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Progress of Theoretical Physics

Exact Time Evolution of a Quantum Harmonic-Oscillator Chain

J. M. Hong

Published Aug 1, 1989• 1 citation
Publisher

Abstract

We generalize the classical time evolution problem of a harmonic-oscillator chain with periodic and fixed-end boundary conditions to the quantum mechanical case. It is shown explicitly that the high-temperature limits of our quantum mechanical treatment give the known classical results. We also show that the finite diffusion constant for the periodic chain arises from the zero-frequency normal mode which represents the rotational mode of the periodic chain, and the diffusion constant vanishes as tempperature approaches zero. The zero-frequency normal mode is also responsible for the divergence of the mean square displacement of a tagged oscillator. On the other hand, the absence of the zero-frequency normal mode in a fixed-end chain gives rise to zero diffusion constant and finite mean square displacement. The finiteness of diffusion constant and the divergence of mean square displacement in this model are responsible for the symmetry of the system.

DOI
10.1143/ptp.82.291

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We generalize the classical time evolution problem of a harmonic-oscillator chain with periodic and fixed-end boundary conditions to the quantum mechanical case. It is shown explicitly that the high-temperature limits of our quantum mechanical treatment give the known classical results. We also show that the finite diffusion constant for the periodic chain arises from the zero-frequency normal mode which represents the rotational mode of the periodic chain, and the diffusion constant vanishes as tempperature approaches zero. The zero-frequency normal mode is also responsible for the divergence of the mean square displacement of a tagged oscillator. On the other hand, the absence of the zero-frequency normal mode in a fixed-end chain gives rise to zero diffusion constant and finite mean square displacement. The finiteness of diffusion constant and the divergence of mean square displacement in this model are responsible for the symmetry of the system.
The nonzero ground-state energy of the quantum mechanical harmonic oscillator implies quantum fluctuations around the minimum of the potential with the mean-square value proportional to Planck's constant. On the other hand, thermal fluctuations occur when the oscillator is coupled to a heat bath. Using quantum statistical mechanics, this paper describes the transition from pure quantum fluctuations at zero temperature to classical thermal fluctuations in the high-temperature limit. It was early pointed out by Peierls that the classical mean-square thermal fluctuations in a harmonic chain—a linear chain of coupled harmonic oscillators—increase linearly with the distance of the masses from the fixed end of the chain, destroying long-range crystalline order. As shown here, the corresponding quantum fluctuations lead to a much slower logarithmic increase with the distance from the end. It is also shown that this behavior implies, for example, the absence of sharp Bragg peaks in x-ray scattering in an infinite chain at zero temperature, which instead shows power-law behavior typical for one-dimensional quantum liquids (called Luttinger liquids).
We investigate the time evolution of a coupled harmonic-oscillator chain under two boundary conditions: two ends fixed and one end fixed. The dynamics of the coupled chain and the steady variances of the coordinates are explicitly analyzed by the entire Hamiltonian using a diagonalization approach. Our result shows the desirable symmetry for the case with two ends fixed. In particular, a Langevin simulation technique is proposed to sample the harmonic chain across the entire equilibrium distribution.
The problem of the quantum description of the damping of a 1D chain of coupled oscillators with periodic boundary conditions in the Markovian regime is solved. The equations of motion of the normal modes are derived, and the evolution in time of their mean values and variances is explicitly studied. Furthermore, we investigate the effect of the diffusion and the friction on the dynamics of the chain. It is shown that when the off-diagonal elements of the diffusion and friction tensors are taken into account, the most notable contribution of these processes is observed in the mode in which all the oscillators undergo an in-phase vibration.
The dynamics of the harmonic oscillator, with a fluctuating, time-dependent frequency, is studied in the quantum framework. For periodic fluctuations of period \ensuremath{\tau}, it is found that the mean energy diverges exponentially in time, provided \ensuremath{\tau} falls into certain ``bands,'' centered around ``quasiresonant'' values, for which the rate of increase is maximum. In agreement with the correspondence principle, this rate is shown to coincide exactly with the one obtained for the classic oscillator, in the same conditions [Loris Ferrari, Phys. Rev. B 56, 593 (1997)]. The notion of adiabaticity has been revisited in the case of exponentially diverging mean energy. It is found that the Heisenberg form of the Hamiltonian operator can be expressed as a new effective Hamiltonian, which is still harmonic, but has exponentially diverging coefficients. It is argued that the same results hold in general (that is, without any quasiresonance condition) if the frequency fluctuation in time is disordered. A brief discussion is presented about nonharmonic interactions leading to dissipative effects. Applications to real physical systems are outlined, with special reference to the electrons in a uniform and constant magnetic field, to which a small fluctuating component is added.
We study the dissipative quantum harmonic oscillator with general nonthermal preparations of the harmonic oscillator bath. The focus is on equilibration of the oscillator in the long-time limit and the additional requirements for thermalization. Our study is based on the exact solution of the microscopic model obtained by means of operator equations of motion, which provides us with the time evolution of the central oscillator density matrix in terms of the propagating function. We find a hierarchy of conditions for thermalization, together with the relation of the asymptotic temperature to the energy distribution in the initial bath state. We discuss the presence and absence of equilibration for the example of an inhomogeneous chain of harmonic oscillators, and we illustrate the general findings about thermalization for the nonthermal environment that results from a quench.
We describe a quantum system consisting of a one-dimensional linear chain of n identical harmonic oscillators coupled by a nearest neighbor interaction. Two boundary conditions are taken into account: periodic boundary conditions (where the nth oscillator is coupled back to the first oscillator) and fixed wall boundary conditions (where the first oscillator and the nth oscillator are coupled to a fixed wall). The two systems are characterized by their Hamiltonian. For their quantization, we treat these systems as Wigner quantum systems (WQSs), allowing more solutions than just the canonical quantization solution. In this WQS approach, one is led to certain algebraic relations for operators (which are linear combinations of position and momentum operators) that should satisfy triple relations involving commutators and anti-commutators. These triple relations have a solution in terms of the Lie superalgebra gl(1|n). We study a particular class of gl(1|n) representations V(p), the so-called ladder representations. For these representations, we determine the spectrum of the Hamiltonian and of the position operators (for both types of boundary conditions). Furthermore, we compute the eigenvectors of the position operators in terms of stationary states. This leads to explicit expressions for position probabilities of the n oscillators in the chain. An analysis of the plots of such position probability distributions gives rise to some interesting observations. In particular, the physical behavior of the system as a WQS is very much in agreement with what one would expect from the classical case, except that all physical quantities (energy, position, and momentum of each oscillator) have a finite spectrum.