Nucleation in turbulent fluids near the critical point

Effects of turbulence on nucleation are investigated in classical fluids near the critical point. Turbulence can suppress the onset of nucleation, enhance the coagulation of droplets, and break up droplets. The final characteristic droplet size strongly depends on the temperature difference T_c − T and the Reynolds number Re.     

Two-scale-factor universality near the critical point of fluids

Experimental evidence is presented in support of the validity of the hypothesis of two-scale-factor universality for the correlation function of fluids near the gas-liquid critical point. For Xe, SF₆, and CO₂, the dimensionless quantity $\text{R = ξ}{}_0\text{(}\text{B}{}^2\text{P}{}_{\mathrm{c}}\text{Γk}{}_{\mathrm{B}}\text{T}{}_{\mathrm{c}}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>3</mtext>}}$ is universal within the experimental accuracy of 10% and agrees with the value predicted by theory.     

Gravity influence on thermal relaxation near the critical point

Holographic interferometry measurements on a critical fluid sample performed during the D1 Spacelab flight reveal unexpected large density inhomogeneities under microgravity conditions. They are sustained by adiabatic temperature gradients and grow when approaching the critical point due to the diverging thermal expansion coefficient. Substantially increased relaxation time scales are found.     

Nuclear spin relaxation and molecular motions in sebacate polyesters

In this study, pulsed NMR techniques were used to probe the molecular motions occurring in poly(hexamethylenesebacate) (HMS), and its amorphous isomer poly(2-methyl-2-ethyl-1,3-propylenesebacate (MEPS), and in particular, to examine the perturbations in the molecular motions of HMS and MEPS that arise as a result of block copolymerization. The results show a low-temperature β-relaxation due to local motion of the methylenes of the chain backbone, an α-relaxation associated with the glass transition, and an α_c-relaxation due to the melting of the crystalline phase. Both α-and β-relaxations in the block copolymer are only slightly perturbed, and this suggests that the blocks are incompatible. The β-relaxation of HMS is confined to the amorphous regions of the polymer. Although sample preparation had little effect on the degree of crystallinity, large differences in the relaxation behavior were observed when the sample preparation was varied.     

Droplet model for sound absorption in fluids near the critical point

The dynamic droplet model of critical fluids is applied to understanding critical fluid sound absorption. The resonance frequency of an oscillating droplet is used to explain the scaling on frequency and temperature observed in sound absorption data.     

Dynamic susceptibility of a spin ice near the critical point

We consider spin ice magnets (primarily, Dy₂Ti₂O₇ in the vicinity of their critical point on the (H, T) plane. We find that the longitudinal susceptibility diverges at the critical point, leading to the behavior qualitatively similar to the one which would result from non-zero conductance of magnetic charges. We show that dynamics of critical fluctuations belongs to the universality class of easy-axis ferroelectric and calculate logarithmic corrections (within two-loop approximation) to the mean-field critical behavior.     

Nuclear spin relaxation rates in two-Leg spin ladders

Using the transfer-matrix density-matrix renormalization group method, we study the nuclear spin relaxation rate 1/T(1) in the two-leg s = 1 / 2 ladder as a function of the interchain (J( perpendicular)) and intrachain (J( parallel)) couplings. In particular, we separate the q(y) = 0 and pi contributions and show that the latter contribute significantly to the copper relaxation rate (63)(1/T(1)) in the experimentally relevant coupling and temperature range. We compare our results to both theoretical predictions and experimental measures on ladder materials.     

Nuclear spin relaxation in CoAl system

The temperature and magnetic field dependence of the nuclear spin relaxation rate $\text{1}\text{T}{}_1$ for ²⁷Al in CoAl has been investigated in the vicinity of exact stoichiometry. The experimental results are interpreted well in terms of the Benoit, de Gennes and Silhoutte (BGS) mechanism which involves a mutual flip of a nuclear spin and a localized electro spin on an excess Co atom via the Rudermann, Kittel, Kasuya and Yosida (RKKY) interaction, as well as the Korringa mechanism. The dynamic behavior of the localized moments thus obtained is discussed in terms of the Kondo effect.     

Nuclear spin relaxation in disordered conductors

The relaxation timesT ₁ andT ₂ for nuclear spins interacting with the conduction electrons of disordered metals are calculated. An explicite expression for Warren's relaxation enhancement in terms of electron spectra is obtained, showing in particular that the proportionality ofT ₁ and the conductivity is a universal feature of high resistivity conductors. A formula for an inhomogeneous line width contribution due to disorder induced static Knight shift fluctuations is found. Two dimensional electron spin diffusion is shown to imply various logarithmic line width corrections.     

Nuclear spin relaxation in liquid hydrogen

The proton longitudinal relaxation time, T₁, has been measured in dilute solutions of orthohydrogen in parahydrogen over a range of temperature and composition. Below 1.75% orthohydrogen the temperature dependence of T₁ observed at higher concentration is reversed. This fact is interpreted in terms of an anisotropic intermolecular force which is of longer range for ortho-ortho than for ortho-para pairs, in keeping with previous interpretations of T₁ in the gaseous state.     

Spin-spin relaxation near the curie point

The electronic spin-spin relaxation time, T₂, is calculated for Heisenberg ferromagnets in the region immediately above the Curie point. The calculation, which pertains to zero field, predicts that 1/T₂ diverges as χ_T^3/4 where χ_T is the isothermal susceptibility. The relative efficiencies of the dipolar spin-spin relaxation and exchange induced spin diffusion are compared. For systems with parameters comparable to EuO spin-spin relaxation dominates for temperatures such that the ratio of the correlation length to lattice parameter is greater than two. In systems having the parameters of iron dipolar effects are much less important.     

Nuclear spin relaxation in strongly disordered superconductors

The influence of nonmagnetic impurity and spin-orbit scattering on the nuclear spin-lattice relaxation rate in strongly disordered superconductors is presented. Using Anderson's exact-eigenstate formalism, it is shown that there exist two effects of disorder onT ₁ ^–1 . Firstly, nonmagnetic impurity and spin-orbit scattering enhances the magnitude of the relaxation rate in the same manner as in the normal dirty metal due to the diffusive nature of quasiparticle motion. Secondly, the Hebel-Slichter peak becomes suppressed due to the disorder enhancement of the quasiparticle inelastic scattering rate due to phonon, Coulomb, and/or spin-fluctuation interactions. Comparison with the available experimental data is made.     

Non-newtonian effect near the critical point

In the presence of a shear flow the order parameter fluctuations are distorted and suppressed by the shear and there appears a new correlation length independent of temperature sufficiently near the critical point.     

129Xe-Xe molecular spin relaxation

We identify the formation of bound 129Xe-Xe molecules as the primary fundamental spin-relaxation process at densities below 14 amagat. Low pressure Xe relaxation rate measurements as a function of gas composition show that Xe-Xe molecular relaxation contributes 1/T1 = 1/4.1 h to the total observed relaxation rate. The measured rate is consistent with theoretical estimates deduced from previously measured NMR chemical shifts. At atmospheric pressure the molecular relaxation is more than an order of magnitude stronger than binary relaxation. Confusion of molecular and wall relaxation mechanisms has historically caused wall relaxation rates to be overestimated.