arXiv subjects

Youichi Yanase

Publications and source records attributed to Youichi Yanase.

177 records · Page 10Linked to original sources

arXiv2005

Ferromagnetic and triplet-pairing instabilities controlled by the trigonal distortion of the CoO_6 octahedra in Na_xCoO_2*yH_2O

In the recently discovered Co-oxide superconductor Na_xCoO_2*yH_2O, the edge-shared CoO6 octahedra are trigonally contracted along the c-axis in the CoO2-plane. We study how this CoO6 distortion affects the magnetic properties and superconductivity in this compound by analyzing the multiorbital Hubbard model using the fluctuation-exchange approximation. It is shown that through generating the trigonal crystal field, the distortion pushes the Co e'g bands up and consequently gives rise to the hole-pocket Fermi surfaces, which have been predicted in the band calculations. As the distortion increases, the hole pockets are enlarged and the ferromagnetic fluctuation as well as the pairing instability increases, which is in good agreement with recent NQR results.

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arXiv2005

Ferromagnetic fluctuation and superconductivity in Na_0.35CoO_2*1.3H_2O: FLEX study of multi-orbital Hubbard model

Spin and charge fluctuations and superconductivity in Na_0.35CoO_2*1.3H_2O are studied based on a multi-orbital Hubbard model. By applying the fluctuation exchange (FLEX) approximation, we show that the Hund's-rule coupling between the Co t2g orbitals causes ferromagnetic spin fluctuation. Triplet pairing is favored by this ferromagnetic fluctuation on the hole-pocket band. We propose that, in Na_0.35CoO_2*1.3H_2O, Co t2g orbitals and inter-orbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor mediated by inter-orbital-interaction-induced ferromagnetic fluctuation.

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arXiv2005

Ferromagnetic fluctuation and possible triplet superconductivity in Na_xCoO_2*yH_2O: Fluctuation-exchange study of multi-orbital Hubbard model

Spin and charge fluctuations and superconductivity in a recently discovered superconductor Na_xCoO_2*yH_2O are studied based on a multi-orbital Hubbard model. Tight-binding parameters are determined to reproduce the LDA band dispersions with the Fermi surface, which consist of a large cylindrical one around the Gamma-point and six hole pockets near the K-points. By applying the fluctuation-exchange (FLEX) approximation, we show that the Hund's-rule coupling between the Co t2g orbitals causes ferromagnetic (FM) spin fluctuation. Triplet f_{y(y^2-3x^2)}-wave and p-wave pairings are favored by this FM fluctuation on the hole-pocket band. We propose that, in Na_xCoO_2*yH_2O, the Co t2g orbitals and inter-orbital Hund's-rule coupling play important roles on the triplet pairing, and this compound can be a first example of the triplet superconductor in which the orbital degrees of freedom play substantial roles.

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arXiv2004

Kinetic Energy, Condensation Energy, Optical Sum Rule and Pairing Mechanism in High-Tc Cuprates

The mechanism of high-Tc superconductivity is investigated with interests on the microscopic aspects of the condensation energy. The theoretical analysis is performed on the basis of the FLEX approximation which is a microscopic description of the spin-fluctuation-induced-superconductivity. Most of phase transitions in strongly correlated electron system arise from the correlation energy which is copmetitive to the kinetic energy. However, we show that the kinetic energy cooperatively induces the superconductivity in the underdoped region. This unusual decrease of kinetic energy below T_c is induced by the feedback effect. The feedback effect induces the magnetic resonance mode as well as the kink in the electronic dispersion, and alters the properties of quasi-particles, such as mass renormalization and lifetime. The crossover from BCS behavior to this unusual behavior occurs for hole dopings. On the other hand, the decrease of kinetic energy below T_c does not occur in the electron-doped region. We discuss the relation to the recent obserbation of the violation of optical sum rule.

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arXiv2004

Multi-orbital analysis on the Superconductivity in Na_{x}CoO_{2} \cdot y H_{2}O

We preform a multi-orbital analysis on the novel superconductivity in Na_{x}CoO_{2} \cdot yH_{2}O. We construct a three-orbital model which reproduces the band structure expected from the LDA calculation. The effective interaction leading to the pairing is estimated by means of the perturbation theory. It is shown that the spin triplet superconductivity is stabilized in the wide parameter region. This is basically owing to the ferromagnetic character of spin fluctuation. The p-wave and f-wave superconductivity are nearly degenerate. The former is realized when the Hund's rule coupling is large, and vice versa. In a part of the parameter space, the d-wave superconductivity is also stabilized. We point out that the orbital degeneracy plays an essential role for these results through the wave function of quasi-particles. The nearly degeneracy of p-wave and f-wave superconductivity is explained by analysing the orbital character of each Fermi surface. We discuss the validity of some reduced models. While the single band Hubbard model reproducing the Fermi surface is qualitatively inappropriate, we find an effective two-orbital model appropriate for studying the superconductivity. We investigate the vertex corrections higher than the third order on the basis of the two-orbital model. It is shown that the vertex correction induces the screening effect but does not affect on the qualitative results.

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arXiv2004

Effects of Fermi surface and superconducting gap structure in the field-rotational experiments: A possible explanation of the cusp-like singularity in YNi$_2$B$_2$C

We have studied the field-orientational dependence of zero-energy density of states (FODOS) for a series of systems with different Fermi surface and superconducting gap structures. Instead of phenomenological Doppler-shift method, we use an approximate analytical solution of Eilenberger equation together with self-consistent determination of order parameter and a variational treatment of vortex lattice. First, we compare zero-energy density of states (ZEDOS) when a magnetic field is applied in the nodal direction ($ν_{node}(0)$) and in the antinodal direction ($ν_{anti}(0)$), by taking account of the field-angle dependence of order parameter. As a result, we found that there exists a crossover magnetic field $H^*$ so that $ν_{anti}(0) > ν_{node}(0)$ for $H < H^*$, while $ν_{node}(0) > ν_{anti}(0)$ for $H > H^*$, consistent with our previous analyses. Next, we showed that $H^*$ and the shape of FODOS are determined by contribution from the small part of Fermi surface where Fermi velocity is parallel to field-rotational plane. In particular, we found that $H^*$ is lowered and FODOS has broader minima, when a superconducting gap has point nodes, in contrast to the result of the Doppler-shift method. We also studied the effects of in-plane anisotropy of Fermi surface. We found that in-plane anisotropy of quasi-two dimensional Fermi surface sometimes becomes larger than the effects of Doppler-shift and can destroy the Doppler-shift predominant region. In particular, this tendency is strong in a multi-band system where superconducting coherence lengths are isotropic. Finally, we addressed the problem of cusp-like singularity in YNi$_2$B$_2$C and present a possible explanation of this phenomenon.

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arXiv2004

Pseudogap and Superconducting Fluctuation in High-Tc Cuprates: Theory beyond 1-loop Approximation

The pseudogap phenomena induced by the SC fluctuation are investigated in details. We perform a calculation beyond the 1-loop approximation. The SC fluctuation is microscopically derived on the basis of the repulsive Hubbard model. The vertex corrections are collected in the infinite order with use of the quasi-static approximation. The single-particle excitations, NMR 1/T_{1}T, spin susceptibility and superconducting transition temperature are discussed. The important role of the vertex correction is pointed out for the single particle spectral function. On the other hand, the validity of the 1-loop order theory is confirmed for other quantities. We shed light on the essential nature of SC fluctuation leading to the pseudogap from the comparison with spin and charge fluctuations.

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arXiv2004

Influence of superconducting gap structure on the quasiparticle spectrum in the vortex state

We study the vortex state of a layered superconductor with vertical line nodes on its Fermi surface when a magnetic field is applied in the ab-plane direction. We rotate the magnetic field within the plane, and analyze the change of low-energy excitation spectrum. Our analysis is based on the microscopic Bogoliubov-de Gennes equation and a convenient approximate analytical method developped by Pesch and Dahm. Both methods give a consistent result. Near the upper critical field H$_{c2}$, we observe a larger zero-energy density of states(ZEDOS) when the magnetic field is applied in the nodal direction, while much below H$_{c2}$, larger ZEDOS is observed under a field in the anti-nodal direction. We give a natural interpretation to this crossover behavior in terms of momentum distribution of low-energy quasiparticles. We examine the recent field angle variation experiments of thermal conductivity and specific heat. Comparison with our results suggest that special care should be taken to derive the position of line nodes from the experimental data. Combining the experimental data of the specific heat and our analyses, we conclude that Sr$_2$RuO$_4$ has vertical line nodes in the direction of the a-axis and the b-axis.

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arXiv2003

Superconductivity in Na_xCoO_2yH_2O by charge fluctuation

A new mechanism for superconductivity in the newly discovered Co-based oxide is proposed by using charge fluctuation. A single-band extended Hubbard model on the triangular lattice is studied within random phase approximation. $f$-wave triplet superconductivity is stabilized in the vicinity of charge-density-wave instability, which is in sharp contrast with the square-lattice case. The physical origin of the realization of the $f$-wave triplet state as well as the relevance to experiments are discussed.

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arXiv2003

Microscopic Identification of the D-vector in Triplet Superconductor Sr_2RuO_4

Triplet superconductivity in Sr_2RuO_4 is investigated with main interest on its internal degree of freedom. We perform a microscopic calculation to investigate how the chiral state d(k) = (k_x \pm ik_y)z is realized among the underlying six degenerate states. Starting from the three band Hubbard model with spin-orbit interaction, we use a perturbation theory in order to calculate the pairing interaction. The p-wave superconductivity with T_c \sim 1.5K is obtained in the moderately weak coupling region. It is shown that the orbital dependent superconductivity (ODS) robustly appears in Sr_2RuO_4. We determine the stabilized state by solving the Eliashberg equations. It is found that the Hund coupling term as well as the spin-orbit interaction is necessary for the ``symmetry breaking interaction''. The main result is that the chiral state is stabilized in case of the p-wave symmetry with the main γ-band, which is obtained in the perturbation theory. When we assume the other pairing symmetry including the f-wave state, the symmetry breaking interaction gives the other D-vector. The electronic structure constructed from the t_2g-orbitals is essential for this result.

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arXiv2001

Theory of Electric Transport in the Pseudogap State of High-Tc Cuprates

We theoretically investigate the electric transport in the pseudogap state of High-Tc cuprates. Starting from the repulsive Hubbard model, we perform the microscopic calculation to describe the pseudogap phenomena which are induced by the superconducting fluctuations. The single particle Green function, spin susceptibility and superconducting fluctuations are self-consistently determined by the SC-FLEX+T-matrix approximation. The longitudinal and transverse conductivities are calculated by using the Eliashberg and Kohno-Yamada formalism. The effects of the spin fluctuations and superconducting fluctuations are estimated, respectively. The vertex corrections arising from the two fluctuations are also calculated. The additional contribution from the Aslamazov-Larkin term is also estimated beyond the Eliashberg formalism. It is shown that the main effect of the superconducting fluctuations is the feedback effect through the spin fluctuations. The correct results are obtained by considering the superconducting fluctuations and the spin fluctuations simultaneously. The temperature and doping dependences of the resistivity and the Hall coefficient are well explained. We point out that the characteristic momentum dependence of the systems plays an essential role in this explanation.

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arXiv2001

Pseudogap Phenomena and Superconducting Fluctuations in Hubbard Model for High-Tc Cuprates

The pseudogap phenomena in High-$T_{\rm c}$ cuprates are investigated on the basis of the Hubbard model which includes only the on-site repulsive interaction $U$. We consider the pairing scenario for the pseudogap. The pseudogap arises from the resonance scattering due to the strong superconducting fluctuations. First, the electronic state and the anti-ferromagnetic spin fluctuations are calculated by using the FLEX approximation. The T-matrix (the propagator of the superconducting fluctuations) is calculated by extending the $\acute{\rm E}$liashberg equation. The self-energy due to the superconducting fluctuations is calculated by the T-matrix approximation. The pseudogap is shown in the single particle properties and the magnetic properties by the microscopic calculation. A comprehensive explanation of the doping dependence of the pseudogap is obtained. Furthermore, we apply the theory to the electron-doped cuprates and obtain the consistent results with the recent experiments. Finally, the self-consistent calculation for the spin fluctuations, superconducting fluctuations and the single particle properties are carried out within the FLEX and the self-consistent T-matrix approximations. The calculated superconducting critical temperature $T_{\rm c}$ is remarkably reduced from the results of the mean field (FLEX) calculation. It is shown that the critical temperature decreases with decreasing doping in the under-doped region with large $U$.

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arXiv2000

Theory on Superconducting Transition from Pseudogap State

The anomalous properties of High-$T_{\rm c}$ cuprates are investigated both in the normal state and in the superconducting state. In particular, we pay atte ntion to the pseudogap in the normal state and the phase transition from the pse udogap state to the superconducting state. The pseudogap phenomena observed in cuprates are naturally understood as a precursor of the strong coupling superconductivity. We have previously show n by using the self-consistent T-matrix calculation that the pseudogap is a resu lt of the strong superconducting fluctuations which are accompanied by the stron g coupling superconductivity in quasi-two dimensional systems [J. Phys. Soc. Jpn . {\bf68} (1999) 2999.]. We extend the scenario to the superconducting state. The close relation between the pseudogap state and the superconducting state is pointed out. Once t he superconducting phase transition occurs, the superconducting order parameter rapidly grows rather than the result of BCS theory. With the rapid growth of the order parameter, the gap structure becomes sharp, while it is remarkably broad in the pseudogap state. The characteristic energy scale of the gap does not chan ge. These results well explain the phase transition observed in the spectroscopi c measurements. Further, we calculate the magnetic and transport properties which show the pseu dogap phenomena. The comprehensive understanding of the NMR, the neutron scatter ing, the optical conductivity and the London penetration depth is obtained both in the pseudogap state and in the superconducting state.

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arXiv2000

Effect of Magnetic field on the Pseudogap Phenomena in High-Tc Cuprates

We theoretically investigate the effect of magnetic field on the pseudogap phenomena in High-Tc cuprates. The obtained results well explain the experimental results including their doping dependences. In our previous paper (J. Phys. Soc. Jpn. 68 (1999) 2999.), we have shown that the pseudogap phenomena observed in High-Tc cuprates are naturally understood as a precursor of the strong coupling superconductivity. On the other hand, there is an interpretation for the recent high field NMR measurements to be an evidence denying the pairing scenarios for the pseudogap. In this paper, we investigate the magnetic field dependence of NMR $1/T_{1}T$ on the basis of our formalism and show the interpretation to be inappropriate. The results indicate that the value of the characteristic magnetic field $B_{\rm ch}$ is remarkably large in case of the strong coupling superconductivity, especially near the pseudogap onset temperature $T^{*}$. Therefore, the magnetic field dependences can not be observed and $T^{*}$ does not vary when the strong pseudogap anomaly is observed. On the other hand, $B_{\rm ch}$ is small in the comparatively weak coupling case and $T^{*}$ varies when the weak pseudogap phenomena are observed. These results properly explain the high magnetic field NMR experiments continuously from under-doped to over-doped cuprates. Moreover, we discuss the transport phenomena in the pseudogap phase. The behaviors of the in-plane resistivity, the Hall coefficient and the c-axis resistivity in the pseudogap phase are naturally understood by considering the d-wave pseudogap.

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arXiv1999

Theory of Pseudogap Phenomena in High-Tc Cuprates Based on the Strong Coupling Superconductivity

In this paper we study the effects of the strong coupling superconductivity on the normal state electronic structure. We point out that the pseudogap phenomena in High-T_c cuprates are naturally explained as a precursor of the strong coupling superconductivity. We expand the reciprocal of the T-matrix with respect to the momentum and the frequency, and we discuss the properties of the expansion parameters. We estimate these parameters and carry out the calculations for the single particle self-energy using the T-matrix and the self-consistent T-matrix approximations, respectively. We show that the superconducting fluctuations, which are strongly diffusive within the conventional weak coupling theory, become propagative and have the character of the pre-formed pairs in the strong coupling case. The spectral weight and the density of states show the gap-like features by the effects of the resonances with the thermally excited weakly-damped pre-formed pairs. The spectral weight at the Fermi energy is strongly suppressed. This suppression starts at the mean field critical temperature $T_{MF}$ and the actual critical temperature T_c is strongly reduced owing to the fluctuations. These features properly explain to the pseudogap phenomena in High-T_c cuprates.

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