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钿胡
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2015 Winter School on Superconductivity @ HPSTAR Superconductivity in FeSe/STO with a Tc above 100K and new evidences for the Majorana mode

1 2015 Winter School on HPSTAR Superconductivity in FeSe/STO with a Tc above 100K and new evidences for the Majorana mode Jinfeng Jia Dept. of Phys. & Astronomy, Shanghai Jiaotong University, Shanghai , China jfjia@sjtu.edu.cn 2015 winter school on SC, HPSTAR

2 Content Introduction Superconductivity in FeSe/STO with a Tc above 100 K Majorana mode in a vortex core Topological superconductor by proximity Evidences for Majorana mode Summary

3 Molecular Beam Epitaxy(MBE) State of art sample growth technique Invented by J. R. Arthur & A. Y. Cho

4 Scanning Tunneling Microscope Ultra low temperature STM

5 QUANTUM CORRAL ( 量子围拦 ) 48 Fe atoms Fe Fe distance:0.94nm D:14.3nm Courtesy: Dr. Eigler (IBM Almaden)

$energy electron diffraction (LEED) J. Phys.$

6 UHV-VT STM-MBE system Experimental methods LEED STM MBE Ultrahigh vacuum (UHV) mbar Scanning tunneling microscopy (VT STM) 30K-1500K Molecular beam epitaxy (MBE) Low energy electron diffraction (LEED) J. Phys. D-Appl. Phys. V44 (46), (2011)

7 Indentical In Nanodot Array Empty States at +1.5V Empty States at +0.6V Phys. Rev. Lett. 88 (2002) Surface mediated

8 Lab for Low Dimensional Physics and Interface Engineering LTSTM-MBE 400mK, 11T SPSTM-MBE 400mK 3D field: 2T, 2T, 7T ARPES-LTSTM-MBE 丹麦 CAP RES A/S 日本 UNI SO KU SJTU, June, 2010 LTSTM-MBE-4probe, 400mK, 11T In-situ transport July, 2013

, 29, 037402 (2012) A STM/STS T c ~85 K, if 2 /k B T c = 5.

10 Introduction on 1 layer FeSe/STO Pioneering Experiment: Xue s group, Chin. Phys. Lett., 29, (2012) A STM/STS T c ~85 K, if 2 /k B T c = 5.5 (bulk FeSe) 1 UCL mv Tunneling spectroscopy Discovered a large SC energy gap, which indicates a high T c

Previous researches on FeSe/STO ex situ Transport Measurements: Wang s, arxiv: 1311.5370 (2013); Chu s, arxiv: 1311.6459 (2013).

11 Previous researches on FeSe/STO ex situ Transport Measurements: Wang s, arxiv: (2013); Chu s, arxiv: (2013). T c zero ~23.5 K 1 UCL 80 K T c zero <10 K 4 UCL Detected zero-resistance at temperatures higher than T c of bulk FeSe, but not as high as expected

12 Previous work on FeSe/STO ARPES Investigations: Zhou s, Nat. Mater., 12, 605 (2013); Feng s, Nat. Mater., 12, 634 (2013); Shen s arxiv: (2013). 70 K 23 K T c ~60 K T c ~55 K T c ~65 K Confirmed the SC-like energy gap, which closes at a relatively high temperature.

13 STM tip holder Piezo tube z y x Piezo stage STM/4PP Tip Stage 4PP tip holder Our In situ 4PP setup FeSe STO 4PP tips Tip separation: New design: MBE+STM+4PP ~10 μm or ~100 μm STM, Unisoku Co. Low Temperature 1 K High Magnetic Field 11 T Vibration isolation system Approaching system UHV system

Ohmic contact In situ 4PP: contacts 90 K Tips touch sample one after another V(mV) V(mV) V(mV) V(mV) 200 100 60 30 0-30 Tip 4 R=53.0 V(mV) -60-1.0-0.5 0.0 0.5 1.0 40 I(mA) Tip 4 20 R=29.

14 Ohmic contact In situ 4PP: contacts 90 K Tips touch sample one after another V(mV) V(mV) V(mV) V(mV) Tip 4 R=53.0 V(mV) I(mA) Tip 4 20 R= V(mV) I(mA) Tip 4 20 R= Tip 4 R= I(mA) I(mA) V(mV) Tip 2 R= I(mA) 0 Tip 2 R= I(mA) No contact No contact No contact Tip 2 R= I(mA) V(mV) V(mV) Tip 1 R= I(mA) 80 Tip 1 40 R= No contact I(mA) V(mV) No contact No contact Tip 3 R= I(mA) Tip 4 Tip 2 Tip 1 Tip 3 1 STO FeSe 2 3 V 4

15 Thermometer calibration Cernox Resistor type provided by Lake Shore Cryotronics (Model: CX-1030-SD-HT-0.3M) RT (296 K) Liquid N 2 Thermometer display: K 77.6 K LN 2 or LHe T Meas. spot Our In situ 4PP setup LN 2 or LHe 4PP Exp. T. Calibration

16 Test: 4PP measurement on BSCCO C1423 Temperature down

17 Test: 4PP measurement on BSCCO C1234 Temperature down

18 Experimental results Sample growth by MBE Substrate: Nb-doped STO 500nm 12nm

19 V 23 (mv) C1423 Experimental results SC I-V curves in two measurement configurations I 14 (ma) V (mv) C1234 Ic=3.7mA Ic=4.1mA 7.8 K 3.6 K I (ma)

20 Experimental results Temperature dependence of SC I-V curves V 34 ( V) R (m I 12 (ma) Cooling by LHe K 108.8K 131.6K 94.0K 113.9K 92.9K 109.8K K T (K) I-V curves I c -T relation Cooling by LN 100 C1423 C Ic (ma) Tc ~ 109 K Tc ~ 111 K T (K) Tc=111K 120

21 Control Experiments Phase transition of STO substrate 1.4 T.Nestler et al., Appl. Phys. A 105, 103 (2011). R (m V 34 ( V) SC I-V curve is not due to STO phase transition! Bare SrTiO 3 3 cubic-tetragonal phase transition 104 T (K) 99.2 K I 12 (ma) 108

22 R-T at one fixed location

23 Experimental results Magnetic field dependence of SC I-V curves 7.8 K

24 Fix Temperature, Measure R-H Curves 2.0 R(m ) Below 95K, SC persists at 10T T=100K, B=0T, no SC, R increases a bit with magnetic field K 99K 98K 96K 97K 95K 90K 80K 70K 60K 50K B (T) 8 10

25 Fix Magnetic Field, Measure R-T Tc =98.5~99K smaller than previous one R(m ) T 2T 4T 6T 8T 10T R(m ) B=10 T T (K) T (K)

26 Estimation of Hc Hc=115.7±12.3 T, Tc=99.3±0.2 K Nature Materials 14, 285 (2015)

27 Majorana fermions Ettore Majorana in 1937 In the high-energy area Neutrinos are MFs Supersymmetric theories Majorana superpartner of bosonic particles may provide one of the keys to the dark matter puzzle Pursuit of MFs in solid state systems Exotic fundamental physics Quantum computing applications Hot topic in modern physics

28 Majorana mode in Superconductors E=0, Q=0, S=1/2 Frank Wilczek, Majorana returns, Nature Phy. 5, 614 (2009)

29 TSC by Proximity effect Proximity effect between SC and TI leads to p x +ip y SC-like-state Majorana Bound States (MBS) at magnetic vertices SC TI L. Fu and C.L. Kane PRL 100, (2008) A. Cook and M. Franz, Phys. Rev. B, 84, (2011)

30 MF observed? PRL 109, (2012) Science 336, 1003 (2012)

31 Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor /science Ali Yazdani, Science , 2 October 2014

32 This suggests that the topological gap is comparable to or smaller than 80 μev.

33 Vortex lines in topological insulator-superconductor heterostructures In most case, the bulk effects can be ignored A Majorana fermion is stable with a spatial extent~40nm Chemical potential & Majorana states Majorana fermions can survive for thick samples Hughes group, PRB 84, (2011) PRB 87, (2013)

34 s-wave superconductor Topological insulator MBE growth to obtain sharp interface. Pb, Nb, Al, NbSe 2, NbS 2 Bi-Sb, Bi 2 Te 3,Sb 2 Te 3,Bi 2 Se 3, heavy atoms Stable at T<300 o C Sharp interface! s wave-sc on TI, difficult to realize in practice.

Our previous work on TI fims High quality

by MBE Control the type of Bi 2 Te 3 films

103, 266803 (2009) Landau levels PRL 105,

35 Our previous work on TI fims High quality Bi 2 Te 3, Bi 2 Se 3,Sb 2 Te 3 thin films by MBE Control the type of Bi 2 Te 3 films Studies by LTSTM & ARPES Standing wave PRL 103, (2009) Landau levels PRL 105, (2010) Formation of DC Nature Phys. 6, 584 (2010) Adv. Mater. 22, (2010) Adv. Mater. 23, (2011) Thinnest limit

36 TI on SC, much easier to achieve Topological insulator s-wave superconductor NbSe 2 Bi 2 Se 3 or Bi 2 Te 3 Sharp interface! Good crystallization? Topological surface state? Cooper pairs on the surface? Topological SC? Vortex? Bound states & Majorana?

37 Bi 2 Se 3 films grown on NbSe 2 SC Sharp interface Less defects

38 A platform for searching Majorana Fermions Topological surface states 2 mev 0-2 mev h + e - Intensity superconductivity Science 336, (2012)

39 Bi 2 Te 3 on NbSe 2

40 Topological superconductor

41 Vortex on topological superconductor

42 Coherence length and core states Coherence length deduced from Vortex Much larger than that in NbSe 2 Saturate at 3QL Change with magnetic field Saturate at ~0.7T Core states observed PRL 112, (2014)

43 Momentum-space imaging of Cooper pairing 3QL 7QL

44 Ef=0, 15SC+5QL TI E f in conduction band => Normal vortex Can host Majorana mode!

45 Tuning Fermi energy Future: For 5QL Bi 2 Te 3 /NbSe 2 Now: Thermo energy ~ mv

46 Splitting of core states

47 Core states splitting in CSC NbSe 2 Hess et al., Phys. Rev. Lett. 62, 214 (1989) F. Gygi, M. Schluter, PRB 43, 7609 (1991)

48 Spatial extent of Mojorana fermion A Majorana fermion in a spatial extent~40nm Hughes group, PRB 84, (2011) PRB 87, (2013) Strong evidence for existence of Majorana mode

49 More evidence

50 Why transition at 4QL? PRL 114, (2015)

Model system T. Kawakami and XH: arxiv.1506.

51 Model system T. Kawakami and XH: arxiv Geometry Hamiltonian s: orbital s: spin Parameters for calculation: * Band gap of Bi 2 Te 3 : 2e 0 ~0.2eV * SC gap: =0.02e 0 ~2meV * chemical potential: =0.5e 0 ~2e 0

Majorana bound states in thick TI film Energy dispersion and distribution of DOS of quasiparticles * typical length * coherence length MBS at small Fermi level

52 Majorana bound states in thick TI film Energy dispersion and distribution of DOS of quasiparticles * typical length * coherence length MBS at small Fermi level thick TI film suppression of MBS bulk conduction bands induce interactions in thin TI film Fu and Kane, PRL 100, (2008). Hosur, et al., PRL 107, (2011).

Evolution of DOS with thickness Energy-space distribution of DOS of quasiparticles: di/dv in experiments Y thick Y V V smearing factor in energy η = 0.

53 Evolution of DOS with thickness Energy-space distribution of DOS of quasiparticles: di/dv in experiments Y thick Y V V smearing factor in energy η = ~4K Y shape w MBS V shape w/o MBS full agreement with experiments! thin Thickness vs. chemical potential theoretically thickness only cannot induce phase transition, but can.

54 In situ is important Tc of single-layer FeSe/STO~109 K Topological superconductor Vortex and core states Evidences for MFs Summary 4 3 C1423 C R (m Cooling by LHe T (K) Cooling by LN R(m ) K 99K 98K 97K 96K 95K B (T) 8 10

55 Acknowledgments QK Xue Zhuan Xu ZJU Xi Chen Y. Liu PSU X.C. Ma S.C.Li Nanjing U C.H. Liu M.X. Wang, J.P. Xu, L. Miao, F. Yang, M.Y. Yao, Z. F. Wang, F.F. Zhu, H.H. Sun Supported by NSFC, MOST and MOE C.L. Gao D. Qian Theoreticians: F. Liu, L. Fu, S.B. Zhang, X.C. Xie, Z. Fang, X. Dai, S. Q. Shen, Xiao-Liang Qi, Shou-Cheng Zhang., F.C. Zhang, Q.H. Wang, Y. Chen

56 Thank you very much!

強相関伝導系のバルクや薄膜の電子状態

強相関伝導系のバルクや薄膜の電子状態 UVSOR 0140130 Outline ARPES ARPES 1 + 9.1110-31 kg Designed by T. Ito m c E v p = m c v (E) 1900 E m v 1 (v), (p), (k) c p m c k m c m f (E) E 1 m f (v), (p), (k) v p m f k m f m f >> m c 4f 4f Ce 3+ :