The thermodynamics of low dimensional spin-1/2 Heisenberg ferromagnets (HFM) in an external magnetic field is investigated within a second-order two-time Green function formalism in the wide temperature and field range. A crucial point of the proposed scheme is a proper account of the analytical properties for the approximate transverse commutator Green function obtained as a result of the decoupling procedure. A good quantitative description of the correlation functions, magnetization, susceptibility, and heat capacity of the HFM on a chain, square and triangular lattices is found for both infinite and finite-sized systems. The dependences of the thermodynamic functions of 2D HFM on the cluster size are studied. The obtained results agree well with the corresponding data found by Bethe ansatz, exact diagonalization, high temperature series expansions, and quantum Monte Carlo simulations.

Experiments on the plastic deformation of LiF ionic monocrystals under uniaxial compression are performed with simultaneous recording of acoustic ͑AE͒ and electromagnetic ͑EME͒ emissions. A strong correlation between AE and EME events has been found, which clearly demonstrates that the observed EME is caused by a dynamical interaction between moving dislocations and charged vacancies in the ionic lattice during work hardening. The mechanism proposed to explain EME is based on the assumption that gliding edge dislocations sweep up the vacancies of a preferable sign. As a result, when a dislocation pileup is formed, a certain nonequilibrium charge density is accumulated at its head, resulting in electric polarization of the deformed crystal. As the external loading increases, a locked dislocation pileup bursts through the stoppers and quickly loses its bounded charge. The relaxation of this charge produces an intrinsic polarization current generating an electric pulse. It is assumed that the relaxation current can be described as an athermic viscous motion of vacancies under the kinetic friction force ϳBv ͑B is the friction coefficient and v is the vacancy velocity͒ in a self-consistent electric field determined by the distribution of the total charge density. A nonlinear integrodifferential equation of motion for the nonequilibrium charge density is derived. For a special form of the initial charge density distribution, an automodel solution of this equation describing the polarization current has been built. The electrical signal generated by an acting slip system has been calculated. By comparing the calculated and experimentally measured electric signal patterns, the friction coefficient for the linear chain of vacancies ͑the analog of an edge dislocation extra plane͒ in LiF has been estimated to be B Ӎ 0.9 ϫ 10 −5 g cm −1 s −1 . This value is in accordance with the corresponding coefficient for dislocations in ionic lattices.

No abstract

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) are calculated for various temperatures. Experimentally measured adsorption isotherms are presented for He4 deposited on bundles of single-walled carbon nanotubes. The measurements were made in the temperature range 2–15 K. Those experiments and previously published data on the adsorption isotherms of methane are interpreted using a theoretical model developed in this paper.