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The lattice distortion of hcp solid He under pressure is calculated using semiempirical and first-principle approaches. While three-body forces tend to flatten the lattice at all compressions, the effect of pair forces changes from the flattening at small compression to elongation at large one. At large compressions, the lattice distortion due to the triple forces is more than twice as large as those due to pair forces and the lattice is slightly flattened. First-principles results show that over approximately fivefold compressions higher-order, many-body forces become important.

The T = 0 K equations of state (EOS) of rare-gas solids (RGS) (He, Ne, Ar, Kr, and Xe) are calculated in the experimentally studied ranges of pressures accounting for two-and three-body interatomic forces. Solid-state corrections to the pure two-body Aziz et al. potentials included the long-range Axilrod-Teller three-body interaction and short-range three-body exchange interaction. The energy-scale and length-scale parameters of the latter were taken as adjustable parameters of theory. The calculated T = 0 K EOS for all RGS are in excellent agreement with experiment in the whole range of pressures. The calculated EOS for Ar, Kr, and Xe exhibit inflection points where the isothermal bulk moduli have non-physical maxima indicating that account of only three-body forces becomes insufficient. These points lie at pressures 250, 200, and 175 GPa (volume compressions of approximately 4.8, 4.1, and 3.6) for Ar, Kr, and Xe, respectively. No such points were found in the calculated EOS of He and Ne. The relative magnitude of the three-body contribution to the ground-state energy with respect to the two-body one as a function of the volume compression was found to be non-monotonic in the sequence NeAr -Kr-Xe. In a large range of compressions, Kr has the highest value of this ratio. This anomally high three-body exchange forces contributes to the EOS so large negative pressure that the EOS for Kr and Ar as a function of compression nearly coincide. At compressions higher approximately 3.5, the curves intersect and further on the EOS of Kr lies lower than that of Ar. PACS: 64.60.Cn Order-disorder transformations; statistical mechanics of model systems; 67.80.-s Solid helium and related quantum crystals; 67.90.+z Other topics in quantum fluids and solids; liquid and solid helium.

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We consider a simple system of interacting rotators and show that such a system could display properties of quantum crystals. The possibility of realizing new quantum system is discussed

This paper reports the results of the first measurements of the thermal conductivity of the a, )3, and y phases of solid oxygen over the temperature range of 1-52 K. A simple qualitative analysis was performed to explain the observed anomalies in thermal conductivity, which manifested themselves by a jump at the a-fi transition, the anomalously weak temperature dependence in the p phase, and an increase of the conductivity with temperature in the y phase.PACS numbers: 66.70.+f, 31.70.Ks, 75.20.Ck, Solid oxygen, a unique crystal combining properties of a molecular crystal and a magnet, has been a subject of intensive experimental and theoretical studies during the last two decades (see, for example [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], and references therein). The following picture has emerged as a result of those studies. Oxygen exists under the equilibrium vapor pressure in three crystalline modifications. The orientationally ordered monoclinic low temperature a phase (C2/m) is a collinear two-sublattice quasi-2D antiferromagnet. The phase transition (7^ = 23.9 K) into the intermediate rhombohedral /3 phase (R3m) is associated with the transformation of the magnetic structure. The nature of this transformation is still a subject for discussion [13].Though none of the measured thermodynamic characteristics as well as the molar volume [5] show any jump at the a-fi transition point and, moreover, the heat of transition has not been detected in precise calorimetric study [3], the a-p transformation is beyond all doubts a first order phase transition. This conclusion follows, for instance, from the observation of hysteresis of the magnetization curves at the transition [11].The long-range magnetic order in /3-oxygen is most probably absent; however the short-range order with the correlation length of 5 A in three-sublattice or incommensurate helicoidal structures persists over the whole range of the existence of the phase [ 11,14,15].Both the low temperature phases have the same orientational structure in which the molecular axes are collinear and perpendicular to the close packed layers. The spectrum of elementary excitations in the a phase consists of (besides acoustic modes) two libron modes at 43 and 78 cm -1 [2] and two magnon ones at 6.4 and 27 cm -1 [1,4]. No magnon excitations were found in the p phase and there is one twofold degenerated libron mode at 50 cm" 1 [2].The p-y transition (7^=43.8 K) is accompanied by a radical rearrangement of the lattice, a considerable jump (5.4%) of volume [5], and high value of the latent heat of the transition [3]. The y phase has an eight-molecule cubic cell with an orientationally disordered structure with Pm3n symmetry. It exhibits paramagnetic properties with a quasi-ID magnetic short-range order [9,10].In this paper we report results of the first measurements of the thermal conductivity of the a, /?, and y phases of solid oxygen.The measurements of the thermal conductivity were carried out by the stationary heat flux method over a temperature ran...

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