Akihide Oguchi

Magnetization Process in the Shastry-Sutherland System SrCu2(BO3)2. Results of Third-Order Dimer Expansion.

The magnetization process in the Shastry-Sutherland system is studied by using the third-order perturbation expansion. It is shown that the 1/3-plateau is realized by the second-order perturbation, which is not prevented by the off-diagonal part. In each subspace whose magnetization per dimer is less than 1/3, the lowest energy state is determined by a small but finite energy-gain due to the third-order correlated flip terms and there exists no plateau originating from the third-order effect. Our results are compared with those of the exact diagonalization method to discuss the validity of truncation of states in our perturbation theory.

Frustrated Ising Model on a Diamond Hierarchical Lattice

A frustrated Ising model on a diamond hierarchical lattice is studied. We obtain the exact partition function of this model and calculate the transition temperature, specific heat, entropy, magnetization, and ferromagnetic correlation function. Depending on the magnitude of a parameter giving the frustration, there exist three types of ground states: ferromagnetic, classical spin-liquid with highly developed short-range order, and paramagnetic. The dependence of the zero-temperature entropy on the frustration parameter has an infinite number of steps. The temperature dependence of the specific heat exhibits many peaks with decreasing temperature and entropy loss. The dominant spin configurations at low temperatures are also specified.

Susceptibility and triplet excitation spectrum for CaV4O9 : Third-order perturbation expansions

The uniform susceptibility and triplet excitation spectrum of the Heisenberg antiferromagnet on the one-fifth depleted square lattice are calculated using the third-order perturbation expansion method from the viewpoint of the weakly coupled meta-plaquette picture. Both the susceptibility and triplet excitation spectrum data for CaV 4 O 9 are simultaneously explained. The origin of the relatively large spin gap in this compound is discussed.

Magnetic susceptibility and ground state energy of mMPYNN-BF4

We treat the organic system m M P Y N N - B F 4 as a network of hexagons. Each hexagon contains six S =1/2 spins coupled alternately with ferro- and antiferromagnetic interactions. The antiferromagnetic coupling between hexagons is treated as a perturbation term. We obtain the ferro- and antiferromagnetic coupling constants in the hexagon and the antiferromagnetic coupling constant between hexagons. The susceptibility calculated by the second-order perturbation method for m M P Y N N - B F 4 agrees well with the experimental results. The ground state energy is also calculated.

Order-disorder transition in the spin 1/2 Heisenberg antiferromagnet on the 4-8 lattice

By means of the modified spin wave method, we study the ground state properties of the spin 1/2 Heisenberg antiferromagnet on the 4-8 lattice, which is composed of a combination of squares and octagons, as a function of the exchange interaction within the square I (>0) and that between the squares J (>0). This model relates to the magnetism of CaV 4 O 9 . It is found that the system is in the ordered state for 0.401< I / J <1.803, in the disordered state with an energy gap and exponentially decaying correlation functions for 0.395< I / J <0.401 and 1.803< I / J <1.836, and in the singlet state for I / J <0.395 and 1.836< I / J . The ground state order-disorder transitions at I / J =0.401 and 1.803 originate in purely quantum mechanical nature since this system contains no frustration. At I / J =1, the ground state energy is -0.370(2 I + J ) and the long-range order parameter is 0.210.

Infinitely Multiple Steps in Magnetization of Ferro- and Antiferromagnetic Ising Models with Frustration on a Diamond Hierarchical Lattice

Magnetizations of ferro- and antiferromagnetic Ising models with frustration on diamond hierarchical lattices are exactly obtained at zero temperature. For the zero-field classical spin-liquid phase found in [Kobayashi et al., J. Phys. Soc. Jpn. 78, 074004 (2009)], for which frustrating interactions play an important role, an infinitely small applied magnetic field can induce an infinitely small magnetization, despite classical Ising models that have discrete energy levels. In antiferromagnetic systems, the magnetization cannot saturate under finite magnetic fields owing to the competition between the unfrustrating antiferromagnetic interaction and the Zeeman interaction and an intrinsic long-range nature of hierarchical lattices.

Spin Dependence of the Critical Concentration for the Neel State of 2D Impure Heisenberg Antiferromagnets.

The spin-wave theory and the coherent potential approximation are applied to a spin S Heisenberg antiferromagnet with nonmagnetic impurities on square lattice. The impurity effects are taken into account by substituting S (1- x ) for S and using the coherent potential approximation to the exchange interaction, where x is the impurity concentration. At T =0 for S =1/2 the critical impurity concentration x c of the Neel state is 0.303 and the percolation threshold x p is 0.500. The ground state in x c < x < x p is the disordered state with the spin gap. For S ≥1 the long range Neel order vanishes at x p =0.500. These results explain qualitatively the experimental results of La 2 Cu 1- x Mg x O 4 ( S =1/2) and K 2 Mn 1- x Mg x F 4 ( S =5/2). The difference of x c between these materials is caused by the decrease in the magnitude of the effective spin with impurity doping. The spin gap is expected to be observed for La 2 Cu 1- x Mg x O 4 in x c < x < x p at low temperatures.

Temperature dependence of the magnetic susceptibility of the spin 1/2 antiferromagnetic XXZ model on a square lattice

The magnetic susceptibility of the X X Z model is studied over a wide temperature range by the Greens function method with the Kondo-Yamaji decoupling scheme. By this method it is possible that both X Y - and Ising-like systems are studied in a unified way. In the X Y -like system, our results of the transverse uniform susceptibility agree with those of a Monte-Carlo simulation. In the Ising-like system, our temperature dependence of the longitudinal uniform susceptibility agrees well with the experimental results of the susceptibility of quasi-2D Ising-like antiferromagnet K 2 CoF 4 above its Neel point.

The ground state of La2(Cu1−mZnm)O4

Abstract It is theoretically shown that for La2(Cu1−mZnm)O4 nonmagnetic impurities stabilize the RVB state with respect to the Neel state in the region 0.376

Elucidation of Ground-State Spin Configurations of Ising Models in a Magnetic Field with Frustration on a Diamond Hierarchical Lattice

To study the ground-state spin configuration as a function of magnetic field, the spin configurations at each stage lattice are determined by analyzing recursion equations. The exact calculation of the magnetization curve by Hirose et al. [J. Phys. Soc. Jpn. 83, 074716 (2014)] shows that an infinitely small applied magnetic field on the zero-field classical spin-liquid phase can induce an infinitely small magnetization, which is as if this Ising system has a gapless spectrum. In this study, we reveal that an infinitely small applied field makes a large number of spins flip upwards with the exchange-energy loss remaining finite. This exotic behavior originates from the frustration effect of diamond structures and an inherent long-range nature of hierarchical lattices.