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ABSTRACT: Crystal point group symmetry is shown to protect Majorana fermions (MFs) in
spinfull superconductors (SCs). We elucidate the condition necessary to obtain
MFs protected by the point group symmetry. We reveal that superconductivity in
Sr2RuO4 hosts a topological phase transition to a topological crystalline SC,
which accompanies a d-vector rotation under a magnetic field along the c-axis.
Taking all three bands and spin-orbit interactions into account,
symmetry-protected MFs in the topological crystalline SC are identified.
Detection of such MFs provides evidence of the d-vector rotation in Sr2RuO4
expected from Knight shift measurements but not yet verified.
03/2013;
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ABSTRACT: We have investigated the proximity effect in superconductor/ferromagnet
junctions in a systematic manner to discuss the relation between the zero
energy peak (ZEP) of the local density of states (LDOS) and spin-triplet
odd-frequency paring. By exactly solving the nonlinear Usadel equations, we
have found that the ZEP is realized in a wide range of geometrical and material
parameters in the case of the noncollinear magnetization. This strongly
suggests the robustness of the ZEP induced by spin-triplet odd-frequency paring
in such systems. We also found that the crossover from singlet to triplet
paring can be detected by measuring the F layer thickness dependence of the ZEP
height. Further, we show how to observe signatures of spin-triplet
odd-frequency paring and the paring crossover by the LDOS measurements. Our
results provide a direct way to experimentally detect signatures of
odd-frequency paring state.
02/2013;
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ABSTRACT: We calculate tunneling conductance of normal metal and superconducting
topological insulator CuxBi2Se3 junctions for the possible pairing symmetries.
In the presence of gapless Andreev bound states (ABSs), the tunneling
conductance shows a zero-bias peak even for the three-dimensional full-gap
superconducting state. This zero-bias conductance peak stems from the
enhancement of the surface density of states induced by the surface-state
transition in momentum space.
11/2012;
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ABSTRACT: We have derived new boundary conditions on wave function at the normal metal
/ superconductor (NS) interface beyond effective mass approximation. These
conditions are based on tight-binding approach and enable one to formulate
quantitative model for tunneling spectroscopy of superconductors with complex
non-parabolic energy spectra. The model is applied to superconductors with
unconventional pairing and with multiband electronic structure. In the case of
single band unconventional superconductors this model provides known
conductance formula (Phys. Rev. Lett. 74 3451 1995), but with generalized
definition of the normal-state conductance. Based on new boundary conditions,
we have calculated conductance in normal metal / superconducting pnictide
junctions for different orientations of the NS interface with respect to the
crystallographic axes of the pnictides. The present approach provides the basis
for quantitative tunneling spectroscopy in multi-orbital superconductors.
10/2012;
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ABSTRACT: We study electronic properties of a superconducting topological insulator
whose parent material is a topological insulator. We calculate the temperature
dependence of the specific heat and spin susceptibility for four promising
superconducting pairings proposed by L. Fu and E. Berg (Phys. Rev. Lett. 105,
097001). Since the line shapes of temperature dependence of specific heat are
almost identical among three of the four pairings, it is difficult to identify
them simply from the specific heat. On the other hand, we obtain wide varieties
of the temperature dependence of spin susceptibility for each pairing
reflecting the spin structure of Cooper pair. We propose that the pairing
symmetry of superconducting topological insulator can be determined from
measurement of Knight shift by changing the direction of applied magnetic
field.
09/2012;
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ABSTRACT: We investigate the effect of helical Majorana fermions at the surface of
superconducting topological insulators (STI) on the Josephson current by
referring to possible pairing states of Cu-doped Bi2Se3. The surface state in
the present STI has a spin helicity because the directions of spin and momentum
are locked to each other. The Josephson current-phase relation in an STI/s-wave
superconductor junction shows robust sin(2{\phi}) owing to mirror symmetry,
where \phi denotes the macroscopic phase difference between the two
superconductors. In contrast, the maximum Josephson current in an STI/STI
junction exhibits a nonmonotonic temperature dependence depending on the
relative spin helicity of the two surface states. Detecting these features
qualifies as distinct experimental evidence for the identification of the
helical Majorana fermion in STIs.
08/2012;
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ABSTRACT: With high-Tc cuprates in mind, properties of correlated dx2-y2-wave
superconducting (SC) and antiferromagnetic (AF) states are studied for the
Hubbard (t-t'-U) model on square lattices, using a variational Monte Carlo
method. We employ simple trial wave functions including only crucial
parameters, such as a doublon-holon binding factor indispensable to describe
correlated SC and normal states as doped Mott insulators. U/t, t'/t and \delta
(doping rate) dependence of relevant quantities are systematically calculated.
As U/t increases, a sharp crossover of SC properties occurs at U_co/t \sim 10
from a conventional BCS type to a kinetic-energy-driven type for any t'/t. As
\delta decreases, U_co/t is smoothly connected to the Mott transition point at
half filling. For U/t\lsim 5, steady superconductivity corresponding to the
cuprates is not found, whereas the d-wave SC correlation function Pd^\infty
rapidly increases for U/t\gsim 6 and becomes maximum at U=U_co. Comparing the
\delta dependence of Pd^\infty with experimentally observed dome-shaped Tc and
condensation energy, we find that the effective value of $U$ for the cuprates
should be larger than the band width, for which the t-J model is valid.
Analyzing the kinetic energy, we reveal that for U>U_co only doped holes
(electrons) become charge carriers, which will make a small Fermi surface (hole
pocket), but for U<U_co all the electrons (holes) contribute to conduction and
will make an ordinary large Fermi surface, which is contradictory to the
feature of cuprates. By introducing a proper negative (positive) t'/t, the SC
(AF) state is stabilized. In the underdoped regime, the strength of SC for
U>U_co is determined by two factors, i.e., the AF spin correlation, which
creates singlet pairs (pseudogap), and the charge mobility dominated by Mott
physics.
08/2012;
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ABSTRACT: Solving the Bogoliubov-de Gennes equation, we study the spatial structure of
odd-frequency s-wave Cooper pair amplitudes at each quantized energy level of
vortex bound states in chiral p-wave superconductors. For zero energy Majorana
states, the odd-frequency s-wave pair amplitude has the same spatial structure
as that of the local density of states even in atomic length scale. This
relation also holds in finite energy bound states for single vortex winding
anti-parallel to the chirality, but not for parallel vortex winding. The double
winding vortex case is also studied.
06/2012;
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ABSTRACT: We discuss the surface impedance (Z=R-iX) of a normal-metal/superconductor
proximity structure taking into account the spin-dependent potential at the
junction interface. Because of the spin mixing transport at the interface,
odd-frequency spin-triplet s-wave Cooper pairs penetrate into the normal metal
and cause the anomalous response to electromagnetic fields. At low temperature,
the local impedance at a surface of the normal metal shows the nonmonotonic
temperature dependence and the anomalous relation R>X. We also discuss a
possibility of observing such anomalous impedance in experiments.
04/2012;
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ABSTRACT: We discuss a strong relationship between Majorana fermions and odd-frequency
Cooper pairs which appear at a disordered normal (N) nano wire attached to a
topologically nontrivial superconducting (S) one. The transport properties in
superconducting nano wire junctions show universal behaviors irrespective of
the degree of disorder: the quantized zero-bias differential conductance at
2e^{2}/h in NS junction and the fractional current-phase (J-\varphi)
relationship of the Josephson effect in SNS junction J\propto \sin(\varphi/2).
Such behaviors are exactly the same as those in the anomalous proximity effect
of odd-parity spin-triplet superconductors. We show that odd-frequency Cooper
pairs support the universal transport properties. The odd-frequency pairs exist
wherever the Majorana fermions stay.
04/2012;
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ABSTRACT: We study the charge conductance of an interface between a normal metal and a
superconducting quantum anomalous Hall system, based on the recursive Green's
function. The angle resolved conductance gamma(ky, eV) with the momentum ky
parallel to the interface and the bias voltage V shows a rich structure
depending on the Chern number N of the system. We find that when the bias
voltage is tuned to the energy dispersion of the edge mode, eV = Eedge(ky), the
angle resolved conductance gamma(ky,Eedge(ky)) shows a pronounced even-odd
effect; the conductance vanishes for N = 0 or 2 while it takes a universal
value 2e^2/h for N = 1. In particular, in N = 2 phase, we find that the
conductance gamma(ky,Eedge(ky)) becomes zero due to interference of two
degenerate Majorana edge modes, although the corresponding surface spectral
weight remains non-zero.
04/2012;
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ABSTRACT: We study Andreev bound states (ABS) and the resulting charge transport of a Rashba superconductor (RSC) where two-dimensional semiconductor (2DSM) heterostructures are sandwiched by spin-singlet s-wave superconductor and ferromagnet insulator. ABS becomes a chiral Majorana edge mode in the topological phase (TP). We clarify two types of quantum criticality about the topological change of ABS near a quantum critical point (QCP), whether or not ABS exists at QCP. In the former type, ABS has an energy gap and does not cross at zero energy in the nontopological phase. These complex properties can be detected by tunneling conductance between normal metal-RSC junctions.
Physical Review Letters 02/2012; 108(8):087003. · 7.37 Impact Factor
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ABSTRACT: We develop a theory of the tunneling spectroscopy for superconducting topological insulators (STIs), where the surface Andreev bound states (SABSs) appear as helical Majorana fermions. Based on the symmetry and topological nature of parent topological insulators, we find that the SABSs in the STIs have a structural transition in the energy dispersions. The transition results in a variety of Majorana fermions, by tuning the chemical potential and the effective mass of the energy band. We clarify that Majorana fermions in the vicinity of the transitions give rise to robust zero bias peaks in the tunneling conductance between normal metal/STI junctions.
Phys. Rev. B. 12/2011; 85(18).
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ABSTRACT: A topological superconductor (TSC) is characterized by the topologically protected gapless surface state that is essentially an Andreev bound state consisting of Majorana fermions. While a TSC has not yet been discovered, the doped topological insulator Cu(x)Bi(2)Se(3), which superconducts below ∼3 K, has been predicted to possess a topological superconducting state. We report that the point-contact spectra on the cleaved surface of superconducting Cu(x)Bi(2)Se(3) present a zero-bias conductance peak (ZBCP) which signifies unconventional superconductivity. Theoretical considerations of all possible superconducting states help us conclude that this ZBCP is due to Majorana fermions and gives evidence for a topological superconductivity in Cu(x)Bi(2)Se(3). In addition, we found an unusual pseudogap that develops below ∼20 K and coexists with the topological superconducting state.
Physical Review Letters 11/2011; 107(21):217001. · 7.37 Impact Factor
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ABSTRACT: Energy gap and wave function in thin films of topological insulator is
studied, based on tight--binding model. It is revealed that thickness
dependence of the magnitude of energy gap is composed of damping and
oscillation. The damped behavior originates from the presence of gapless
surface Dirac cone in the infinite thickness limit. On the other hand, the
oscillatory behavior stems from electronic properties in the thin thickness
limit.
11/2011;
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ABSTRACT: Ferromagnetic-insulator (FI) based Josephson junctions are promising
candidates for a coherent superconducting quantum bit as well as a classical
superconducting logic circuit. Recently the appearance of an intriguing
atomic-scale 0-pi transition has been theoretically predicted. In order to
uncover the mechanism of this phenomena, we numerically calculate the spectrum
of Andreev bound states in a FI barrier by diagonalizing the Bogoliubov-de
Gennes equation. We show that Andreev spectrum drastically depends on the
parity of the FI-layer number L and accordingly the pi (0) state is always more
stable than the 0 (pi) state if L is odd (even).
09/2011;
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ABSTRACT: We discuss the dynamic response of odd-frequency Cooper pairs to an electromagnetic field. By using the quasiclassical Green function method, we calculate the impedance (Z=R-iX) of a normal-metal thin film which covers a superconductor. In contrast with the standard relation (i.e., R≪X), the impedance in spin-triplet proximity structures shows anomalous behavior (i.e., R>X) in the low frequency limit. This unusual relation is a result of the penetration of odd-frequency pairs into the normal metal and reflects the negative Cooper pair density.
Physical Review Letters 08/2011; 107(8):087001. · 7.37 Impact Factor
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ABSTRACT: Tunneling spectroscopy has been performed on Sr(2)RuO(4) searching for the edge states peculiar to topological superconductivity. Conductance spectra exhibit broad humps with three types of peak shape: domelike peak, split peak, and two-step peak. By comparing the experiments with predictions for unconventional superconductivity, these varieties are shown to originate from multiband chiral p-wave symmetry with weak anisotropy of pair amplitude. The broad hump in the conductance spectrum is a direct manifestation of the edge state due to chiral p-wave superconductivity.
Physical Review Letters 08/2011; 107(7):077003. · 7.37 Impact Factor
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ABSTRACT: A topological superconductor (TSC) is characterized by the
topologically-protected gapless surface state that is essentially an Andreev
bound state consisting of Majorana fermions. While a TSC has not yet been
discovered, the doped topological insulator CuxBi2Se3, which superconducts
below ~3 K, has been predicted to possess a topological superconducting state.
We report that the point-contact spectra on the cleaved surface of
superconducting CuxBi2Se3 present a zero-bias conductance peak (ZBCP) which
signifies unconventional superconductivity. Theoretical considerations of all
possible superconducting states help us conclude that this ZBCP is due to
Majorana Fermions and gives evidence for a topological superconductivity in
CuxBi2Se3. In addition, we found an unusual pseudogap that develops below ~20 K
and coexists with the topological superconducting state.
08/2011;
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ABSTRACT: We investigate Meissner effect in normal-metal-superconductor junctions where the interface is spin active. We find that orbital magnetic susceptibility of the normal metal shows highly nontrivial behavior. In particular, the magnetic susceptibility depends on the temperature in an oscillatory fashion, accompanied by its sign change. Correspondingly, the magnetic field and current density can spatially oscillate in the normal metal. These results are attributed to the generation of odd-frequency pairing due to the spin-active interface.
Physical Review Letters 06/2011; 106(24):246601. · 7.37 Impact Factor
