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1.
Fermi-Pasta-Ulam (FPU) β格点链中能量输运的载流子是孤子还是声子一直存在较多的争议. 本文通过单脉冲方法, 明确了一个能量波包在该格点链系统中从声子波包转变成为孤子波包的条件, 即波包能量达到一定阈值. 基于纯四次势链的声子真空效应, 构造了由FPU- β链与纯四次势链构成的双段链系统. 通过对比研究双段链系统和单段FPU- β链中的热流, 发现低温下声子是FPU- β链中能量的主要载流子, 而随着温度的升高孤子逐步取代声子成为能量的主要载流子.
关键词:
Fermi-Pasta-Ulam格点链
声子
孤子
热传导 相似文献
2.
The problem of how single central spins interact with a nuclear spin bath is essential for understanding decoherence and relaxation in many quantum systems, yet is highly nontrivial owing to the many-body couplings involved. Different models yield widely varying time scales and dynamical responses (exponential, power-law, gaussian, etc.). Here we detect the small random fluctuations of central spins in thermal equilibrium [holes in singly charged (In,Ga)As quantum dots] to reveal the time scales and functional form of bath-induced spin relaxation. This spin noise indicates long (400 ns) spin correlation times at a zero magnetic field that increase to ~5 μs as dominant hole-nuclear relaxation channels are suppressed with small (100 G) applied fields. Concomitantly, the noise line shape evolves from Lorentzian to power law, indicating a crossover from exponential to slow [~1/log(t)] dynamics. 相似文献
3.
We investigate energy localization and transport in the form of discrete breathers and their movability in two-dimensional
Fermi–Pasta–Ulam(FPU) lattices. We study the dynamics of the two-dimensional Fermi–Pasta–Ulam(FPU) lattice, incorporating
the complicated effects of geometry, long-range interactions as well as nonlinear dispersion. We obtain several exact discrete
breather(DB) solutions, such as the odd-parity and even-parity DBs, compact-like DBs and moving DBs for various geometries
of the two-dimensional FPU chain. We show that DBs also exist in the same lattice in presence of next-nearest neighbour interaction.
Large-amplitude exact subsonic travelling kink-soliton solutions are obtained in such a periodic chain in presence of long-range
nonlinear dispersive interaction in the long-wavelength and weakly nonlinear limit. Such a two-dimensional FPU lattice admits
finite amplitude nonlinear sinusoidal wave (NSW) solutions with short commensurate as well as incommensurate wavelengths for
different geometries of the chain. The usefulness of these solutions for energy localization and transport in various physical
systems are discussed. 相似文献
4.
We investigate how generic the onset of chaos in interacting many-body classical systems is in the context of lattices of classical spins with nearest-neighbor anisotropic couplings. Seven large lattices in different spatial dimensions were considered. For each lattice, more than 2000 largest Lyapunov exponents for randomly sampled Hamiltonians were numerically computed. Our results strongly suggest the absence of integrable nearest-neighbor Hamiltonians for the infinite lattices except for the trivial Ising case. In the vicinity of the Ising case, the largest Lyapunov exponents exhibit a power-law growth, while further away they become rather weakly sensitive to the Hamiltonian anisotropy. We also provide an analytical derivation of these results. 相似文献
5.
We study a model in which a Hubbard Hamiltonian is coupled to the dispersive phonons in a classical nonlinear lattice. Our calculations are restricted to the case where we have only two quasi-particles of opposite spins, and we investigate the dynamics when the second quasi-particle is added to a state corresponding to a minimal energy single quasi-particle state. Depending on the parameter values, we find a number of interesting regimes. In many of these, discrete breathers (DBs) play a prominent role with a localized lattice mode coupled to the quasiparticles. Simulations with a purely harmonic lattice show much weaker localization effects. Our results support the possibility that DBs are important in HTSC.Received: 14 August 2004, Published online: 26 November 2004PACS:
71.38.-k Polarons and electron-phonon interactions - 63.20.Pw Localized modes - 63.20.Ry Anharmonic lattice modes 相似文献
6.
We study a two-dimensional (2D) diatomic lattice of anharmonic
oscillators with only quartic nearest-neighbor interactions, in
which discrete breathers (DBs) can be explicitly constructed by an
exact separation of their time and space dependence. DBs can stably
exist in the 2D discrete diatomic Klein--Gordon lattice with hard
and soft on-site potentials. When a parametric driving term is
introduced in the factor multiplying the harmonic part of the
on-site potential of the system, we can obtain the stable
quasiperiodic discrete breathers (QDBs) and chaotic discrete
breathers (CDBs) by changing the amplitude of the driver. But the
DBs and QDBs with symmetric and anti-symmetric profiles that are
centered at a heavy atom are more stable than at a light atom,
because the frequencies of the DBs and QDBs centered at a heavy atom
are lower than those centered at a light atom. 相似文献
7.
The self-consistent harmonic approximation is applied to the classical two-dimensional anisotropic XY (planar rotator) model on the square lattice. From the effective spin-wave stiffness constant data the phase diagram is constructed and the corresponding lattice dimensionally crossover exponents are defined and found. The variation of the stiffness constant jump with the anisotropy magnitude is obtained and the scaling functions ε( X) are extracted. 相似文献
8.
The fully symmetric A 1g phonons of bismuth have been investigated at liquid-helium temperature by the coherent control method for various crystal
excitation levels. It has been found that large-amplitude coherent phonons exhibit the “rigidity” of the phase, which is absent
at a small amplitude. The impossibility of changing the phase of phonon oscillations appears at the excitation levels at which
their amplitude relaxation law changes from exponential to power. The modification of the phase properties and relaxation
law of the excitations of the crystal lattice can be understood in terms of the concept of the condensation of phonons, which
occurs with an increase in the crystal excitation level. 相似文献
9.
We study the temperature dependence of bottomonium for temperatures in the range 0.4T(c) < T < 2.1T(c), using nonrelativistic dynamics for the bottom quark and full relativistic lattice QCD simulations for Nf = 2 light flavors on a highly anisotropic lattice. We find that the Υ is insensitive to the temperature in this range, while the χb propagators show a crossover from the exponential decay characterizing the hadronic phase to a power-law behavior consistent with nearly free dynamics at T ? 2T(c). 相似文献
10.
We consider diffusion of vibrations in random lattices with translational invariance. Above the frequency ω IR corresponding to the Ioffe-Regel crossover (and depending on the strength of disorder), phonons cannot propagate through the lattice and transfer energy. At the same time, most of the vibrations in this range are not localized. We show that these delocalized excitations are similar to diffusons introduced by P. B. Allen, J. L. Feldman, J. Fabian, and F. Wooten (see, e.g., Phil. Mag. B 79, 1715 (1999)) to describe heat transport in glasses. In this range the energy in the lattice is transferred by means of diffusion of vibrational excitations. We have calculated the diffusivity of the modes D(ω) using both the direct numerical solution of Newton equations and the Edwards-Thouless formula. It is nearly constant above ω IR and goes to zero at the localization threshold. 相似文献
11.
Energy relaxation time of hot electrons in Hg 0.8Cd 0.2Te at a lattice temperature of 1.5 K is calculated for quantizing magnetic flux densities of 4 and 6 T considering acoustic scattering via deformation potential and piezoelectric couplings. Band nonparabolicity, nonequipartition of phonons, screening of the scattering rates due to carrier space charge, and low-temperature broadening of the Landau level are incorporated. The calculated results agree qualitatively with experimental data. 相似文献
12.
Heat conduction in three-dimensional anharmonic lattices was numerically studied by the Green-Kubo theory. For a given lattice width W, a dimensional crossover is generally observed to occur at a W-dependent threshold of the lattice length. Lattices shorter than W will display a 3D behavior while lattices longer than W will display a 1D behavior. In the 3D regime, the heat current autocorrelation function was found to show a power-law decay as a function of the time lag τ as τ^{β} with β=-1.2. This indicates normal heat conduction. However, the decay exponent deviates significantly from the conventional theoretical value of β=-1.5. A flat power spectrum S(ω) of the global heat current in the low-frequency limit was also observed in the 3D regime. This provides not only an alternative verification of normal heat conduction but also a clear physical insight into its origin. 相似文献
13.
Structural transformations in nickel films exposed to N + ions were investigated through the use of electron diffraction, electron microscopy, Rutherford backscattering (RBS) of He + ions, thermal desorption spectroscopy (TDS). With an increasing exposure dose, there occurred a smooth transition of the FCC structure of Ni to the HCP phase -Ni3N with the lattice parameters a=0.266 nm and c=0.430 nm. The completion of the transition was observed once the implanted nitrogen concentration corresponding to the stoichiometric ratio of Ni3N was attained, this giving evidence for the chemical nature of the structural change observed. During annealing, structure variations were traced, enabling one to observe the following four nickel nitrides: (1) -Ni3N with a disordered arrangement of nitrogen atoms (HCP-phase with a=0.266 nm and c=0.430 nm), (2) β-Ni3N with an ordered arrangement of nitrogen atoms (two hexagonal lattices: one with a=0.266 nm, c=0.430 nm, and the other with a=0.466 nm and c=0.43 nm), (3) Ni4N having a primitive cubic lattice structure with a=0.377 nm, (4) Ni8N having the FCC structure with a=0.725 nm. The results of an investigation have led to building up a phase diagram for the Ni–N system. 相似文献
14.
研究了原子在光晶格偶极势依赖原子动量情况下的运动, 特别考虑了偶极势对原子动量的依赖特性. 对动量和位置的方差研究表明, 原子的动量方差呈现压缩性质, 位置方差呈现放大性质.据此我们预言光晶格动量依赖偶极势中的单粒子态可能接近动量压缩线态. 研究结果还表明, 红失谐情况下原子的动量演化可分为三个过程: 第一个过程是慢减速过程, 初始动量较大的原子, 动量以近似阻尼振荡的形式衰减; 第二个过程是快减速过程, 当动量被减速到接近到光子动量时, 动量迅速减小到 hk( Ω/ γ) 2, 其中 hk为光子动量, Ω为拉比频率, γ为原子波函数衰减函数; 第三个过程是原子被囚禁过程, 当原子动能被降低到小于势井深度时, 原子被囚禁在晶格波腹附近.
关键词:
动量演化
光晶格
压缩态 相似文献
15.
We report 63,65Cu spin–lattice relaxation rates measured by nuclear quadrupole resonance (NQR) in the delafossite compound CuYO 2 and CuYO 2:Ca over a temperature range from 200 to 450 K. CuYO 2:Ca is a prototype transparent oxide exhibiting p-type electrical conductivity. Relaxation rates in CuYO 2:Ca are enhanced by one to two orders of magnitude relative to undoped material, exhibit much stronger temperature dependence, and contain contributions from magnetic and quadrupolar relaxation mechanisms with roughly equal strengths. Relaxation in undoped CuYO 2 is of purely quadrupolar origin and is attributed to interactions with lattice phonons. The main focus of this paper is the magnetic contribution to the relaxation rate in CuYO 2:Ca which is attributed to the hyperfine fields of carriers. It is argued that the dynamics of the hyperfine field are dominated by the hopping rate for carrier transfers between neighboring atoms in the copper planes of the delafossite structure. Comparison of the magnetic relaxation rates with the DC conductivity permits an estimate of the carrier concentration and mobility. 相似文献
16.
Under investigation in this paper is a relativistic Toda lattice system with one perturbation parameter α abbreviated as RTL_(α) system by Suris, which may describe the motions of particles in lattices interacting through an exponential interaction force. First of all, an integrable lattice hierarchy associated with an RTL_(α) system is constructed, from which some relevant integrable properties such as Hamiltonian structures, Liouville integrability and conservation laws are investigated. Secondly, the discrete generalized(m, 2 N-m)-fold Darboux transformation is constructed to derive multi-soliton solutions, higher-order rational and semirational solutions, and their mixed solutions of an RTL_(α) system. The soliton elastic interactions and details of rational solutions are analyzed via the graphics and asymptotic analysis. Finally, soliton dynamical evolutions are investigated via numerical simulations,showing that a small noise has very little effect on the soliton propagation. These results may provide new insight into nonlinear lattice dynamics described by RTL_(α) system. 相似文献
17.
Anharmonically coupled, high-and low-frequency branches of vibrational excitations of a two-dimensional dipole system on a
substrate are studied. It is shown that independent low-frequency resonance modes characterized by a two-dimensional wave
vector belonging to the first Brillouin zone of the dipole lattice arise as a result of the harmonic interaction of phonons
in the dipole system and in the substrate. The positions and widths of the spectral lines of the high-frequency local vibrations,
which in the general case depend on the dispersion laws and lifetimes of the resonance modes, are calculated for low-temperature
orientational states of degenerate dipoles on triangular and square lattices.
Fiz. Tverd. Tela (St. Petersburg) 40, 152–155 (January 1998) 相似文献
18.
Studies on thermal diffusion of lattice solitons in Fermi-Pasta-Ulam (FPU)-like lattices
were recently generalized to the
case of dispersive long-range interactions (LRI) of the Kac-Baker form.
The variance of the soliton position shows a stronger than linear
time-dependence (superdiffusion) as found earlier for lattice solitons on FPU chains with
nearest-neighbour interactions (NNI). Since the superdiffusion seems to be generic for nontopological solitons, we want to
illuminate the role of the soliton shape on the superdiffusive mechanism.
Therefore, we concentrate on an FPU-like lattice with a certain class of power-law long-range interactions where the solitons
have algebraic tails instead of
the exponential tails in the case of FPU-type interactions (with or without Kac-Baker LRI).Despite of structurally similar
Langevin equations which hold for the soliton position and width of the two soliton types, the
algebraic solitons reach the superdiffusive long-time limit with a
characteristic t 3/2 time-dependence much faster than
exponential solitons. The soliton shape determines the diffusion constant in the long-time limit that is approximately a factor
of π smaller for algebraic
solitons. Our results appear to be generic for nonlinear excitaitons in
FPU-chains, because the same superdiffusive time-dependence was also observed in
simulations with discrete breathers. 相似文献
19.
Dimensionality serves as an indispensable ingredient in any attempt to formulate low-dimensional physics, and studying the dimensional crossover at a fundamental level is challenging. The purpose of this work is to study the hierarchical dimensional crossovers, namely the crossover from three dimensions (3D) to quasi-2D and then to 1D. Our system consists of a 3D Bose–Einstein condensate trapped in an anisotropic 2D optical lattice characterized by the lattice depths V1 along the x direction and V2 along the y direction, respectively, where the hierarchical dimensional crossover is controlled via V1 and V2. We analytically derive the ground-state energy, quantum depletion and the superfluid density of the system. Our results demonstrate the 3D-quasi-2D-1D dimensional crossovers in the behavior of quantum fluctuations. Conditions for possible experimental realization of our scenario are also discussed. 相似文献
20.
Uranium is an important radioactive material used in the field of nuclear energy and it is interesting from the scientific point of view because it possesses unique structure and properties. There exist several experimental reports on anomalies of physical properties of uranium that have not been yet explained. Manley et al. [Phys. Rev. Lett. 96, 125501 (2006); Phys. Rev. B 77, 214305 (2008)] speculate that the excitation of discrete breathers (DBs) could be the reason for anisotropy of thermal expansion and for the deviation of heat capacity from the theoretical prediction in the high temperature range. In the present work, with the use of molecular dynamics, the existence of DBs in α-uranium is demonstrated and their properties are studied. It is found that DB frequency lies above the phonon band and increases with DB amplitude. DB is localized on half a dozen of atoms belonging to a straight atomic chain. DB in uranium, unlike DBs in fcc, bcc and hcp metals, is almost immobile. Thus, the DB reported in this study cannot contribute to thermal conductivity and the search for other types of DBs in α-uranium should be continued. Our results demonstrate that even metals with low-symmetry crystal lattices such as the orthorhombic lattice of α-uranium can support DBs. 相似文献
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