arXiv subjects

Hiroshi Kontani

Publications and source records attributed to Hiroshi Kontani.

130 records · Page 8Linked to original sources

arXiv1996

Effect of Quantum Fluctuations on Magnetic Ordering in CaV$_3$O$_7$

We present a theoretical model for CaV$_3$O$_7$: the $1/4$-depleted square spin-$1/2$ Heisenberg model which includes both the nearest-neighbor coupling ($J$) and the next-nearest-neighbor coupling ($J'$), where $J$ and $J'$ are antiferromagnetic. Recent experiments of the neutron diffraction by Harashina et.al. report the magnetic ordering at low temperatures, which may be called as a stripe phase. It is shown that the observed spin structure is not stable in the classical theory. By employing the modified spin wave theory, we show that the stripe phase is stabilized by the quantum fluctuations for $J'/J > 0.69$. In CaV$_3$O$_7$, the coupling constants are estimated as $J \sim J'$ by comparing the theoretical and experimental results.

cond-mat↗

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↗

arXiv1995

Phase diagram of depleted Heisenberg model for CaV4O9

We have numerically investigated the 1/5-depleted Heisenberg square lattice representing CaV4O9 using the Quantum Monte Carlo loop algorithm. We have determined the phase diagram of the model as a function of the ratio of the two different couplings: bonds within a plaquette and dimer bonds between plaquettes. By calculating both the spin gap and the staggered magnetization we determine the range of stability of the long range ordered (LRO) phase. At isotropic coupling LRO survives the depletion. But the close vicinity of the isotropic point to the spin gap phase leads us to the conclusion that already a small frustrating next nearest neighbor interaction can drive the system into the quantum disordered phase and thus explain the spin gap behavior of CaV4O9.

cond-mat↗

arXiv1995

Plaquette Resonating-Valence-Bond Ground State of CaV$_4$O$_9$

A theoretical model is presented to explain the spin gap observed for CaV$_4$O$_9$. The underlying lattice of the 1/5-depleted square lattice favors a formation of plaquette resonating valence bond state. Inclusion of the frustrating second neighbor interaction enhances this tendency, leading to a quantum disordered state of a two dimensional spin-1/2 Heisenberg model with a sufficiently big spin gap compatible with experiments.

cond-mat↗