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Yukio Tanaka

Publications and source records attributed to Yukio Tanaka.

At least 253 records · Page 14Linked to original sources
arXiv2004

Competition between singlet and triplet pairings in Na_xCoO_2 yH_2O

We discuss the pairing symmetry of a cobaltate superconductor Na$_x$CoO$_2\cdot y$ H$_2$O by adopting an effective single band model that takes into account the $e_g'$ hole pockets, as discussed in our previous paper [to appear in Phys. Rev. Lett.] Here we consider the off-site repulsions in addition to the on-site repulsion considered in our previous study. We show that the spin-triplet f-wave pairing proposed in our previous study is robust to some extent even in the presence of off-site repulsions. However, f-wave pairing gives way to singlet pairings for sufficiently large values of off-site repulsions. Among the singlet pairings, i-wave and extended s-wave pairings are good candidates which do not break time reversal symmetry below $T_c$ in agreement with the experiments.

cond-mat.str-el↗
arXiv2004

Possible spin-triplet $f$-wave pairing due to disconnected Fermi surfaces in Na$_x$CoO$_2\cdot y$H$_2$O

We propose that spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [Phys. Rev. B {\bf 63} 174507 (2001)] may be at work in a recently discovered superconductor Na$_x$CoO$_2$ $\cdot y$ H$_2$O. We introduce a single band effective model that takes into account the pocket-like Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying fluctuation exchange method and solving the linearized {É}liashberg equation, the most dominant pairing is found to have spin-triplet $f$-wave symmetry, where the nodes of the gap function do not intersect the pocket Fermi surfaces. Presence of finite $T_c$ is suggested in sharp contrast with cases when the gap nodes intersect the Fermi surface.

cond-mat.str-el↗
arXiv2004

Anomalous charge transport in mesoscopic triplet superconductor junctions

Charge transport properties of a diffusive normal metal /triplet superconductor (DN/TS) junction are studied based on the Keldysh-Nambu quasiclassical Green's function formalism. % Contrary to the unconventional singlet superconductor junction case, the mid gap Andreev resonant state (MARS) at the interface of the TS is shown to enhance the proximity effect in the DN. % The total resistance of the DN/TS junction is drastically reduced and is completely independent of the resistance of the DN in the extreme case. % Such anomalous transport accompanies a giant zero-bias peak in the conductance spectra and a zero-energy peak of the local density of states in the DN region. % These striking features manifest the presence of novel proximity effect peculiar to triplet superconductor junctions.

cond-mat.supr-con↗
arXiv2003

A phenomenological theory of zero-energy Andreev resonant states

A conceptual consideration is given to a zero-energy state (ZES) at the surface of unconventional superconductors. The reflection coefficients in normal-metal / superconductor (NS) junctions are calculated based on a phenomenological description of the reflection processes of a quasiparticle. The phenomenological theory reveals the importance of the sign change in the pair potential for the formation of the ZES. The ZES is observed as the zero-bias conductance peak (ZBCP) in the differential conductance of NS junctions. The split of the ZBCP due to broken time-reversal symmetry states is naturally understood in the present theory. We also discuss effects of external magnetic fields on the ZBCP.

cond-mat.supr-con↗
arXiv2003

Crossover of superconducting properties and kinetic-energy gain in two-dimensional Hubbard model

Superconductivity in the Hubbard model on a square lattice near half filling is studied using an optimization (or correlated) variational Monte Carlo method. Second-order processes of the strong-coupling expansion are considered in the wave functions beyond the Gutzwiller factor. Superconductivity of d_x^2-y^2-wave is widely stable, and exhibits a crossover around U=U_co\sim 12t from a BCS type to a new type. For U\gsim U_co (U\lsim U_co), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and \propto exp(-t/J) [tiny] on the strong [weak] coupling side of U_co. Cuprates belong to the strong-coupling regime.

cond-mat.str-el↗
arXiv2003

Quantum Monte Carlo study of the pairing symmetry competition in the Hubbard model

To shed light into the pairing mechanism of possible spin-triplet superconductors (TMTSF)$_2$X and Sr$_2$RuO$_4$, we study the competition among various spin singlet and triplet pairing channels in the Hubbard model by calculating the pairing interaction vertex using the ground state quantum Monte Carlo technique. We model (TMTSF)$_2$X by a quarter-filled quasi-one dimensional (quasi-1D) Hubbard model,and the $γ$ band of Sr$_2$RuO$_4$ by a two dimensional (2D) Hubbard model with a band filling of $\sim 4/3$. For the quasi-1D system, we find that triplet $f$-wave pairing not only dominates over triplet p-wave in agreement with the spin fluctuation theory, but also looks unexpectedly competitive against d-wave. For the 2D system, although the results suggest presence of attractive interaction in the triplet pairing channels, the d-wave pairing interaction is found to be larger than those of the triplet channels.

cond-mat.str-el↗
arXiv2003

Nature of carriers in one-dimensional correlated electron systems

Electron densities at the edge site in one-dimensional Hubbard model and t-J model are studied by using the Bethe ansatz solutions and exact diagonalization method. It is found that the boundary is electron-attractive, or equivalently hole-repulsive, near half-filling. We propose a new criterion to determine whether the carriers in a strongly correlated system are electron-like or hole-like depending on the electronic behavior at the boundary.

cond-mat.str-el↗
arXiv2003

Electronic states around a vortex core in high-Tc superconductors based on the t-J model

Electronic states around vortex cores in high-Tc superconductors are studied using the two-dimensional t-J model in order to treat the d-wave superconductivity with short coherence length and the antiferromagnetic (AF) instability within the same framework. We focus on the disappearance of the large zero-energy peak in the local density of states observed in high-Tc superconductors. When the system is near the optimum doping, we find that the local AF correlation develops inside the vortex cores. However, the detailed doping dependence calculations confirm that the experimentally observed reduction of the zero-energy peak is more reasonably attributed to the smallness of the core size rather than to the AF correlation developed inside the core. The correlation between the spatial dependence of the core states and the core radius is discussed.

cond-mat.supr-con↗
arXiv2001

d_{x^2-y^2}- vs. d_{xy}- like pairings in organic superconductors κ-(BEDT-TTF)_2 X

Pairing symmetry in $κ$-(BEDT-TTF)$_2$X is re-examined by applying fluctuation exchange method to the Hubbard model on the original four band lattice for the first time. Our study is motivated by the discrepancy between a recent thermal conductivity/tunneling spectroscopy experiments and the previous theories based on the effective single or two band model. In contrast to the previous theories, we find that $d_{x^2-y^2}$-like pairing dominates over $d_{xy}$ in agreement with recent experiments. Our study provides a general lesson that even if the Fermi surface seems to be reproduced by an effective model, the pairing symmetry can be different from that of the original system.

cond-mat.str-el↗
arXiv2000

Local density of states around a magnetic impurity in high-Tc superconductors based on the t-J model

The local density of states (LDOS) around a magnetic impurity in high-Tc superconductors is studied using the two-dimensional t-J model with a realistic band structure. The order parameters are determined in a self-consistent way within the Gutzwiller approximation and the Bogoliubov-de Gennes theory. In sharp contrast with the nonmagnetic impurity case, the LDOS near the magnetic impurity shows two resonance peaks reflecting the presence of spin-dependent resonance states. It is also shown that these resonance states are approximately localized around the impurity. The present results have an large implication on the scanning tunneling spectroscopy observation of Bi_{2}Sr_{2}Ca(Cu_{1-x}Ni[Zn]_{x})_{2}O_{8+delta}.

cond-mat.supr-con↗
arXiv1999

Theory of Spin polarized Tunneling in Superconducting Sr2RuO4

A theory of tunneling conductance in ferromagnetic metal/insulator/triplet - supercondcutor junctions is presented for unitary and non-unitary spin triplet pairing states which are promising candidates for the superconducting paring symmetry of Sr2RuO4. As the magnitude of the exchange interaction in the ferromagnetic metal is increased, the conductance for the unitary pairing state below the energy gap is reduced in contrast to the case for the non-unitary pairing state.

cond-mat.supr-con↗
arXiv1999

Quasiparticle properties around a nonmagnetic impurity in the superconducting state of the two-dimensional t-J model

The superconducting state around a well isolated nonmagnetic impurity in the high-Tc superconductors is studied using the two-dimensional t-J model. The spatial dependence of the order parameter and the local density of states are obtained from the numerical diagonalization of the Bogoliubov-de Gennes equation derived using the Gutzwiller approximation. We find a zero-energy peak in the local density of states around the impurity. Different from the previous results on a vortex or surfaces in the t-J model, the splitting of zero-energy peak is not found. The zero-energy states corresponding to the zero-energy peak is approximately localized around the impurity.

cond-mat.supr-con↗
arXiv1999

Intrinsic Josephson Effect in the Layered Two-dimensional t-J Model

The intrinsic Josephson effect in the high-Tc superconductors is studied using the layered two-dimensional t-J model. The d.c.Josephson current which flows perpendicular to the t-J planes is obtained within the mean-field approximation and the Gutzwiller approximation. We find that the Josephson current has its maximum near the optimum doping region as a function of the doping rate.

cond-mat.supr-con↗
arXiv1999

Theory of the Josephson effect in superconductor / one-dimensional electron gas / superconductor junction

We present a theory for the Josephson effect in an unconventional superconductor / one-dimensional electron gas / unconventional superconductor (s/o/s) junction, where the Josephson current is carried by components injected perpendicular to the interface. When superconductors on both sides have triplet symmetries, the Josephson current is enhanced at low temperature due to the zero-energy states formed near the interface. Measuring Josephson current in this s/o/s junction, we can identify parity of the superconductor.

cond-mat.supr-con↗
arXiv1998

Theory of tunneling conductance for normal metal/insulator/triplet superconductor junctions

Tunneling conductance spectra of normal metal/insulator/triplet superconductor junctions are investigated theoretically. As triplet paring states we select several types of symmetries that are promising candidates for the superconducting states in UPt$_{3}$ and in Sr$_{2}$RuO$_{4}$. The calculated conductance spectra are sensitive to the orientation of the junction which reflects the anisotropy of the pairing states. They show either zero-bias conductance peaks or gap-like structures depending on the orientation of the junctions. The existence of a residual density of states, peculiar to nonunitary states, is shown to have a significant influence on the properties of the conductance spectra. Present results serve as a guidefor the experimental determination of the symmetry of the pair potentials in UPt$_{3}$ and Sr$_{2}$RuO$_{4}$.

cond-mat.supr-con↗
arXiv1998

Influences of broken time-reversal symmetry on the d.c. Josephson effects in d-wave superconductors

In order to examine the influences of the spatial dependence of the pair potential, the d.c. Josephson current in d-wave superconductors is calculated using self-consistently determined pair potentials. The results show that the suppression of the d-wave pair potential near the insulator does not have serious effect on the properties of the Josephson current. On the other hand, drastic changes are obtained due to the inducement of a subdominant s-wave component, which spontaneously breaks time reversal symmetry. Especially, a rapid enhancement of the Josephson current at low temperature predicted in previous formulas is strongly suppressed.

cond-mat.supr-con↗
arXiv1998

Theory of the d.c. Josephson effect in Pb / Sr2RuO4 / Pb

To clarify the origin of anomalous behaviors in Pb/Sr2RuO4/Pb junctions in terms of the pairing symmetry, a theory of the d.c. Josephson current in s-wave superconductor / p-wave superconductor / s-wave superconductor junctions is developed. Calculated results on the temperature dependence of the critical Josephson current exhibit non-monotonous behaviors when the thickness of the p-wave superconductor is comparable to the coherence length. The consistency between present results with recent experimental measurement supports the possibility of a unitary p-wave pairing state in Sr2RuO4.

cond-mat.supr-con↗