arXiv subjects

Yukio Tanaka

Publications and source records attributed to Yukio Tanaka.

At least 199 records · Page 11Linked to original sources

arXiv2010

DC Current Driven Critical Current Variation in Sr2RuO4-Ru Junction Proved by Local Transport Measurements

To search for new evidence of the chiral p-wave superconducting domain in Sr2RuO4, we investigated the unconventional local transport characteristics of a microfabricated Sr2RuO4-Ru eutectic junction. We found that the anomalous hysteresis in voltage-current characteristics [as reported in H. Kambara et al.: Phys. Rev. Lett. 101 (2008) 267003.] is strongly affected by dc currents, but not by magnetic fields. This suggests that dc current acts as a driving force to move chiral p-wave domain walls; a domain wall trapped at a pinning potential is forced to shift to the next stable position, thereby forming a larger critical current path and resulting in the anomalous hysteresis.

cond-mat.supr-con↗

arXiv2010

Tunneling between Two Helical Superconductors via Majorana Edge Channels

We discuss electric transport through a point contact which bridges Majorana fermion modes appearing at edges of two helical superconductors. The contents focus on effects of interference and interaction unique to the Majorana fermions and role of spin-orbit interaction (SOI). Besides the Josephson current, the quasi-particle conductance depends sensitively on phase difference and relative helicity between the two superconductors. The interaction among the Majorana fermions causes the power-law temperature dependences of conductance for various tunneling channels. Especially, in the presence of SOI, the conductance always increases as the temperature is lowered.

cond-mat.supr-con↗

arXiv2010

Theory of Josephson transport through spintronics nano-structure

We study the Josephson transport through ferromagnetic insulators (FIs) by taking into account its band structure explicitly. In the case of the fully polarized FIs (FPFIs), we found the formation of a pi-junction and an atomic-scale 0-pi transition induced by increasing the FI thickness. More remarkably, in the Josephson junction through spin-filter materials such as Eu chalcogenides, the orbital hybridization between the conduction d and the localized f electron gives rise to the pi-junction behavior. Such FI-based pi-junctions can be used to implement highly-coherent solid-state quantum bits.

cond-mat.mes-hall↗

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

Josephson pi-state in a ferromagnetic insulator

We predict anomalous atomic-scale 0-pi transitions in a Josephson junction with a ferromagnetic-insulator (FI) barrier. The ground state of such junction alternates between 0- and pi-states when thickness of FI is increasing by a single atomic layer. We find that the mechanism of the 0-pi transition can be attributed to thickness-dependent phase-shifts between the wave numbers of electrons and holes in FI. Based on these results, we show that stable pi-state can be realized in junctions based on high-Tc superconductors with La$_2$BaCuO$_5$ barrier.

cond-mat.supr-con↗

arXiv2010

Effect of d-f hybridization on the Josephson current through Eu-chalcogenides

A superconducting ring with a pi junction made from superconductor/ferromagnetic-metal/superconductor (S-FM-S) exhibits a spontaneous current without an external magnetic field in the ground state. Such pi ring provides so-called quiet qubit that can be efficiently decoupled from the fluctuation of the external field. However, the usage of the FM gives rise to strong Ohmic dissipation. Therefore, the realization of pi junctions without FM is expected for qubit applications. We theoretically consider the possibility of the pi coupling for S/Eu-chalcogenides/S junctions based on the d-f Hamiltonian. By use of the Green's function method we found that pi junction can be formed in the case of the finite d-f hybridization between the conduction d and the localized f electrons.

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

Theoretical aspects of Andreev spectroscopy and tunneling spectroscopy in non-centrosymmetric superconductors: a topical review

Tunneling spectroscopy at surfaces of unconventional superconductors has proven an invaluable tool for obtaining information about the pairing symmetry. It is known that mid gap Andreev bound states manifest itself as a zero bias conductance peak in tunneling spectroscopy. The zero bias conductance peak is a signature for a non-trivial pair potential that exhibits different signs on different regions of the Fermi surface. Here, we review recent theoretical results on the spectrum of Andreev bound states near interfaces and surfaces in non-centrosymmetric superconductors. We introduce a theoretical scheme to calculate the energy spectrum of a non-centrosymmetric superconductor. Then, we discuss the interplay between the spin orbit vector field on the Fermi surface and the order parameter symmetry. The Andreev states carry a spin supercurrent and represent a helical edge mode along the interface. We study the topological nature of the resulting edge currents. If the triplet component of the order parameter dominates, then the helical edge mode exists. If, on the other hand, the singlet component dominates, the helical edge mode is absent. A quantum phase transition occurs for equal spin singlet and triplet order parameter components. We discuss the tunneling conductance and the Andreev point contact conductance between a normal metal and a non-centrosymmetric superconductor.

cond-mat.supr-con↗

arXiv2009

Pairing competition in a quasi-one-dimensional model of organic superconductors (TMTSF)$_{2}X$ in magnetic field

We microscopically study the effect of the magnetic field (Zeeman splitting) on the superconducting state in a model for quasi-one-dimensional organic superconductors (TMTSF)$_{2}X$. We investigate the competition between spin singlet and spin triplet pairings and the Fulde-Ferrell-Larkin-Ovchinnikov(FFLO) state by random phase approximation. While we studied the competition by comparison with the eigenvalue of the gap equation at a fixed temperature in our previous study (Phys. Rev. Lett. \textbf{102} (2009) 016403), here we obtain both the $T_c$ for each pairing state and a phase diagram in the $T$(temperature)-$h_z$(field)-$V_y$(strength of the charge fluctuation) space. The phase diagram shows that consecutive transitions from singlet pairing to the FFLO state and further to $S_z=1$ triplet pairing can occur upon increasing the magnetic field when $2k_{F}$ charge fluctuations coexist with $2k_{F}$ spin fluctuations. In the FFLO state, the singlet d-wave and $S_{z}=0$ triplet $f$-wave components are strongly mixed especially when the charge fluctuations are strong.

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↗

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

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

Surface density of states of s+-wave Cooper pairs in a two-band model

We calculate surface density of state (SDOS) of s+-wave Cooper pair in two-band superconductor model, where gap functions have different signs between two bands. We find that Andreev bound state appears at surface due to the sign change in the gap function in the interband quasiparticle scattering. However, we do not obtain the zero-energy peak of SDOS in contrast to the d-wave case. The tunneling spectroscopy of s+-wave is much more complex as compared to the d-wave case realized in high-Tc cuprates.

cond-mat.supr-con↗

arXiv2009

Interacting two helical edge modes in quantum spin Hall systems

We study theoretically the two interacting one-dimensional helical modes at the edges of the quantum spin Hall systems. A new type of inter-edge correlated liquid (IECL) without the spin gap is found. This liquid shows the diverging density wave (DW) and superconductivity (SC) correlations much stronger than those of the spinfull electrons. Possible experimental observations are also discussed.

cond-mat.supr-con↗

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

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.

cond-mat.mes-hall↗