arXiv subjects

Hiroaki Ikeda

Publications and source records attributed to Hiroaki Ikeda.

62 records · Page 4Linked to original sources

arXiv2004

Perturbation Theory for a Repulsive Hubbard Model in Quasi-One-Dimensional Superconductors

We investigate pairing symmetry and a transition temperature in a quasi-one-dimensional repulsive Hubbard model. We solve the Eliashberg equation using the third-order perturbation expansion with respect to the on-site repulsion $U$. We find that when the electron number density is shifted from the half-filled, a transition into unconventional superconductivity is expected. When one dimensionality is weak, a spin-singlet state is favorable. By contrast, when one dimensionality is strong and electron number density is far from the half-filled, a spin-triplet state is stabilized. Finally, we discuss the possibility of unconventional superconductivity caused by the on-site Coulomb repulsion in $β$-Na$_{0.33}$V$_2$O$_5$.

cond-mat.supr-con↗

arXiv2003

Theory of Superconductivity in Strongly Correlated Electron Systems

In this article we review essential natures of superconductivity in strongly correlated electron systems (SCES) from a universal point of view. After summarizing experimental results on typical materials such as high-$T_{\rm c}$ cuprates, BEDT-TTF organic superconductors, and ruthenate Sr$_2$RuO$_4$, we review theoretical results for the analyses of superconducting properties of these materials based on the Fermi-liquid framework in the single- and multi-band Hubbard model. It is emphasized that the Coulomb interaction induces various types of anisotropic superconductivity, $d$- or p-wave, through the momentum dependence of quasi-particle interaction. While some inter-orbital interactions exist in the multi-orbital system, anisotropic superconductivity is induced by essentially the same mechanism, namely the momentum dependence of quasi-particle interaction. This is the understanding of the mechanism of superconductivity in SCES. Another important purpose of this article is to review anomalous electronic properties of SCES near the Mott transition. Especially, we focus on pseudogap phenomena observed in under-doped cuprates and organic superconductors. According to the recent theory, superconducting fluctuations, inherent in the quasi-two-dimensional and strong-coupling superconductors, are the origin of the pseudogap formation. Based on the microscopic theory of the superconducting fluctuations, we discuss the magnetic and transport properties as well as the single-particle spectra in the pseudogap state. As for heavy-fermion superconductors, experimental results are reviewed and several theoretical analyses on the mechanism are provided based on the same viewpoint as explained above.

cond-mat.str-el↗

arXiv2003

Possibility of f-wave spin-triplet superconductivity in the CoO superconductor: a case study on a 2D triangular lattice in the repulsive Hubbard model

Stimulated by the recent finding of Na$_{0.35}$CoO$_2$.1.3H$_2$O superconductor, we investigate superconducting instabilities on a 2D triangular lattice in the repulsive Hubbard model. Using the third-order perturbation expansion with respect to the on-site repulsion $U$, we evaluate the linearized Dyson-Gor'kov equation. We find that an $f$-wave spin-triplet pairing is the most stable in a wide range of the next nearest neighbor hopping integral $t'$ and an electron number density $n$. The introduction of $t'$ is crucial to adjust the van Hove singularities to the neighborhood of the Fermi surface crossing around K point. In this case, the bare spin susceptibility shows the broad peak around $Γ$ point. These conditions stabilize the $f$-wave pairing. Although the $f$-wave pairing is also given by the fluctuation-exchange approximation, the transition temperature is too low to be observed. This is because the depairing effect by the spin fluctuation is over-estimated. Thus, the third-order vertex corrections are important for the spin-triplet superconductivity, like the case in Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2002

Role of Vertex Correction and Dimensionality on Superconductivity Induced by On-site Coulomb Repuls ion

We evaluate the dominant superconducting pairing symmetry in some 3-dimensional cubic lattice struct ures within the third-order perturbation theory with respect to the on-site coulomb repulsion. We cl arify whether the vertex correction term, which has a critical contribution to the p-wave state in t he 2-dimensional systems, is also important in the 3-dimensional system. We finally investigate the interplay between dimensionality and the vertex correction to grasp common features independent of t he lattice structures in the mechanism of the superconductivity.

cond-mat.str-el↗

arXiv1999

Reply to the Comment by Kiselev and Bouis

The possibility of the ferromagnetic state modifies the phase diagram in Phys.Rev.Lett.81,3723(1998), as Kiselev and Bouis have pointed out. However, the unconventional SDW that has been proposed in the letter above is still a candidate for the curious magnetism in URu_2Si_2.

cond-mat.str-el↗

arXiv1999

Theory of Unconventional Spin Density Wave: A Possible Mechanism of the Micromagnetism in U-based Heavy Fermion Compounds

We propose a novel spin density wave (SDW) state as a possible mechanism of the anomalous antiferromagnetism, so-called the micromagnetism, in URu_2Si_2 below 17.5[K]. In this new SDW, the electron-hole pair amplitude changes its sign in the momentum space as in the case of the unconventional superconductivity. It is shown that this state can be realized in an extended Hubbard model within the mean field theory. We also examine some characteristic properties of this SDW to compare with the experimental results. All these properties well explain the unsolved problem of the micromagnetism.

cond-mat.str-el↗

arXiv1996

Impurity Effects in Strongly Correlated Metals: Large Pressure Dependence of Residual Resistivity of Heavy Fermions

It is shown on the basis of the Fermi liquid theory that the s-wave scattering potential due to nonmagnetic impurity is strongly enhanced by the mass enhancement factor $1/z$. As a result the impurity potential with moderate strength, of the order of the bandwidth of conduction electrons, gives scattering in the unitarity limit in strongly correlated metals as heavy fermions. This effect is embodied by large pressure dependence of residual resistivities in heavy fermion systems, since the pressure decreases the degree of correlations, which makes the Kondo temperature increase rapidly. This accounts for the large pressure dependence of the residual resistivity observed in heavy fermion metals, such as CeInCu$_{2}$, CeCu$_{6}$, CeAl$_{3}$.

cond-mat↗

arXiv1996

A Theory of Anisotropic Semiconductor of Heavy Fermions

It is demonstrated that a {\veck}-dependence of the hybridization matrix element between $f$- and conduction electrons can give rise to an anisotropic hybridization gap of heavy fermions if the filling of electrons corresponds to that of the band insulator. The most interesting case occurs when the hybridization vanishes along some symmetry axis of the crystal reflecting a particular symmetry of the crystal field. The results of a model calculation are consistent with wide range of anomalous properties observed in CeNiSn and its isostructural compounds, the anisotropic semiconductor of heavy fermions. In particular, highly sensitive effect of impurity scattering on the residual density of states for zero energy excitation and the anisotropic temperature dependence of the resistivity are well explained. It is also discussed that a weak semimetallic behavior arises through the weak $\veck$-dependence of the $f$-electron self-energy $Σ_{f}(\veck,0)$.

cond-mat↗