Seiji Miyashita

Quantum Tunneling in Half-Integer Spin Systems

Motivated by the experimental observation of resonant tunneling in systems of halfinteger spin, such as V15 and Mn4, we study the mechanism of the adiabatic change of the magnetization in systems with time-reversal symmetry. Although tunneling between the degenerate ground states is suppressed in a simple Kramers doublet, some types of perturbational interactions cause an avoided level crossing structure in the energy level structures when the system possesses additional degeneracy of the ground state. We show that a nona

Magnetic Foehn effect in adiabatic transition

The magnetization process as a response to a sweeping magnetic field in the thermal environment is investigated by using the quantum master equation. In a slow sweeping velocity region where the system shows quasi-adiabatic motion, a magnetic plateau in the magnetization process has been observed in the recent experiment of V 15 [Phys. Rev. Lett. 84 (2000) 3454]. Although the experiment was explained from a view of small capacity of the phonon heat bath, i.e., the phonon bottleneck effect, we found that a magnetic plateau appears almost regardless of the properties of the heat bath in the quasi-adiabatic transition and the appearance of the plateau is quite universal. We call it `Magnetic Foehn effect. On the other hand, the magnetic plateau also appears in a fast sweeping velocity region by the Landau–Zener–Stuckelberg mechanism which is different from the above mechanism in a slow sweeping region, and we study the crossover between them. We also propose an experiment which clarifies the structure of co...

Magnetization process of nanoscale iron cluster

Low-temperature magnetization process of the nanoscale iron cluster in linearly sweeped fields is investigated by a numerical analysis of time-dependent Schrodinger equation and the quantum master equation. We introduce an effective basis method extracting important states, by which we can obtain the magnetization process effectively. We investigate the structure of the field derivative of the magnetization. We find out that the antisymmetric interaction determined from the lattice structure reproduces well the experimental results of the iron magnets and that this interaction plays an important role in the iron cluster. Deviations from the adiabatic process are also studied. In the fast sweeping case, our calculations indicate that the nonadiabatic transition dominantly occurs at the level crossing for the lowest field. In slow sweeping case, due to the influence of the thermal environment to the spin system, the field derivative of the magnetization shows an asymmetric behavior, the magnetic Fohn effect...

ESR of Antiferromagnetic Cluster

Electron spin resonance in an antiferromagnetic cluster is studied by a direct numerical method to obtain the dynamical susceptibility χ(ω) expressed by the Kubo formula. Temperature dependence of the dipolar broadening is investigated in the isotropic Heisenberg model. The width decreases monotonically as the temperature decreases in the case of high frequency. In the case of low frequency, however it is found to show non-monotonic dependence. The frequency- and temperature-dependence of the resonant field of Ising-like Heisenberg model is studied, and it is compared with the conventional antiferromagnetic resonance (AFMR). It is found that the paramagnetic resonance (PMR) coexists with the antiferromagnetic resonance in the wide range of the temperature when the field is applied parallel to the easy axis. A branch of the conventional AFMR is found to split into double peaks. In the case of the field perpendicular to the easy axis, PMR is very broad and disappears at low temperature.

Number partitioning on a quantum computer

Abstract We present an algorithm to compute the number of solutions of the (constrained) number partitioning problem. A concrete implementation of the algorithm on an Ising-type quantum computer is given.

Feedback effect on Landau-Zener-Stückelberg transitions in magnetic systems

We examine the effect of the dynamics of the internal magnetic field on the staircase magnetization curves observed in large-spin molecular magnets. We show that the size of the magnetization steps depends sensitively on the intermolecular interactions, even if these are very small compared to the intramolecular couplings.

Temperature Dependence of Spin and Bond Ordering in a Spin-Peierls System

We investigate thermodynamic properties of a one-dimensional S =1/2 antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum Monte Carlo method. In particular we study how sp...

Nonexponential Relaxation of Magnetization at the Resonant Tunneling Point under a Fluctuating Random Noise

At resonant tunneling points of nanoscale molecular magnets, a fluctuating random field causes successive Landau–Zener–Stuckelberg transitions. We have studied dynamics of magnetization by a Langevin type Schrodinger equationxa0and found that the successive transitions result in a stretched exponential decay with square-root time when the fluctuation of the field is regarded as an random walk, i.e., the Wiener process. When the fluctuation is bounded by a restoring force, i.e., the Ornstein–Uhlenbeck process, the relaxation obeys the exponential decay at the late stage, while it shows the stretched exponential decay at the early stage. The scaling relations of the relaxation functions at both stages are also studied as functions of the parameters: the strength of the fluctuation, the restoring force, and the energy gap of the system at the resonant point. Roles of the observed nonexponential decay are also discussed in various kinds of square-root time behavior found in experiments.At resonant tunneling points of nanoscale molecular magnets, a fluctuating random field causes successive Landau–Zener–Stuckelberg transitions. We have studied dynamics of magnetization by a Langev...

Noise effect on the nonadiabatic transition and correction to the tunneling energy gap estimated by the Landau-Zener-Stückelberg formula

We study the effect of noise on the tunneling process of the magnetization in a sweeping field from the viewpoint of the nonadiabatic transition. The tunneling probability depends on the properties of the noise, e.g. the direction, the amplitude, and the relaxation time of the autocorrelation function. We investigate the tunneling probability in the presence of noise by solving the time-dependent Schrodinger equation numerically. We estimate the effective tunneling gap making use of the Landau-Zener-Stuckelberg (LZS) formula. The dependence of the gap on the sweeping velocity reproduces the corresponding experimental data qualitatively. The estimated gap has no velocity dependence for a fast sweeping field and this gap seems to give the true gap. However, we show that a correction for the estimated energy gap is necessary and we propose a correction formula.

Spin-Peierls transition of the first order in S = 1 antiferromagnetic Heisenberg chains

We investigate a one-dimensional S=1 antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum Monte Carlo method. Investigating the ground state energy of the static bond-alternating chain, we find that the instability to a dimerized chain depends on the value of the spin-phonon coupling, unlike the case of S=1/2. The spin state is the dimer state or the uniform Haldane state depending on whether the lattice distorts or not, respectively. At an intermediate value of the spin-phonon coupling, we find the first-order transition between the two states. We also find the coexistence of the two states.