arXiv subjects

Takashi Yanagisawa

Publications and source records attributed to Takashi Yanagisawa.

At least 55 records · Page 3Linked to original sources

arXiv2011

Unrestricted Hartree-Fock Analysis of Sr$_{3-x}$Ca$_x$Ru$_2$O$_7$

We investigated the electronic and magnetic structure of Sr$_{3-x}$Ca$_x$Ru$_2$O$_7$ ($0 \leq x \leq 3$) on the basis of the double-layered three-dimensional multiband Hubbard model with spin-orbit interaction. In our model, lattice distortion is implemented as the modulation of transfer integrals or a crystal field. The most stable states are estimated within the unrestricted Hartree-Fock approximation, in which the colinear spin configurations with five different spin-quantization axes are adopted as candidates. The obtained spin structures for some particular lattice distortions are consistent with the neutron diffraction results for Ca$_3$Ru$_2$O$_7$. Also, some magnetic phase transitions can occur due to changes in lattice distortion. These results facilitate the comprehensive understanding of the phase diagram of Sr$_{3-x}$Ca$_x$Ru$_2$O$_7$.

cond-mat.str-el↗

arXiv2011

Superconducting Condensation Energy of the Two-Dimensional Hubbard Model in the Large-Negative-t' Region

We compute the superconducting condensation energies Econd of Hg1201 and Tl2201 cuprates by applying the variational Monte Carlo method to the two-dimensional Hubbard model, first, with the specific band parameters t' = -2t" = -0.25t appropriate for these highest-Tc single-CuO2-layer cuprates; t, t' and t" are the first-, second- and third-neighbor transfer energies, respectively. In the range of on-site Coulomb energy U of (7\sim10)t with optimal doping, we succeed in obtaining the bulk-limit Econd values by extrapolating the results. They sharply increase with increasing U and become comparable to experimental values when U \approx 9t. Next, Econd values at t' = -2t" = -0.18t demonstrate that with a fixed U the bulk-limit Econd quickly increases with a decrease in |t'| = 2t" in the large-|t'| region. Finally, we argue that the remoteness of the apex oxygen from the planar Cu in the two cuprates is considered to increase U and bring about the experimental magnitude of Econd. The present scheme explains the observed correlation between Tc and t' among cuprates.

cond-mat.supr-con↗

arXiv2011

Reply to "Comment on 'Isotope effect in multi-band and multi-channel attractive systems and inverse isotope effect in iron-based superconductors'"

The Comment insists on the following: in our model it is assumed that the effective interactions have specific energy ranges within the single band with a cutoff at ω_1 for the phononic part and a range from ω_1 to ω_2 in the AF channel. Our reply is that we assume that V_i(k,k')\neq 0 if |ξ_k|<ω_i and |ξ_{k'}|<ω_i, and otherwise V_i(k,k')= 0 (i=1,2), as stated in our paper. This is the model of BCS type with two attractive interactions, and this assumption is the characteristic of the BCS approximation. The claim "the integration limits have been modified such that the AF channel mediated pairing sets in where the ph-channel pairing terminates and is limited at an energy given by ω_j=ω_{AF}" in the Comment is wrong. We describe the model and the method to solve the gap equation in more detail.

cond-mat.supr-con↗

arXiv2010

Three-Dimensional Multiband d-p Model of Superconductivity in Spin-Chain Ladder Cuprate

We study the superconductivity in the three-dimensional multiband d-p model, in which a Cu$_2$O$_3$-ladder layer and a CuO$_2$-chain layer are alternately stacked, as a model of the superconducting spin-chain ladder cuprate. $p_z$-Wave-like triplet superconductivity is found to be the most stable, and its dependence on interlayer coupling can explain the superconducting transition temperature dependence on pressure in real superconducting spin-chain ladder cuprates. The superconductivity may be enhanced if hole transfer from the chain layer to the ladder layer can be promoted beyond the typical transfer rate.

cond-mat.supr-con↗

arXiv2009

Inverse Iron Isotope Effect on the transition temperature of the (Ba,K)Fe2As2 superconductor

We report that (Ba,K)Fe2As2 superconductor (a transition temperature, Tc = 38 K) shows inverse Iron isotope effect (-0.18) (the sample including the larger atomic weight of Fe depicts higher Tc). Measurements of both temperature dependent magnetization and resistivity reveal a clear inverse shift by systematic studies on Tc using three types of Fe-isotopes (Fe-54, natural Fe and Fe-57). This indicates the first evidence of the inverse isotope effect in high-Tc superconductors. This atomic mass dependence on Tc implies the exotic coupling mechanism.

cond-mat.supr-con↗

arXiv2009

Incommensurate Antiferromagnetism Coexisting with Superconductivity in Two-Dimensional d-p Model

Numerical studies of the two-dimensional d-p model using the Gutzwiller ansatz have exhibited the incommensurate antiferromagnetic state coexisting with superconductivity in the under- and lightly doped regions. Our results are based on the variational Monte Carlo method for the three-band Hubbard model with d and p orbitals. We obtained the finite superconducting condensation energy for the coexistent sate at the doping rate $x=1/8$, 1/12, and 1/16, up to the systems of 256 unit cells with 768 atoms (oxygen and copper atoms). The phase diagram for the hole-doped case is consistent with recent results reported for layered high temperature cuprates.

cond-mat.str-el↗

arXiv2008

Metallic State of the Three-band Hubbard Model with Super-lattice Structure

We investigate the dynamical superlattice correlation in the two-dimensional three-band Hubbard model on the basis of the unrestricted fluctuation exchange approximation. We calculate the one-particle spectral function, the spin correlation function and the charge correlation function at finite temperature. We find that some experimental results can be reproduced consistenly by taking inhomogenous distribution of Cu 3d electrons into account. The correlation functions suggest that several kinds of instabilities with spatial inhomogenieties exist in some regions, where these instabilities significantly affect the one-particle spectral fuctions.

cond-mat.str-el↗

arXiv2008

Superconductivity in Multilayer Perovskite: Weak Coupling Analysis

We investigate the superconductivity of a three-dimensional d-p model with a multilayer perovskite structure on the basis of the second-order perturabation theory within the weak coupling framework. Our model has been designed with multilayer high-$T_c$ superconducting cuprates in mind. In our model, multiple Fermi surfaces appear, and the component of a superconducting gap function develops on each band. We have found that the multilayer structure can stabilize the superconductivity in a wide doping range.

cond-mat.supr-con↗

arXiv2008

Phase diagram of the t-U^2 Hamiltonian of the weak coupling Hubbard model

We determine the symmetry of Cooper pairs, on the basis of the perturbation theory in terms of the Coulomb interaction $U$, for the two-dimensional Hubbard model on the square lattice. The phase diagram is investigated in detail. The Hubbard model for small $U$ is mapped onto an effective Hamiltonian with the attractive interaction using the canonical transformation: $H_{eff}=e^S He^{-S}$. The gap equation of the weak coupling formulation is solved without numerical ambiguity to determine the symmetry of Cooper pairs. The superconducting gap crucially depends on the position of the van Hove singularity. We show the phase diagram in the plane of the electron filling $n_e$ and the next nearest-neighbor transfer $t'$. The d-wave pairing is dominant for the square lattice in a wide range of $n_e$ and $t'$. The d-wave pairing is also stable for the square lattice with anisotropic $t'$. The three-band $d$-$p$ model is also investigated, for which the d-wave pairing is stable in a wide range of $n_e$ and $t_{pp}$ (the transfer between neighboring oxygen atoms). In the weak coupling analysis, the second-neighbor transfer parameter $-t'$ could not be so large so that the optimum doping rate is in the range of $0.8 <n_e <0.9$.

cond-mat.supr-con↗

arXiv2007

Madelung Energy of the Valence Skipping Compound BaBiO$_3$

Several elements show valence skip fluctuation, for instance, Tl forms the compounds in valence states +1 and +3, and Bi forms in +3 and +5 states. This kind of fluctuation gives rise to a negative effective attractive interaction and the Kondo effect. In the compounds of valence skipping elements, the carrier doping will induce superconductivity with high critical temperature. For example, Ba$_{1-x}$K$_x$BiO$_3$ shows high $T_c$ which is unlikely from the conventional electron-phonon mechanism. The reason for the missing of some valence states in such valence skip compounds remains a mystery. We have performed the evaluation of the Madelung potential for BaBiO$_3$, and have shown for the first time that charge-ordered state is stabilized if we take into account the polarization of the oxygen charge. We argue that the effective Coulomb interaction energy $U$ may be negative evaluating the local excitation energy.

cond-mat.str-el↗

arXiv2007

Quantum Monte Carlo diagonalization for many-fermion systems

In this study we present an optimization method based on the quantum Monte Carlo diagonalization for many-fermion systems. Using the Hubbard-Stratonovich transformation, employed to decompose the interactions in terms of auxiliary fields, we expand the true ground-state wave function. The ground-state wave function is written as a linear combination of the basis wave functions. The Hamiltonian is diagonalized to obtain the lowest energy state, using the variational principle within the selected subspace of the basis functions. This method is free from the difficulty known as the negative sign problem. We can optimize a wave function using two procedures. The first procedure is to increase the number of basis functions. The second improves each basis function through the operators, $e^{-ΔτH}$, using the Hubbard-Stratonovich decomposition. We present an algorithm for the Quantum Monte Carlo diagonalization method using a genetic algorithm and the renormalization method. We compute the ground-state energy and correlation functions of small clusters to compare with available data.

cond-mat.str-el↗

arXiv2006

Fermi arc in doped high-Tc cuprates

We propose a $d$-density wave induced by the spin-orbit coupling in the CuO plane. The spectral function of high-temperature superconductors in the under doped and lightly doped regions is calculated in order to explain the Fermi arc spectra observed recently by angle-resolved photoemission spectroscopy. We take into account the tilting of CuO octahedra as well as the on-site Coulombrepulsive interaction; the tilted octahedra induce the staggered transfer integral between $p_{x,y}$ orbitals and Cu $t_{2g}$ orbitals, and bring about nontrivial effects of spin-orbit coupling for the $d$ electrons in the CuO plane. The spectral weight shows a peak at around ($π/2$,$π/2$) for light doping and extends around this point forming an arc as the carrier density increases, where the spectra for light doping grow continuously to be the spectra in the optimally doped region. This behavior significantly agrees with that of the angle-resolved photoemissionspectroscopy spectra. Furthermore, the spin-orbit term and staggered transfer effectively induce a flux state, a pseudo-gap with time-reversal symmetry breaking. We have a nodal metallic state in the light-doping case since the pseudogap has a $d_{x^2-y^2}$ symmetry.

cond-mat.str-el↗

arXiv2004

Optical properties of unconventional superconductors

The optical conductivity measurements give a powerful tool to investigate the nature of the superconducting gap for conventional and unconventional superconductors. In this article, first, general analyses of the optical conductivity are given stemmed from the Mattis-Bardeen formula for conventional BCS superconductors to unconventional anisotropic superconductors. Second, we discuss the reflectance-transmittance (R-T) method which has been proposed to measure far-infrared spectroscopy. The R-T method provides us precise measurements of the frequency-dependent conductivity. Third, the optical conductivity spectra of the electron-doped cuprate superconductor Nd_{2-x}Ce_xCuO_4 are investigated based on the anisotropic pairing model. It is shown that the behavior of optical conductivity is consistent with an anisotropic gap and is well explained by the formula for d-wave pairing in the far-infrared region. The optical properties of the multiband superconductor MgB_2, i n which the existence of superconductivity with relatively high-T_c (39K) was recently announced, is also examined to determine the symmetry of superconducting gaps.

cond-mat.supr-con↗

arXiv2004

Uniaxial-Pressure induced Ferromagnetism of Enhanced Paramagnetic Sr3Ru2O7

We report a uniaxial pressure-dependence of magnetism in layered perovskite strontium ruthenate Sr3Ru2O7. By applying a relatively small uniaxial pressure, greater than 0.1 GPa normal to the RuO2 layer, ferromagnetic ordering manifests below 80 K from the enhanced-paramagnet. Magnetization at 1 kOe and 2 K becomes 100 times larger than that under ambient condition. Uniaxial pressure dependence of Curie temperature T_C suggests the first order magnetic transition. Origin of this uniaxial-pressure induced ferromagnetism is discussed in terms of the rotation of RuO6 octahedra within the RuO2 plane.

cond-mat.str-el↗

arXiv2003

Orbital-dependent two-band superconductivity in MgB2

We show that a two-band model with ${\bf k}$-dependent superconducting gaps well describes the transmission and optical conductivity measured for MgB$_2$ thin films. It is also shown that the two-band anisotropic model consistently describes the specific-heat jump and thermodynamic critical magnetic field $H_c$. A single-gap anisotropic model is shown to be insufficient to understand consistently optical and thermodynamic behaviors. In our model, the pairing symmetry in each band has an anisotropic characteristic which is determined almost uniquely; the superconducting gap in the $σ$-band has anisotropy in the ab-plane and the gap in the $π$-band has a prolate form exhibiting anisotropy in the c-direction. The anisotropy in the $σ$-band produces rather small effects on the physical properties compared to the anisotropy in the $π$-band.

cond-mat.supr-con↗

arXiv2003

Lattice distortions, incommensurability, and stripes in the electric model for high-Tc cuprates

Striped superconductivity with lattice distortions is investigated based on the three-band Hubbard model for high-Tc cuprates. A stable inhomogeneous striped state is determined in the low-temperature tetragonal (LTT) phase with lattice distortions using a quantum variational Monte Carlo method. The ground state has vertical or horizontal hole-rich arrays coexisting with incommensurate magnetism and superconductivity induced by several percent lattice distortions. The superconductivity order parameter oscillates according to the inhomogeneity in the antiferromagnetic background with its maxima in the hole-rich regions, and the superconductivity condensation energy is reduced as the doping rate decreases.

cond-mat.str-el↗

arXiv2003

Superconductivity in Sr$_2$RuO$_4$ Mediated by Coulomb Scattering

We investigate the superconductivity in Sr$_2$RuO$_4$ on the basis of the three-dimensional three-band Hubbard model. We propose a model with Coulomb interactions among the electrons on the nearest-neighbor Ru sites. In our model the intersite Coulomb repulsion and exchange coupling can work as the effective interaction for the spin-triplet paring. This effective interaction is enhanced by the band hybridization, which is mediated by the interlayer transfers. We investigate the possibility of this mechanism in the ground state and find that the orbital dependent spin-triplet superconductivity is more stable than the spin-singlet one for realistic parameters. This spin-triplet superconducting state has horizontal line nodes on the Fermi surface.

cond-mat.supr-con↗

arXiv2002

Ground state of the three-band Hubbard model

The ground state of the two-dimensional three-band Hubbard model in oxide superconductors is investigated by using the variational Monte Carlo method. The Gutzwiller-projected BCS and spin- density wave (SDW) functions are employed in the search for a possible ground state with respect to dependences on electron density. Antiferromagnetic correlations are considerably enhanced near half-filling. It is shown that the d-wave state may exist away from half-filling for both the hole and electron doping cases. The overall structure of the phase diagram obtained by the calculations qualitatively agrees with experimental indications. The superconducting condensation energy is in reasonable agreement with the experimental value obtained from specific heat and critical magnetic field measurements for optimally doped samples. The inhomogeneous SDW state is also examined near 1/8-hole doping.

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