Vacancies in a strongly strained crystal at low temperatures

It is shown that an equilibrium vacancy concentration can be noticeable even at a temperature close to absolute zero. This will be the case if the volume deformation of a crystal is higher than a critical value, which is a second-order phase transition point.     

Nonlinear diffusion of vacancies in a crystal

The correlated motion of vacancies breaks the translational symmetry of the jump probability if the concentraton of the vacancies varies from place to place. This leads to a significant deviation from the result of linear diffusion theory. The problem is treated by using the model of a generalized random walk in which the probabilities of a jump by the walker depend on the position.On leave of absence from Department of Applied Science, Faculty of Engineering, Tohoku University, Sendai 980, Japan.     

On the interaction of vacancies in a two-dimensional rare-gas crystal

We study the interaction between vacancies in a two-dimensional van der Waals crystal by molecular dynamics simulation. We find that the vacancy–vacancy interaction is attractive at short distances, but its binding energy is not enough to keep the pair bounded at the melting region where the vacancies are more likely to be created.     

On the propagation of hypersonic solitons in a strained paramagnetic crystal

Nonlinear dynamics of a subnanosecond transverse elastic pulse in a low-temperature paramagnetic crystal placed into a magnetic field and statically strained in the same direction is investigated. Paramagnetic impurities implanted into the crystal have an effective spin of 3/2, and the pulse propagates at right angles to the magnetic field. In the general case, the structure of the pulse is such that the approximation of slowly varying envelopes, which is standard for quasi-monochromatic signals, is inapplicable. Under certain conditions, the pulse propagation in the 1D case is described by the Konno-Kameyama-Sanuki integrable wave equation for strain, which is transformed into the Hirota equation for the envelope of the given strain in the quasi-monochromatic limit. The effect of transverse perturbations on extremely short and quasi-monochromatic solitons is studied in detail. The conditions and features of self-focusing and defocusing of acoustic solitons in the form of extremely short pulses and envelope solitons are revealed. The propagation of an extremely short “half-wave” hypersonic pulse in the “acoustic bullet” regime in the medium with a quasiequilibrium population of quantum sublevels of effective spins is predicted.     

Analysis of interdiffusion via isolated vacancies in strongly ionic crystals

In this paper, we analyse chemical interdiffusion in strongly ionic crystals for diffusion couples AY _m –BY _m , where A and B have the same charge numbers. We employ the exact sum rule given by Moleko and Allnatt relating the phenomenological coefficients for diffusion in the multicomponent random alloy via the agency of monovacancies. It is shown that the ratio of the intrinsic diffusivities can be expressed very simply in terms of the atom–vacancy exchange frequencies without correlation terms. For the case of an immobile anion sublattice and making use of a highly accurate diffusion kinetics theory due to Moleko et al., it is shown that the interdiffusivity is principally proportional only to the off-diagonal phenomenological coefficient relating the two cations.     

Ring defects in a strongly confined chiral liquid crystal

We find numerically that a regular array of isolated ring defects can exist as a stable state in a highly chiral liquid crystal confined in a thin cell imposing fixed planar anchoring at the parallel confining surfaces. This peculiar defect structure can be stable when the cell thickness d is around 3/4 of the helical pitch p. A cell of thickness 3p/4 with parallel surface anchoring is incompatible with helical alignment that favors d=mp/2 (with m being an integer). Formation of ring defects can thus be regarded as a result of frustrations between the helical alignment with a specific pitch and the confining surfaces that prevent it.     

Magnetic structure of a CaMnO2.75 crystal with ordered oxygen vacancies

The magnetic state of a CaMnO_{3 ? δ} crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn⁴⁺ and Mn³⁺ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO_2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a _c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T _N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T _N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn³⁺-Mn⁴⁺ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO_2.75 crystal are governed directly by the competition of the Mn³⁺-Mn⁴⁺ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.     

Elastic behavior of a two-dimensional crystal near melting

Using positional data from video microscopy, we determine the elastic moduli of two-dimensional colloidal crystals as a function of temperature. The moduli are extracted from the wave-vector-dependent normal-mode spring constants in the limit q-->0 and are compared to the renormalized Young's modulus of the Kosterlitz-Thouless-Halperin-Nelson-Young theory. An essential element of this theory is the universal prediction that Young's modulus must approach 16 pi at the melting temperature. This is indeed observed in our experiment.     

Two-stage melting in systems of strongly interacting Rydberg atoms

We analyze the ground state properties of a one-dimensional cold atomic system in a lattice, where Rydberg excitations are created by an external laser drive. In the classical limit, the ground state is characterized by a complete devil's staircase for the commensurate solid structures of Rydberg excitations. Using perturbation theory and a mapping onto an effective low-energy Hamiltonian, we find a transition of these commensurate solids into a floating solid with algebraic correlations. For stronger quantum fluctuations the floating solid eventually melts within a second quantum phase transition and the ground state becomes paramagnetic.     

Collective mode in melting of a crystal with a face-centered lattice

Physics of the Solid State - The collective mechanism, which can lead to the loss of long-range order during melting of an ideal crystal with a face-centered lattice, has been studied using the...     

The molecular dynamics of a naphthalene crystal near melting

It is suggested that atom-atom potential functions become more appropriate for molecular dynamics calculations for systems as they approach melting. A simulation for naphthalene shows the molecular rate of reorientation about the axis of greatest inertia to be approaching 100 MHz within 20 K from melting. Self-correlations for this motion are highly significant, though neighbour correlations appear to be random in this case. Such behaviour is contrasted with plastic crystal behaviour where neighbour correlations are high, and it is suggested that this characterises the difference between a true crystal and a plastic crystal. For naphthalene this result contrasts with an experimental conclusion where the motion about the axis of least inertia is though to be responsible for the onset of melting.     

不同晶系应变Si状态密度研究

应变Si技术是当前微电子领域研究发展重点,态密度是其材料的重要物理参量.本文基于应力相关KP理论,建立了(001),(101)和(111)晶面施加双轴应力形成的四方、单斜及三角晶系应变Si导带、价带态密度模型.结果表明,除单斜和三角晶系导带底态密度外,应力对其余各态密度均有显著影响.本文所得模型数据量化,可为应变Si材料物理的理解及其他物理参数模型的建立奠定重要理论基础. 关键词： 应变Si KP 态密度     

On the applicability of the Frenkel-Kontorova model to describing the dynamics of vacancies in a polymeric crystal chain

The necessary conditions of the applicability of the Frenkel-Kontorova one-dimensional model [the approximation of immobile neighboring chains plus sine-Gordon (continual) equation for nonlinear dynamics of the chain under consideration] to describing the dynamics of vacancies in a polymeric crystal chain are determined. It is shown that these conditions are satisfied for polyethylene crystals. The physical mechanism of model applicability limitations is established.     

Modification of the parabolic approximation in the theory of ultrasonic-beam diffraction in a strongly anisotropic crystal

A modification of the parabolic approximation of the diffraction theory is based on the approximation of the wave surface with allowance for the shape of the angular spectrum of the ultrasonic beam. It is demonstrated that such an approach is more accurate in the analysis of the acoustic field at a relatively high diffraction divergence and/or high acoustic anisotropy. An acoustic field is simulated in paratellurite, which is a leading acoustic material with anomalously high acoustic anisotropy.