arXiv subjects

Keiichiro Nasu

Publications and source records attributed to Keiichiro Nasu.

17 recordsLinked to original sources

arXiv2018

Real Time Quantum Dynamics of Spontaneous Translational Symmetry Breakage in the Early Stage of Photo-induced Structural Phase Transitions

Real time quantum dynamics of the spontaneous translational symmetry breakage in the early stage of photoinduced structural phase transitions is reviewed and supplementally explained, under the guide of the Toyozawa theory, which is exactly in compliance with the conservation laws of the total momentum and energy. At the Franck Condon state, an electronic excitation just created by a visible light, is in a plane wave state, extended all over the crystal. While, after the lattice relaxation having been completed, it is localized around a certain lattice site of the crystal, as a new excitation. Is there a sudden shrinkage of the excitation wave function, in between. The wave function never shrinks, but only the spatial, or inter lattice site quantum coherence, interference of the excitation disappears, as the lattice relaxation proceeds. This is nothing but the spontaneous breakage of translational symmetry.

cond-mat.mtrl-sci↗

arXiv2017

Direct Determination of Exciton Wave Function Amplitudes by the Momentum-Resolved Photo-Electron Emission Experiment

We study conceptional problems of a photo-electron emission (PEE) process from a free exciton in insulating crystals. In this PEE process, only the electron constituting the exciton is suddenly emitted out of the crystal, while the hole constituting the exciton is still left inside and forced to be recoiled back to its original valence band. This recoil on the hole is surely reflected in the spectrum of the PEE with a statistical distribution along the momentum-energy curve of the valence band. This distribution is nothing but the square of the exciton wave function amplitude, since it shows how the electron and the hole are originally bound together. Thus, the momentum-resolved PEE can directly determine the exciton wave function. These problems are clarified, taking the $Γ$ and the saddle point excitons in GaAs, as typical examples. New PEE experiments are also suggested.

cond-mat.mtrl-sci↗

arXiv2015

Instantaneous Intervalley Transition just at the Franck-Condon State in the Conduction Band of GaAs

We propose two possible quantum microscopic mechanisms for intervalley transitions just at the Franck-Condon state in GaAs. The first is the simultaneously combined coulombic elastic transitions of two electrons,from the originally photo-excited $Γ$ valley to the mutually opposite L and -L valleys in the Brillouin zone. The second is the "elastic" scattering of an electron from the $Γ$ valley to the L one by the frozen phonon, which is not newly created in the excited state, but already exists, being inherited from the starting ground state, according to the Franck-Condon principle of the photo-excitation. Surprisingly, we can theoretically show that such the elastic electron-phonon scattering gives extremely fast time constant of the order of a few tens femtoseconds, as well as the above elastic electron-electron scattering does.

cond-mat.mtrl-sci↗

arXiv2015

Real time relaxation dynamics of macroscopically photo-excited electrons toward the Fermi degeneracy formation in the conduction band of semiconductors

Concerning with the recent experiment of time-resolved two-photon photo-emission spectral measurements on semiconductors (GaAs, InP), we theoretically study real time relaxation dynamics of macroscopically photo-excited electrons, toward the Fermi degeneracy formation in an originally vacant conduction band of these semiconductors. Very soon after the photo-excitation, the whole electrons are shown to exhibit a quite rapid relaxation, like an avalanching phenomenon, mainly due to successive multi-(optical and acoustic) phonon emission from them. Repeating this multi-phonon process, the whole energy distribution of the electrons is shown to become a multi-peaked structure largely elongated over the lower part of the wide conduction band. However, after around 1 ps from the excitation, this relaxation critically slows down, since the emission of a long-wave acoustic phonon from electrons around the Fermi level becomes prohibitively difficult. By using the electron temperature approximation, we show that this slow relaxation is inversely proportional to time. Thus, the formation of the complete Fermi degeneracy takes an infinite time. These theoretical results are quite consistent to the aforementioned recent experiment.

cond-mat.mes-hall↗

arXiv2010

Proposal of Two-Pulse Excitation to Efficiently Photoinduced Novel $sp^3$ Nano Domain with Frozen Shear in Graphite Crystal

We propose a two-pulse excitation to efficiently photoinduced a novel $sp^3$-bonded nano-domain with a frozen shear in a graphite crystal. This $sp^3$ structure is known to be well stabilized by shear displacement between neighboring two graphite layers. This shear motion is generated only as a transient and unfrozen one by the first visible laser pulse, shone over the graphite crystal. While, the second pulse is proved to freeze it before it disappears, resulting in an efficient interlayer $σ$ ($sp^3$) bond formation.

cond-mat.mtrl-sci↗

arXiv2010

The dynamics of a qubit in a spin-boson environment: a comparison between analytical and numerical method

The dynamics of a qubit under the decoherence of a two level fluctuator (TLF) in addition to its coupling to a bosonic bath is investigated theoretically. Two different methods are applied and compared for this problem. One is a perturbation method based on a unitary transformation. With the merit of our unitary transformation, non-adiabatic effect can be taken into account efficiently. And the other one is the numerically exact method, namely the quasi-adiabatic path-integral (QUAPI) propagator technique. We find that the analytical method works well for a wide parameter range and show good agreement with QUAPI. On the other hand, The enhancement and the reduction of quantum decoherence of the qubit is checked with varying bath temperature $T$ and TLF-bath coupling.

quant-ph↗

arXiv2010

Comparison of analytical and numerical methods and the effect of bath coupling on the quantum decoherence

The dynamics of a qubit in a structured environment is investigated theoretically. One point of view of the model is the spin-boson model with a Lorentz shaped spectral density. An alternative view is a qubit coupled to harmonic oscillator (HO), which in turn coupled to a Ohmic environment. Two different methods are applied and compared for this problem. One is a perturbation method based on a unitary transformation. Since the transformed hamiltonian is of rotating wave approximation (RWA) form, we call it the transformed rotating wave approximation (TRWA) method. And the other one is the numerically exact method of the quasi-adiabatic propagator path-integral (QUAPI) method. TRWA method can be applied from the first point of view. And the QUAPI method can applied from both points of views. We find that from the 1st point of view QUAPI only works well for large $Γ$. Since the memory time is too long for the practical evaluation of QUAPI when $Γ$ is small. We call this treatment as QUAPI1. And from the 2nd point of view, QUAPI works well for small $Γ$, since the non-adiabatic effect become more important as $Γ$ increases, one need smaller time-step and more steps to obtain accurate result which also quickly runs out the computational resources. This treatment is called QUAPI2. We find that the TRWA method works well for the whole parameter range of $Γ$ and show good agreement with QUAPI1 and QUAPI2. On the other hand, we find that the decoherence of the qubit can be reduced with increasing coupling between HO and bath. This result may be relevant to the design of quantum computer.

quant-ph↗

arXiv2010

Cooperative domain type interlayer $sp^3$-bond formation in graphite

Using the classical molecular dynamics and the semiempirical Brenner's potential, we theoretically study the interlayer sigma bond formation, as cooperative and nonlinear phenomena induced by visible light excitations of a graphite crystal. We have found several cases, wherein the excitations of certain lattice sites result in new interlayer bonds even at non-excited sites. We have also found that, a new interlayer bond is easier to be formed around a bond, if it is already existing. As many more sites are going to be excited, the number of interlayer bonds increases nonlinearly with the number of excited sites. This nonlinearity shows 1.7 power of the total number of excited sites, corresponding to about three- or four-photon process.

cond-mat.mtrl-sci↗

arXiv2009

Quantum Monte Carlo study on speckle variation due to photorelaxation of ferroelectric clusters in paraelectric barium titanate

Time-dependent speckle pattern of paraelectric barium titanate observed in a soft x-ray laser pump-probe measurement is theoretically investigated as a correlated optical response to the pump and probe pulses. The scattering probability is calculated based on a model with coupled soft x-ray photon and ferroelectric phonon mode. It is found that the speckle variation is related with the relaxation dynamics of ferroelectric clusters created by the pump pulse. Additionally, critical slowing down of cluster relaxation arises on decreasing temperature towards the paraelectric-ferroelectric transition temperature. Relation between critical slowing down, local dipole fluctuation and crystal structure are revealed by quantum Monte Carlo simulation.

cond-mat.mtrl-sci↗

arXiv2008

Dynamics of ferroelectric nano cluster in BaTiO3 observed as a real time correlation between two soft X-ray laser pulses

We carry out a theoretical investigation to clarify the dynamic property of photo-created nano-sized ferroelectric cluster observed in the paraelectric BaTiO3 as a real time correlation of speckle pattern between two soft X-ray laser pulses, at just above the paraelectric-ferroelectric phase transition temperature. Based on a model with coupled soft X-ray photon and ferroelectric phonon mode, we study the time dependence of scattering probability by using a perturbative expansion approach. The cluster-associated phonon softening as well as central peak effects are well reproduced in the phonon spectral function via quantum Monte Carlo simulation. Besides, it is found that the time dependence of speckle correlation is determined by the relaxation dynamics of ferroelectric clusters. Near the transition point, cluster excitation is stable, leading to a long relaxation time. While, at high temperature, cluster structure is subject to the thermal fluctuation, ending up with a short relaxation time.

cond-mat.mtrl-sci↗

arXiv2008

Quantum pattern formation dynamics of photoinduced nucleation process

We study the dynamics of quantum pattern formation processes in molecular crystals which is a concomitant with photoinduced nucleation. Since the nucleation process in coherent regime is driven by the nonadiabatic transition in each molecule followed by the propagation of phonons, it is necessary to take into account the quantum nature of both electrons and phonons in order to pursue the dynamics of the system. Therefore, we employ a model of localized electrons coupled with a quantized phonon mode and solved the time-dependent Schrödinger equation numerically. We found that there is a minimal size of clusters of excited molecules which triggers the photoinduced nucleation process, i.e., nucleation does not take place unless sufficient photoexcitation energy is concentrated within a narrow area of the system. We show that this result means that the spatial distribution of photoexcited molecules plays an important role in the nonlinearity of the dynamics and also of the optical properties observed in experiments. We calculated the conversion ratio, the nucleation rate, and correlation functions to reveal the dynamical properties of the pattern formation process, and the initial dynamics of the photoinduced structural change is discussed from the viewpoint of pattern formation.

cond-mat.other↗

arXiv2007

Novel theoretical approach in photoemission spectroscopy: application to isotope effect and boron-doped diamond

A new path-integral theory is developed to calculate the photoemission spectra (PES) of correlated many-electron systems. The application to the study on Bi2Sr2CaCu2O8 (Bi2212) and boron-doped diamond (BDD) is discussed in details. It is found that the isotopic shift in the angle-resolved photoemission spectra of Bi2212 is due to the off-diagonal quadratic electron-phonon (e-ph) coupling, whereas the presence of electron-electron repulsion partially suppresses this effect. For the BDD, a semiconductor-metal phase transition, which is induced by increasing the e-ph coupling and dopant concentration, is reproduced by our theory. Additionally, the presence of Fermi edge and phonon step-like structure in PES is found to be due to a co-existence of itinerant and localized electronic states in BDD.

cond-mat.str-el↗

arXiv2007

Theoretical study on isotopic shift in angle-resolved photoemission spectra of Bi2Sr2CaCu2O8

We develop a path-integral theory to study the angle-resolved photoemission spectra (ARPES) of high-T_c superconductors based on a two-dimensional model for the CuO2 conduction plane, including both electron-electron (e-e) and electron-phonon (e-ph) interactions. Comparing our result with the experimental one of Bi2Sr2CaCu2O8, we find that the experimentally observed isotopic band shift in ARPES is due to the off-diagonal quadratic e-ph coupling, whereas the presence of e-e repulsion partially suppresses this effect.

cond-mat.str-el↗

arXiv2007

Isotopic shift in angle-resolved photoemission spectra of Bi2Sr2CaCu2O8 due to quadratic electron-phonon coupling

In connection with the experiment on oxygen isotope effect of Bi2Sr2CaCu2O8 with the angle-resolved photoemission spectroscopy (ARPES), we theoretically study the isotope-induced band shift in ARPES by the Hartree-Fork and quantum Monte Carlo methods. We find that this band shift can be clarified based on a quadratically coupled electron-phonon (e-ph) model. The large ratio of band shift versus phonon energy change is connected with the softening effect of phonon, and the positive-negative sign change is due to the momentum dependence of the e-ph coupling.

cond-mat.str-el↗

arXiv2007

Nonlinearity in the dynamics of photoinduced nucleation process

Nonlinear dynamics of photoinduced cooperative phenomena is studied by numerical calculations on a model of molecular crystals. We found that the photoinduced nucleation process is triggered only when certain amount of excitation energy is supplied in a narrow part of the system, i.e., there exists a smallest cluster of excited molecules which makes the nucleation possible. As a result, the portion of the cooperatively converted molecules is nonlinearly dependent on the photoexcitation strength, which has been observed in various materials.

physics.comp-ph↗

arXiv2007

Coherent dynamics of photoinduced nucleation processes

We study the dynamics of initial nucleation processes of photoinduced structural change of molecular crystals. In order to describe the nonadiabatic transition in each molecule, we employ a model of localized electrons coupled with a fully quantized phonon mode, and the time-dependent Schrödinger equation for the model is numerically solved. We found a minimal model to describe the nucleation induced by injection of an excited state of a single molecule in which multiple types of intermolecular interactions are required. In this model coherently driven molecular distortion plays an important role in the successive conversion of electronic states which leads to photoinduced cooperative phenomena.

cond-mat.mtrl-sci↗

arXiv2005

Theory of Photoinduced Phase Transitions

Theories of photoinduced phase transitions have developed along with the progress in experimental studies, especially concerning their nonlinear characters and transition dynamics. At an early stage, paths from photoinduced local structural distortions to global ones are explained in classical statistical models. Their dynamics are governed by transition probabilities and inevitably stochastic, but they were sufficient to describe coarse-grained time evolutions. Recently, however, a variety of dynamics including ultrafast ones are observed in different electronic states. They are explained in relevant electronic models. In particular, a coherent lattice oscillation and coherent motion of a macroscopic domain boundary need appropriate interactions among electrons and lattice displacements. Furthermore, some transitions proceed almost in one direction, which can be explained by considering relevant electronic processes. We describe the history of theories of photoinduced phase transitions and discuss a future perspective.

cond-mat.str-el↗