arXiv subjects

Yasuhiro Asano

Publications and source records attributed to Yasuhiro Asano.

At least 55 records · Page 3Linked to original sources

arXiv2016

Quantization of Conductance Minimum and Index Theorem

We discuss the minimum value of the zero-bias differential conductance $G_{\textrm{min}}$ in a junction consisting of a normal metal and a nodal superconductor preserving time-reversal symmetry. Using the quasiclassical Green function method, we show that $G_{\textrm{min}}$ is quantized at $ (4e^2/h) N_{\mathrm{ZES}}$ in the limit of strong impurity scatterings in the normal metal. The integer $N_{\mathrm{ZES}}$ represents the number of perfect transmission channels through the junction. An analysis of the chiral symmetry of the Hamiltonian indicates that $N_{\mathrm{ZES}}$ corresponds to the Atiyah-Singer index in mathematics.

cond-mat.supr-con↗

arXiv2016

All-electrical generation and control of odd-frequency s-wave Cooper pairs in double quantum dots

We propose an all-electrical experimental setup to detect and manipulate the amplitude of odd-frequency pairing in a double quantum dot. Odd-frequency pair amplitude is induced from the breakdown of orbital symmetry when Cooper pairs are injected in the double dot with electrons in different dots. When the dot levels are aligned with the Fermi energy, i.e., on resonance, nonlocal Andreev processes are directly connected to the presence of odd-frequency pairing. Therefore, their amplitude can be manipulated by tuning the level positions. Detection of nonlocal Andreev processes by conductance measurements contributes a direct proof of the existence of odd-frequency pair amplitude and is available using current experimental techniques.

cond-mat.mes-hall↗

arXiv2015

Odd-frequency Cooper pairs in two-band superconductors and their magnetic response

We discuss odd-frequency Cooper pairs appearing in two-band superconductors by solving the Gor'kov equation analytically. We introduce the equal-time $s$-wave pair potentials as realized in MgB$_2$ and iron pnictides. Although the order parameter symmetry is conventional, the band degree of freedom enriches the symmetry variety of pairing correlations. The hybridization and the asymmetry between the two conduction bands induce odd-frequency pairs as a subdominant pairing correlation in the uniform ground state. To study the magnetic response of odd-frequency Cooper pairs, we analyze the Meissner kernel represented by the Gor'kov Green function. In contrast to the even-frequency pairs linked to the pair potential, the induced odd-frequency Cooper pairs indicate a paramagnetic property. We also discuss the relation between the amplitude of the odd-frequency pairing correlation and the stability of superconducting states in terms of the self-consistent equation for the pair potential.

cond-mat.supr-con↗

arXiv2015

Inversion symmetry of Josephson current as test of chiral domain wall motion in Sr$_{2}$RuO$_{4}$

Clarifying the chiral domains structure of superconducting Sr$_{2}$RuO$_{4}$ has been a long-standing issue in identifying its peculiar topological superconducting state. We evaluated the critical current $I_{c}$ versus the magnetic field $H$ of Nb/Sr$_{2}$RuO$_{4}$ Josephson junctions, changing the junction dimension in expectation of that the number of domains in the junction is controlled. $I_{c}(H)$ exhibits a recovery from inversion symmetry breaking to invariance when the dimension is reduced to several microns. This inversion invariant behavior indicates the disappearance of domain walls; thus, the size of a single domain is estimated at approximately several microns.

cond-mat.supr-con↗

arXiv2015

Effects of surface roughness on the paramagnetic response of small unconventional superconductors

We theoretically study effects of surface roughness on the magnetic response of small unconventional superconductors by solving the Eilenberger equation for the quassiclassical Green function and the Maxwell equation for the vector potential simultaneously and self-consistently. The paramagnetic phase of spin-singlet $d$-wave superconducting disks is drastically suppressed by the surface roughness, whereas that of spin-triplet $p$-wave disks is robust even in the presence of the roughness. Such difference derives from the orbital symmetry of paramagnetic odd-frequency Cooper pairs appearing at the surface of disks. The orbital part of the paramagnetic pairing correlation is $p$-wave symmetry in the $d$-wave disks, whereas it is $s$-wave symmetry in the $p$-wave ones. Calculating the free-energy, we also confirm that the paramagnetic state is more stable than the normal state, which indicates a possibility of detecting the paramagnetic effect in experiments. Indeed our results are consistent with an experimental finding on high-$T_c$ thin films.

cond-mat.supr-con↗

arXiv2015

Anomalous Proximity Effect and Theoretical Design for its Realization

We discuss the stability of zero-energy states appearing in a dirty normal metal attached to a superconducting thin film with Dresselhaus [110] spin-orbit coupling under the in-plane Zeeman field. The Dresselhaus superconductor preserves an additional chiral symmetry and traps more than one zero-energy state at its edges. All the zero-energy states at an edge belong to the same chirality in large Zeeman field due to the effective $p$-wave pairing symmetry. The pure chiral nature in the wave function enables the penetration of the zero-energy states into the dirty normal metal with keeping their high degree of degeneracy. By applying a theorem, we prove the the perfect Andreev reflection into the dirty normal metal at the zero-energy. This paper gives a microscopic understanding of the anomalous proximity effect.

cond-mat.supr-con↗

arXiv2014

Consequences of bulk odd-frequency superconducting states for the classification of Cooper pairs

We analyze symmetries and magnetic properties of Cooper pairs appearing as subdominant pairing correlations in inhomogeneous superconductors, on the basis of the quasiclassical Green-function theory. The frequency symmetry, parity, and the type of magnetic response of such subdominant correlations are opposite to those of the dominant pairing correlations in the bulk state. Our conclusion is valid even when we generalize the theory of superconductivity to recently proposed diamagnetic odd-frequency superconductors. As a consequence, Cooper pairs are classified into eight classes in terms of their symmetries and magnetic properties. Anomalous magnetic properties of subdominant components can be probed by studying the Meissner effect.

cond-mat.supr-con↗

arXiv2014

Majorana Bound States in Proximity Junctions of Superconducting Nanowires with Dresselhaus Spin-orbit Coupling

We theoretically study transport properties of nanowires with the Dresselhaus [110] spin-orbit coupling under the in-plane Zeeman potential and the proximity-induced s-wave pair potential. In the topologically nontrivial phase, the nanowire hosts the Majorana fermions at its edges and the number of the Majorana bound states is equal to the number propagating channels (Nc). When we attach a normal metal to the superconductor, such Majorana bound states penetrate into the dirty normal segment and form the Nc resonant transmission channels there. We show that chiral symmetry of the electronic states protects the Majorana bound states at the zero energy even in the presence of impurities. As a result, we find that the zero-bias conductance of normal-nanowire/superconducting-nanowire junctions is quantized at 2e2Nc/h independent of the random potentials.

cond-mat.supr-con↗

arXiv2014

Paramagnetic instability of small topological superconductors

Although the diamagnetism is of the essence in the superconducting phenomena, small enough topological (or unconventional) superconductors are paramagnetic. By solving the quasiclassical Eilenberger equation and the Maxwell equation self-consistently in two-dimensional disks, we theoretically study the response of small superconductors to weak magnetic field. Because of the topologically nontrivial character of the wave function, the unconventional superconductor hosts the zero-energy surface Andreev bound states which always accompany the odd-frequency Cooper pairs. The paramagnetic property of the odd-frequency pairs explains the unusual Meissner effect at low temperature.

cond-mat.supr-con↗

arXiv2014

Andreev Reflection in Weyl Semimetals

We theoretically study low energy electric transport in a junction consisting of a Weyl semimetal and a metallic superconductor. The characteristic features of the differential conductance depend on the relative directions between the current and the vector connecting the two Weyl points. When the electric current is perpendicular to the vector, the conductance spectra are sensitive to the direction and the amplitude of magnetic moment at the junction interface. This is a direct consequence of the chiral spin configuration on the fermi surface near the Weyl points.

cond-mat.supr-con↗

arXiv2014

Purely Surface Electric Transport Phenomenon on a Three-Dimensional Topological Insulator

We theoretically study the electric current induced by the temporally oscillating magnetic field onto a three-dimensional topological insulator based on the linear response theory. The electric current flows on its surface because of the unique characters of the surface states: the linear dispersion in two-dimension and the chiral spin texture on the Fermi surface. In the bulk region, on the other hand, the current is absent because of the degeneracy for the bulk bands. The response function has a peak structure when the frequency of the magnetic field is equal to twice of the Fermi energy of the surface states measured from the Dirac node. This feature is unique to the surface states on the three-dimensional topological insulator.

cond-mat.mes-hall↗

arXiv2013

Robustness of Spin-Triplet Pairing and Singlet-Triplet Pairing Crossover in Superconductor/Ferromagnet Hybrids

We have investigated the proximity effect in superconductor/ferromagnet junctions in a systematic manner to discuss the relationship between the zero-energy peak (ZEP) of the local density of states (LDOS) and spin-triplet odd-frequency pairing. 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 pairing in such systems. We also found that the crossover from singlet pairing to triplet pairing can be detected by measuring the F layer thickness dependence of the ZEP height. Furthermore, we show how to observe signatures of spin-triplet odd-frequency pairing and the pairing crossover by LDOS measurement. Our results provide a direct way to experimentally detect signatures of the odd-frequency pairing state.

cond-mat.supr-con↗

arXiv2013

Robustness of Gapless Interface State in a Junction of Two Topological Insulators

We theoretically study subgap states appearing at the interface between two three-dimensional topological insulators which have different configurations in the spin-orbit interactions from each other. The coupling of spin $\boldsymbolσ$ with momenta $\boldsymbol{p}$ is configured by a material dependent $3\times 3$ matrix $\boldsymbolΛ$ as $σ^μΛ_μ^νp_ν$. We show that the spectra of the interface suggap states depend strongly on the relative choices of $\boldsymbolΛ$ in the two topological insulators. In particular, we focus on properties of gapless states which appear when $\boldsymbolΛ$ in two topological insulators are connected by the inversion in momentum space. We also discuss the robustness of the gapless states under perturbations breaking the time-reversal symmetry or the band-inversion symmetry by the numerical simulation.

cond-mat.mes-hall↗

arXiv2013

Quantum Landau Spin Hall Insulator

We study theoretically the two-dimensional topological electric state in a single band semiconductor with strong spin-orbit interactions under harmonic scalar electrostatic potentials. The electronic states described by the spin Landau levels are insulating in the bulk and host gapless edge states in the presence of time-reversal symmetry. Such topological states show the properties of the quantized electric conductivity and the quantized spin Hall conductivity characterized by the spin Chern number $\mathbb{Z}$. The quantization of the two conductivities are robust under various perturbations such as the potential disorder, the Zeeman field, and the spin-orbit scatterings. Existing semiconductor technologies would realize the topological states discussed in the present paper.

cond-mat.mes-hall↗

arXiv2013

Charge and spin supercurrents in triplet superconductor--ferromagnet--singlet superconductor Josephson junctions

We study the Josephson effect in a triplet superconductor--ferromagnet--singlet superconductor junction. We show that the interaction of tunneling Cooper pairs with the interface magnetization can permit a Josephson current at the lowest order of a tunneling Hamiltonian perturbation theory. Two conditions must be satisfied for this to occur: the magnetization of the ferromagnet has a component parallel to the ${\bf d}$-vector of the triplet superconductor, and the gaps of the superconductors have the same parity with respect to the interface momentum. The resulting charge current displays an unconventional dependence on the orientation of the magnetic moment and the phase difference. This is accompanied by a phase-dependent spin current in the triplet superconductor, while a phase-independent spin current is always present. The tunneling perturbation theory predictions are confirmed using a numerical Green's function method. An analytical treatment of a one-dimensional junction demonstrates that our conclusions are robust far away from the tunneling regime, and reveals signatures of the unconventional Josephson effect in the critical currents.

cond-mat.supr-con↗

arXiv2013

Three-dimensional symmetry breaking topological matters

We discuss topological electronic states described by the Dirac Hamiltonian plus an additional one in three-dimension. When the additional Hamiltonian is an element of an Abelian group, electronic states become topologically nontrivial even in the absence of fundamental symmetries such as the time-reversal and the particle-hole symmety. The symmetry-breaking topological states are charercterized by the Chern number defined in the two-dimensional partial Brillouin zone. The topological insulators under Zeeman field are an example of the symmetry-breaking topological matters. We show the transision from the topological insulating phase to the topological semimetal one under the strong Zeeman field.

cond-mat.mes-hall↗

arXiv2013

Majorana Fermions and Odd-frequency Cooper Pairs in a Nano Wire

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-φ) relationship of the Josephson effect in SNS junction J\propto \sin(φ/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.

cond-mat.supr-con↗

arXiv2013

Gapped energy spectra around the Dirac node at the surface of a 3D topological insulator in the presence of the time-reversal symmetry

We discuss the excitation spectra around the Dirac node on a surface of a three-dimensional topological insulator. By using the diagrammatic expansion, we show that the coupling of an electron with the gauge field in the presence of impurity scatterings opens a gap around the Dirac node. The results are consistent with a recent experimental finding by T. Sato,et. al.[ Nature Phys. \textbf{7}, 840 (2011)]. We also discuss the consistency between the present results and the bulk-boundary correspondence of the topological insulator in the presence of time reversal symmetry. The conclusion can be applied any two dimensional massless fermions.

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