arXiv subjects

Yukio Tanaka

Publications and source records attributed to Yukio Tanaka.

At least 217 records · Page 12Linked to original sources

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

Strong parity mixing in the FFLO superconductivity in systems with coexisting spin and charge fluctuations

We study the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of spin fluctuation mediated pairing, and focus on the effect of coexisting charge fluctuations. We find that (i) consecutive transitions from singlet pairing to FFLO and further to $S_z=1$ triplet pairing can generally take place upon increasing the magnetic field when strong charge fluctuations coexist with spin fluctuations, and (ii) the enhancement of the charge fluctuations lead to a significant increase of the parity mixing in the FFLO state, where the triplet/singlet component ratio in the gap function can be close to unity. We propose that such consecutive pairing state transition and strong parity mixing in the FFLO state may take place in a quasi-one-dimensional organic superconductor (TMTSF)$_2X$.

cond-mat.supr-con↗

arXiv2008

Topological spin-current in non-centrosymmetric superconductors

We study the spin transport properties of the non-centrosymmetric superconductor with time-reversal-symmetry where spin-triplet $(p_{x} \pm i p_{y})$-wave and spin-singlet s-wave pair potential can mix each other. We show that when the amplitude of $(p_{x} \pm ip_{y})$-wave pair potential is larger than that of s-wave one, the superconducting state belongs to the topologically nontrivial class analogous to the quantum spin Hall system, and the resulting helical edge modes as Andreev bound states are topologically protected. We find that the incident angle dependent spin polarized current flows through the interface due to the presence of the helical edge modes. With a weak magnetic field, also the angle-integrated current is strongly spin polarized.

cond-mat.supr-con↗

arXiv2008

Unconventional pairing originating from disconnected Fermi surfaces in superconducting LaFeAsO$_{1-x}$F$_x$}

For a newly discovered iron-based high $T_c$ superconductor LaFeAsO$_{1-x}$F$_x$, we have constructed a minimal model, where inclusion of all the five Fe $d$ bands is found to be necessary. Random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.

cond-mat.supr-con↗

arXiv2008

Predominant Magnetic States in Hubbard Model on Anisotropic Triangular Lattices

Using an optimization variational Monte Carlo method, we study the half-filled-band Hubbard model on anisotropic triangular lattices, as a continuation of the preceding study [J. Phys. Soc. Jpn 75, 074707 (2006)]. We introduce two new trial states: (i) A coexisting state of (π,π)-antiferromagnetic (AF) and a d-wave singlet gaps, in which we allow for a band renormalization effect, and (ii) a state with an AF order of 120^\circ spin structure. In both states, a first-order metal-to-insulator transition occurs at smaller U/t than that of the pure d-wave state. In insulating regimes, magnetic orders always exist; an ordinary (π,π)-AF order survives up to t'/t\sim 0.9 (U/t=12), and a 120^\circ-AF order becomes dominant for t'/t \gsim 0.9. The regimes of the robust superconductor and of the nonmagnetic insulator the preceding study proposed give way to these magnetic domains.

cond-mat.supr-con↗

arXiv2008

Strongly spin-polarized current generated in Zeeman-split unconventional superconductors

We consider a thin-film normal metal/superconductor junction in the presence of an externally applied in-plane magnetic field for several symmetries of the superconducting order parameter. For p-wave superconductors, a strongly spin-polarized current emerges due to an interplay between the nodal structure of the superconducting order parameter, the existence or non-existence of zero-energy surface states, and the Zeeman-splitting of the bands which form superconductivity. Thus, the polarization depends strongly on the orbital symmetry of the superconducting state. Our findings suggest a mechanism for obtaining fully spin-polarized currents crucially involving zero-energy surface states, not present in s-wave superconductors.

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

Quantum transport in a normal metal/odd-frequency superconductor junction

Recent experimental results indicate the possible realization of a bulk odd-frequency superconducting state in the compounds CeCu$_2$Si$_2$, and CeRhIn$_5$. Motivated by this, we present a study of the quantum transport properties of a normal metal/odd-frequency superconductor junctions in a search for probes to unveil the odd-frequency symmetry. From the Eliashberg equations, we perform a quasiclassical approximation to account for the transport formalism of an odd-frequency superconductor with the Keldysh formalism. Specifically, we consider the tunneling charge conductance and tunneling thermal conductance. We find qualitatively distinct behaviour in the odd-frequency case as compared to the conventional even-frequency case, in both the electrical and thermal current. This serves as a useful tool to identify the possible existence of a bulk odd-frequency superconducting state.

cond-mat.supr-con↗

arXiv2008

Theory of pairing symmetry inside the Abrikosov vortex core

We show that the Cooper pair wave function at the center of an Abrikosov vortex with vorticity m has different parity with respect to frequency from that in the bulk if m is an odd number and has the same parity if m is an even number. As a result, in a conventional vortex with m=1, the local density of states at the Fermi energy has a maximum (minimum) at the center of the vortex core in even(odd)-frequency superconductor. We propose a scanning tunneling microscope experiment using a superconducting tip to explore odd-frequency superconductivity.

cond-mat.supr-con↗

arXiv2007

Odd-frequency Pairs and Josephson Current through a Strong Ferromagnet

We study Josephson current in superconductor / diffusive ferromagnet /superconductor junctions by using the recursive Green function method. When the exchange potential in a ferromagnet is sufficiently large as compared to the pair potential in a superconductor, an ensemble average of Josephson current is much smaller than its mesoscopic fluctuations. The Josephson current vanishes when the exchange potential is extremely large so that a ferromagnet is half-metallic. Spin-flip scattering at junction interfaces drastically changes the characteristic behavior of Josephson current. In addition to spin-singlet Cooper pairs, equal-spin triplet pairs penetrate into a half metal. Such equal-spin pairs have an unusual symmetry property called odd-frequency symmetry and carry the Josephson current through a half metal. The penetration of odd-frequency pairs into a half metal enhances the low energy quasiparticle density of states, which could be detected experimentally by scanning tunneling spectroscopy. We will also show that odd-frequency pairs in a half metal cause a nonmonotonic temperature dependence of the critical Josephson current.

cond-mat.supr-con↗

arXiv2007

Chirality sensitive effect on surface states in chiral p-wave superconductors

We study the local density of states at the surface of a chiral p-wave superconductor in the presence of a weak magnetic field. As a result, the formation of low-energy Andreev bound states is either suppressed or enhanced by an applied magnetic field, depending on its orientation with respect to the chirality of the p-wave superconductor. Similarly, an Abrikosov vortex, which is situated not too far from the surface, leads to a zero-energy peak of the density of states, if its chirality is the same as that of the superconductor, and to a gap structure for the opposite case. We explain the underlying principle of this effect and propose a chirality sensitive test on unconventional superconductors.

cond-mat.supr-con↗

arXiv2007

Spin-Dependent Mass Enhancement under Magnetic Field in the Periodic Anderson Model

In order to study the mechanism of the mass enhancement in heavy fermion compounds in the presence of magnetic field, we study the periodic Anderson model using the fluctuation exchange approximation. The resulting value of the mass enhancement factor z^{-1} can become up to 10, which is significantly larger than that in the single-band Hubbard model. We show that the difference between the magnitude of the mass enhancement factor of up spin (minority spin) electrons z^{-1}_up and that of down spin (majority spin) electrons z^{-1}_down increases by the applied magnetic field B//z, which is consistent with de Haas-van Alphen measurements for CeCoIn_5, CeRu_2Si_2 and CePd_2Si_2. We predict that z^{-1}_up >z^{-1}_down in many Ce compounds, whereas z^{-1}_up < z^{-1}_down in Yb compounds.

cond-mat.str-el↗

arXiv2007

Theory of Macroscopic Quantum Tunneling in High-T_c c-Axis Josephson Junctions

We study macroscopic quantum tunneling (MQT) in c-axis twist Josephson junctions made of high-T_c superconductors in order to clarify the influence of the anisotropic order parameter symmetry (OPS) on MQT. The dependence of the MQT rate on the twist angle $γ$ about the c-axis is calculated by using the functional integral and the bounce method. Due to the d-wave OPS, the $γ$ dependence of standard deviation of the switching current distribution and the crossover temperature from thermal activation to MQT are found to be given by $\cos2γ$ and $\sqrt{\cos2γ}$, respectively. We also show that a dissipative effect resulting from the nodal quasiparticle excitation on MQT is negligibly small, which is consistent with recent MQT experiments using Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8 + δ}$ intrinsic junctions. These results indicate that MQT in c-axis twist junctions becomes a useful experimental tool for testing the OPS of high-T_c materials at low temperature, and suggest high potential of such junctions for qubit applications.

cond-mat.supr-con↗

arXiv2007

Theory of Fulde-Ferrell-Larkin-Ovchinnikov state of superconductors with and without inversion symmetry: Hubbard model approach

We study Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of superconductors with and without inversion symmetry based on the Hubbard model on the square lattice near half-filling, using the random phase approximation. We show that center of mass momentum $Q$ tends to be parallel to $x$- or y-axis in the presence of inversion symmetry, while $Q$ vector is likely to be perpendicular to the magnetic field in the absence of inversion symmetry. We also clarify that $d+f$-wave pairing is favored and the hetero spin triplet $f$-wave state is present in the FFLO state unlike state in the superconductors only with the Rashba type spin-orbit coupling (RSOC) originating from the broken inversion symmetry. The triplet $f$-wave state is enhanced by magnetic field and the RSOC. This stems from the reduction of the spin susceptibilities by the magnetic field and the RSOC.

cond-mat.supr-con↗

arXiv2007

Strong magnetic field enhancement of spin triplet pairing arising from coexisting $2k_F$ spin and $2k_F$ charge fluctuations

We study the effect of the magnetic field (Zeeman splitting) on the triplet pairing. We show generally that the enhancement of spin triplet pairing mediated by coexisting $2k_F$ spin and $2k_F$ charge fluctuations can be much larger than in the case of triplet pairing mediated by ferromagnetic spin fluctuations. We propose that this may be related to the recent experiment for (TMTSF)$_2$ClO$_4$, in which a possibility of singlet to triplet pairing transition has been suggested.

cond-mat.supr-con↗

arXiv2007

Conductance Spectroscopy of Spin-triplet Superconductors

We propose a novel experiment to identify the symmetry of superconductivity on the basis of theoretical results for differential conductance of a normal metal connected to a superconductor. The proximity effect from the superconductor modifies the conductance of the remote current depending remarkably on the pairing symmetry: spin-singlet or spin-triplet. The clear-cut difference in the conductance is explained by symmetry of Cooper pairs in a normal metal with respect to frequency. In the spin-triplet case, the anomalous transport is realized due to an odd-frequency symmetry of Cooper pairs.

cond-mat.supr-con↗

arXiv2007

Enhanced triplet superconductivity in noncentrosymmetric systems

We study pairing symmetry of noncentrosymmetric superconductors based on the extended Hubbard model on square lattice near half-filling, using the random phase approximation. We show that d+f-wave pairing is favored and the triplet f-wave state is enhanced by Rashba type spin-orbit coupling originating from the broken inversion symmetry. The enhanced triplet superconductivity stems from the increase of the effective interaction for the triplet pairing and the reduction of the spin susceptibility caused by the Rashba type spin-orbit coupling which lead to the increase of the triplet component and the destruction of the singlet one, respectively.

cond-mat.supr-con↗

arXiv2007

Theory of Macroscopic Quantum Tunneling and Dissipation in High-Tc Josephson Junctions

We have investigated macroscopic quantum tunneling (MQT) in in-plane high-Tc superconductor Josephson junctions and the influence of the nodal-quasiparticle and the zero energy bound states (ZES) on MQT. We have shown that the presence of the ZES at the interface between the insulator and the superconductor leads to strong Ohmic quasiparticle dissipation. Therefore, the MQT rate is noticeably suppressed in comparison with the c-axis junctions in which ZES are completely absent.

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