arXiv subjects

Masahito Mochizuki

Publications and source records attributed to Masahito Mochizuki.

102 records · Page 6Linked to original sources

arXiv2005

Role of spin-orbit coupling on the spin triplet pairing in Na_{x}CoO_{2}yH_{2}O I: d-vector under zero magnetic field

The d-vector in possibile spin triplet superconductor Na_{x}CoO_{2}yH_{2}O is microscopically investigated on the basis of the multi-orbital Hubbard model including the atomic spin-orbit coupling. As a result of the perturbation theory, we obtain the stable spin triplet superconductivity where the p-wave and f-wave states can be stabilized. If we neglect the spin-orbit coupling, superconducting state has 6-fold (3-fold) degeneracy in the p-wave (f-wave) state. This degeneracy is lifted by the spin-orbit coupling. We determine the d-vector within the linearlized Dyson-Gorkov equation. It is shown that the d-vector is always along the plane when the pairing symmetry is p-wave, while it depends on the parameters in case of the f-wave state. The lifting of degeneracy is significant in the p-wave state while it is very small in the f-wave state. This is because the first order term with respect to the spin-orbit coupling is effective in the former case, while it is ineffective in the latter case. The consistency of these results with NMR and \muSR measurements are discussed.

cond-mat.supr-con↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.str-el↗

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.

cond-mat.supr-con↗

arXiv2004

Orbital Physics in the Perovskite Ti Oxides

In the perovskite Ti oxide RTiO3 (R=rare-earth ions), the Ti t2g orbitals and spins in the 3d^1 state couple each other through the strong electron correlations, resulting in a rich variety of orbital-spin phases. The origin and nature of orbital-spin states of these Mott insulators have been intensively studied. In this article, we review the studies on orbital physics in the perovskite titanates. We focus on the following three topics: (1) the origin and nature of the ferromagnetism as well as the orbital ordering in the compounds with relatively small R ions such as GdTiO3 and YTiO3, (2) the origin of the G-type antiferromagnetism and the orbital state in LaTiO3, and (3) the orbital-spin structures in other AFM(G) compounds with relatively large R ions (R=Ce, Pr, Nd and Sm). On the basis of these discussions, we discuss the whole phase diagram together with mechanisms of the magnetic phase transition. We also show that the Ti t2g degeneracy is inherently lifted in the titanates, which allows the single-band descriptions of the ground-state and low-energy electronic structures as a good starting point. Our analyses indicate that these compounds offer touchstone materials described by the single-band Hubbard model on the cubic lattice. From this insight, we also reanalyze the hole-doped titanates. Experimentally revealed filling-dependent and bandwidth-dependent properties and the critical behavior of the metal-insulator transitions are discussed in the light of theories based on the single-band Hubbard models.

cond-mat.str-el↗

arXiv2003

G-type antiferromagnetism and orbital ordering due to the crystal field from the rare-earth ions induced by the GdFeO_3-type distortion in RTiO_3 with R=La, Pr, Nd and Sm

The origin of the antiferromagnetic order and puzzling properties of LaTiO_3 as well as the magnetic phase diagram of the perovskite titanates are studied theoretically. We show that in LaTiO_3, the t_{2g} degeneracy is eventually lifted by the La cations in the GdFeO_3-type structure, which generates a crystal field with nearly trigonal symmetry. This allows the description of the low-energy structure of LaTiO_3 by a single-band Hubbard model as a good starting point. The lowest-orbital occupation in this crystal field stabilizes the AFM(G) state, and well explains the spin-wave spectrum of LaTiO_3 obtained by the neutron scattering experiment. The orbital-spin structures for RTiO_3 with R=Pr, Nd and Sm are also accounted for by the same mechanism. We point out that through generating the R crystal field, the GdFeO_3-type distortion has a universal relevance in determining the orbital-spin structure of the perovskite compounds in competition with the Jahn-Teller mechanism, which has been overlooked in the literature. Since the GdFeO_3-type distortion is a universal phenomenon as is seen in a large number of perovskite compounds, this mechanism may also play important roles in other compounds of this type.

cond-mat.str-el↗

arXiv2003

Orbital-Spin Structure and Coupling to Lattice in RTiO$_3$ with R=La,Pr,Nd and Sm

The origin of the G-type antiferromagnetism (AFM(G)) and puzzling properties of RTiO$_3$ with R=La are studied. We clarify that the crystal field from La caused by the GdFeO$_3$-type distortion lifts the $t_{2g}$ degeneracy at Ti 3d orbitals. The lift stabilizes the AFM(G) with spin-exchange constant in agreement with neutron scattering results. The orbital-spin structures for R=Pr, Nd and Sm are also consistent with experiments. We propose that the GdFeO$_3$-type distortion has a universal mechanism of controlling orbital-spin structure competing with the Jahn-Teller (JT) mechanism.

cond-mat.str-el↗

arXiv2002

Spin and Orbital States and Their Phase Transitions of the Perovskite-Type Ti Oxides: Weak coupling Approach

The magnetic phase diagram of the perovskite-type Ti oxides as a function of the GdFeO3-type distortion is examined by using the Hartree-Fock analysis of a multiband d-p Hamiltonian from a viewpoint of competitions of the spin-orbit interaction, the Jahn-Teller (JT) level-splitting and spin-orbital superexchange interactions. Near the antiferromagnetic (AFM)-to-ferromagnetic (FM) phase boundary, A-type AFM [AFM(A)] and FM states accompanied by a certain type of orbital ordering are lowered in energy at large JT distortion, which is in agreement with the previous strong coupling study. With increasing the GdFeO3-type distortion, their phase transition occurs. Through this magnetic phase transition, the orbital state hardly changes, which induces nearly continuous change in the spin coupling along the c-axis from negative to positive. The resultant strong two-dimensionality in the spin coupling near the phase boundary is attributed to the strong suppression of T_N and T_C, which is experimentally observed. On the other hand, at small GdFeO3-type without JT distortions, which correspond to LaTiO3, the most stable solution is not G-type AFM [AFM(G)] but FM. Although the spin-orbit interaction has been considered to be relevant at the small or no JT distortion of LaTiO3 in the literature, our analysis indicates that the spin-orbit interaction is irrelevant to the AFM(G) state in LaTiO3 and superexchange-type interaction dominates. On the basis of further investigations on the nature of this FM state and other solutions, this discrepancy is discussed in detail.

cond-mat.str-el↗

arXiv2001

Origin of G-type Antiferromagnetism and Orbital-Spin Structures in ${\rm LaTiO}_3$

The possibility of the $D_{3d}$ distortion of ${\rm TiO}_6$ octahedra is examined theoretically in order to understand the origin of the G-type antiferromagnetism (AFM(G)) and experimentally observed puzzling properties of ${\rm LaTiO}_3$. By utilizing an effective spin and pseudospin Hamiltonian with the strong Coulomb repulsion, it is shown that AFM(G) state is stabilized through the lift of the $t_{2g}$-orbital degeneracy accompanied by a tiny $D_{3d}$-distortion . The estimated spin-exchange interaction is in agreement with that obtained by the neutron scattering. Moreover, the level-splitting energy due to the distortion can be considerably larger than the spin-orbit interaction even when the distortion becomes smaller than the detectable limit under the available experimental resolution. This suggests that the orbital momentum is fully quenched and the relativistic spin-orbit interaction is not effective in this system, in agreement with recent neutron-scattering experiment.

cond-mat.str-el↗

arXiv2001

Magnetic and Orbital States and Their Phase Transition of the Perovskite-Type Ti Oxides: Strong Coupling Approach

The properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically by using the effective spin and pseudospin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic (AFM(A)) to ferromagnetic (FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state changes only little whereas the spin-exchange coupling along the c-axis is expected to change from positive to negative nearly continuously and approaches zero at the phase boundary. The resultant strong two-dimensionality in the spin coupling causes rapid suppression of the critical temperature, as observed experimentally. It may induce large quantum fluctuations in this region.

cond-mat.str-el↗

arXiv2000

Magnetic Phase Transition of the Perovskite-type Ti Oxides

Properties and mechanism of the magnetic phase transition of the perovskite-type Ti oxides, which is driven by the Ti-O-Ti bond angle distortion, are studied theoretically by using the effective spin and pseudo-spin Hamiltonian with strong Coulomb repulsion. It is shown that the A-type antiferromagnetic(AFM(A)) to ferromagnetic(FM) phase transition occurs as the Ti-O-Ti bond angle is decreased. Through this phase transition, the orbital state is hardly changed so that the spin-exchange coupling along the c-axis changes nearly continuously from positive to negative and takes approximately zero at the phase boundary. The resultant strong two-dimensionality in the spin coupling causes a rapid suppression of the critical temperature as is observed experimentally.

cond-mat.str-el↗