arXiv subjects

Kosaku Yamada

Publications and source records attributed to Kosaku Yamada.

29 records · Page 2Linked to original sources

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↗

arXiv2002

Third Order Perturbation Analysis of Pairing Symmetry in Two-Dimensional Hubbard Model

We study the superconducting instabilities of singlet and triplet pairing in a two-dimensional Hubbard model on the basis of the third-order perturbation theory (TOPT). We investigate the effect of the vertex correction that is given by TOPT, comparing with the study with only the second-order effective interaction. In our results, a stable p-wave pairing state spreads from low to intermediate electron density. A dx2-y2-wave pairing is dominant for the high density near a half-filling. It is shown that the vertex correction plays an essential role in making the p-wave pairing state dominant.

cond-mat.str-el↗

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 $U$.

cond-mat.str-el↗

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.

cond-mat.str-el↗

arXiv2000

Spin Susceptibility and Specific Heat in the d-p Model

We analyze the two-dimensional {\it d-p} model, considering both antiferromagnetic spin fluctuation and $d_{x^2-y^2}$-wave superconducting fluctuation. We adopt the fluctuation-exchange approximation in order to derive both normal and anomalous vertices composed only of the renormalized {\it d}-electron Green function and the on-site repulsive interaction among the {\it d}-electrons. Using these vertices, we derive a $t$-matrix as a superconducting fluctuation propagator. Then, we obtain self-consistent solutions in which the system is close to antiferromagnetic instability. In our solutions, the superconducting fluctuation couples strongly with the quasiparticle state and this causes the anomalous behavior in the temperature dependences of spin susceptibility and specific heat.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.supr-con↗

arXiv1998

Strong Coupling Approach to the d-p Model on the Basis of Fermi Liquid Theory

We study the superconducting transition temperature $T_c$ of the bilayer d-p model with $d_{x^2-y^2}$-wavelike attractive interaction based on the formalism first employed by Nozières and Schmitt-Rink. In the strong coupling regime, $T_c$ obtained through this formalism are much suppressed, compared with those through Thouless criterion only. We also find that, whether the interlayer coupling exsists or not, $T_c$ is almost propotional to the Fermi energy in the strong coupling regime. Thus, we can reproduce the essential nature in the underdoped region.

cond-mat↗

arXiv1997

Electronic State and Magnetic Susceptibility in Orbitally Degenerate (J=5/2) Periodic Anderson Model

Magnetic susceptibility in a heavy fermion systemis composed of the Pauli term (χ_P) and the Van-Vleck term (χ_V). The latter comes from the interband excitation, where f-orbital degeneracy is essential. In this work, we study χ_P and χ_V in the orbitally degenerate (J=5/2) periodic Anderson model for both the metallic and insulating cases. The effect of the correlation between f-electrons is investigated using the self-consistent second-order perturbation theory. The main results are as follows. (i) Sixfold degenerate model: both χ_P and χ_V are enhanced by a factor of 1/z (z is the renormalization constant). (ii) Nondegenerate model: only χ_P is enhanced by 1/z. Thus, orbital degeneracy is indispensable for enhancement of χ_V. Moreover, orbital degeneracy reduces the Wilson ratio and stabilizes a nonmagnetic Fermi liquid state.

cond-mat.str-el↗

arXiv1997

Theory of Anomalous Hall Effect in a Heavy fermion System with a Strong Anisotropic Crystal Field

In a heavy fermion system, there exists the anomalous Hall effect caused by localized $f$-orbital freedom, in addition to the normal Hall effect due to the Lorentz force. In 1994, we found that the Hall coefficient caused by the anomalous Hall effect ($R_H^{AHE}$) is predominant and the relation $R_H^{AHE} \propto ρ^2$ ($ρ$ is the electrical resistivity) holds at low temperatures in many compounds. In this work, we study the system where the magnetic susceptibility is highly anisotropic due to the strong crystalline electric field on $f$-orbitals. Interestingly, we find that $R_H^{AHE}$ is nearly isotropic in general. This tendency is frequently observed experimentally, which has casted suspicion that the anomalous Hall effect may be irrelevant in real materials. Our theory corresponds to corrections and generalizations of the pioneering work on ferromagnetic metals by Karplus and Luttinger.

cond-mat.str-el↗

arXiv1996

Magnetic Susceptibility of the Orbitally Degenerate (J= 5/2) Periodic Anderson Model - Analysis on the Basis of the Fermi Liquid Theory

In the orbitally degenerate ($J=5/2$) Periodic Anderson Model, the magnetic susceptibility is composed of both the Pauli term and the Van Vleck term, as is well known. The former is strongly enhanced by the strong correlation between $f$-electrons. But, for the latter, the influence of the strong correlation has been obscure for years. In this paper we give the solution of the longstanding problem. With the aid of the $d=\infty$ approximation, we study this problem on the basis of the Fermi liquid theory with degenerate orbitals, taking account of all the vertex corrections in a consistent way. As a result, we obtain the simple expression for the magnetic susceptibility, and show unambiguously that the Van Vleck term is also highly enhanced} in the strong correlation regime. This fact explains naturally the enhanced magnetic susceptibility observed in many insulating systems (i.e., Kondo insulator). Moreover, we show that the Wilson ratio takes a value around 1 in the metallic system, in good agreement with experiments.

cond-mat↗