arXiv subjects

Keiji Yada

Publications and source records attributed to Keiji Yada.

At least 37 records · Page 2Linked to original sources

arXiv2022

Spin susceptibility for orbital-singlet Cooper pair in the three-dimensional Sr$_2$RuO$_4$ superconductor

We study the spin susceptibility of the orbital-singlet pairings, including the spin-triplet/orbital-singlet/$s$-wave $E_g$ representation proposed by Suh \textit{et al}., [H.\ G.\ Suh \textit{et al}., Phys.\ Rev.\ Research \textbf{2}, 032023 (2020)], for a three-orbital model of superconducting Sr$_2$RuO$_4$ in three dimensions. For the pseudospin-singlet states represented in the band basis, the spin susceptibility decreases when reducing the temperature, irrespective of the direction of the applied magnetic fields, even if they are spin-triplet/orbital-singlet pairings in the spin-orbital space. However, because the pseudospin-triplet \textbf{d}-vector in the band basis is not completely aligned in the $xy$-plane (along $z$-axis) owing to the strong atomic spin-orbit coupling, the spin susceptibility for spin-singlet/orbital-singlet/odd-parity pairings is reduced around $5$-$10$ percent with the decrease of the temperature along the $z$ ($x$) axis. We can determine the symmetry of the pseudospin structure of the Cooper pair by the temperature dependence of the spin susceptibility measured by nuclear magnetic resonance experiments. Our obtained results serve as a guide to determine the pairing symmetry of Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2021

Theory of pair density wave on a quasi-one-dimensional lattice in the Hubbard model

In this study, we examine the superconducting instability of a quasi-one-dimensional lattice in the Hubbard model based on the random-phase approximation (RPA) and the fluctuation exchange (FLEX) approximation. We find that a spin-singlet pair density wave (PDW-singlet) with a center-of-mass momentum of $2k_F$ can be stabilized when the one-dimensionality becomes prominent toward the perfect nesting of the Fermi surface. The obtained pair is a mixture of even-frequency and odd-frequency singlet ones. The dominant even-frequency component does not have nodal lines on the Fermi surface. This PDW-singlet state is more favorable as compared to RPA when self-energy correction is introduced in the FLEX approximation.

cond-mat.supr-con↗

arXiv2020

Theory of Tunneling Effect in 1D AIII-class Topological Insulator (Nanowire) Proximity Coupled with a Superconductor

We study the tunneling effect in an AIII-class insulator proximity coupled with a spin-singlet $s$-wave superconductor, in which three phases are characterized by the integer topological invariant $\mathcal{N}$. By solving the Bogoliubov-de Gennes equation explicitly, we analytically obtain a normal reflection coefficient $R_{σσ'}$ and an Andreev reflection coefficient $A_{σσ'}$, and derive a charge conductance formula,where $σ(σ')$ is the spin index of a reflected (injected) wave. The resulting conductance indicates a wide variety of line shapes: (i)gap structure without coherence peaks for $\mathcal{N}=0$, (ii)quantized zero-bias conductance peak (ZBCP) with height $2e^{2}/h$ for $\mathcal{N}=1$, and (iii)ZBCP spitting for $\mathcal{N}=2$. At zero bias voltage $eV=0$, $\sum_{σσ'} R_{σσ'} = \sum_{σσ'} A_{σσ'}$ is satisfied and the spin direction of an injected electron is rotated at approximately $90^\circ$ for the $\mathcal{N}=1$ state. Meanwhile, $A_{σσ'}=0$ is satisfied for the $\mathcal{N}=2$ state, and the spin rotation angle can become $180^\circ$.

cond-mat.supr-con↗

arXiv2019

Time-Reversal Invariant Superconductivity of Sr$_{2}$RuO$_{4}$ Revealed by Josephson Effects

Sr$_{2}$RuO$_{4}$ is one of the most promising candidates of a topological superconductor with broken time-reversal symmetry, because a number of experiments have revealed evidences for a spin-triplet chiral $p$-wave superconductivity. In order to clarify the time-reversal symmetry of Sr$_{2}$RuO$_{4}$, we introduce a novel test that examines the invariance of the Josephson critical current under the inversion of both the current and magnetic fields, in contrast to the detection of a spontaneous magnetic field employed in past experiments. Analyses of the transport properties of the planar and corner Josephson junctions formed between Sr$_{2}$RuO$_{4}$ and Nb reveal the time-reversal invariant superconductivity, most probably helical $p$-wave, of Sr$_{2}$RuO$_{4}$. This state corresponds to a yet-to-be confirmed $topological crystalline superconductivity$ that can host two Majorana edge modes at the surface protected by crystalline mirror symmetry.

cond-mat.supr-con↗

arXiv2019

Spin-Orbital Hallmarks of Unconventional Superconductors Without Inversion Symmetry

The spin-orbital polarization of superconducting excitations in momentum space is shown to provide distinctive marks of unconventional pairing in the presence of inversion symmetry breaking.Taking the prototypical example of an electronic system with atomic spin-orbit and orbital-Rashba couplings, we provide a general description of the spin-orbital textures and their most striking changeover moving from the normal to the superconducting state. We find that the variation of the spin-texture is strongly imprinted by the combination of the misalignment of spin-triplet d-vector with the inversion asymmetry g-vector coupling and the occurrence of superconducting nodal excitations. Remarkably, the multi-orbital character of the superconducting state allows to unveil a unique type of topological transition for the spin-winding around the nodal points. This finding indicates the fundamental topological relation between chiral and spin-winding in nodal superconductors. By analogy between spin- and orbital-triplet pairing we point out how orbital polarization patterns can be also employed to assess the character of the superconducting state.

cond-mat.supr-con↗

arXiv2018

Influence of edge magnetization and electric fields on zigzag silicene, germanene and stanene nanoribbons

Using a multi-orbital tight-binding model, we have studied the edge states of zigzag silicene, germanene, and stanene nanoribbons (ZSiNRs, ZGeNRs and ZSnNRs, respectively) in the presence of the Coulomb interaction and a vertical electric field. The resulting edge states have non-linear energy dispersions due to multi-orbital effects, and the nanoribbons show induced magnetization at the edges. Owing to this non-linear dispersion, ZSiNRs, ZGeNRs and ZSnNRs may not provide superior performance in field effect transistors, as has been proposed from single-orbital tight-binding model calculations. We propose an effective low-energy model that describes the edge states of ZSiNRs, ZGeNRs, and ZSnNRs. We demonstrate that the edge states of ZGeNR and ZSnNR show anti-crossing of bands with opposite spins, even if only out-of-plane edge magnetization is present. The ability to tune the spin polarizations of the edge states by applying an electric field points to future opportunities to fabricate silicene, germanene and stanene nanoribbons as spintronics devices.

cond-mat.mes-hall↗

arXiv2018

Inter-orbital topological superconductivity in spin-orbit coupled superconductors with inversion symmetry breaking

We study the superconducting state of multi-orbital spin-orbit coupled systems in the presence of an orbitally driven inversion asymmetry assuming that the inter-orbital attraction is the dominant pairing channel. Although the inversion symmetry is absent, we show that superconducting states that avoid mixing of spin-triplet and spin-singlet configurations are allowed, and remarkably, spin-triplet states that are topologically nontrivial can be stabilized in a large portion of the phase diagram. The orbital-dependent spin-triplet pairing generally leads to topological superconductivity with point nodes that are protected by a nonvanishing winding number. We demonstrate that the disclosed topological phase can exhibit Lifshitz-type transitions upon different driving mechanisms and interactions, e.g., by tuning the strength of the atomic spin-orbit and inversion asymmetry couplings or by varying the doping and the amplitude of order parameter. Such distinctive signatures of the nodal phase manifest through an extraordinary reconstruction of the low-energy excitation spectra both in the bulk and at the edge of the superconductor.

cond-mat.supr-con↗

arXiv2017

Josephson effect in a multi-orbital model for Sr$_{2}$RuO$_{4}$

We study Josephson current between s-wave/spin-triplet superconductor junctions by taking into account details of band structures in Sr$_{2}$RuO$_{4}$ such as three conduction bands, spin-orbit interaction in the bulk and that at the interface. We assume five superconducting order parameters in Sr$_{2}$RuO$_{4}$: a chiral p-wave symmetry and four helical p-wave symmetries. We calculate current-phase relationship $I(φ)$ in these junctions, where $φ$ is the macroscopic phase difference between two superconductors. The results for a chiral p-wave pairing symmetry show that $\cos(φ)$ term appears in the current-phase relation due to time-reversal symmetry (TRS) breaking. On the other hand, $\cos(φ)$ term is absent in the helical pairing states which preserve the TRS. We also study the dependence of maximum Josephson current $I_c$ on an external magnetic flux $Φ$ in a corner junction. The calculated results of $I_c(Φ)$ show a relation $I_{c}(Φ) \neq I_{c}(-Φ)$ in a chiral state and $I_{c}(Φ)=I_{c}(-Φ)$ in a helical state. We calculate $I_c(Φ)$ in a corner and a symmetric SQUIDs geometry. In a symmetric SQUID geometry, the relation $I_{c}(Φ)=I_{c}(-Φ)$ is satisfied for all the pairing states and it is impossible to distinguish chiral state from helical one. On the other hand, results for a corner SQUID always show $I_{c}(Φ) \neq I_{c}(-Φ)$ and $I_{c}(Φ)=I_{c}(-Φ)$ for a chiral and a helical states, respectively. Experimental tests of these relations in a corner junctions and SQUIDs may serve as a tool for unambiguous determination of the pairing symmetry in Sr$_{2}$RuO$_{4}$.

cond-mat.supr-con↗

arXiv2016

Edge states of hydrogen terminated monolayer materials: silicene, germanene and stanene ribbons

We investigate the energy dispersion of the edge states in zigzag silicene, germanene and stanene nanoribbons with and without hydrogen termination based on a multi-orbital tight-binding model. Since the low buckled structures are crucial for these materials, both the $π$ and $σ$ orbitals have a strong influence on the edge states, different from the case for graphene nanoribbons. The obtained dispersion of helical edge states is nonlinear, similar to that obtained by first-principles calculations. On the other hand, the dispersion derived from the single-orbital tight-binding model is always linear. Therefore, we find that the non-linearity comes from the multi-orbital effects, and accurate results cannot be obtained by the single-orbital model but can be obtained by the multi-orbital tight-binding model. We show that the multi-orbital model is essential for correctly understanding the dispersion of the edge states in tetragen nanoribbons with a low buckled geometry.

cond-mat.mes-hall↗

arXiv2016

Pairing mechanism of unconventional superconductivity in doped Kane-Mele model

We study the pairing symmetry of a doped Kane-Mele model on a honeycomb lattice with on-site Coulomb interaction. The pairing instability of Cooper pair is calculated based on the linearized Eliashberg equation within the random phase approximation (RPA). When the magnitude of the spin-orbit coupling is weak, even-frequency spin-singlet even-parity (ESE) pairing is dominant. On the other hand, with the increase of the spin-orbit coupling, we show that the even-frequency spin-triplet odd-parity (ETO) f-wave pairing exceeds ESE one. ETO f-wave pairing is supported by the longitudinal spin fluctuation. Since the transverse spin fluctuation is strongly suppressed by spin-orbit coupling, ETO f-wave pairing becomes dominant for large magnitude of spin-orbit coupling.

cond-mat.supr-con↗

arXiv2016

Tunneling and Josephson effects in odd-frequency superconductor junctions: A study on multi-channel Kondo chain

Junction systems of odd-frequency (OF) superconductors are investigated based on a mean-field Hamiltonian formalism. One-dimensional two-channel Kondo lattice (TCKL) is taken as a concrete example of OF superconductors. Properties of normal and Andreev reflections are examined in a normal metal/superconductor junction. Unlike conventional superconductors, normal reflection is always present due to the normal self energy that necessarily appears in the present OF pairing state. The conductance reflects the difference between repulsive and attractive potentials located at the interface, which is in contrast with the preexisting superconducting junctions. Josephson junction is also constructed by connecting TCKL with the other types of superconductors. The results can be understood from symmetry of the induced Cooper pairs at the edge in the presence of spin/orbital symmetry breaking. It has also been demonstrated that the symmetry argument for Cooper pairs is useful in explaining Meissner response in bulk.

cond-mat.supr-con↗

arXiv2015

Surface electronic state of superconducting topological crystalline insulator

We study the surface state of a doped topological crystalline insulator in the superconducting state. Motivated by Sn$_{1-x}$In$_x$Te, we consider fully gapped pair potentials and calculate the surface spectral function. It is found that mirror-protected zero-energy Andreev bound states appear at the (001) surface and that these states can move along the mirror symmetric line on the surface Brillouin zone. We also show that the surface Andreev bound state changes systematically with doping due to the presence of the Dirac surface state in the normal state.

cond-mat.supr-con↗

arXiv2015

Anomalous Josephson effect in d-wave superconductor junctions on TI surface

We study Josephson effect of $d$-wave superconductor (DS)/ferromagnet insulator(FI)/DS junctions on a surface of topological insulator (TI). We calculate Josephson current $I\left(φ\right) $ for various orientations of the junctions where $φ$ is the macroscopic phase difference between two DSs. In certain configurations, we find anomalous current-phase relation $I(φ)=-I\left( -φ+π\right)$ with $2π$ periodicity. In the case where the first order Josephson coupling is absent without magnetization in FI, $I(φ)$ can be proportional to $\cos φ$. The magnitude of the obtained Josephson current is enhanced due to the zero energy states on the edge of DS on TI. Even if we introduce an $s$-wave component of pair potential in DS, we can still expect the anomalous current-phase relation in asymmetric DS junctions with $I\left( φ=0\right) \neq 0$. This can be used to probe the induced $d$-wave component of pair potential on TI surface in high-$T_{c}$ cuperate/TI hybrid structures.

cond-mat.supr-con↗

arXiv2015

Tunneling spectroscopy and Josephson current of superconductor-ferromagnet hybrids on the surface of a 3D TI

We investigate the charge transport property of superconductor (S) /normal metal (N) / ferromagnet insulator (FI) /(normal metal) N' and S/N/FI/N'/S Josephson junctions on a three-dimensional topological insulator surface. We find asymmetric local density of states (LDOSs) in a S/N/FI/N' junction and show that the finite length of the N interlayer gives rise to subgap resonant spikes in the differential conductance and LDOSs. In a S/N/FI/N'/S junction, the Josephson current shows a non-sinusoidal current-phase relation and the N (or N') interlayer decreases the magnitude of the critical current monotonically.

cond-mat.supr-con↗

arXiv2015

Odd-frequency pair in topological superconductivity of 1D magnetic chain

A chain of magnetic atoms with non-collinear spin configuration on a superconductor is a promising new system that can host Majorana Fermions (MFs). In this study, we clarify that in the presence of MFs, an odd-frequency Cooper pair is generated at the edge of the chain. Furthermore, it is revealed that this feature is robust against the distance between magnetic atoms. We also elucidate the close relationship between the pair amplitude of the odd-frequency pair, the parity of the wave function, and the direction of the MF spin. If Rashba-type spin-orbit coupling is included, MFs can be realized even in a collinear alignment of magnetic atoms, $i.e.$, in a ferromagnetic or anti-ferromagnetic chain on a superconductor. Then, the odd-frequency pairing is generated at the edge, similar to the non-collinear case. Based on our results, it can be concluded that the detection of the zero energy peak of the local density of states by scanning tunneling microscopy at the edge of the magnetic chain is strong evidence for the generation of odd-frequency pairing.

cond-mat.supr-con↗

arXiv2014

Crossed surface flat bands of Weyl semimetal superconductors

It has been noted that certain surfaces of Weyl semimetals have bound states forming open Fermi arcs, which are never seen in typical metallic states. We show that the Fermi arcs enable them to support an even more exotic surface state with crossed flat bands in the superconducting state. We clarify the topological origin of the crossed flat bands and the relevant symmetry that stabilizes the cross point. We also discuss their possible experimental verification by tunneling spectroscopy.

cond-mat.supr-con↗

arXiv2014

Effect of Fermi surface evolution on superconducting gap in superconducting topological insulator

We study bulk electronic states of superconducting topological insulator, which is the promising candidate for topological superconductor. Recent experiments suggest that the three-dimensional Fermi surface evolves into two-dimensional one. We show that the superconducting energy gap structure on the Fermi surface systematically changes with this evolution. It is clarified that the bulk electronic properties such as spin-lattice relaxation rate and specific heat depend on the shape of the Fermi surface and the type of the energy gap function. These results serve as a guide to determine the pairing symmetry of Cu$_x$Bi$_2$Se$_3$.

cond-mat.supr-con↗

arXiv2014

Quasi-classical theory of tunneling spectroscopy in superconducting topological insulator

We develop a theory of tunneling spectroscopy in superconducting topological insulator (STI), i.e. superconducting state of a carrier-doped topological insulator. Based on the quasi-classical approximation, we obtain an analytical expression of the energy dispersion of surface Andreev bound states (ABSs) and a compact formula for tunneling conductance of normal metal/STI junctions. The obtained compact formula of tunneling conductance makes the analysis of experimental data easy. As an application of our theory, we study tunneling conductance in Cu$_x$Bi$_2$Se$_3$. We reveal that our theory reproduces the previous results by Yamakage {\it et al} [Phys. Rev. B 85, 180509(R) (2012)]. We further study magneto-tunneling spectroscopy in the presence of external magnetic fields, where the energy of quasiparticles is shifted by the Doppler effect. By rotating magnetic fields in the basal plane, we can distinguish between two different topological superconducting states with and without point nodes, in their magneto-tunneling spectra.

cond-mat.supr-con↗