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Kazuya Yuasa

Publications and source records attributed to Kazuya Yuasa.

80 records · Page 5Linked to original sources
arXiv2003

Modified Fowler-Nordheim Field-Emission Formulae from a Nonplanar-Emitter Model

Field emission formulae, current-voltage characteristics and energy distribution of emitted electrons, are derived analytically for a nonplanar (hyperboloidal) metallic emitter model. The traditional Fowler-Nordheim formulae, which are derived from a planar emitter model, are modified, and the assumption of the planar emitter in the F-N model is reconsidered. Our analytical calculation also reveals the backgrounds of the previous numerical discussion by He et al. on the effect of the geometry of emitter on field emission. The new formulae contain a parameter which characterizes the sharpness of the hyperboloidal emitter, and experimental data of field emissions from clean tungsten emitters and nanotip emitters are analyzed by making use of this feature.

cond-mat.mtrl-sci↗
arXiv2003

Optimization of a Neutron-Spin Test of the Quantum Zeno Effect

A neutron-spin experimental test of the quantum Zeno effect (QZE) is discussed from a practical point of view, when the nonideal efficiency of the magnetic mirrors, used for filtering the spin state, is taken into account. In the idealized case the number N of (ideal) mirrors can be indefinitely increased, yielding an increasingly better QZE. By contrast, in a practical situation with imperfect mirrors, there is an optimal number of mirrors, N_opt, at which the QZE becomes maximum: more frequent measurements would deteriorate the performance. However, a quantitative analysis shows that a good experimental test of the QZE is still feasible. These conclusions are of general validity: in a realistic experiment, the presence of losses and imperfections leads to an optimal frequency N_opt, which is in general finite. One should not increase N beyond N_opt. A convenient formula for N_opt, valid in a broad framework, is derived as a function of the parameters characterizing the experimental setup.

quant-ph↗
arXiv2003

Purification through Zeno-like Measurements

A series of frequent measurements on a quantum system (Zeno-like measurements) is shown to result in the ``purification'' of another quantum system in interaction with the former. Even though the measurements are performed on the former system, their effect drives the latter into a pure state, irrespectively of its initial (mixed) state, provided certain conditions are satisfied.

quant-ph↗
arXiv2001

Bifurcation Phenomenon in a Spin Relaxation

Spin relaxation in a strong-coupling regime (with respect to the spin system) is investigated in detail based on the spin-boson model in a stochastic limit. We find a bifurcation phenomenon in temperature dependence of relaxation constants, which is never observed in the weak-coupling regime. We also discuss inequalities among the relaxation constants in our model and show the well-known relation 2Γ_T >= Γ_L, for example, for a wider parameter region than before.

quant-ph↗
arXiv2000

Stochastic limit approximation for rapidly decaying systems

The stochastic limit approximation method for ``rapid'' decay is presented, where the damping rate γis comparable to the system frequency Ω, i.e., γ\sim Ω, whereas the usual stochastic limit approximation is applied only to the weak damping situation γ<< Ω. The key formulas for rapid decay are very similar to those for weak damping, but the dynamics is quite different. From a microscopic Hamiltonian, the spin-boson model, a Bloch equation containing two independent time scales is derived. This is a useful method to extract the minimal dissipative dynamics at high temperature kT >> \hbarΩand the master equations obtained are of the Lindblad form even for the Caldeira-Leggett model. The validity of the method is confirmed by comparing the master equation derived through this method with the exact one.

quant-ph↗
arXiv1999

Quantum stochastic resonance in driven spin-boson system with stochastic limit approximation

After a brief review of stochastic limit approximation with spin-boson system from physical points of view, amplification phenomenon-stochastic resonance phenomenon-in driven spin-boson system is observed which is helped by the quantum white noise introduced through the stochastic limit approximation. The shift in frequency of the system due to the interaction with the environment-Lamb shift-has an important role in these phenomena.

quant-ph↗
arXiv1999

Stochastic Quantization of Bottomless Systems: Stationary quantities in a diffusive process

By making use of the Langevin equation with a kernel, it was shown that the Feynman measure exp(-S) can be realized in a restricted sense in a diffusive stochastic process, which diverges and has no equilibrium, for bottomless systems. In this paper, the dependence on the initial conditions and the temporal behavior are analyzed for 0-dim bottomless systems. Furthermore, it is shown that it is possible to find stationary quantities.

hep-th↗