K. A. Chishko

Thermodynamics of quasi-one-dimensional deposits on carbon nanobundles

The low-temperature thermodynamics of helium adsorbed in the outer grooves of carbon nanobundles is investigated theoretically in a lattice-gas model with the use of the Green’s function formalism. The proposed model describes both the formation of a one-dimensional (1D) condensate on the bottoms of the grooves and also the formation of two secondary chains in the groove (a three-chain structure) and thus is adequate for interpreting the behavior of the adsorbate over a rather wide range of coverages in the initial stage of deposition. The temperature dependence of the density of the deposit is obtained for the primary chain and secondary chains. The energy, heat capacity, and heat of adsorption are found as functions of temperature, and the total density of the 1D adsorbate is obtained for several different values of the binding energy with the substrate for atoms deposited in the primary and secondary positions. The adsorption isotherms (the total density of adsorbate as a function of external pressure)...

Lattice dynamics and heat capacity of a two-dimensional monoatomic crystal on a substrate

A model is proposed which gives an analytical description of the dynamics of collective excitations of two-dimensional close-packed atomic crystal lattices (atomic monolayers on substrates). The model takes into account both the interaction between atoms of the layer and the interaction of the layer with the substrate. The phonon spectra are found for an ideal triangular lattice and for a triangular lattice with a uniform distortion along one of the close-packed directions in the plane of the layer. The temperature dependence of the heat capacity is constructed for crystalline structures of both the commensurate and incommensurate types (in relation to the substrate). The theoretical results obtained are used for a detailed discussion and interpretation of the published experimental data on the spectra of lattice excitations and the heat capacity of monolayers of rare gases, including 3He and 4He, on various types of substrates.

Hysteresis of the bcc–hcp transition in a solid mixture of He3 in He4

Hysteresis of the bcc–hcp transition is observed by precision barometry in samples of a solid mixture of 1% He3 in He4 grown by the capillary blocking method. It is shown that the lines of equilibrium bcc–hcp and hcp–bcc transitions on the P–T diagram on cooling and heating do not coincide (the cooling line corresponds to higher pressure). In the process of thermocycling in the two-phase region the system executes a closed thermodynamic cycle, two branches of which correspond to the “normal” bcc–hcp transformation, with a slope of dP∕dT=6–12bar∕K, and the other two, to an “anomalous” transformation with a slope of dP∕dT=2.5–7bar∕K (the slope increases with decreasing molar volume). This effect is not observed in crystals of pure He4, and it can therefore be attributed to properties of the He3 impurity subsystem. A hierarchy of relaxation times for the pressure in the system is established which indicates that the mechanisms of transformation on different branches of the cycle are different. An interpretat...

Characteristic behavior of the specific heat of exsolving 3He-4He solid solutions

A theory that includes a substantial contribution of the correlation effects to the specific heat of exsolving 3He-4He solid solutions is constructed on a lattice-gas model. This theory makes it possible to construct a unified description of the temperature variation of the specific heat of these systems for a wide range of values of the 4He impurity density n0. It is shown that, to obtain quantitative agreement of the proposed theory with published experimental data, the effective coordination number z for the impurity in the solution must be treated as an adjustable parameter. For concentrated solutions (5%<n0<50%) the optimal value z⋍250 is independent of density, while for dilute solutions (n0<5%) it decreases with n0. Specifically, z=5 for n0=0.28% and z=3 for n0=0.11%. This suggests that a second phase can precipitate in the form of low-dimensional fractal structures during the exsolution of dilute solutions.

Detection of fluctuation effects near the phase separation temperature of concentrated 3He-4He solid solutions

A precision barometric study has revealed unusual behavior of the pressure of 3He−4He solid solutions with a concentration of around 30% 3He: in the pre-separation region the pressure increases with decreasing temperature long before the start of the phase transition. It is established that such an anomaly is due to correlation effects in the impurity subsystem which give rise to large-scale fluctuations of the impurity concentration, and the fluctuation contribution to the pressure is much greater than the phonon contribution. Quantitative agreement between the experimental data and the proposed theory is obtained, and it is shown that the observed temperature dependence of the pressure in the pre-separation and metastable regions can be explained only when the long-range character of the interaction between impurities is taken into account.

The Nature of the Glassy Phase in 4He Crystals

A model of a close-packed polytype with a random stacking fault structure is used to interpret the anomalies of the thermodynamic properties of the disordered (glassy) phase in solid HCP 4He in the so-called supersolid state. The temperature dependence of the phonon pressure is calculated theoretically, and compared to experimental data. A quantitative agreement between the theory and the experiment is achieved.

Interaction of translational and rotational modes of a molecular impurity in two-dimensional atomic crystals

The interaction between the translational and rotational degrees of freedom of a diatomic homonu-clear molecule that executes a motion at the site of a two-dimensional close-packed atomic matrix located on a close-packed atomic substrate (a molecular substitutional impurity in the crystal field of the two-dimensional lattice of a solidified rare gas) is investigated theoretically. The relationships describing the effective dynamic properties of an impurity rotator in the presence of translational excitations of its center of inertia are derived in the framework of consistent procedures on the basis of the Lagrangian (the functional-integral method) and Hamiltonian (the canonical-transformation method) formalisms. The inclusion of the translational-rotational interaction leads to a radical change in the inertial properties of the molecule. This manifests itself in a change in the form of the operator for the rotational kinetic energy as compared to the corresponding expression for a free rotator. The inertia tensor components for the molecule become functions of molecular orientation, and the molecule, in terms of rotational motion, transforms into a “parametric rotator” whose effective kinetic energy is represented as a generalized quadratic form of the angular momentum (or the angular velocity) components with a symmetry corresponding to the symmetry of the external crystal field. The translational-rotational interaction also results in the renormalization of the parameters of the crystal potential without a change in its initial form.

Electromagnetic emission of mobile dislocation segments in an ionic crystal

The problem of electromagnetic emission of an edge dislocation segment moving in an ionic lattice with a NaCl-type structure is considered. The proposed mechanism of electromagnetic emission is associated with the appearance of macroscopic alternating polarization currents along the extraplane edge of the edge dislocation in the course of its motion between adjacent valleys of the Peierls relief. The relationships for electro-magnetic radiation fields of an arbitrarily moving segment are derived, and the problem of electromagnetic emission of a segment that executes harmonic oscillations in the field of an external quasi-stationary elastic wave with a frequency (where l is the segment length and c is the velocity of sound) is treated in detail. The power of the emitted electromagnetic signal and the “acoustoelectromagnetic transformation” coefficient (the ratio between the electromagnetic radiation power and the mechanical power required for setting the segment in motion) are determined.

Translational–rotational interaction in the dynamics and thermodynamics of a 2D atomic crystal with a molecular impurity

The interaction between the rotational degrees of freedom of a diatomic molecular impurity and the phonon excitations of a two-dimensional atomic matrix commensurate with a substrate is investigated theoretically. It is shown that the translational–rotational interaction changes the form of the rotational kinetic energy operator as compared to the corresponding expression for a free rotator, and also renormalizes the parameters of the crystal field without altering in its initial form. The contribution of the impurity rotational degrees of freedom to the low-temperature heat capacity for a dilute solution of diatomic molecules in an atomic two-dimensional matrix is calculated. The possibility of experimental observation of the predicted effects is discussed.

Thermodynamics of an atomic monolayer with diatomic substitutional impurities

The low-temperature thermodynamics of a two-dimensional monatomic crystalline matrix containing a diatomic molecular impurity is investigated theoretically. Typical examples of this type of system are monatomic layers of rare gases (Ne, Ar, Kr, Xe) with included molecules of the type N2 and O2. Another example is a hydrogen film, which is a mixture of ortho and para components. Expressions are obtained which describe the crystalline field for a diatomic impurity with allowance for both the contribution of the atoms of the 2D matrix and the field created by the atoms of the substrate. Thus the effective crystalline field is a complicated function of the orientation of the diatomic rotator. In particular, the equilibrium orientation of the rotator depends substantially on the relative amplitudes of the crystalline fields of the matrix and substrate. For example, if the attraction exerted by the substrate is dominant, then the rotator in the equilibrium state will be oriented perpendicular to the layer, and ...