arXiv subjects

Shinji Watanabe

Publications and source records attributed to Shinji Watanabe.

529 records · Page 30Linked to original sources

arXiv2002

Absence of Translational Symmetry Breaking in Nonmagnetic Insulator Phase on Two-Dimensional Lattice with Geometrical Frustration

The ground-state properties of the two-dimensional Hubbard model with nearest-neighbor and next-nearest-neighbor hoppings at half filling are studied by the path-integral-renormalization-group method. The nonmagnetic-insulator phase sandwiched by the the paramagnetic-metal phase and the antiferromagnetic-insulator phase shows evidence against translational symmetry breaking of the dimerized state, plaquette singlet state, staggered flux state, and charge ordered state. These results support that the genuine Mott insulator which cannot be adiabatically continued to the band insulator is realized generically by Umklapp scattering through the effects of geometrical frustration and quantum fluctuation in the two-dimensional system.

cond-mat.str-el↗

arXiv2002

Coupled CDW and SDW Fluctuations as an Origin of Anomalous Properties of Ferromagnetic Superconductor UGe_2

It is shown that anomalous properties of UGe_2 can be understood in a unified way on the basis of a single assumption that the superconductivity is mediated by the coupled SDW and CDW fluctuations induced by the imperfect nesting of the Fermi surface with majority spins at T=T_x(P) deep in the ferromagnetic phase. Excess growth of uniform magnetization is shown to develop in the temperature range T<T_x(P) as a mode-coupling effect of coupled growth of SDW and CDW orderings, which has been observed by two different types of experiments. The coupled CDW and SDW fluctuations are shown to be essentially ferromagnetic spin fluctuations which induce a spin-triplet p-wave attraction. These fluctuations consist of two modes, spin and charge fluctuations with large momentum transfer of the nesting vector. An anomalous temperature dependence of the upper critical field H_c2(T) such as crossing of H_c2(T) at P=11.4 kbar and P=13.5 kbar, can be understood by the strong-coupling-superconductivity formalism. Temperature dependence of the lattice specific heat including a large shoulder near T_x is also explained quite well as an effect of a kind of Kohn anomaly associated with coupled SDW-CDW transition.

cond-mat.str-el↗

arXiv2002

Nonmagnetic Insulating States near the Mott Transitions on Lattices with Geometrical Frustration and Implications for $κ$-(ET)$_2$Cu$_2(CN)_3$

We study phase diagrams of the Hubbard model on anisotropic triangular lattices, which also represents a model for $κ$-type BEDT-TTF compounds. In contrast with mean-field predictions, path-integral renormalization group calculations show a universal presence of nonmagnetic insulator sandwitched by antiferromagnetic insulator and paramagnetic metals. The nonmagnetic phase does not show a simple translational symmetry breakings such as flux phases, implying a genuine Mott insulator. We discuss possible relevance on the nonmagnetic insulating phase found in $κ$-(ET)$_2$Cu$_2(CN)_3$.

cond-mat.str-el↗

arXiv1999

Magnetization Process in the One-Dimensional Doped Kondo Lattice Model

The magnetization process in the one-dimensional Kondo lattice model for the doped (n_{c}<1) case is studied by the density matrix renormalization group (DMRG) method. A rapid increase of the magnetization is caused by the collapse of the intersite incommensurate correlation of f spins. On the contrary, the intrasite f-c singlet correlation survives in the larger magnetic field. The crossover from large to small Fermi surfaces for majority and minority spins is observed, whereas the Fermi surfaces are always contributed by f spins. A magnetization plateau appears with the magnitude of 1-n_{c}. Both ends of the plateau are related to the coherence temperature and the Kondo temperature which are characteristic energies essential in heavy electron systems.

cond-mat.str-el↗

arXiv1999

Solar Neutrinos and Leptonic CP Violation

We examine the possibility of detecting effects of leptonic CP violation by precise measurement of the solar neutrinos within the framework of standard electroweak theory minimally extended to include neutrino masses and mixing. We prove a ``no-go theorem'' which states that effects of CP violating phase disappear in the nu_e survival probability to the leading order in electroweak interaction. The effects due to the next-to-leading order correction is estimated to be extremely small, effectively closing the door to the possibility we intended to pursue.

hep-ph↗

arXiv1999

Transition from Haldane Phase to Spin Liquid and Incommensurate Correlation in Spin-1/2 Heisenberg Chains

The bulk properties and impurity effect in the one-dimensional S=1/2 Heisenberg model with dimerization and next-nearest-neighbor exchange are studied by the density matrix renormalization group (DMRG) and exact diagonalization methods. This model widely covers the S=1 Heisenberg model, two-leg ladders, bond-alternating chains and the S=1/2 Heisenberg model. Almost the whole parameter space belongs to the Haldane phase. In the non-dimerizing limit a transition to a kind of spin liquid takes place, characterized by the delocalized spin polarization. This transition causes the distinct impurity effects between a spin-Peierls compound CuGeO_{3} and the Haldane systems. Experimental aspects of a bond-alternating compound CuNb_{2}O_{6} are also discussed. Furthermore, the incommensurate correlation arising in this model is extensively studied, and a phase diagram is constructed.

cond-mat.str-el↗

arXiv1997

Competition between crystalline electric field singlet and itinerant states of f electrons

A new kind of phase transition is proposed for lattice fermion systems with simplified f^2 configurations at each site. The free energy of the model is computed in the mean-field approximation for both the itinerant state with the Kondo screening, and a localized state with the crystalline electric field (CEF) singlet at each site. The presence of a first-order phase transition is demonstrated in which the itinerant state changes into the localized state toward lower temperatures. In the half-filled case, the insulating state at high temperatures changes into a metallic state, in marked contrast with the Mott transition in the Hubbard model. For comparison, corresponding states are discussed for the two-impurity Kondo system with f^1 configuration at each site.

cond-mat.str-el↗