arXiv subjects

Hideo Hasegawa

Publications and source records attributed to Hideo Hasegawa.

At least 19 recordsLinked to original sources

arXiv2015

Effects of applied fields on quantum coupled double-well systems

Effects of time-dependent applied fields on quantum coupled double-well (DW) systems with Razavy's hyperbolic potential have been studied. By solving the Schrödinger equation for the DW system, we have obtained time-dependent occupation probabilities of the eigenstates, from which expectation values of positions $x_1$ and $x_2$ of particles ($ $), the correlation ($Γ(t)$) and the concurrence ($C(t)$) expressing a degree of the entanglement of the coupled DW system, are obtained. Analytical expressions for $ $, $Γ(t)$ and $C(t)$ are derived with the use of the rotating-wave approximation (RWA) for sinusoidal fields. Model calculations have indicated that $ $, $Γ(t)$ and $C(t)$ show very complicated time dependences. Results of the RWA are in good agreement with exact ones evaluated by numerical methods for cases of weak couplings and small applied fields in the near-resonant condition. Applications of our method to step fields are also studied.

quant-ph↗

arXiv2014

Dynamical properties of an exactly solvable coupled quantum double-well system: The evolution speed and entanglement

We have studied dynamical properties of an exactly solvable quantum coupled double-well (DW) systems with Razavy's hyperbolic potential. With the use of four kinds of initial wavepackets, the correlation function $Γ(t)$ and the concurrence $C(t)$ which is a typical measure of the entanglement in two qubits, are calculated. We obtain the orthogonality time $τ$ which signifies a time interval for an initial state to evolve to its orthogonal state, and the temporal average of $C(t)$, $C_{av}$ $(=\sqrt{\langle C(t)^2 \rangle})$. The coupling dependence of $τ$ and the concurrence [$C_{av}$ or $C(0)$], and the relation between $τ$ and the concurrence are investigated. Our calculations have shown that the evolution speed measured by $τ^{-1}$ is not necessarily increased with increasing the concurrence in coupled DW systems.

quant-ph↗

arXiv2014

Quantum tunneling and evolution speed in an exactly solvable coupled double-well system

Exact analytical calculations of eigenvalues and eigenstates are presented for quantum coupled double-well (DW) systems with Razavy's hyperbolic potential. With the use of four kinds of initial wavepackets, we have calculated the tunneling period $T$ and the orthogonality time $τ$ which signifies a time interval for an initial state to evolve to its orthogonal state. We discuss the coupling dependence of $T$ and $τ$, and the relation between $τ$ and the concurrence $C$ which is a typical measure of the entanglement in two qubits. Our calculations have shown that it is not clear whether the speed of quantum evolution may be measured by $T$ or $τ$ and that the evolution speed measured by $τ$ (or $T$) is not necessarily increased with increasing $C$. This is in contrast with the earlier study [V. Giovannetti, S. Lloyd and L. Maccone, Europhys. Lett. {\bf 62} (2003) 615] which pointed out that the evolution speed measured by $τ$ is enhanced by the entanglement in the two-level model.

quant-ph↗

arXiv2014

Wavepacket dynamics in a quantum double-well system with Razavy's potential coupled to a harmonic oscillator

We have studied wavepacket dynamics in the Razavy hyperbolic double-well (DW) potential which is coupled to a harmonic oscillator (HO) by linear and quadratic interactions. Taking into account the lowest two states of DW and $(N+1)$ states of HO ($N=1$ to 10), we evaluate eigenvalues and eigenfunctions of the composite system. An analytical calculation is made for $N=1$ and numerical calculations are performed for $1 < N \leq 10$. Quantum tunneling of wavepackets is realized between two bottoms of composite potential $U(x,y)$ where $x$ and $y$ denote coordinates in DW and HO potentials, respectively. It has been shown that with increasing $N$ and/or the coupling strength, the tunneling period is considerably increased. Phase space plots of $\langle x \rangle$ vs. $\langle p_x \rangle$ and $\langle y \rangle$ vs. $\langle p_y \rangle$ are elliptic, where $\langle \cdot \rangle$ denotes an expectation value for the two-term wavepacket. This result is quite different from the relevant one previously obtained for the quartic DW potential with the use of the quantum phase space representation [Babyuk, arXiv:0208070]. Similarity and difference between results calculated for linear and quadratic couplings, and the uncertainty relation in the model are discussed.

quant-ph↗

arXiv2013

Validity of the time-dependent variational approximation to the Gaussian wavepacket method applied to double-well systems

We have examined the validity of the time-dependent variational approximation (TDVA) to the Gaussian wavepacket method (GWM) for quantum double-well (DW) systems, by using the quasi-exact spectral method (SM). Comparisons between results of wavefunctions, averages of position and momentum, the auto-correlation function, and an uncertainty product calculated by SM and TDVA have been made. It has been shown that a given initial Gaussian wavepacket in SM is quickly deformed at $t > 0$ where a wavepacket cannot be expressed by a {\it single} Gaussian, and that assumptions on averages of higher-order fluctuations in TDVA are not justified. These results cast some doubt on an application of TDVA to DW systems. Gaussian wavepacket dynamics in anharmonic potential systems is studied also.

quant-ph↗

arXiv2013

Bound states of the one-dimensional Dirac equation for scalar and vector double square-well potentials

We have analytically studied bound states of the one-dimensional Dirac equation for scalar and vector double square-well potentials (DSPs), by using the transfer-matrix method. Detailed numerical calculations of the eigenvalue, wave function and density probability have been performed for the three cases: (1) vector DSP only, (2) scalar DSP only, and (3) scalar and vector DSPs with equal magnitudes. We discuss the difference and similarity among results of the cases (1)-(3) in the Dirac equation and that in the Schrödinger equation. Motion of a wave packet is calculated for a study on quantum tunneling through the central barrier in the DSP.

quant-ph↗

arXiv2013

Gaussian wavepacket dynamics and quantum tunneling in asymmetric double-well systems

We have studied dynamical properties and quantum tunneling in asymmetric double-well (DW) systems, by solving Schrödinger equation with the use of two kinds of spectral methods for initially squeezed Gaussian wavepackets. Time dependences of wavefunction, averages of position and momentum, the auto-correlation function, an uncertainty product and the tunneling probability have been calculated. Our calculations have shown that (i) the tunneling probability is considerably reduced by a potential asymmetry $ΔU$, (ii) a resonant tunneling with $|ΔU| \simeq κ\:\hbar ω$ is realized for motion starting from upper minimum of asymmetric potential wells, but not for motion from lower minimum, ($κ=0,1,2,...$; $ω$: oscillator frequency at minima), (iii) the reduction of the tunneling probability by an asymmetry is less significant for the Gaussian wavepacket with narrower width, and (iv) the uncertainty product $<δx^2> <δp^2>$ in the resonant tunneling state is larger than that in the non-resonant tunneling state. The item (ii) is in contrast with the earlier study [Mugnai {\it et al.}, Phys. Rev. A {\bf 38} (1987) 2182] which showed the symmetric result for motion starting from upper and lower minima.

quant-ph↗

arXiv2013

Stochastic resonance in bistable systems with nonlinear dissipation and multiplicative noise: A microscopic approach

The stochastic resonance (SR) in bistable systems has been extensively discussed with the use of {\it phenomenological} Langevin models. By using the {\it microscopic}, generalized Caldeira-Leggett (CL) model, we study in this paper, SR of an open bistable system coupled to a bath with a nonlinear system-bath interaction. The adopted CL model yields the non-Markovian Langevin equation with nonlinear dissipation and state-dependent diffusion which preserve the fluctuation-dissipation relation (FDR). From numerical calculations, we find the following: (1) the spectral power amplification (SPA) exhibits SR not only for $a$ and $b$ but also for $τ$ while the stationary probability distribution function is independent of them where $a$ ($b$) denotes the magnitude of multiplicative (additive) noise and $τ$ expresses the relaxation time of colored noise; (2) the SPA for coexisting additive and multiplicative noises has a single-peak but two-peak structure as functions of $a$, $b$ and/or $τ$. These results (1) and (2) are qualitatively different from previous ones obtained by phenomenological Langevin models where the FDR is indefinite or not held.

cond-mat.stat-mech↗

arXiv2012

Specific heats of quantum double-well systems

Specific heats of quantum systems with symmetric and asymmetric double-well potentials have been calculated. In numerical calculations of their specific heats, we have adopted the combined method which takes into account not only eigenvalues of $ε_n$ for $0 \leq n \leq N_m$ obtained by the energy-matrix diagonalization but also their extrapolated ones for $N_m+1 \leq n < \infty$ ($N_m=20$ or 30). Calculated specific heats are shown to be rather different from counterparts of a harmonic oscillator. In particular, specific heats of symmetric double-well systems at very low temperatures have the Schottky-type anomaly, which is rooted to a small energy gap in low-lying two-level eigenstates induced by a tunneling through the potential barrier. The Schottky-type anomaly is removed when an asymmetry is introduced into the double-well potential. It has been pointed out that the specific-heat calculation of a double-well system reported by Feranchuk, Ulyanenkov and Kuz'min [Chem. Phys. 157, 61 (1991)] is misleading because the zeroth-order operator method they adopted neglects crucially important off-diagonal contributions.

cond-mat.stat-mech↗

arXiv2012

Classical double-well systems coupled to finite baths

We have studied properties of a classical $N_S$-body double-well system coupled to an $N_B$-body bath, performing simulations of $2(N_S+N_B)$ first-order differential equations with $N_S \simeq 1 - 10$ and $N_B \simeq 1 - 1000$. A motion of Brownian particles in the absence of external forces becomes chaotic for appropriate model parameters such as $N_B$, $c_o$ (coupling strength), and $\{ω_n\}$ (oscillator frequency of bath): For example, it is chaotic for a small $N_B$ ($\lesssim 100$) but regular for a large $N_B$ ($\gtrsim 500$). Detailed calculations of the stationary energy distribution of the system $f_S(u)$ ($u$: an energy per particle in the system) have shown that its properties are mainly determined by $N_S$, $c_o$ and $T$ (temperature) but weakly depend on $N_B$ and $\{ω_n \}$. The calculated $f_S(u)$ is analyzed with the use of the $Γ$ distribution. Difference and similarity between properties of double-well and harmonic-oscillator systems coupled to finite bath are discussed.

cond-mat.stat-mech↗

arXiv2012

Responses of small quantum systems subjected to finite baths

We have studied responses to applied external forces of the quantum $(N_S+N_B)$ model for $N_S$-body interacting harmonic oscillator (HO) system subjected to $N_B$-body HO bath, by using canonical transformations combined with Husimi's method for a driven quantum HO [K. Husimi, Prog. Theor. Phys. {\bf 9}, 381 (1953)]. It has been shown that the response to a uniform force expressed by the Hamiltonian: $H_f= -f(t) \sum_{k=1}^{N_S} Q_k$ is generally not proportional to $N_S$ except for no system-bath couplings, where $f(t)$ expresses its time dependence and $Q_k$ denotes a position operator of $k$th particle of the system. We have calculated also the response to a space- and time-dependent force expressed by $H_f= -f(t) \sum_{k=1}^{N_S} Q_k \: e^{i 2 πk u/N_S}$, where the wavevector $u$ is $u=0$ and $u=-N_S/2$ for uniform and staggered forces, respectively. The spatial correlation $Γ_m$ for a pair of positions of $Q_k$ and $Q_{k+m}$ has been studied as functions of $N_S$ and the temperature. Our calculations have indicated an importance of taking account of finite $N_S$ in studying quantum open systems which generally include arbitrary numbers of particles.

cond-mat.stat-mech↗

arXiv2011

The Jarzynski equality in van der Pol and Rayleigh oscillators

We have studied the Jarzynski equality (JE) in van der Pol and Rayleigh oscillators which are typical deterministic non-Hamiltonian models, but not expected to rigorously satisfy the JE because they are not microscopically reversible. Our simulations that calculate the contribution to the work $W$ of an applied ramp force with a duration $τ$ show that the JE approximately holds for a fairly wide range of $τ$ including $τ\rightarrow 0 $ and $τ\rightarrow \infty$, except for $τ\sim T$ where $T$ denotes the period of relaxation oscillations in the limit cycle. The work distribution function (WDF) is shown to be non-Gaussian with the $U$-shaped structure for a strong damping parameter. The $τ$ dependence of $R$ $(=- k_B T \ln )$ obtained by our simulations is semi-quantitatively elucidated with the use of a simple expression for limit-cycle oscillations, where the bracket $<\cdot>$ expresses an average over the WDF. The result obtained in self-excited oscillators is in contrast with the fact that the JE holds in the Nosé-Hoover oscillator which also belongs to deterministic non-Hamiltonian models.

cond-mat.stat-mech↗

arXiv2011

Specific heat anomalies of small quantum systems subjected to finite baths

We have studied the specific heat of the $(N_S+N_B)$ model for an $N_S$-body harmonic oscillator (HO) system which is strongly coupled to an $N_B$-body HO bath without dissipation. The system specific heat of $C_S(T)$ becomes $N_S k_B$ at $T \rightarrow \infty$ and vanishes at $T = 0$ in accordance with the third law of thermodynamics. The calculated $C_S(T)$ at low temperatures is not proportional to $N_S$ and shows an anomalous temperature dependence, strongly depending on $N_S$, $N_B$ and the system-bath coupling. In particular at very low (but finite) temperatures, it may become {\it negative} for a strong system-bath coupling, which is in contrast with {\it non-negative} specific heat of an HO system with $N_S=1$ reported by G-L. Ingold, P. Hänggi and P. Talkner [Phys. Rev. E {\bf 79}, 061105 (2005)]. Our calculation indicates an importance of taking account of finite $N_S$ in studying open quantum systems which may include an arbitrary number of particles in general.

cond-mat.stat-mech↗

arXiv2011

Classical open systems with nonlinear nonlocal dissipation and state-dependent diffusion: Dynamical responses and the Jarzynski equality

We have studied dynamical responses and the Jarzynski equality (JE) of classical open systems described by the generalized Caldeira-Leggett model with the nonlocal system-bath coupling. In the derived non-Markovian Langevin equation, the nonlinear nonlocal dissipative term and state-dependent diffusion term yielding multiplicative colored noise satisfy the fluctuation-dissipation relation. Simulation results for harmonic oscillator systems have shown the following: (a) averaged responses of the system $< x >$ to applied sinusoidal and step forces significantly depend on model parameters of magnitudes of additive and multiplicative noises and the relaxation time of colored noise, although stationary marginal probability distribution functions are independent of them, (b) a combined effect of nonlinear dissipation and multiplicative colored noise induces enhanced fluctuations $< [x-< x>]^2 >$ for an applied sinusoidal force, and (c) the JE holds for an applied ramp force independently of the model parameters with a work distribution function which is (symmetric) Gaussian and asymmetric non-Gaussian for additive and multiplicative noises, respectively. It has been shown that the non-Markovian Langevin equation in the local and over-damped limits is quite different from the widely adopted phenomenological Markovian Langevin equation subjected to multiplicative noise.

cond-mat.stat-mech↗

arXiv2011

Comment on "Energy and information in Hodgkin-Huxley neurons"

In a recent paper [A. Moujahid, A. d'Anjou, F. J. Torrealdea and F. Torrealdea, Phys. Rev. E {\bf 83}, 031912 (2011)], the authors have calculated the energy consumed in firing neurons by using the Hodgkin-Huxley (HH) model. The energy consumption rate adopted for the HH model yields a {\it negative} energy consumption meaning an energy transfer from an HH neuron to a source which is physically strange, although they have interpreted it as a biochemical energy cost. I propose an alternative expression for the power consumption which leads to a {\it positive} energy consumed in an HH neuron, presenting some model calculations which are compared to those in their paper.

cond-mat.dis-nn↗

arXiv2011

Responses to applied forces and the Jarzynski equality in classical oscillator systems coupled to finite baths: An exactly solvable non-dissipative non-ergodic model

Responses of small open oscillator systems to applied external forces have been studied with the use of an exactly solvable classical Caldeira-Leggett (CL) model in which a harmonic oscillator (system) is coupled to finite $N$-body oscillators (bath) with an identical frequency ($ω_n=ω_o$ for $n=1$ to $N$). We have derived exact expressions for positions, momenta and energy of the system in nonequilibrium states and for work performed by applied forces. Detailed study has been made on an analytical method for canonical averages of physical quantities over the initial equilibrium state, which is much superior than numerical averages commonly adopted in simulations of small systems. The calculated energy of the system which is strongly coupled to finite bath is fluctuating but non-dissipative. It has been shown that the Jarzynski equality (JE) is valid in non-dissipative, non-ergodic open oscillator systems regardless of the rate of applied ramp force.

cond-mat.stat-mech↗

arXiv2010

Classical small systems coupled to finite baths

We have studied the properties of a classical $N_S$-body system coupled to a bath containing $N_B$-body harmonic oscillators, employing an $(N_S+N_B)$ model which is different from most of the existing models with $N_S=1$. We have performed simulations for $N_S$-oscillator systems, solving $2(N_S+N_B)$ first-order differential equations with $N_S \simeq 1 - 10$ and $N_B \simeq 10 - 1000$, in order to calculate the time-dependent energy exchange between the system and the bath. The calculated energy in the system rapidly changes while its envelope has a much slower time dependence. Detailed calculations of the stationary energy distribution of the system $f_S(u)$ ($u$: an energy per particle in the system) have shown that its properties are mainly determined by $N_S$ but weakly depend on $N_B$. The calculated $f_S(u)$ is analyzed with the use of the $Γ$ and $q$-$Γ$ distributions: the latter is derived with the superstatistical approach (SSA) and microcanonical approach (MCA) to the nonextensive statistics, where $q$ stands for the entropic index. Based on analyses of our simulation results, a critical comparison is made between the SSA and MCA. Simulations have been performed also for the $N_S$-body ideal-gas system. The effect of the coupling between oscillators in the bath has been examined by additional ($N_S+N_B$) models which include baths consisting of coupled linear chains with periodic and fixed-end boundary conditions.

cond-mat.stat-mech↗

arXiv2010

Thermal entanglement of Hubbard dimers in the nonextensive statistics

The thermal entanglement of the Hubbard dimer (two-site Hubbard model) has been studied with the nonextensive statistics. We have calculated the auto-correlation ($O_q$), pair correlation ($L_q$), concurrence ($Γ_q$) and conditional entropy ($R_q$) as functions of entropic index $q$ and the temperature $T$. The thermal entanglement is shown to considerably depend on the entropic index. For $q < 1.0$, the threshold temperature where $Γ_q$ vanishes or $R_q$ changes its sign is more increased and the entanglement may survive at higher temperatures than for $q=1.0$. Relations among $L_q$, $Γ_q$ and $R_q$ are investigated. The physical meaning of the entropic index $q$ is discussed with the microcanonical and superstatistical approaches. The nonextensive statistics is applied also to Heisenberg dimers.

cond-mat.stat-mech↗