arXiv subjects

Takehito Yokoyama

Publications and source records attributed to Takehito Yokoyama.

At least 73 records · Page 4Linked to original sources

arXiv2010

Signature of odd-frequency pairing correlations induced by a magnetic interface

We investigate the mutual proximity effect in a normal metal contacted to a superconductor through a magnetic interface. Analytical and self-consistent numerical results are presented, and we consider both the diffusive and ballistic regimes. We focus on the density of states in both the normal and superconducting region, and find that the presence of spin-dependent phase-shifts occurring at the interface qualitatively modifies the density of states. In particular, we find that the proximity-induced pairing amplitudes in the normal metal region undergo a conversion at the Fermi level from pure even-frequency to odd-frequency. Above a critical value of the interface spin-polarization (or, equivalently, for fixed interface spin-polarization, above a critical interface resistance), only odd frequency correlations remain. This is accompanied by the replacement of the familiar proximity minigap or pseudogap in the normal layer by an enhancement of the density of states above its normal state value for energies near the chemical potential. The robustness of this effect towards inelastic scattering, impurity scattering, and the depletion of the superconducting order parameter close to the interface is investigated. We also study the inverse proximity effect in the diffusive limit. We find that the above-mentioned conversion persists also for thin superconducting layers comparable in size to the superconducting coherence length $ξ_\text{S}$, as long as the inverse proximity effect is relatively weak. Concomitantly, we find a shift in the critical interface resistance where the pairing conversion occurs. Our findings suggest a robust and simple method for producing purely odd-frequency superconducting correlations, that can be tested experimentally.

cond-mat.supr-con↗

arXiv2010

Interplay between Superconductivity and Ferromagnetism on a Topological Insulator

We study theoretically proximity-induced superconductivity and ferromagnetism on the surface of a topological insulator. In particular, we investigate how the Andreev-bound states are influenced by the interplay between these phenomena, taking also into account the possibility of unconventional pairing. We find a qualitative difference in the excitation spectrum when comparing spin-singlet and spin-triplet pairing, leading to non-gapped excitations in the latter case. The formation of surface-states and their dependence on the magnetization orientation is investigated, and it is found that these states are Majorana fermions in the $d_{xy}$-wave case in stark contrast to the topologically trivial high-$T_c$ cuprates. The signature of such states in the conductance spectra is studied, and we also compute the supercurrent which flows on the surface of the topological insulator when a Josephson junction is deposited on top of it. It is found that the current exhibits an anomalous current-phase relation when the region separating the superconducting banks is ferromagnetic, and we also show that in contrast to the metallic case the exchange field in such a scenario does not induce 0-$π$ oscillations in the critical current. Similarly to the high-$T_c$ cuprates, the presence of zero-energy surface states on the topological surface leads to a strong low-temperature enhancement of the critical current.

cond-mat.supr-con↗

arXiv2010

Dynamics of magnetization on the topological surface

We investigate theoretically the dynamics of magnetization coupled to the surface Dirac fermions of a three dimensional topological insulator, by deriving the Landau-Lifshitz-Gilbert (LLG) equation in the presence of charge current. Both the inverse spin-Galvanic effect and the Gilbert damping coefficient $α$ are related to the two-dimensional diagonal conductivity $σ_{xx}$ of the Dirac fermion, while the Berry phase of the ferromagnetic moment to the Hall conductivity $σ_{xy}$. The spin transfer torque and the so-called $β$-terms are shown to be negligibly small. Anomalous behaviors in various phenomena including the ferromagnetic resonance are predicted in terms of this LLG equation.

cond-mat.mes-hall↗

arXiv2010

Gate-controlled one-dimensional channel on the topological surface

We investigate the formation of the one-dimensional channels on the topological surface under the gate electrode. The energy dispersion of these channels is almost linear in the momentum with the velocity sensitively depending on the strength of the gate voltage. The energy is also restricted to be positive or negative depending on the strength of the gate voltage. Consequently, the local density of states near the gated region has an asymmetric structure with respect to zero energy. In the presence of the electron-electron interaction, the correlation effect can be tuned by the gate voltage. We also suggest a tunneling experiment to verify the presence of these bound states.

cond-mat.mes-hall↗

arXiv2010

Unconventional superconductivity on a topological insulator

We study proximity-induced superconductivity on the surface of a topological insulator (TI), focusing on unconventional pairing. We find that the excitation spectrum becomes gapless for any spin-triplet pairing, such that both subgap bound states and Andreev reflection is strongly suppressed. For spin-singlet pairing, the zero-energy surface state in the $d_{xy}$-wave case becomes a Majorana fermion, in contrast to the situation realized in the topologically trivial high-$T_c$ cuprates. We also study the influence of a Zeeman field on the surface states. Both the magnitude and direction of this field is shown to strongly influence the transport properties, in contrast to the case without TI. We predict an experimental signature of the Majorana states via conductance spectroscopy.

cond-mat.supr-con↗

arXiv2010

Pure spin-current generated by reflection at a normal metal/2DEG interface

The concept of a spin-current is a useful tool in understanding spin-transport in hybrid systems, but its very definition is problematic in systems where spin-orbit coupling effects are strong. In the absence of spin-dependent scattering, the spin-current remains well-defined. We here propose a method for generating pure spin-currents in a normal metal where the spin-current consequently does not suffer from the aforementioned problems pertaining to its very definition or spin-relaxation processes. More specifically, we show how an unpolarized incident charge-current can induce a pure transverse spin-current by means of scattering at a normal metal/2DEG interface. This occurs for both Rashba and Dresselhaus spin-orbit coupling. An experimental setup for observation of this effect is proposed.

cond-mat.mes-hall↗

arXiv2009

Anomalous Finite Size Effects on Surface States in the Topological Insulator Bi$_2$Se$_3$

We study how the surface states in the strong topological insulator Bi$_2$Se$_3$ are influenced by finite size effects, and compare our results with those recently obtained for 2D topological insulator HgTe. We demonstrate two important distinctions: \textit{(i)} contrary to HgTe, the surface-states in Bi$_2$Se$_3$ display a remarkable robustness towards decreasing the width $L$ down to a few nm, thus ensuring that the topological surface states remain intact, and \textit{(ii)} the gapping due to the hybridization of the surface states features an oscillating exponential decay as a function of $L$ in Bi$_2$Se$_3$ in sharp contrast to HgTe. Our findings suggest that Bi$_2$Se$_3$ is suitable for nanoscale applications in quantum computing or spintronics. Also, we propose a way to experimentally detect both of the predicted effects.

cond-mat.mes-hall↗

arXiv2009

Spin-polarized Josephson current in SFS junctions with inhomogeneous magnetization

SHORTENED ABSTRACT: We study numerically the properties of spin- and charge-transport in a nanoscale diffusive superconductor$\mid$ferromagnet$\mid$superconductor junction when the magnetization texture is non-uniform. Specifically, we incorporate the presence of a Bloch/Neel domain walls and conical ferromagnetism, including spin-active interfaces. The superconducting leads are assumed to be of s-wave type. We investigate how the 0-$π$ transition is influenced by the inhomogeneous magnetization texture and focus on the particular case where the charge-current vanishes while the spin-current is non-zero. In the case of a Bloch/Neel domain-wall, the spin-current can be seen only for one component of the spin polarization, whereas in the case of conical ferromagnetism the spin-current has the three components. We explain all of these results in terms of the interplay between the triplet anomalous Green's function induced in the ferromagnetic region and the local direction of the magnetization vector in the ferromagnet. Interestingly, we find that the spin-current exhibits discontinuous jumps at the 0--$π$ transition points of the critical charge-current. We explain this result in terms of the different symmetry obeyed by the current-phase relation when comparing the charge- and spin-current. Specifically, we find that whereas the charge-current obeys the well-known relation $I_c(ϕ) = -I_c(2π-ϕ)$, the spin-current satisfies $I_s(ϕ) = I_s(2π-ϕ)$, where $ϕ$ is the superconducting phase difference.

cond-mat.supr-con↗

arXiv2009

Anomalous magnetoresistance on the topological surface

We investigate charge transport in two-dimensional ferromagnet/feromagnet junction on a topological insulator. The conductance across the interface depends sensitively on the directions of the magnetizations of the two ferromagnets, showing anomalous behaviors compared with the conventional spin-valve. It is found that the conductance depends strongly on the in-plane direction of the magnetization. Moreover, in sharp contrast to the conventional magnetoresistance effect, in the p-n junction, the conductance at the parallel configuration is much smaller than that at the antiparallel configuration. This stems from the way how the wavefunctions connect between both sides.

cond-mat.mes-hall↗

arXiv2009

Manipulation of Majorana fermion, Andreev reflection and Josephson current on topological insulators

We study theoretically charge transport properties of normal metal (N) / ferromagnet insulator (FI) / superconductor (S) junction and S/FI/S junction formed on the surface of three-dimensional topological insulator (TI), where chiral Majorana mode (CMM) exists at FI/S interface. We find that CMM generated in N/FI/S and S/FI/S junctions are very sensitively controlled by the direction of the magnetization ${\bm m}$ in FI region. Especially, the current-phase relation of Josephson current in S/FI/S junctions has a phase shift neither 0 nor $π$, which can be tuned continuously by the component of ${\bm m}$ perpendicular to the interface.

cond-mat.supr-con↗

arXiv2009

Josephson current in graphene: the role of unconventional pairing symmetries

We investigate the Josephson current in a graphene superconductor/normal/superconductor junction, where superconductivity is induced by means of the proximity effect from external contacts. We take into account the possibility of anisotropic pairing by also including singlet nearest-neighbor interactions, and investigate how the transport properties are affected by the symmetry of the superconducting order parameter. This corresponds to an extension of the usual on-site interaction assumption, which yields an isotropic s-wave order parameter near the Dirac points. Here, we employ a full numerical solution as well as an analytical treatment, and show how the proximity effect may induce exotic types of superconducting states near the Dirac points, e.g. $p_x$- and $p_y$-wave pairing or a combination of s-wave and $p+ıp$-wave pairing. We find that the Josephson current exhibits a weakly-damped, oscillatory dependence on the length of the junction when the graphene sheet is strongly doped. The analytical and numerical treatments are found to agree well with each other in the s-wave case when calculating the critical current and current-phase relationship. For the scenarios with anisotropic superconducting pairing, there is a deviation between the two treatments, especially for the effective $p_x$-wave order parameter near the Dirac cones which features zero-energy states at the interfaces. This indicates that a numerical, self-consistent approach becomes necessary when treating anisotropic superconducting pairing in graphene.

cond-mat.supr-con↗

arXiv2009

Tuning odd triplet superconductivity by spin pumping

We study proximity effect in diffusive ferromagnet/normal-metal/superconductor junction with precessing magnetization of the ferromagnet. We find that the odd-frequency pairing induced in the normal metal is modified by spin pumping from the ferromagnet and hence can be tuned by changing the precessional frequency. At the frequency corresponding to twice the superconducting gap, the odd-frequency pairing is strongly enhanced. We find a crossover from the even- to the odd-frequency superconductivity in the normal metal by tuning the precessional frequency. This gives a clearcut signature of the odd-frequency superconductivity observable by scanning tunneling microscopy. According to the pairing symmetries in the normal metal, we find a crossover from the gap to the peak structure in the tunneling conductance between the normal metal and a scanning tunneling microscope tip.

cond-mat.supr-con↗

arXiv2009

Theory of charge transport in ferromagnetic semiconductor/s-wave superconductor junction

We study tunneling conductance in ferromagnetic semiconductor/insulator/s-wave superconductor junction where Rashba spin-orbit interaction (RSOI) and exchange field are taken into account in the ferromagnetic semiconductor. We show that normalized conductance at zero voltage has a maximum as a function of RSOI for high transparent interface and finite exchange field. This is because Andreev reflection probability shows a nonmonotonic dependence on RSOI in the presence of the exchange field. On the other hand, for intermediate transparent interface, normalized conductance at zero voltage has a reentrant shape at zero or small exchange field with increasing RSOI but is monotonically increasing by RSOI at large exchange field.

cond-mat.mes-hall↗

arXiv2009

Theory of pairing symmetry in Fulde-Ferrell-Larkin-Ovchinnikov vortex state and vortex lattice

We investigate pairing symmetry in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) vortex state and vortex lattice, and explain the electronic structure in these states in terms of pairing symmetry. We show analytically that at the intersection point of FFLO nodal plane and vortex line, only even frequency pairing is present if the Zeeman splitting is negligibly small. With increasing Zeeman splitting, odd frequency pairing emerges there. This makes it possible to interpret the gap structure of the density of states at the intersection point as a manifestation of the even frequency pairing. In the vortex lattice, we find that only odd frequency pairing is present at the core centers, while at the midpoint of the vortex lines, only even frequency pairing exists. Thus, the odd and even frequency pairings also form the lattice in the vortex lattice state.

cond-mat.supr-con↗

arXiv2009

Pairing symmetry conversion by spin-active interfaces in superconducting junctions

We study the proximity-induced superconducting correlations in a normal metal connected to a superconductor when the interface between them is spin-active and the normal metal is ballistic or diffusive. Remarkably, for any interface spin polarization there is a critical interface resistance, above which the conventional even-frequency proximity component vanishes completely at the chemical potential, while the odd-frequency component remains finite. We propose a way to unambiguously observe the odd-frequency component.

cond-mat.supr-con↗

arXiv2009

Theory of superconducting and magnetic proximity effect in S$\mid$F structures with inhomogeneous magnetization textures and spin-active interfaces

We present a study of the proximity effect and the inverse proximity effect in a superconductor$\mid$ferromagnet bilayer, taking into account several important factors which mostly have been ignored in the literature so far. These include spin-dependent interfacial phase shifts (spin-DIPS) and inhomogeneous textures of the magnetization in the ferromagnetic layer, both of which are expected to be present in real experimental samples. Our approach is numerical, allowing us to access the full proximity effect regime. In Part I of this work, we study the superconducting proximity effect and the resulting local density of states in an inhomogeneous ferromagnet with a non-trivial magnetic texture. Our two main results in Part I are a study of how Bloch and Néel domain walls affect the proximity-induced superconducting correlations and a study of the superconducting proximity effect in a conical ferromagnet. The latter topic should be relevant for the ferromagnet Ho, which was recently used in an experiment to demonstrate the possibility to generate and sustain long-range triplet superconducting correlations. In Part II of this work, we investigate the inverse proximity effect with emphasis on the induced magnetization in the superconducting region as a result of the "leakage" from the ferromagnetic region. It is shown that the presence of spin-DIPS modify conclusions obtained previously in the literature with regard to the induced magnetization in the superconducting region. In particular, we find that the spin-DIPS can trigger an anti-screening effect of the magnetization, leading to an induced magnetization in the superconducting region with \textit{the same sign} as in the proximity ferromagnet.

cond-mat.supr-con↗

arXiv2009

Tunneling conductance and local density of states in time-reversal symmetry breaking superconductors under the influence of an external magnetic field

We consider different effects that arise when time-reversal symmetry breaking superconductors are subjected to an external magnetic field, thus rendering the superconductor to be in the mixed state. We focus in particular on two time-reversal symmetry breaking order parameters which are believed to be realized in actual materials: $p+ıp'$-wave and $d+ıs$- or $d+ıd'$-wave. The first order parameter is relevant for Sr$_2$RuO$_4$, while the latter order parameters have been suggested to exist near surfaces in some of the high-$T_c$ cuprates. We investigate the interplay between surface states and vortex states in the presence of an external magnetic field and their influence on both the tunneling conductance and the local density of states. Our findings may be helpful to experimentally identify the symmetry of unconventional time-reversal symmetry breaking superconducting states.

cond-mat.supr-con↗

arXiv2009

Giant spin rotation in the normal metal/quantum spin Hall junction

We study theoretically reflection problem in the junction between a normal metal and an insulator characterized by a parameter $M$, which is a usual insulator for $M>0$ or a quantum spin Hall system for $M<0$. The spin rotation angle $α$ at the reflection is obtained in the plane of $M$ and the incident angle $θ$ measured from the normal to the interface. The $α$ shows rich structures around the quantum critical point M=0 and $θ=0$, i.e., $α$ can be as large as $\sim π$ at an incident angle in the quatum spin Hall case $M<0$ because the helical edge modes resonantly enhance the spin rotation, which can be used to map the energy dispersion of the helical edge modes. As an experimentally relevant system, we also study spin rotation effect in quantum spin Hall/normal metal/quantum spin Hall trilayer junction.

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