Ken-ichi Noba

Self-consistent theory for ferromagnetism induced by photo-excited carriers

Abstract The paramagnetic–ferromagnetic phase transition in diluted magnetic semiconductors induced by spin carriers is studied theoretically. A self-consistent formalism to describe the phase transition is developed on the basis of the mean field approximation and the coherent potential approximation. It is predicted that the Curie temperature is maximized at a relatively low concentration of carrier density, and decreases to zero for over-doping.

Wiener–Hermite expansion formalism for the stochastic model of a driven quantum system

Abstract The time evolution of a simple two-level system with a periodic modulation of the energy levels superimposed with a stochastic fluctuation is investigated. The Wiener–Hermite expansion method is successfully applied to the numerical solution of the equation of motion. In this method, we transform the stochastic equation into an infinite set of simultaneous deterministic equations, which can be solved by truncating it into a finite one. Two types of mechanisms of phase relaxation are exhibited to modify the population dynamics. The one is the homogeneous phase relaxation in the rapid fluctuation limit, and the other is the inhomogeneous phase relaxation in the slow fluctuation limit.

Quasi bound states in continuum induced by an external oscillating field

The presence of quasi bound states in continuum is demonstrated in a system represented by a time-dependent Fano–Anderson Hamiltonian where an impurity coupled with a one-dimensional chain is driven by an external oscillating field. The QBIC state is a metastable state with an extremely small decay rate caused by the competition between a stabilizing effect by the Van Hove singularity in the density of states and a destabilizing effect by the resonance instability. This competition occurs in systems with multiple overlapping energy bands. Although there is only one energy band in our system, multiple bands emerge in the spectrum of the Floquet Hamiltonian. As a result of the overlapping bands in the Floquet spectrum, the QBIC state is induced in the presence of an oscillating field. In terms of the Greenʼs function method, the analytical expression of the complex energy for the QBIC state is obtained as expansion near the band edge in powers of the coupling constant. Our results indicate that the appearance and the disappearance of the QBIC state repeatedly occur as the amplitude of the external field increases.

Optical responses of one-dimensional exciton system in correlated random potentials

Urbach tails at absorption edge and photon echo signals are theoretically investigated for one-dimensional exciton systems which have correlated random potentials. As two limits, these correlated random potentials include inhomogeneous distribution of energy levels and random distribution of site-energies for excitons. It is shown that spectral profile of the optical responses reflects the continuous change of the correlated potentials between the two limits.

Fano absorption spectrum with the complex spectral analysis

A new aspect of understanding a Fano absorption spectrum is presented in terms of the complex spectral analysis. The absorption spectrum of an impurity embedded in semi-infinite superlattice is investigated. The boundary condition on the continuum causes a large energy dependence of the self-energy, enhances the nonlinearity of the eigenvalue problem of the effective Hamiltonian, yielding several nonanalytic resonance states. The overall spectral features is perfectly reproduced by the direct transitions to these discrete resonance states. Even with a single optical transition path the spectrum exhibits an asymmetric Fano profile, which is enhanced for the transition to the nonanalytic resonance states. Since this is the genuine eigenstates of the total Hamiltonian, there is no ambiguity in the interpretation of the absorption spectrum, avoiding the arbitrary interpretation based on the quantum interference. The spectral change around the exceptional point is well understood when we extract the resonant state component.

Theory of Urbach tail for ferroelectric materials with an order-disorder-type phase transition

Abstract A new model describing the Urbach tail of exciton absorption spectra in ferroelectric materials with an order-disorder-type phase transition is proposed. In addition to the thermal effect of the lattice vibration, the internal Stark effect for excitons by the local electric field due to the displacement of protons is taken into account. It turns out that the randomness of the orientation of the polarization associated with the proton ordering decreases the steepness of the exponential tail in absorption spectra. By the present model, the anomalous temperature dependence of the steepness parameter for the Urbach tail observed in PbHPO 4 is reproduced.