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Tunneling Conductance and Surface States Transition in Superconducting Topological Insulators Yukio Tanaka (Nagoya University) Chernogolovka June 17 (2012)

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Main collaborators Theory Y. Asano （ Hokkaido ） A. Golubov (Enshede) A. Yamakage (Nagoya) K. Yada (Nagoya) M. Sato （ Nagoya ） T. Yokoyama （ Tokyo ） N. Nagaosa （ Tokyo ） M. Ueda （ Tokyo ） Y. Tanuma(Akita ） Y. Nazarov(Delft) M. Sigrist (ETH) Y. Fominov (Landau Institute) J. Linder (Tronheim) S. Kawabata(AIST) Experiment S. Kashiwaya （ AIST ） Y. Maeno (Kyoto) Y. Ando (Osaka) M. Koyanagi (AIST)

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(1) Theory of Tunneling Conductance in Superconducting Topological Insulator A. Yamakage, K. Yada, M. Sato and Y. Tanaka (2) Majorana fermion and odd-frequency Cooper pair Y. Asano and Y. Tanaka Phys. Rev. B (R) 2012 arXiv:

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Surface Andreev bound state (ABS) up to now (1)d-wave (cuprate) (2)chiral p-wave (Sr 2 RuO 4 ) (3)helical (NCS superconductor) (4)3d superconductor (superfluid 3 He) The presence of ABS is supported by the bulk topological invariant. Y. Tanaka, M. Sato and N. Nagaosa, J. Phys. Soc. Jpn (2012)

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Tunneling effect in unconventional superconductors s-wave Normal metal Cuprate Unconventional superconductor ？ Important issue of cuprate in the 90s.

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Tunneling conductance in d-wave junction Normal metal d-wave superconductor angle between the normal to the interface and the lobe direction Bulk ldos (blue line ） Zero bias conductance peak Andreev bound state Surface zero energy state L. Buchholtz & G. Zwicknagl : Phys. Rev. B 23 (1981) J. Hara & K. Nagai : Prog. Theor. Phys. 74 (1986) C.R. Hu : Phys. Rev. Lett. 72 (1994) Y. Tanaka & S. Kashiwaya: Phys. Rev. Lett. 74 (1995) 3451.

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Conductance formula in unconventional superconductor Condition for ABS surface Flat zero energy band C.R. Hu : Phys. Rev. Lett. 72 (1994) transparency （ Tanaka and Kashiwaya PRL ) Bruder (1990) Blonder Tinkham Klapwijk (1982)

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Surface Phase change of pair potential is π Tanaka Kashiwaya PRL (1995), Kashiwaya, Tanaka, Rep. Prog. Phys (2000) Hu(1994) Matsumoto Shiba(1995) ー ー ＋ ＋ Well known example of Andreev bound states in d-wave superconductor kyky ABS in d-wave y Flat dispersion!! Zero energy (110)direction

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Surface Andreev bound state (ABS) up to now (1)d-wave (cuprate) (2)chiral p-wave (Sr 2 RuO 4 ) (3)helical (NCS superconductor) (4)3d superconductor (superfluid 3 He) The presence of ABS is supported by the bulk topological invariant. Y. Tanaka, M. Sato and N. Nagaosa, J. Phys. Soc. Jpn (2012)

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Extension to spin-triplet superconductor J. Phys. Soc. Jpn. 67, 3224 (1998) Phys. Rev. B. 56, 7847 (1997) Normal metal superconductor L. Buchholtz & G. Zwicknagl : Phys. Rev. B 23 (1981) J. Hara & K. Nagai : Prog. Theor. Phys. 74 (1986) 1237

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Condition for ABS chiral p pxpx surface flat dispersion linear dispersion

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Chiral superconductor Sr 2 RuO 4 Maeno (1994) Similar structure to cuprate Edge surface current

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Recent experiment of Sr 2 RuO 4 Experiment Sr 2 RuO 4 Au S/I/N SiO 2 It is possible to fit experimental data taking into account of anisotropy of pair potential. Phys. Rev. Lett. 107, (2011) S. Kashiwaya, et al,

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Tunneling spectrum in two-dimensional topological superconductors d x2-y2 -wave nodal gap chiral p-wave full gap chiral edge state broad zero-bias peak due to linear dispersion E/ E/ theory expt. S.Kashiwaya, 1995 Injected angle Angle resolved conductance Injected angle Kashiwaya et al, Phys. Rev. Lett. 107, (2011) YBCO(110) zero energy flat band of surface states Sr 2 RuO 4

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Surface Andreev bound state (ABS) up to now (1)d-wave (cuprate) (2)chiral p-wave (Sr 2 RuO 4 ) (3)helical (NCS superconductor) (4)3d superconductor (superfluid 3 He) The presence of ABS is supported by the bulk topological invariant. Y. Tanaka, M. Sato and N. Nagaosa, J. Phys. Soc. Jpn (2012)

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Andreev bound state in the presence of spin-orbit coupling Iniotakis, Tanaka et al, Phys. Rev. B 76, (2007) Spin-singlet （ s-wave ） s spin-triplet(p-wave ） p Andreev bound state CePt 3 Si Zero bias conductance peak by Andreev bound state No Andreev bound state Bulk energy gap Helical superconductor Gap closes No Andreev bound state Calculated conductance

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Feature of the Andreev bound states d xy -wave Chiral p-wave NCS (Helical) Hu(94) Tanaka Kashiwaya (95) Tanaka Kashiwaya (97) Sigrist Honerkamp (98) Non-centrosymmetric superconductor (NCS) Iniotakis (07) Eschrig(08) Tanaka (09) Helical Chiral Flat -wave p+s -wave

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Flat dispersion of ABS in NCS superconductor P. M. R. Brydon et al, PRB11 3d case LaAlO 3 SrTiO 3 Edge (mixing of d and p-wave pairing) 2d case K. Yada, et al, Phys. Rev. B Vol (2011) Flat ABS one of the Fermi surface is absent by SO coupling

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Superconducting Materials where zero bias conductance peak by ABS is observed YBa 2 CuO 7- (Geerk, Kashiwaya, Iguchi, Greene, Yeh,Wei..) Bi 2 Sr 2 CaCu 2 O y (Ng, Suzuki, Greene….) La 2-x Sr x CuO 4 (Iguchi) La 2-x Ce x CuO 4 (Cheska) Pr 2-x Ce x CuO 4 (R.L.Greene) Sr 2 RuO 4 (Mao, Maeno, Laube,Kashiwaya) (BEDT-TTF) 2 X, X=Cu[N(CN) 2 ]Br (Ichimura) UBe 13 (Ott) CeCoIn 5 (Wei Greene) PrOs 4 Sb 12 (Wei) PuCoGa 5 (Daghero) Superfluid 3 He (Okuda, Nomura, Higashitani, Nagai)

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Surface Andreev bound state (ABS) up to now (1)d-wave (cuprate) (2)chiral p-wave (Sr 2 RuO 4 ) (3)helical (NCS superconductor) (4)3d superconductor (superfluid 3 He) The presence of ABS is supported by the bulk topological invariant. Y. Tanaka, M. Sato and N. Nagaosa, J. Phys. Soc. Jpn (2012)

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ABS in B-phase of superfluid 3 He 21 Y. Asano et al, PRB ’ 03 tunneling conductance bias-voltage barrier no zero-bias peak due to linear dispersion of surface states BW state (B-phase in 3 He) full gap superconductor Metal z x y z0z0 BW Chung, S.C. Zhang (2009) Volovik (2009) Salomaa Volovik (1988) Schnyder (2008) Roy (2008) Nagai (2009) Qi (2009) Kitaev(2009) perpendicular injection ZES: Buchholtz and Zwicknagle (1981) Dirac Cone type ABS

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ABS and tunneling conductance space dimension gap structure surface state tunneling conductance 2D nodalflat band zero-bias peak full chiral/helical 3D nodalflat band full BW helical double peak superconducting topological insulator ? To clarify tunneling conductance in new type of three-dimensional topological superconductor (superconducting topological insulator). Motivation

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Superconducting topological insulator topological insulator ……metallic surface states surface states Y. S. Hor et al, PRL ’ superconducting topological insulator Cu x Bi 2 Se 3 S. Sasaki et al, PRL ’ 11 tunneling conductance (point contact) zero-bias peak ⇒ gapless surface states new type of three-dimensional topological superconductor L. A. Wray et al, Nature Phys. 10

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Superconductivity on the surface states spin-triplet superconducting gap in bulk not in surface L. Hao and T. K. Lee, PRB 2011, T. H. Hsieh and L. Fu, PRL 2012 energy bulk surface momentum

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Electronic states of Bi 2 Se 3 25 energy levels of the atomic orbitals in Bi 2 Se 3 two low-energy effective orbitals Se1 Se3 Se2 Bi1 Bi2 unit cell of Bi 2 Se 3 Zhang et al, Nature 09

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Hamiltonian of a superconducting topological insulator 26 Hamiltonian of a superconducting topological insulator ： orbital (spin) ： spin full gappoint nodes L. Fu and E. Berg, PRL ’ 10 s-wave spin-triplet (orbital-singlet) superconductor （ supporting gapless surface states ） [111] // z for Bi 2 Se 3 Hamiltonian of the parent topological insulator

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Pair potential proposed by Fu and Berg Energy gap spinOrbit Δ1Δ1 full gap singletintra Δ2Δ2 full gap tripletinter Δ3Δ3 point node along k z direction singletintra Δ4Δ4 point node along k x direction tripletinter or Se Bi Se Bi Se unit cell Se Bi Se Bi Se Cu x Bi 2 Se 3 Effective orbital p z orbital (No momentum dependence) Cu Intra-orbital Liang Fu, Erez Berg, PRL,105, (2010) p z orbital Candidate of Cu x Bi 2 Se 3 Inter-orbital (orbital triplet)(orbital singlet)

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Pairing function in superconducting topological insulator 28 L. Fu and E. Berg, PRL ’ 10 spin singlet s-wave pairing topological insulator: two orbitals spin triplet (orbital singlet) no surface states gapless surface states full gapnodal gapfull gapnodal gap

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Surface states in topological insulators in the normal phase 29 surface states at the Fermi level L. Hao and T. K. Lee, PRB 2011, T. H. Hsieh and L. Fu, PRL 2012 Orbital degrees of freedom is quenched. s-wave spin-triplet superconducting gap is impossible J. Linder et al, PRL 10 (momentum-dependent case) on the surface helical surface states

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Superconductivity on the surface states energy spectrum of topological insulator L. Hao and T. K. Lee, PRB 2011, T. H. Hsieh and L. Fu, PRL 2012 energy bulk surface momentum

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Superconductivity on the surface states 31 spin-triplet superconducting gap in bulk not in surface L. Hao and T. K. Lee, PRB ’ 11, T. H. Hsieh and L. Fu, PRL ’ 12 energy bulk surface energy bulk surface spin-triplet superconductor twisted spectrum momentum

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Structural transition of ABS 32 A. Yamakage, Y, K. Yada, M. Sato, and Y. Tanaka, PRB 12 L. Hao and T. K. Lee, PRB ’ 11 T. H. Hsieh and L. Fu, PRL ’ 12 energy momentum energy large chemical potential cone

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Structural transition of ABS 33 AY, K. Yada, M. Sato, and Y. Tanaka, PRB 12 L. Hao and T. K. Lee, PRB ’ 11 T. H. Hsieh and L. Fu, PRL ’ 12 energy momentum energy at transition group velocity=0

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Structural transition of ABS 34 A.Yamakage, K. Yada, M. Sato, and Y. Tanaka, 2012 L. Hao and T. K. Lee, PRB ’ 11 T. H. Hsieh and L. Fu, PRL ’ 12 energy momentum energy small chemical potential caldera

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Structural transition of ABS 35 AY, K. Yada, M. Sato, and Y. Tanaka, 2012 transition L. Hao and T. K. Lee, PRB ’ 11 T. H. Hsieh and L. Fu, PRL ’ 12 energy transition point: group velocity = 0

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Tunneling conductance in full-gap superconducting topological insulators 36 structural transition -> group velocity ~ zero -> large surface DoS full-gap case eV/ Metal z x y z0z0 STI zero-bias peak even in the full gap case A. Yamakage, K. Yada, M. Sato, and Y. Tanaka, PRB2012

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Summary: Theory of tunneling spectroscopy of superconducting topological insulators 1. Zero-bias conductance peak is possible even in full-gap topological 3d superconductors, differently from the case of BW states. 2. This originates from the structural transition of energy dispersion of ABS. Yamakage, Yada, Sato, and Tanaka, Physical Review B (R) 2012

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Josephson effect in s-wave/STI s-wave singlet STI full gap triplet Josephson current Fu and Berg, PRL 10

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Josephson effect in d-wave/N/STI Josephson current irrespective of anisotropic pairings

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(1) Theory of Tunneling Conductance in Superconducting Topological Insulator A. Yamakage, K. Yada, M. Sato and Y. Tanaka (2) Majorana fermion and odd-frequency Cooper pair Y. Asano and Y. Tanaka Phys. Rev. B (R) 2012 arXiv:

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Majorana Fermion and odd-frequency pairing Kitaev(01); Lutchyn(10), Oleg(10) Beenakker(11), … Kouwnehoven(12) Science Nature, News, March(2012) Spin-orbit coupling Zeeman Proximity coupling to s-wave Superconductivity on Nanowire in topological phase is similar to spin-triplet p -wave Kitaev 01

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What is odd-frequency pairing spin - singlet triplet orbital even odd Time (frequency) even odd Preexisting Cooper pair (even-frequency ） Spin-singlet even-parity (BCS, Cuprate ) Odd-frequency Cooper pair Spin-triplet odd-parity ( 3 He,Sr 2 RuO 4,UPt 3 ) Spin-triplet even-parity Berezinskii (1974) Spin-singlet odd-parity Balatsky Abraham(1992)

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Generation of odd-frequency pairing by symmetry breaking (1)Translational invariance (inversion symmetry) is broken ESE OSO ETO OTE (inhomogeneous system, junction, vortex..) (2)Spin rotational symmetry is broken (exchange field) (Efetov, Volkov, Bergeret, Eschrig) ESE OTE ETO OSO Fermi Dirac statistics ESE (Even-frequency spin-singlet even-parity) ETO (Even-frequency spin-triplet odd-parity) OTE (Odd-frequency spin-triplet even-parity) OSO (Odd-frequency spin-singlet odd-parity)

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(1) (2) (3) (4) ESE (Even-frequency spin-singlet even-parity) ETO (Even-frequency spin-triplet odd-parity) OTE (Odd-frequency spin-triplet even-parity) Berezinskii OSO (Odd-frequency spin-singlet odd-parity) Balatsky,Abraham Bulk state ESE (s,d x2-y2 -wave) ESE (d xy -wave) ETO (p x -wave) ETO (p y -wave) Sign change (MABS) No Yes Interface-induced symmetry (subdominant component ) Yes No ESE + (OSO) OSO +(ESE) OTE + (ETO) ETO + (OTE) Symmetry of the Cooper pair in junctions (No spin flip) Phys. Rev. Lett (2007) (1)(2)(3) (4)

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Mid gap Andreev bound state （ MABS ） Surface ＋ー ー ー ＋ Odd-frequency pairing ＋ MABS Low transparent limit (Surface state) Y. Tanaka, et al Phys. Rev. Lett (2007)

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(1) (2) (3) (4) Proximity into DN (Diffusive normal metal) even-parity (s-wave)○ Odd-parity × Bulk state ESE (s,dx2-y2 -wave) ESE (d xy -wave) ETO (p x -wave) ETO (p y -wave) Sign change No Yes Interface-induced state (subdominant) Proximity into DN Yes No ESE + (OSO) OSO +(ESE) OTE + (ETO) ETO + (OTE) ESE No Proximity effect into DN (No spin flip) Y. Tanaka, et al Phys. Rev. Lett (2007) OTE Y. Tanaka and Golubov, PRL. 98, (2007) (1)(2)(3) (4) ESE (Even-frequency spin-singlet even-parity) ETO (Even-frequency spin-triplet odd-parity) OTE (Odd-frequency spin-triplet even-parity) OSO (Odd-frequency spin-singlet odd-parity Case (3) is very interesting!!

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Density of states in DN Conventional proximity effect with Even-frequency Cooper pair in DN Unconventional proximity effect with Odd-frequency Cooper pair in DN Tanaka, Kashiwaya PRB (2004) Peak(dip) width, Thouless energy In the actual calculation, DN is attached to normal electrode.

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Anomalous proximity effect expected in chiral p- wave superconductor Asano PRL 99, (2007) DN RDRD Odd-frequency triplet s-wave in diffusive normal metal (DN) LDOS in DN Tanaka PRB(2005)

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Majorana fermion in Nano-wire Topological (Majorana) Non Topological Nano wire on the insulator (diffusive) (a): non topological (b): topological Robust zero bias conductance peak independent of disorder normalsuperconductor Charge conductance in nano wire Similar anomalous charge transport has been clarified in Diffusive normal metal/p x -wave superconductor junction in Tanaka and Kashiwaya, PRB 2004 arXiv:

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(Conventional proximity effect) Zero voltage resistance of the junction R is independent of R D (3) p x -wave (No proximity effect) (Anomalous proximity effect) Anomalous proximity effect in DN/p x -wave junction Tanaka and Kashiwaya PRB (2004)

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Majorana fermion in Nano-wire Topological Non Topological non topological topological normal superconductor Local density of state in nano wire Similar anomalous charge transport has been clarified in diffusive normal metal/p-wave superconductor junction in Tanaka and Kashiwaya, PRB 2004 robust zero energy peak of LDOS arXiv:

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Anomalous current phase relation of Josephson current 52 topological non-topological static Josephson current 2 Non-static Josephson current: 4 Similar anomalous current phase relation appears in d-wave junction (Tanaka 96, Barash 96) and p-wave junction (Yakovenko 04). arXiv:

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Induced odd-frequency pairing in topological phase 53 Non TopologicalTopological Odd-frequency pairing is hugely enhanced in topological phase arXiv:

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Summary: Nano wire hosting Majorana fermion 1.Majorana fermion should be always hosting odd- frequency pairing. 2.Anomalous proximity effect, anomalous charge transport are expected similar to spin-triplet p-wave superconductor junctions. 3.Nano wire is an idealistic system to study anomalous proximity effect expected for spin-triplet p x -wave superconductor. Y. Asano and Y. Tanaka arXiv:

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Calculation of surface states 55 STI z x y z0z0 1. construct the wave function in the STI 2. the coefficient t is determined by the confined condition : wave function of evanescent state with energy E

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Energy Gap function Full Gap Point Node Fu and Berg, Phys. Rev. Lett (2010) Yamakage et al., PRB R(2012)

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Local density of state full gap point node E2E2 Δ 1 :singlet, full gap Δ 2 :triplet, full gap Δ 3 :singlet, point node Δ 4 :triplet, point node Ldos Energy ( E/Δ )

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Surface state generated at z=0 STI (Superconducting topological insulator) vacuum z-axis STI

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Andreev bound state Hsieh and Fu PRL (2012); arXiv: Normal Cone Caldera Cone Helical Majorana (Surface state) Deformed Cone (Only positive spin helicity k x s y – k y s x = +k states are shown.) (Only negative energy states are shown.) (solution of confinement condition (z=0)=0) Yamakage et al., arXiv:

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Charge transport in normal metal / STI junctions STI (Superconducting topological insulator) Normal metal z-axis STI

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Tunneling conductance between normal metal / superconducting topological insulator junction Similar to conventional s-wave superconductor Zero bias conductance peak is possible even for 2 case with full gap Hsieh and Fu PRL (2012); arXiv: Yamakage et al., arXiv: (2011)

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Tunneling conductance between normal metal / superconducting topological insulator junction (2) Full gap case Point node case Tunneling conductance strongly depends on the direction of nodes. Yamakage et al., arXiv: (2011)

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Tunneling conductance 63 Andreev bound state (Majorana Fermion ） Full Gap Point Node Yamakage et al., arXiv: (2011)

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64 Structural transition of Andreev bound state Transition line Yamakage et al., arXiv: (2011)

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65 Velocity of Majorana fermion along x-direction Transition line

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Josephson effect in singlet/triplet junction first order Josephson current singlettriplet Josephson current in the absence of spin-dependent H’ Geshkenbein Larkin 88, Y. Asano et al, PRB 03

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Josephson effect in s-wave/STI s-wave singlet STI full gap triplet Josephson current Fu and Berg, PRL 10

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Josephson effect in d-wave/N/STI Josephson current irrespective of anisotropic pairings

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Absence of spin-dependent tunneling STI Assumption: The left system has the same or the higher symmetry as STI (D 3d ). Rotational symmetryMirror symmetry

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Absence of spin-dependent tunneling STI Assumption: The left system has the same or the higher symmetry as STI (D 3d ). 3D TSCs show a robust sin2 protected by the symmetry cf. A spin-dependent tunneling is possible in Sr 2 RuO 4 since the electronic state has higher angular momentum in lower point group symmetry. (Asano)

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