Nonasymptotic critical thermodynamical behavior of fluids

The paper is concerned with some theoretical and empirical attempts to obtain a unified equation for the thermodynamic properties of fluids that incorporates the nonclassical scaling laws near the critical point and crosses over to a classical analytic equation far away from the critical point. Specifically we investigate to what extent the various proposed crossover models agree with the known theoretical predictions for the universal ratios among the asymptotic and correction-to-scaling amplitudes in the power-law expansions of the thermodynamic properties around the critical point.     

Effect of impurity on the nematic-smectic-A transition

The divergence of the bend elastic constant above the nematic-smectic-A transition has been measured in undoped 80CB and in mixtures of 80CB and 70CB by Rayleigh scattering. The apparent critical exponent is found to be insensitive to the doping and has a nonclassical result.     

Director dynamics near the nematic-smectic-A phase transition

The Landau-de Gennes model for the free energy of a nematic liquid crystal near the phase transition to the smectic A-phase is used to determine the frequency dependence of the fluctuation corrections to the Frank elastic constants. It is shown that the interaction of the fluctuations of the smectic order parameter and the director results in corrections to all the Frank elastic constants. In the low-frequency limit (ω→0), the corrections to the Frank elastic constants K ₂₂ and K ₃₃ are the largest, and decrease to zero in the infinite-frequency limit. The correction to K ₁₁ is negative, and vanishes in both limits. The absolute value of the correction to K ₁₁ is the largest at frequencies in the megahertz range. It is shown that in oriented nematics the interaction of the smectic fluctuations and the director limits deviations of the director from the direction of preferred orientation, as a result of which relaxation of both inhomogeneous and homogeneous distortions of the director field can be observed. It is also shown that this gives rise to a frequency interval in the megahertz range in which shear waves begin to propagate in the nematic. The propagation speed of these waves is roughly a hundred times smaller than that of sound and strongly depends on the direction of propagation. Zh. éksp. Teor. Fiz. 114, 2022–2033 (December 1998)     

On the critical exponents for the λ transition in liquid helium

The use of a new method for summing divergent series makes it possible to significantly increase the accuracy of determining the critical exponents from the field theoretical renormalization group. The exponent value ν = 0.6700 ± 0.0006 for the λ transition in liquid helium is in good agreement with the experiment, but contradicts the last theoretical results based on using high-temperature series, the Monte Carlo method, and their synthesis.     

Nonasymptotic critical dynamics of the asymmetric-spin model in two-loop order

The crossover behaviour in the dynamics near the Lambda transition of⁴He is analyzed on the basis of the complete renormalization-group flow equations for the asymmetric-spin model (F) in two-loop order. Our results confirm the conclusions and refine the results obtained from an earlier analysis based on the symmetric model (E) and invalidate part of a recent analysis treating model F in an unsystematic approximation. Excellent agreement is found with measurements of second-sound damping over four decades in relative temperature. Closer toT our analysis predicts a further increase of the effective amplitudeR ^eff of the thermal conductivity.Supported in part by the Fonds zur Förderung der wissenschaftlichen Forschung     

A specific heat study of the nematic-smectic-A transition in octyloxycyanobiphenyl

A high-resolution ac calorimetry measurement of the nematic to smectic-A transition has been carried out carefully on octyloxycyanobiphenyl (80CB). The measured critical exponents α = α′ = .24 ± .03 are consistent with X-ray results through the hyperscaling relation ν∥ + 2ν_⊥ = 2 - α.     

Freedericksz transition in an anticlinic liquid crystal

It is shown experimentally that a very-long-pitch, surface-stabilized, anticlinic liquid crystal undergoes a two-step electric-field-induced transition to the synclinic phase. The liquid crystal remains undistorted below a threshold field E(th). For E>E(th), a Freedericksz transition occurs, wherein molecules in adjacent smectic layers undergo unequal azimuthal rotations about the layer normal. At higher fields a transition to the synclinic phase occurs via solitary waves.     

双轴性向列相液晶的相变理论

假设一双轴性向列相液晶分子简单相互作用模型，利用平均场理论，得到从各向同性相至单轴向列相、单轴向列相至双轴向列相的相变. 结果表明，在一定温度范围内，分子形状的非轴对称性可导致系统双轴向列相的产生. 在核磁共振实验中，导出双轴向列相序参数与谱线分裂频率间的一般关系，并讨论序参数测量性的可能性. 关键词： 向列相液晶 相变     

Tricritical-like behavior of the nonlinear optical refraction at the nematic-isotropic transition in the E7 thermotropic liquid crystal

We use Z-scan technique to investigate the nonlinear optical response of the thermotropic liquid crystal E7 in the neighborhood of the nematic-isotropic phase transition. The analysis of the data for the nonlinear optical birefringence is compatible with an effective critical exponent of the order parameter, β = 0.28 ± 0.03, which is close to the classical value, β = 0.25 , for a tricritical point. The nonlinear optical absorption in the nematic range depends on the geometrical configuration of the nematic director with respect to the polarization beam, and vanishes in the isotropic phase.     

A relation between critical exponents for the lambda phase transition in liquid helium

A relation between critical exponents is derived which permits the experimental determination of a so far unknown combination of critical exponents for theλ phase transition in liquid helium.