External field in Landau theory of the weakly-first-order phase transition. Effect of uniaxial pressure on the orientational ordering in solid C70

The effect of the external conjugate field on a weakly discontinuous first-order phase transition is analyzed within Landau theory. The third-degree term in the Ginzburg-Landau expansion is shown to preserve the phase transition for some external fields, in contrast with the second-order case. The free-energy expansion is shown to correspond to a second-order phase transition under the influence of an “effective” external field that depends on both the temperature and the real field. The case of the orientational phase transition in solid C₇₀ is considered, and the transition thermoelastic phenomena caused by the coupling of order parameter with elastic strain are analyzed. It is shown that the uniaxial mechanical pressure along the direction of the 3-fold axis appears to be an external conjugate field for orientational ordering in crystalline C₇₀. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 594–599 (25 April 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Analytic approach to the motion of cosmological phase transition fronts

We consider the motion of planar phase-transition fronts in first-order phase transitions of the Universe. We find the steady state wall velocity as a function of a friction coefficient and thermodynamical parameters, taking into account the different hydrodynamic modes of propagation. We obtain analytical approximations for the velocity by using the thin wall approximation and the bag equation of state. We compare our results to those of numerical calculations and discuss the range of validity of the approximations. We analyze the structure of the stationary solutions. Multiple solutions may exist for a given set of parameters, even after discarding non-physical ones. We discuss which of these will be realized in the phase transition as the stationary wall velocity. Finally, we discuss on the saturation of the friction at ultra-relativistic velocities and the existence of runaway solutions.     

Local structure and orientational ordering in liquid bromoform

The neutron diffraction data of liquid bromoform (CHBr₃) at 25°C was analysed using the Empirical Potential Structure Refinement technique in combination with H/D isotopic substitution. Compared to liquid chloroform (CHCl₃), CHBr₃ displays more spatially defined intermolecular contacts. A preference for polar stacking with collinear alignment of dipole moments is observed for the most closely approaching CHBr₃ molecules, although to a lesser extent than in chloroform. Consistent with this, and in line with dielectric spectroscopy, the Kirkwood correlation factor from the structural model of CHBr₃ is smaller than that of CHCl₃. The net antiparallel alignment of dipole moments in CHBr₃, as suggested by dielectric spectroscopy, must be due to weak but persistent long-range orientation correlations in CHBr₃, which counteract the local polar stacking.     

Specific heat of the superconducting high pressure phase Ga II

The high pressure modification gallium II, which becomes superconducting near 6.4 K, has for the first time been investigated calorimetrically at about 35 kbar in the temperature range of liquid helium. From the data a Debye temperature _D = 178K and an electronic specific-heat coefficient =1.75 mJ/mole K² are obtained. These values are in good qualitative accord with the increased superconductingT _c with respect to that of Ga I.Work supported by the Deutsche Forschungsgemeinschaft.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthday.     

The change in phase behaviour and orientational order of solid N2 at high pressure under the influence of Ar: a simulation study

We have studied the influence of Ar atoms on the high-pressure phase behaviour of N₂ by a Monte Carlo study of Ar _x (N₂)_1-x mixed solids. The results were obtained on the N₂ rich side (0 ≤ x ≤ 0.25), in the region where the ? and δ phases exist in pure N₂. The ??—δ? transition shifts rapidly to lower temperatures with x: it has vanished for x ? 0.05. For x ? 0.12, a cubic orientationally ordered phase appears. The transition from this phase to the δ? phases shifts to higher temperatures with increasing x. The temperature of the onset as well as the completion of the cascade process, responsible for the δ?_loc—δ?_rot transition, is independent of composition. This transition disappears in the x-region where the ordered phase is present for all temperatures below the transition to the δ?_rot phase. Therefore, the influence of the isotropic diluents on the orientational behaviour of the N₂ molecules is threefold. In the δ?_loc phase, the vibron frequencies show a splitting that shifts as a function of x, which might be related to the occurrence of a tetragonal modification of this phase.     

Shear thinning and orientational ordering of wormlike micelles

Shear thinning and orientation of cylindrical surfactant and block copolymer micelles was investigated by rheo-SANS (small-angle neutron scattering) experiments. Shear thinning and orientation occur for shear rates (.)gamma tau(dis)>1, where tau(dis) is the disentanglement time of the micelles. Micelles align in the flow direction with an orientational distribution that can be well described by an Onsager-type distribution function. Over nearly the whole range of concentrations and for all cylindrical micelles investigated, the shear viscosity eta follows a simple eta approximately e(-aS) behavior as a function of the orientational order parameter S with the same prefactor a.     

Overview of partial orientational and positional ordering in concentrated systems

We present experimental results for Taylor-Couette flow subjected to a Coriolis force. We used an apparatus consisting of two concentric cylinders with the inner one rotating, and with a radius ratio near 0.75. It was mounted with its axis horizontal on a table which rotated with angular velocity about a vertical axis. For sufficiently low, the first bifurcation upon increasing the inner-cylinder rotation rate was to tilted vortices. With further increase in this bifurcation was followed by a secondary one to time-periodic tilted vortices. The two bifurcation lines met at higher. The initial bifurcation then became one to tilted traveling vortices. For even larger values of, the flow immediately above the initial transition was disordered, and for sufficiently large the initial bifurcation was to a featureless turbulent state. We studied these transitions with three different outer cylinders. Two had symmetric spatial ramps terminating both ends of a straight section to reduce the effect of the rigid, nonrotating ends, and one had no ramps. The transition to featureless turbulence in the apparatus with ramps became hysteretic over a range of.     

The orientational relaxation of methane molecules in the solid phase II at low temperatures

A model for the dynamics of the coupling between the orientations of the ordered CH₄ molecules in phase II of solid methane at low temperatures is proposed. The model is equivalent to the dynamics of disordered solid hydrogen. The effective interaction strength is determined by the overlap of the librational ground states in the molecular field potential and vanishes in the classical limit. An approximate expression for the effective interaction strength is derived, showing an exponential dependence on the uncertainty of orientation in the librational ground states. This parameter is estimated from the experimental values of the tunnel energies. The second moments of the spectral densities of several anisotropic operators are evaluated in the infinite temperature limit. The resulting gaussian approximations for the spectra are applied in a derivation of the spin lattice relaxation time. The calculated values of the spin lattice relaxation time are compared to experiment.     

Structure and orientational ordering in a fluid of elongated quadrupolar molecules

A second-order density-functional theory is used to study the effect of quadrupolar interactions on the isotropic–nematic transition in a system of fluids of elongated molecules interacting via the Gay–Berne potential. The direct pair-correlation functions of the coexisting isotropic fluid that enter in the theory as input information are obtained by solving the Ornstein–Zernike equation using the Percus–Yevick integral equation theory in the (reduced) temperature range of 1.6≤T^∗≤3.0$1.6\le T^{\ast }\le 3.0$ for different densities, temperatures and quadrupole moments. Using the harmonic coefficients of the direct pair-correlation functions, isotropic–nematic phase coexistence and thermodynamic parameters have been calculated. The theoretical results have been compared with the available computer simulation results.     

Structure and orientational ordering of nitrogen molecules physisorbed on graphite

The structure and orientational ordering of nitrogen molecules physisorbed on graphite have been studied by low-energy diffraction (LEED). A two-sublattice in-plane herringbone structure with glide lines along two perpendicular directions is inferred from LEED patterns at T < 30 K from the monolayer where the molecular centers have the commensurate $\text{(}\text{3}\text{×}\text{3}\text{)}$ 30° structure. The orientational order-disorder transition of this commensurate phase was examined by superlattice spot intensity and angular profile measurements for 20 < T < 38 K. A rapid drop in superlattice intensity is observed near 27 K. The persistence of some intensity to 38 K. is suggestive of residual short-range orientational ordering and perhaps finite size or heterogeneity effects. For increasing coverage at T = 15 K, there is first a transition to a previously unobserved uniaxial incommensurate phase and then a transition to an apparently triangular incommensurate phase. The orientational superlattice spots are clearly present in the uniaxial phase, but are much weaker in the triangular incommensurate phase. At 31 < T < 35 K, an apparently triangular incommensurate phase with no detectable orientational superlattice spots is observed. The lattice constant versus equilibrium vapor pressure curve has been determined in the latter case assuming a continuous transition. The lattice constants of the incommensurate phases are used to place limits on the extent of possible phase-coexistence regions between the commensurate, uniaxial incommensurate, and triangular incommensurate phases. The LEED patterns from the bilayer at T = 15 K indicate a double-period superlattice structure of the triangular incommensurate phase which does not have the glide line symmetries of the commensurate monolayer. Some effects of heterogeneity on these phase transitions are discussed. A phase diagram for 10 < T < 40 K is proposed.     

Efficient characterization of the orientational ordering of ESR-active probes in supermolecular fluids

Supermolecular fluids often exhibit organized arrangements which order the orientations of solved ESR-active probe molecules. Due to the high microviscosity, the ESR line-shape of the probe virtually reflects the distribution of the resonating frequencies of the involved spin packets and then the degree of orientational order of the probe. An efficient strategy to get precise estimates of both the magnetic parameters of the probe molecule and the width of the spin packet from the ESR spectrum has been developed. This allows us to adjust only the strength of the effective orienting potential in the fit procedure. A study of the orientational ordering of cholestane probe molecule dissolved in side-chain polymeric mesophases provides accurate testing of the procedure.     

Study under high pressure on the Hall effect in the magnetically ordering Kondo lattice material CeAl2

Measurements of the Hall effect in CeAl₂ have been performed between 1.5 K and 4.2 K under pressures up to 2 GPa. The temperature dependence of the Hall coefficient has been found to show increasingly distinct structure with rising pressure, which is obviously correlated to the magnetic ordering transition in this material. Under pressures exceeding about 1.6 GPa the Hall coefficient changes its sign in the temperature range under consideration.     

Superfluidity and quantum melting of p-H2 clusters

Structural and superfluid properties of p-H2 clusters of size up to N=40 molecules, are studied at low temperature (0.5 K相似文献