Volume 29, Issue 5, May 2003
 QUANTUM LIQUIDS AND QUANTUM CRYSTALS


nanoclusters around vacancies in solid solutions of in
View Description Hide DescriptionPrecise measurements of the pressure in solid solutions of in are performed with temperature cycling in the phaseseparation range. It is found that as a result of such cycling the difference of the pressures between the minimum and maximum temperatures decreases by a factor of approximately 3 and then remains unchanged for a long time. The initial values of and the pressure in a uniform solid solution are restored only after the sample is heated to a temperature substantially above the phaseseparation point. The data obtained are explained on the basis of the hypothesis that under these conditions clusters of pure form around quasiequilibrium vacancies. A thermodynamic calculation of such clusters is performed. The computational results made it possible to give a quantitative explanation of the distinctive features observed. Comparing the calculations with experiment made it possible to determine the cluster concentration, and the cluster radius, which is 3–4 interatomic distances at low temperatures. The characteristic equilibration time under these conditions is determined, and it is suggested that processes occurring at cluster boundaries determine its value.

Phonon scattering in orthopara hydrogen solid solutions (role of configurational relaxation)
View Description Hide DescriptionThe experimental data on the thermal conductivity of orthopara hydrogen solutions are analyzed on the basis of a relaxationtime model taking account of configurational relaxation of the ortho subsystem. The influence of configurational relaxation on the thermal conductivity is analyzed using resonance scattering of phonons by pair clusters of orthomolecules taking account of their rotational spectrum.

Motion of a probe nanoparticle in a quantum crystal with a narrow vacancy band
View Description Hide DescriptionThe temperature dependence of the diffusion coefficient of a probe particle with a diameter of several nanometers and moving in a quantum crystal with a narrow vacancy band changes substantially, as a result of interactions with thermal vacancies, with decreasing temperature in the range where a transition occurs from classical thermally activated vacancy hopping to coherent motion of delocalized vacancies. Moreover, in the transitional range the diffusion coefficient of a probe particle in a viscous vacancion gas can increase if the effective vacancion meanfree path length is small compared to the particle diameter and increases with decreasing temperature more rapidly than the concentration of thermal vacancies For in a rarefied vacancion gas where the particle diffusion coefficient decreases as if the cross section for the inelastic scattering of a vacancion by a probe particle is a weak function of the temperature. The model developed in this paper can be used to describe the diffusion of positive charges in hcp crystals, grown at pressures above the lowest solidification pressure of helium, and negative charges in parahydrogen crystals.

Orientational ordering in solid hydrogens with in the presence of a crystal field
View Description Hide DescriptionA system of quantum linear rotators with quantum rotational angular momentum in a crystal field is investigated. An equation is derived for the orientational order parameter. The critical parameters, the phaseseparation curve, and the line of superheating and supercooling points are found. It is shown that the thermodynamic behavior of a system of linear rotators in the classical case is identical to that of the limiting quantum cases studied in this work. For positive crystal fields there is an analogy between orientational phase transitions in a system of rotators with and phase transformations in a liquid–vapor system. It is shown that states with radically change the character the phase transitions in the system of rotators: instead of one critical pointcharacteristic for a system of now there is a line of critical points.

On the phase diagram of the development of an instability of a massive charged surface of liquid helium
View Description Hide DescriptionThe existing interpretation of the development of an instability of a massive charged heliumsurface requires a definite correction that identifies this phenomenon more closely with the known spinodal and binodal decomposition processes in the theory of firstorder phase transformations. The distinctive features of the development of an instability of a charged heliumsurface, exhibiting indications of spinodal (binodal) decomposition, are discussed and a qualitative phase diagram for such transitions is constructed in the plane surface electron density—electric field above a plane charged with electrons.
 Top

 LOWDIMENSIONAL AND DISORDERED SYSTEMS


Atomic dynamics and the problem of the structural stability of free clusters of solidified inert gases
View Description Hide DescriptionThe dependence of the rms amplitudes of atoms in free clusters of solidified inert gases on the cluster size is investigated theoretically and experimentally. Free clusters are produced by homogeneous nucleation in an adiabatically expanding supersonic stream. Electron diffraction is used to measure the rms amplitudes of the atoms; the Jacobimatrix method is used for theoretical calculations. A series of distinguishing features of the atomic dynamics of microclusters was found. This was necessary to determine the character of the formation and the stability conditions of the crystal structure. It wass shown that for clusters consisting of less than as the cluster size decreases, the rms amplitudes grow much more rapidly than expected from the increase in the specific contribution of the surface. It is also established that an fcc structure of a free cluster, as a rule, contains twinning defects (nuclei of an hcp phase). One reason for the appearance of such defects is the socalled vertex instability (anomalously large oscillation amplitudes) of the atoms in coordination spheres.

Nonlinear Rayleigh waves in a medium with a monatomic nonlinear coating
View Description Hide DescriptionThe nonlinear dynamics of surface acoustic waves at the surface of a linear elastic halfspace coated with a monolayer of a nonlinear material is investigated. A onedimensional nonlinear integrodifferential equation describing the dynamics of such a system is derived. The model proposed is used to study Rayleigh solitons with a stationary profile. The possible phenomenological generalizations of the equations derived and their exact solitonsolutions are discussed.
 Top

 PHYSICAL PROPERTIES OF CRYOCRYSTALS


Local excitations in the conduction band of crystalline xenon
View Description Hide DescriptionA detailed experimental investigation of the basic characteristics of the radiation of crystalline xenon with band) is performed as a function of temperature, impurity concentration, lattice perfection, and irradiation dose. The radiation parameters of this band are compared with the same parameters of the radiation of free excitons, localized holes and impurity centers whose bands were recorded in parallel. The photoexcitation spectra of the band and the time decay curves of luminescence are analyzed. Radiation with similar structure with is also found in the binary crystals with high xenon concentrations. It is concluded that the observed radiation is due to intrinsic moleculartype excited states of localized in the interior volume of the crystal and lyingnear 10 eV in the conduction band.
 Top

 LATTICE DYNAMICS


Surface and quasisurface states in a strongly anisotropic layered crystal
View Description Hide DescriptionIn Part 1 a relatively simple model of a strongly anisotropic layered crystal is use to study various types of surface waves in the longwavelength approximation. Particular solutions are obtained for the latticedynamics equations in the presence of a surface impurity monolayer. The solutions have frequencies that lying outside (surface states) and inside (quasisurface states) a band in the continuous spectrum of an ideal lattice. In Part 2 shear waves localized near a monatomic layer adsorbed on the free surface of a strongly anisotropic layered crystal are studied at the microscopic level using a vector laltice model with translational and rotational invariance and elastic stability. It is shown that in some cases a noncentral interatomic interaction results in the appearance of surface shear waves of a special type with one or two termination points and a damping parameter that is a nonmonotonic function the twodimensional wave vector.

Heat transfer by lowfrequency phonons and “diffusive” modes in molecular crystals
View Description Hide DescriptionA model where lowfrequency phonons and, above the mobility limit, “diffusive” modes migrating randomly from site to site transfer heat is proposed to describe the behavior of the isochoric thermal conductivity of molecular crystals in orientationally ordered phases. The mobility limit is found from the condition that the phonon meanfree path length determined by umklapp processes cannot become less than half the wavelength. The Bridgman coefficient is the weightedmean over these modes, whose volume dependence differs strongly. The model proposed here is used to analyze the thermal conductivity of naphthalene anthracene and hexamethylenetetramine It is shown that sitetosite rotational energy transfer must be taken into account in order to calculate the lower limit of the thermal conductivity of molecular crystals.

On the orientational relief of the intermolecular potential and the structure of domain walls in fullerite
View Description Hide DescriptionA simple planar model for an orientational ordering of threefold molecules on a triangular lattice modeling a closepacked (111) plane of fullerite is considered. The system has 3sublattice ordered ground state which includes 3 different molecular orientations. There exist 6 kinds of orientational domains, which are related with a permutation or a mirror symmetry. Interdomain walls are found to be rather narrow. The model molecules have twowell orientational potential profiles, which are slightly effected by a presence of a straight domain wall. The reason is a stronger correlation between neighbor molecules in the triangular lattice versus the square lattice previously considered. A considerable reduction (up to one order) of the orientational interwell potential barrier is found in the core regions of essentially twodimensional potential defects, such as a threedomain boundary or a kink in the domain wall. For ultimately uncorrelated nearest neighbors the height of the interwell barrier can be reduced even by a factor of

Interaction between and gases under pressure
View Description Hide DescriptionA brief review is given of the interaction between fullerite and various gases under elevated pressure. Subjects discussed include the formation of ordered interstitial gasfullerene compounds,reactions between intercalated gases and fullerene molecules to form new endohedral and exohedral compounds, and changes in the structure and properties of because of intercalated gas atoms or molecules.

Intercalation of fullerite with helium and argon at normal temperature and pressure
View Description Hide DescriptionPowder xray diffractometry was used to study the effect of intercalating fullerite with helium and argon atoms on the fullerite structure. The samples were saturated at room temperature and normal pressure. The dependences obtained for the lattice parameter and halfwidth of certain reflections on the intercalation time with helium atoms showed that the voids in the lattice were filled in two stages. Helium filled the octahedral voids relatively rapidly first and then the tetrahedral subsystem, but much more slowly. Both intercalants affected the halfwidth and intensity of the reflections, the matrix lattice parameters, the phase transition temperature, the volume jump at the transition, and the thermal expansion coefficients of fullerite.

Lowtemperature thermal conductivity of solid carbon dioxide
View Description Hide DescriptionPreliminary measurements of the thermal conductivity of pure carbon dioxide in the temperature range 1.5–35 K are reported. The first data below 25 K have been obtained. The thermal conductivity reaches very high values, about 700 W/(m⋅K), which is unusual for simple molecular crystals. A straightforward analysis of the data shows a coarsegrained sample.
