Induced antiferroelectric smectic- C(*)(A) phase by doping ferroelectric- C* phase with bent-shaped molecules

Doping of the ferroelectric Sm-C(*) phase with bent-shaped molecules induces the antiferroelectric Sm-C(*)(A) phase. The effect was observed by means of electro-optic and dielectric measurements in systems with weak interlayer interactions in which the relative strength of anticlinic-synclinic order between molecules in adjacent layers is easily controlled by external factors. Fourier-transform infrared spectroscopy studies suggest that the bent-shaped molecules are not flat. They reorient upon the electric field-induced antiferroelectric-ferroelectric transition to adopt a position in which the average direction of the carbonyl groups is in the smectic plane and a bending tip along the C2 symmetry axis.     

Critical behavior of polar modes in lead phosphate near the ferroelastic phase transition

The temperature dependence of IR reflection spectra (15–4000 cm^?1) of A_u and B_u modes in the α phase and of E_u modes in the β phase of lead phosphate has been studied with a Fourier transform scattering interferometer. Frequencies, dampings and oscillator strengths of all modes show critical behavior, more or less marked, in the vicinity of the transition temperature. Correlations between the modes in both phases, including Raman results, are made. The temperature at which most of the modes lose their polar character is found more than ten degrees above that found by means of other techniques. Raman and X-ray data also indicate that characteristics of the monoclinic structure still persist well above T_c = 180°.     

Behavior of the layer compression modulus as a function of frequency near the nematic-smectic-A and re-entrant nematic-smectic-A phase transitions

A progressive cross-over of the layer compression modulus B from a simple power law behavior to a saturation behavior when the frequency is increased from 1 Hz to Hz is observed for the first time near the nematic-smectic-A and the re-entrant nematic-smectic-A phase transitions of a mixture of 4-cyano-4-(n-hexyloxy)biphenyl and 4-cyano-4-(n-octyloxy)biphenyl (6OCB-8OCB). This result shows that the saturation of B determined by second sound near the nematic-smectic-A phase transition of a similar mixture is dynamic in nature, and cannot be associated with the static saturation effect predicted by the dislocation loop model, as proposed previously.PACS: 64.70.Md Transitions in liquid crystals - 64.60.Ht Dynamic critical phenomena - 62.20.Dc Elasticity, elastic constants     

Critical behavior of the de gennes elastic constants near the nematic-smectic- A transition of TBBA

We have studied the behavior of elastic constants A, B, and C deduced from ultrasound velocity anisotropies in the vicinity of the nematic-smectic- A transition of terephthal-bis- p- p(')-butylaniline. A is associated with compressibility, B with layer compression, and C with the coupling between compressibility and layer compression. We show that the exponent of A is of the preasymptotic 3D- XY type, whereas those of B and C are in between the 3D- XY values and those associated with the anisotropic fixed point. This behavior is consistent with the extended crossover regime predicted by Patton and Andereck [Phys. Rev. Lett. 69, 1556 (1992)].     

Simple Landau model of the smectic- A-isotropic phase transition

A simple Landau-type free energy function is presented to describe the smectic-A-isotropic phase transition. Varying the coupling between orientational and positional order parameters, a smectic-A-isotropic or a nematic-isotropic phase transition occurs. Within this model the smectic-A-isotropic phase transition is found to be always more strongly first order than the nematic-isotropic phase transition. The theoretical results are found to be in good agreement with all published experimental results. Received 27 June 2000     

Critical behavior of the one-dimensional commensurate-incommensurate transition

We study the low-temperature critical behavior of one-dimensional charge-density-waves coupled to an underlying lattice, using the McMillan free energy. For weak coupling, the incommensurate CDW orders at T = 0 as a lattice of phase slip solutions with XY critical behavior. For strong coupling, the commensurate CDW orders at T = 0 with Ising critical behavior. Analytic expressions for the low-temperature inverse correlation length and average phase change are obtained for all values of the coupling to the lattice.     

Critical field enhancement near a superconductor-insulator transition

We have discovered a phenomenon where the orbital pair breaking effect is reduced, if not eliminated. It appears as a striking enhancement in the upper critical field H(c2) for (TMTSF)2PF6 and a strong upward curvature in the critical field versus temperature in the region of pressure-temperature phase space near the superconductor-spin density wave insulator boundary. A simple model based on self-consistently dividing the superconductor into layers explains the observations remarkably well and provides a unique way around orbital frustration and toward higher critical fields.     

Critical behaviour near the incommensurate-commensurate phase transition in dysprosium

Neutron scattering measurements of the helical turnangle indicate that the incommensurate-commensurate transition in Dy obeys a logarithmic mean-field behaviour and not a β-exponent power law.     

Critical behavior of the double layer classical XY-model

The partition function of a double layer classical nearest neighbor XY-model is expressed in terms of three coupled Coulomb gases. Renormalization-group equations, based on this representation, are derived and used in combination with a Griffiths inequality, to discuss the phase diagram for arbitrary inter-layer coupling.     

Critical line near the zero-density critical point of the Kosterlitz-Thouless transition

Using low-fugacity expansions, we study the exact behavior of the Kosterlitz-Thouless critical line near the zero-density critical point. We show that the critical temperature deviates from its zero-density value by a term proportional to the square root of the density.     

Critical behaviour near the metal-insulator transition of a doped Mott insulator

We have studied the critical behaviour of a doped Mott insulator near the metal-insulator transition for the infinite-dimensional Hubbard model using a linearized form of dynamical mean-field theory. The discontinuity in the chemical potential in the change from hole to electron doping, for U larger than a critical value U _c, has been calculated analytically and is found to be in good agreement with the results of numerical methods. We have also derived analytic expressions for the compressibility, the quasiparticle weight, the double occupancy and the local spin susceptibility near half-filling as functions of the on-site Coulomb interaction and the doping. Received 15 March 2001 and Received in final form 22 May 2001