arXiv subjects

Katsunori Kubo

Publications and source records attributed to Katsunori Kubo.

32 records · Page 2Linked to original sources

arXiv2008

Ferromagnetism and orbital order in the two-orbital Hubbard model

We investigate spin and orbital states of the two-orbital Hubbard model on a square lattice by using a variational Monte Carlo method at quarter-filling, i.e., the electron number per site is one. As a variational wave function, we consider a Gutzwiller projected wave function of a mean-field type wave function for a staggered spin and/or orbital ordered state. Then, we evaluate expectation value of energy for the variational wave functions by using the Monte Carlo method and determine the ground state. In the strong Coulomb interaction region, the ground state is the perfect ferromagnetic state with antiferro-orbital (AF-orbital) order. By decreasing the interaction, we find that the disordered state becomes the ground state. Although we have also considered the paramagnetic state with AF-orbital order, i.e., purely orbital ordered state, and partial ferromagnetic states with and without AF-orbital order, they do not become the ground state.

cond-mat.str-el↗

arXiv2008

Fulde-Ferrell-Larkin-Ovchinnikov State in the absence of a Magnetic Field

We propose that in a system with pocket Fermi surfaces, a pairing state with a finite total momentum q_tot like the Fulde-Ferrell-Larkin-Ovchinnikov state can be stabilized even without a magnetic field. When a pair is composed of electrons on a pocket Fermi surface whose center is not located at Gamma point, the pair inevitably has finite q_tot. To investigate this possibility, we consider a two-orbital model on a square lattice that can realize pocket Fermi surfaces and we apply fluctuation exchange approximation. Then, by changing the electron number n per site, we indeed find that such superconducting states with finite q_tot are stabilized when the system has pocket Fermi surfaces.

cond-mat.supr-con↗

arXiv2007

Pairing symmetry in a two-orbital Hubbard model on a square lattice

We investigate superconductivity in a two-orbital Hubbard model on a square lattice by applying fluctuation exchange approximation. In the present model, the symmetry of the two orbitals are assumed to be that of an s orbital. Then, we find that an s-wave spin-triplet orbital-antisymmetric state and a p-wave spin-singlet orbital-antisymmetric state appear when Hund's rule coupling is large. These states are prohibited in a single-orbital model within states with even frequency dependence, but allowed for multi-orbital systems. We also discuss pairing symmetry in other models which are equivalent to the two-orbital Hubbard model except for symmetry of orbitals. Finally, we show that pairing states with a finite total momentum, even without a magnetic field, are possible in a system with two Fermi-surfaces.

cond-mat.supr-con↗

arXiv2006

Orbital-Controlled Superconductivity in f-Electron Systems

We propose a concept of superconductivity controlled by orbital degree of freedom taking CeMIn5 (M= Co, Rh, and Ir) as typical examples. A microscopic multiorbital model for CeMIn5 is analyzed by fluctuation exchange approximation. Even though the Fermi-surface structure is unchanged, the ground state is found to change significantly among paramagnetic, antiferromagnetic, and d-wave superconducting phases, depending on the dominant orbital component in the band near the Fermi energy. We show that our picture naturally explains the different low-temperature properties of CeMIn5 by carefully analyzing the crystalline electric field states.

cond-mat.supr-con↗

arXiv2005

Magnetic Susceptibility of Multiorbital Systems

Effects of orbital degeneracy on magnetic susceptibility in paramagnetic phases are investigated within a mean-field theory. Under certain crystalline electric fields, the magnetic moment consists of two independent moments, e.g., spin and orbital moments. In such a case, the magnetic susceptibility is given by the sum of two different Curie-Weiss relations, leading to deviation from the Curie-Weiss law. Such behavior may be observed in d- and f-electron systems with t_{2g} and Gamma_8 ground states, respectively. As a potential application of our theory, we attempt to explain the difference in the temperature dependence of magnetic susceptibilities of UO_2 and NpO_2.

cond-mat.str-el↗

arXiv2005

Multipole Ordering and Fluctuations in f-Electron Systems

We investigate effects of multipole moments in f-electron systems both from phenomenological and microscopic viewpoints. First, we discuss significant effects of octupole moment on the magnetic susceptibility in a paramagnetic phase. It is found that even within mean-field approximation, the magnetic susceptibility deviates from the Curie-Weiss law due to interactions between dipole and octupole moments. Next, we proceed to a microscopic theory for multipole ordering on the basis of a j-j coupling scheme. After brief explanation of a method to derive multipole interactions from the $f$-electron model, we discuss several multipole ordered phases depending on lattice structure. Finally, we show our new development of the microscopic approach to the evaluation of multipole response functions. We apply fluctuation exchange approximation to the f-electron model, and evaluate multipole response functions.

cond-mat.str-el↗

arXiv2005

Analysis of f-p model for octupole ordering in NpO2

In order to examine the origin of octupole ordering in NpO2, we propose a microscopic model constituted of neptunium 5f and oxygen 2p orbitals. To study multipole ordering, we derive effective multipole interactions from the f-p model by using the fourth-order perturbation theory in terms of p-f hopping integrals. Analyzing the effective model numerically, we find a tendency toward Γ_{5u} antiferro-octupole ordering.

cond-mat.str-el↗

arXiv2005

Multipole ordering in f-electron systems on the basis of a j-j coupling scheme

We investigate microscopic aspects of multipole ordering in f-electron systems with emphasis on the effect of lattice structure. For the purpose, first we construct f-electron models on three kinds of lattices, simple cubic (sc), bcc, and fcc, by including f-electron hopping through (ffσ) bonding in a tight-binding approximation on the basis of a j-j coupling scheme. Then, an effective model is derived in the strong-coupling limit for each lattice structure with the use of second-order perturbation theory with respect to (ffσ). By applying mean-field theory to such effective models, we find different types of multipole ordered state depending on the lattice structure. For the sc lattice, a Γ_{3g} antiferro-quadrupole transition occurs at a finite temperature and as further lowering temperature, we find another transition to a ferromagnetic state. For the bcc lattice, a Γ_{2u} antiferro-octupole ordering occurs first, and then, a ferromagnetic phase transition follows it. Finally, for the fcc lattice, we find a single phase transition to the longitudinal triple-q Γ_{5u} octupole ordering.

cond-mat.str-el↗

arXiv2005

Multipole ordering in f-electron systems

In order to investigate multipole ordering in f-electron systems from a microscopic viewpoint, we study the so-called Γ_8 models on three kinds of lattices, simple cubic (sc), bcc, and fcc, based on a j-j coupling scheme with f-electron hopping integrals through (ffσ) bonding. From the Γ_8 model, we derive an effective model for each lattice structure by using the second-order perturbation theory with respect to (ffσ). By further applying mean-field theory to the effective model, we find a Γ_{3g} antiferro-quadrupole transition for the sc lattice, a Γ_{2u} antiferro-octupole transition for the bcc lattice, and a longitudinal triple-q Γ_{5u} octupole transition for the fcc lattice.

cond-mat.str-el↗

arXiv2005

Microscopic theory of multipole ordering in NpO2

In order to examine the mysterious ordered phase of NpO2 from a microscopic viewpoint, we investigate an f-electron model on an fcc lattice constructed based on a j-j coupling scheme. First, an effective model with multipole interactions is derived in the strong-coupling limit. Numerical analysis of the model clearly indicates that the interactions for Γ_{4u} and Γ_{5u} moments are relevant to the ground state. Then, by applying mean-field theory to the simplified model including only such interactions, we conclude that longitudinal triple-q Γ_{5u} octupole order is realized in NpO2 through the combined effects of multipole interactions and anisotropy of the Γ_{5u} moment.

cond-mat.str-el↗

arXiv2004

Theory of Coupled Multipole Moments Probed by X-ray Scattering in CeB$_6$

A minimal model for multipole orders in CeB$_6$ shows that degeneracy of the quadrupole order parameters and strong spin-orbit coupling lead to peculiar temperature and magnetic-field dependences of the X-ray reflection intensity at superlattice Bragg points. Furthermore, the intensity depends sensitively on the surface direction. These theoretical results explain naturally recent X-ray experiments in phases II and III of CeB$_6$. It is predicted that under weak magnetic field perpendicular to the (111) surface, the reflection intensity should change non-monotonically as a function of temperature.

cond-mat.str-el↗

arXiv2003

Octupole Ordering Model for the Phase IV of CexLa1-xB6

An octupole ordering model is studied by the mean field theory, and its relevance to the phase IV of CexLa1-xB6 is discussed. The observed lattice distortion along the [111] direction is interpreted in terms of the Γ_{5g}-type ferro-quadrupole moment induced by an antiferro-octupole ordered state with Γ_{5u} symmetry. The octupole model also accounts for the cusp in the magnetization as in the Néel transition, and the softening of the elastic constant C_{44} below the ordering temperature. However, the internal magnetic field due to the octupole moment is smaller than the observed one by an order of magnitude. Also discussed is the possibility of a pressure induced antiferromagnetic moment in the octupole-ordered state.

cond-mat.str-el↗

arXiv2003

Lattice Distortion and Octupole Ordering Model in CexLa1-xB6

Possible order parameters of the phase IV in CexLa1-xB6 are discussed with special attention to the lattice distortion recently observed. A Γ_{5u}-type octupole order with finite wave number is proposed as the origin of the distortion along the [111] direction. The Γ_8 crystalline electric field (CEF) level splits into three levels by a mean field with the Γ_{5u} symmetry. The ground and highest singlets have the same quadrupole moment, while the intermediate doublet has an opposite sign. It is shown that any collinear order of Γ_{5u}-type octupole moment accompanies the Γ_{5g}-type ferro-quadrupole order, and the coupling of the quadrupole moment with the lattice induces the distortion. The cusp in the magnetization at the phase transition is reproduced, but the internal magnetic field due to the octupole moment is smaller than the observed one by an order of magnitude.

cond-mat.str-el↗

arXiv2002

Interpocket polarization model for magnetic structures in rare-earth hexaborides

The origin of peculiar magnetic structures in cubic rare-earth (R) hexaborides RB_6 is traced back to their characteristic band structure. The three sphere-like Fermi surfaces induce interpocket polarization of the conduction band as a part of a RKKY-type interaction. It is shown for the free-electron-like model that the interpocket polarization gives rise to a broad maximum in the intersite interaction I(q) around q=(1/4,1/4,1/2) in the Brillouin zone. This maximum is consistent with the superstructure observed in R=Ce, Gd and Dy. The wave-number dependence of I(q) is independently extracted from analysis of the spin-wave spectrum measured for NdB_6. It is found that I(q) obtained from fitting the data has a similarly to that derived by the interpocket polarization model, except that the absolute maximum now occurs at (0,0,1/2) in consistency with the A-type structure. The overall shape of I(q) gives a hint toward understanding an incommensurate structure in PrB_6 as well.

cond-mat.str-el↗