arXiv subjects

Keiji Yada

Publications and source records attributed to Keiji Yada.

At least 55 records · Page 3Linked to original sources

arXiv2014

Microscopic theory of tunneling spectroscopy in Sr$_2$RuO$_4$

We study the surface Andreev bound state (ABS) of superconducting Sr$_2$RuO$_4$, which is a candidate material for the realization of the chiral $p$-wave superconducting state. In order to clarify the role of chiral edge modes as ABSs, the surface density of states and the tunneling conductance is calculated in the normal metal/Sr$_2$RuO$_4$ junction within the framework of recursive Green's function method, while taking into account the orbital degrees of freedom (including Spin-Orbit interactions) with realistic material parameters. In Sr$_2$RuO$_4$, there are two bands $α$ and $β$ originating from quasi-one-dimensional orbitals $d_{yz}$ and $d_{zx}$ and a two-dimensional band $γ$ originating from $d_{xy}$ orbital. We discuss about the contributions of various electronic bands to LDOS and the influence of atomic spin-orbit interaction (SOI). In the light of our calculations, quasi-one-dimensional model with dominant pair potentials in $α$ and $β$ bands is consistent with conductance measurements in Au/Sr$_{2}$RuO$_{4}$ junctions.

cond-mat.supr-con↗

arXiv2014

Majorana edge states and topological properties in 1D/2D Rashba semiconductor proximity coupled to iron-based superconductor

We study Majorana edge states and their topological properties of one-dimensional(1D) and two-dimensional(2D) Rashba semiconductor deposited on iron-based superconductor under the applied Zeeman field for various directions. Using the recursive Green's function method, we calculate the local density of states(LDOS) both for $s_{\pm}$ and $s_{++}$-wave pairings. We elucidate that it shows anisotropic response to the applied Zeeman field specific to Majorana edge states. This anisotropy can be understood by the winding number, which shows whether the present system is topological or not. The resulting LDOS and winding numbers for $s_{\pm}$ and $s_{++}$-wave pairings are significantly different at the lower Zeeman field. These results serve as a guide to determine the pairing symmetry of iron-pnictide.

cond-mat.supr-con↗

arXiv2013

Theory of tunneling spectroscopy in a superconducting topological insulator

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.

cond-mat.supr-con↗

arXiv2013

Bulk electronic state of superconducting topological insulator

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.

cond-mat.supr-con↗

arXiv2013

Anomalous Josephson current in superconducting topological insulator

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(2ϕ) owing to mirror symmetry, where ϕdenotes 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.

cond-mat.supr-con↗

arXiv2012

Theory of tunneling spectroscopy for chiral topological superconductors

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.

cond-mat.supr-con↗

arXiv2012

New approach to theory of tunneling spectroscopy in unconventional superconductors

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.

cond-mat.supr-con↗

arXiv2012

Tunneling Conductance and Surface States Transition in Superconducting Topological Insulators

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 profound 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.

cond-mat.supr-con↗

arXiv2012

Finite Size Effects of the Surface States in a Lattice Model of Topological Insulator

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.

cond-mat.mes-hall↗

arXiv2011

Topological Superconductivity in CuxBi2Se3

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.

cond-mat.supr-con↗

arXiv2011

Theory of edge states in a quantum anomalous Hall insulator/ spin-singlet s-wave superconductor hybrid system

We study the edge states for a quantum anomalous Hall system (QAHS) coupled with a spin-singlet s-wave superconductor through the proximity effect, and clarify the topological nature of them. When we consider a superconducting pair potential induced in the QAHS, there appear topological phases with nonzero Chern numbers, $i.e.$, ${\cal N}=1$ and ${\cal N}=2$, where Andreev bound states appear as chiral Majorana edge modes. We calculate the energy spectrum of the edge modes and the resulting local density of states. It is found that the degenerate chiral Majorana edge modes for ${\cal N}=2$ are lifted off by applying Zeeman magnetic field along the parallel to the interface or the shift of the chemical potential by doping. The degeneracy of the chiral Majorana edge modes and its lifting are explained by two different winding numbers defined at the time-reversal invariant point of the edge momentum.

cond-mat.supr-con↗

arXiv2011

Topology of Andreev Bound States with Flat Dispersion

A theory of dispersionless Andreev bound states on surfaces of time-reversal invariant unconventional superconductors is presented. The generalized criterion for the dispersionless Andreev bound state is derived from the bulk-edge correspondence, and the chiral spin structure of the dispersionless Andreev bound states is argued from which the Andreev bound state is stabilized. Then we summarize the criterion in a form of index theorems. The index theorems are proved in a general framework to certify the bulk-edge correspondence. As concrete examples, we discuss (i) dxy-wave superconductor (ii) px-wave superconductor, and (iii) noncentrosymmetric superconductors. In the last example, we find a peculiar time-reversal invariant Majorana fermion. The time-reversal invariant Majorana fermion shows an unusual response to the Zeeman magnetic field, which can be used to identify it experimentally.

cond-mat.supr-con↗

arXiv2010

Surface density of states and topological edge states in non-centrosymmetric superconductors

We study Andreev bound state (ABS) and surface density of state (SDOS) of non-centrosymmetric superconductor where spin-singlet $d$-wave pairing mixes with spin-triplet $p$ (or $f)$-wave one by spin-orbit coupling. For $d_{xy} + p$-wave pairing, ABS appears as a zero energy state. The present ABS is a Majorana edge mode preserving the time reversal symmetry. We calculate topological invariant number and discuss the relevance to a single Majorana edge mode. In the presence of the Majorana edge mode, the SDOS depends strongly on the direction of the Zeeman field.

cond-mat.supr-con↗

arXiv2010

Anomalous Andreev bound state in non-centrosymmetric superconductors

We study edge states of non-centrosymmetric superconductors where spin-singlet d-wave pairing mixes with spin-triplet p (or f)-wave one by spin-orbit coupling. For dxy-wave pairing, the obtained Andreev bound state has an anomalous dispersion as compared to conventional helical edge modes. A unique topologically protected time-reversal invariant Majorana fermion appears at the edge. The charge conductance in the non-centrosymmetric superconductor junctions reflects the anomalous structures of the dispersions, particularly the time-reversal invariant Majorana fermion is manifested as a zero bias conductance peak.

cond-mat.supr-con↗

arXiv2009

Electrically controlled superconducting states at the heterointerface SrTiO$_3$/LaAlO$_3$

We study the symmetry of Cooper pair in a two-dimensional Hubbard model with the Rashba-type spin-orbit interaction as a minimal model of electron gas generated at a heterointerface of SrTiO$_3$/LaAlO$_3$. Solving the Eliashberg equation based on the third-order perturbation theory, we find that the gap function consists of the mixing of the spin-singlet $d_{xy}$-wave component and the spin-triplet $(p_x\pm ip_y)$-wave one due to the broken inversion symmetry originating from the Rashba-type spin-orbit interaction. The ratio of the d-wave and the p-wave component continuously changes with the carrier concentration. We propose that the pairing symmetry is controlled by tuning the gate voltage.

cond-mat.supr-con↗

arXiv2008

Odd-frequency Superconductivity on a Quasi-one-dimensional Triangular Lattice in the Hubbard Model

In order to clarify whether the odd-frequency superconductivity can be realized or not, we study a quasi-one-dimensional triangular lattice in the Hubbard model using the random phase approximation (RPA) and the fluctuation exchange (FLEX) approximation. We find that odd-frequency spin-singlet p-wave pairing can be enhanced on a quasi-one-dimensional isosceles triangular lattice.

cond-mat.supr-con↗

arXiv2008

A Microscopic Theory of Odd-freqeuncy Pairing in the Two-dimensional Extended Hubbard Model

We present a microscopic theory of fluctuation-mediated pairing mechanism in a two-dimensional extended Hubbard model. In contrast to conventional wisdom, odd-frequency spin-triplet pairing can be stabilized near the spin-density-wave critical point. Favorable conditions for the odd-frequency pairing are the presence of geometrical frustration and off-site Coulomb interaction.

cond-mat.supr-con↗

arXiv2008

s-wave Superconductivity due to Suhl-Kondo Mechanism in Na$_x$CoO$_2\cdot y$H$_2$O: Effect of Coulomb Interaction and Trigonal Distortion

To study the electron-phonon mechanism of superconductivity in NaxCoO2, we perform semiquantitative analysis of the electron-phonon interaction (EPI) between relevant optical phonons (breathing and shear phonons) and t_{2g} electrons (a_{1g} and e_g' electrons) in the presence of trigonal distortion. We consider two kinds of contributions to the EPI; the EPI originating from the Coulomb potential of O ions and that originating from the d-p transfer integral between Co and O in CoO_6 octahedron. We find that the EPI for shear phonons, which induces the interorbital hopping of electrons, is large in NaxCoO_2 because of the trigonal distortion of CoO_2 layer. For this reason, Tc for s-wave pairing is prominently enlarged owing to interorbital hopping of Cooper pairs induced by shear phonons, even if the top of e_g' electron band is close to but below the Fermi level as suggested experimentally. This mechanism of superconductivity is referred to as the valence-band Suhl-Kondo (SK) mechanism. Since the SK mechanism is seldom damaged by the Coulomb repulsion, s-wave superconductivity is realized irrespective of large Coulomb interaction U~5 eV at Co sites. We also study the oxygen isotope effect on Tc, and find that it becomes very small due to strong Coulomb interaction. Finally, we discuss the possible mechanism of anisotropic s-wave superconducting state in NaxCoO2, resulting from the coexistence of strong EPI and the antiferromagnetic fluctuations.

cond-mat.supr-con↗