On orientational relief of inter-molecular potential and the structure of domain walls in fullerite C60

A simple planar model for an orientational ordering of threefold molecules on a triangular lattice modelling a close-packed (111) plane of fullerite is considered. The system has 3-sublattice 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 two-well orientational potential profiles, which are slightly effected by a presence of a straight domain wall. The reason is a stronger correlation between neighbour molecules in triangular lattice versus previously considered square lattice A considerable reduction (up to one order) of orientational interwell potential barrier is found in the core regions of essentially two-dimentional potential defects, such as a three-domain boundary or a kink in the domain wall. For ultimately uncorrelated nearest neighbours the height of the interwell barrier can be reduced even by a factor of 100.Comment: 11 pages, 13 figures, LaTeX, to appear in Low Temperature Physic

Comment on ``BCS to Bose-Einstein crossover phase diagram at zero temperature for a d_{x^2-y^2} order parameter superconductor: Dependence on the tight-binding structure''

The work by Soares et al. [Phys. Rev. B 65, 174506 (2002)] investigates the BCS-BE crossover for d-wave pairing in the 2-dimensional attractive Hubbard model. Contrary to their claims, we found that a non-pairing region does not exist in the density vs coupling phase diagram. The gap parameter at T=0, as obtained by solving analytically as well as numerically the BCS equations, is in fact finite for any non-zero density and coupling, even in the weak-coupling regime.Comment: 7 pages, 1 figur

Two character formulas for sl2^\hat{sl_2} spaces of coinvariants

We consider $\hat{sl_2}$ spaces of coinvariants with respect to two kinds of ideals of the enveloping algebra U(sl_2\otimes\C[t]). The first one is generated by $sl_2\otimes t^N$, and the second one is generated by $e\otimes P(t), f\otimes R(t)$ where $P(t), R(t)$ are fixed generic polynomials. (We also treat a generalization of the latter.) Using a method developed in our previous paper, we give new fermionic formulas for their Hilbert polynomials in terms of the level-restricted Kostka polynomials and $q$-multinomial symbols. As a byproduct, we obtain a fermionic formula for the fusion product of $sl_3$-modules with rectangular highest weights, generalizing a known result for symmetric (or anti-symmetric) tensors.Comment: LaTeX, 22 pages; very minor change

Low-temperature orientational order and possible domain structures in C(_{60}) fullerite

Based on a simple model for ordering of hexagons on square planar lattice, an attempt has been made to consider possible structure of C(_{60}) fullerite in its low temperature phase. It is shown that hexagons, imitating fullerens oriented along (C_{3}) axes of \emph{sc} lattice, can be ordered into an ideal structure with four non-equivalent molecules in unit cell. Then the energy degeneracy for each hexagon rotations by (\pi /3) around its (C_{3}) axis leaves the translational and orientational order in this structure, but leads to a random distribution of (\pi /3) rotations and hence to {}``averaged{}'' unit cell with two molecules. However the most relevant structural defects are not these intrinsic \char`\"{}misorientations\char`\"{} but certain walls between the domains with different sequencies of the above-mentioned two (non-ideal) sublattices. Numeric estimates have been made for the anisotropic inter-molecular potential showing that the anisotropy is noticeably smaller for molecules in walls than in domains

Theory of a Narrow roton Absorption Line in the Spectrum of a Disk-Shaped SHF Resonator

We calculate the probability of the birth of a circular phonon (c-phonon) in He II by a c-photon of the resonator. It is shown that this probability has sharp maxima at frequencies, where the effective group velocity of the c-phonon is equal to zero; the density of states of c-phonons strongly grows at such frequencies. For He II, these frequencies correspond to a roton and a maxon. From the probability of the c-roton birth, we calculate the roto line width which is found to approximately agree with the experimental one. We conclude that the roton line observed in the super-high-frequency (SHF) absorption spectrum of helium is related to the birth of c-rotons. A possible interpretation of the Stark effect observed for the roton line is also proposed.Comment: 13 pages, 1 figure, v2: journal variant, several minor correction

Magnetic structures of δ\delta-O2_2 resulting from competition of interplane exchange interactions

Solid oxygen is a unique molecular crystal whose phase diagram is mostly imposed by magnetic ordering, i.e., each crystal phase has a specific magnetic structure. However, recent experiments showed that high-pressure $\delta$-phase is implemented in different magnetic structures. In the present paper we study the role of interplane exchange interactions in formation of the magnetic structures with different stacking sequences of the close-packed planes. We show that temperature-induced variation of intermolecular distances can give rise to compensation of the exchange coupling between the nearest close-packed planes and result in the phase transition between different magnetic structures within $\delta$-O$_2$. Variation of the magnetic ordering is, in turn, accompanied by the step-wise variation of interplane distance governed by space and angular dependence of interplane exchange constants.Comment: 16 pages, 6 figure

Temperature-doping phase diagram of layered superconductors

The superconducting properties of a layered system are analyzed for the cases of zero- and non-zero angular momentum of the pairs. The effective thermodynamic potential for the quasi-2D XY-model for the gradients of the phase of the order parameter is derived from the microscopic superconducting Hamiltonian. The dependence of the superconducting critical temperature T_c on doping, or carrier density, is studied at different values of coupling and inter-layer hopping. It is shown that the critical temperature T_c of the layered system can be lower than the critical temperature of the two-dimensional Berezinskii-Kosterlitz-Thouless transition T_BKT at some values of the model parameters, contrary to the case when the parameters of the XY-model do not depend on the microscopic Hamiltonian parameters.Comment: To be published in Phys. Rev.

Phase Fluctuations and Pseudogap Properties: Influence of Nonmagnetic Impurities

The presence of nonmagnetic impurities in a 2D ``bad metal'' depresses the superconducting Berezinskii-Kosterlitz-Thouless transition temperature, while leaving the pairing energy scale unchanged. Thus the region of the pseudogap non-superconducting phase, where the modulus of the order parameter is non-zero but its phase is random, and which opens at the pairing temperature is substantially bigger than for the clean system. This supports the premise that fluctuations in the phase of the order parameter can in principle describe the pseudogap phenomena in high-$T_c$ materials over a rather wide range of temperatures and carrier densities. The temperature dependence of the bare superfluid density is also discussed.Comment: 11 pages, LaTeX, 1 EPS figure; final version to appear in Low.Temp.Phy