T. N. Antsygina

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)...

Anharmonic effects in librational motion of N2-type crystals

The dynamics of librational motion in N2-type crystals (α-N2, α-CO, N2O, CO2) is treated by taking into account both anharmonic and correlation effects. The method used is similar to Tyablikovs method in the theory of magnetism. The main thermodynamic characteristics of the librational subsystem are calculated: the order parameter, rms librational angle, librational mode frequencies and corresponding Grüneisen parameters, librational heat capacity, and internal and free energies. The librational isotope effects for α-14N2 and α-15N2 are considered. An explanation of the anomalous isotope effects in the heat capacity is proposed. A theory of the phase transition into the orientationally disordered state is developed.

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.

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.

Lattice Dynamics of 2D Monoatomic Crystals: Application to 3He on Graphite

The dynamics of a two-dimensional rare gas crystal with triangular lattice is investigated theoretically. Phonon spectra of the 2D crystal are calculated within the model which takes into account the interaction between nearest neighbours in the layer and interaction of layer atoms with the substrate. On the basis of obtained results the temperature dependence of lattice heat capacity has been calculated and a comparison is made with experimental data.

Fluctuation Effects in the Heat Capacity of 3He–4He Solid Solutions

The impurity heat capacity of solid3He–4He mixtures is theoretically investigated both below and above phase separation temperature Ts. It is shown that at T > Tsthe temperature behavior of the heat capacity is completely defined by correlation effects in the impuriton subsystem. The proposed theory is in good agreement with experimental data.

Lattice distortion of quantum cryocrystals under pressure

The hcp lattice distortion parameter δ, the deviation of the ratio c∕a from the ideal hcp value 8∕3, is calculated for solid He under pressure, taking into account two- and three-body interatomic forces. The resulting lattice distortion parameter is small and negative, i.e., the lattice is slightly flattened compared with the ideal hcp lattice. It increases monotonically in absolute value with pressure and reaches 10−3 for a molar volume of ∼2.5cm3∕mol. Such small distortions are most likely beyond the capabilities of x-ray or neutron experiments but can be detected by optical methods based on measurements of the birefringence. The data on δ can be used as a probe of the many-body forces.

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.