /(2N) Due to quantum spin Hall effect? C.X. Liu, T.L. Hughes, X.L. Qi, K. Wang, and S.C. Zhang, Phys. Rev. Lett. 100, (2008). I. Knez, R.R. Du, and G. Sullivan, Phys. Rev. Lett. 107, (2011).

22 Part I summary: (1) well-developed integer quantum Hall effect states at Landau level fillings =1, 2 in the hole regime and =1, 2, 3 in the electron regime. (2) a chaotic quantum transport behavior at extremely high magnetic fields around the charge neutrality point (CNP). (3) a circular conductivity law in s xx versus s xy.

23 Part II Transport studies of a superconductor- InAs/GaSb bilayer junction

24 Motivation Theoretic calculation: inverted InAs/GaSb exhibits quantum spin Hall effect C. Liu et al., PRL 100, (2008)

25 Motivation Experiments: inverted InAs/GaSb is a quantum spin Hall insulator I. Knez et al., PRL 107, (2011)

26 Motivation Experiments: Andreev reflection in S-N-S junction I. Knez et al., PRL 109, (2012)

27 InAs/GaSb with critical width 8-band calculation with critical QW width (d=10 nm) K. Chang, unpublished Shubnikov-de Hass oscillations: Inverted sample: 2 Critical width: 4

28 S-N-S junction R (Ohm) Ta InAs 2 nm 240 nm AlSb 50 nm GaSb 5 nm InAs 10 nm AlSb 50 nm Ta 240 nm Ta-InAs/GaSb-Ta junction Wet etch then immediately transfer into sputter chamber S-N-S junction Critical InAs QW width: d=10 nm Junction: W=12 mm L= 2 mm I bilayer Ta Ta V T c =1.55K T (K)

29 di/dv (e 2 /h) Differential conductance 600 T=0.3K, B=0 500 DV=0.3 mv DCV on sample (mv) Two SC gaps in Fe 1+y Te 1-x Se x Complicated structure Need for two superconducting gaps Peng et al., J. Phys.: Cond. Matt, 24, (2012)

30 Two-gaps BTK fit Blonder-Tinkham-Klapwijk (BTK) fit Two-gaps BTK fit di dv V = Ne h df 0 (E ev) dv 1 + A E B(E) de s = w s 1 + (1 - w) s 2 N: number of modes in InAs QW f 0 (E): Fermi Dirac function A(E): probabilities of Andreev reflection B(E): probabilities of ordinary reflection A(E) and B(E) depend on Δ: superconducting gap Z: interfacial barrier strength Γ: depairing parameter Blonder et al., PRB, 25, 4515 (1982) Pleceník et al., PRB, 49, (1994) s 1, s 2 : differential conductance for two SC gaps w: weight of the contribution of the first gap Peng et al., J. Phys.: Cond. Matt, 24, (2012)

31 Two-gaps BTK fit di/dv (e 2 /h) di/dv (e 2 /h) T=0.3K, B=0 2 gaps fit T=0.3K, B=0 2 gaps fit 500 DV=0.3 mv DCV on sample (mv) DCV on sample (mv) 2 gaps fit: D 1 = 0.158meV Γ 1 =0.026 mev D 2 = 0.528meV Γ 2 =0.11 mev w=0.70 Z=0.01 (+/- 0.05)

32 di/dv (e 2 /h) Two-gaps BTK fit R (Ohm) 600 T=0.3K, B=0 2 gaps fit DV=0.3 mv 160 T c =1.55K B=0T DCV on sample (mv) 2 gaps fit: D 1 = 0.158meV Γ 1 =0.026 mev D 2 = 0.528meV Γ 2 =0.11 mev w=0.70 Z=0.01 (+/- 0.05) T (K) Ta: T c =1.55K -> D(0)= 0.236meV Bulk T c =4.47K -> D(0)= 0.68meV Note: BCS gap: D(0)=1.76 k B T c

33 Part II Summary di/dv (e 2 /h) Ta-InAs/GaSb-Ta junction with critical width of QWs: two gaps revealed in di/dv 600 T=0.3K, B=0 2 gaps fit DCV on sample (mv)