D-brane worlds and the cosmological constant

The cosmological constant on a D-brane is analyzed. This D-brane is in the background produced by the p-brane solutions. The energy-momentum tensor in this model has been found and the form of the cosmological constant has been derived. This energy-momentum tensor is interpreted as an energy-momentum tensor for a perfect fluid on the D-brane. The energy density and the pressure for this fluid have been derived. As it turned out the pressure is negative but the speed of sound is real

Thermodynamics of the incommensurate state in Rb_2WO_4: on the Lifshitz point in A`A``BX_4 compounds

We consider the evolution of the phase transition from the parent hexagonal phase $P6_{3}/mmc$ to the orthorhombic phase $Pmcn$ that occurs in several compounds of $A'A''BX_{4}$ family as a function of the hcp lattice parameter $c/a$. For compounds of $K_{2}SO_{4}$ type with $c/a$ larger than the threshold value 1.26 the direct first-order transition $Pmcn-P6_{3}/mmc$ is characterized by the large entropy jump $Rln2$. For compounds $Rb_{2}WO_{4}$, $K_{2}MoO_{4}$, $K_{2}WO_{4}$ with $c/a<1.26$ this transition occurs via an intermediate incommensurate $(Inc)$ phase. DSC measurements were performed in $Rb_{2}WO_{4}$ to characterize the thermodynamics of the $Pmcn-Inc-P6_{3}/mmc$ transitions. It was found that both transitions are again of the first order with entropy jumps $0.2Rln2 and$0.3Rln2$. Therefore, at$c/a ~ 1.26$the$A'A''BX_{4}$compounds reveal an unusual Lifshitz point where three first order transition lines meet. We propose the coupling of crystal elasticity with$BX_{4}$ tetrahedra orientation as a possible source of the transitions discontinuity.Comment: 13 pages,1 Postscript figure. Submitted as Brief Report to Phys. Rev. B, this paper reports a new work in Theory and Experiment, directed to Structural Phase Transition

On the relationship between the temperature and the geometry of the two-dimensional superconducting system

A set of vortices in the superconducting system being a two-dimensional region with a boundary has been considered. Here the system under study is described by the model of the Ginzburg-Landau potential in the dual point. This model predicts that in the bounded superconducting system non-interacting vortices appear. These vortices make the absolute minima of this potential. It turned out that in the thermodynamic equilibrium for the fixed number of vortices, the temperature of the system and the geometry of the boundary are related to each other. The simultaneous change of the temperature of the system and of the geometry of the boundary has been investigated under the assumption that the number of vortices is fixed. In the case of the flat disc the explicit form of the temperature vs. area relation has been obtained for two different boundary conditions

EXCHANGE INTEGRALS FOR THE FIRST AND SECOND COORDINATION SPHERES IN THE NEW SPINEL SERIES Cu1-xZnxCr2Te4 (where x = 0.00, 0.01, 0.02)

The exchange integrals for the first and second coordination spheres of the spinels under study were determined. The former turned out to be positive and the latter-negative. Thus, the ferromagnetic structure of these spinels is a result of the competitition of the ferro- and antiferro-interactions

TRANSPORT PHENOMENA IN THE MAGNETICALLY MODULATED SPINELS : Zn1-xCuxCr2Se4 (WHERE 0.0 ≤x≤ 1.0)

The spinels under study are p-type semiconductors showing a change of the conductivity character from the conductive into metallic one at x ~ 0.3. An increase of the electrical conductivity with the copper concentration in a sample is linked both with the increase of the hole mobility and the hole concentration. For x ≥ 0.05 partial degeneracy of the semiconductor state is observed

DOUBLE-EXCHANGE INTERACTION AS THE MAIN MECHANISM DRIVING A VERY STRONG FERROMAGNETIC COUPLING IN THE SPINEL SYSTEM Cd1-xCuxCr2Se4

Each Cu+ ion introduced in place of a Cd++ ion in the tetrahedral sublattice of the spinels under study causes a transformation of a Cr3+ ion into a Cr4+ ion in the octahedral sublattice. It follows, that apart from the superexchange interaction there appears in these compounds a double exchange interaction which consists in a jump of an electron between Cr3+ and Cr4+ and brings a strong ferromagnetic coupling

On the Critical Points of the Structure Amplitude for the Case of One-Dimensional Displacive Modulation

The behavior of the structure amplitude is analyzed in terms of the harmonics of a modulation function for the case of one-dimensional longitudinal displacive modulation. This modulation function is given by a finite series of N sine functions. It turns out that: (1) for the large values of the amplitudes of the modulation function the structure amplitude tends to zero for the arbitrary values of the order m of the satellite reflections; (2) for certain values of the amplitudes of the modulation function the modulus of the structure amplitude assumes its maximum value and (3) for the latter case the intensity of an arbitrary satellite reflection is a function of the intensities of both the closest main reflection and the intensities of the finite set of the neighbor satellite reflections