Table of contents

Recent theoretical and experimental advances in the study of homogeneous nucleation are reviewed, with emphasis placed on phase transitions involving single-component liquids (condensation, cavitation, and crystallization from the melt). Extensions of classical nucleation theory are described and compared with new experiments that now directly measure nucleation rates. Novel methods of statistical mechanics, including density-functional theory and computer simulations, are presented. The recent rapid evolution of this field has opened up many new questions for further research.

The first successful calculations for the liquid structure S(q) of the early 3d transition metals are presented; they use effective pair interionic potentials and an integral equation theory of liquids. The effective pair potentials are obtained from a local form that assumes the superposition of the nearly free electron band, described by the empty-core model, and the d-band contribution, which is calculated by using an inverse scattering approach. The liquid state theory used in the calculations is the variational modified hypernetted chain (VMHNC) approximation. The agreement between the calculated S(q) and the available X-ray diffraction data for vanadium is reasonably good, but for titanium is only fair.

It is proposed that the first sharp diffraction peak (FSDP) in the structure factor of network glasses and liquids is a pre-peak in the concentration-concentration structure factor due to the chemical ordering of interstitial voids around cation-centred clusters in the structure. This model can predict quantitatively the positions of the FSDP for a wide range of oxide and chalcogenide glasses and liquids, and can also successfully rationalize the anomalous temperature and pressure behaviour of the FSDP intensity, as well as the effect of the incorporation of network modifiers.

Yttrium hydrates with eight-coordinated water molecules seem to be the predominant species in concentrated aqueous solutions of yttrium bromide and chloride. For molarities from approximately 2.1 down to 0.6M in H₂O EXAFS experiments yield an average Y³⁺-O distance of 2.33+or-0.02 AA for the YBr₃ and 2.34+or-0.02 AA for the YCl₃. Investigations by means of Raman spectroscopy with substitution of the solvent by D₂O for a 2.1M solution of YCl₃ lead to an isotopic shift of 1.052+or-0.007 for the polarized band assigned to the stretching vibrational mode of the complex, which corroborates the idea of the presence of a (relatively) stable hydrate of the yttrium ion.

A spinning-drop technique was used to study the thermal variation in the surface tension at a liquid crystal-isotropic liquid interface. A polymesomorphic liquid crystal with nematic and smectic A phases bounded by an isotropic liquid (glycerine) was used for the present study. The surface tension-temperature characteristics, unlike the monotonically decreasing dependence found in most isotropic liquids, showed regions with a positive slope for both the smectic A and the nematic phases. A sharp increase in slope of the surface tension-temperature characteristic was observed near the smectic A-nematic and nematic-isotropic transition temperatures, indicating that excess surface order is developing near these transitions. The observed results are in compliance with the available theoretical predictions.

The crystalline-to-glass transition in dilute charge polydisperse colloidal suspensions is investigated using Monte Carlo simulations assuming pairwise additivity of the interaction. The time averaging of coordinates is used to reduce the broadening of the pair correlation function due to random thermal motion. The suspension is found to remain crystalline at low polydispersities and becomes disordered at large charge polydispersities (CPDs). The behaviour of particle diffusion in the suspension is used to characterize the disordered state as glass. The dependence of structural parameters such as S_max, g_min/g_max and particle diffusion suggests that the transition occurs at 26% CPD. The polydispersity of effective hard-sphere diameters is obtained and compared with that of a size polydisperse system.

The mode coupling theory for supercooled liquid dynamics finds a beta -relaxation regime on mesoscopic timescales. It is caused by the interplay between nonlinear interactions of density fluctuations and phonon-assisted hopping transport. In this regime all correlation functions and spectra can be expressed in terms of a single beta -correlator G, which is a homogeneous function of time and two relevant control parameters. For temperatures T sufficiently above the critical value T_c hopping effects can be neglected and a stretched susceptibility minimum, is found as a crossover from von Schweidler decay to critical decay. For T near T_c hopping effects balance the cage effect and this results on logarithmic scales in a rather abrupt crossover from the high-frequency alpha -peak tail to the critical spectrum. For T below T_c there appears a frequency window between two knees in the susceptibility spectrum, where hopping effects suppress the enhanced fractal spectra. There occurs a crossover from Debye relaxation to white noise. The resulting susceptibility minimum in the strongly supercooled state exhibits a subtle power law dependence on the separation parameter T-T_c.

The scattering of high-frequency (h(cross) omega /k_B>10 K) phonons injected into superfluid ⁴He with low-frequency (h(cross) omega /k_B<1 K) thermal phonons in the liquid is studied both experimentally and theoretically. Quantum evaporation enables the selective study of only the high-frequency phonons. The attenuation of evaporation signals as the temperature is increased from 70 mK to 250 mK for various liquid path lengths is interpreted in terms of four-phonon scattering involving the high-frequency injected phonons and the low-frequency thermal phonons. Monte Carlo simulations of the signal variation with temperature show that the measured scattering is much weaker than the hydrodynamic theory has previously predicted. However, when this theory is extended to include diagrams representing further possible routes of the four-phonon scattering process, there are significant cancellations between these extra diagrams and those considered earlier. This leads to a weaker interaction and to a much improved agreement with the experimental results.

A series of nearly stoichiometric a-GaP films has been prepared by RF sputtering on to substrates held at temperatures from 20-200 degrees C. Extended X-ray fine absorption (EXAFS) at both the Ga and P K edges, infrared absorption, and optical edge data have been used to understand the structures of these films. The stoichiometric amorphous network of GaP, which was found to be fourfold coordinated as expected, showed no chemical disorder. The development of the atomic order of the network with increasing deposition temperature and annealing, from the amorphous to the crystalline state, has been followed. The results are discussed in terms of the bond lengths and angles and the strength of the Ga-P bonds.

Spinless fermions with local disorder and nearest neighbour repulsion are investigated on a Bethe lattice with infinite branching. Two phases are studied: a homogeneous phase and a checkerboard charge-density wave. Within this framework the model is exactly solved for all values of disorder, interaction and temperature. The transition between the two phases is described in detail. The density of states rho ( omega ), critical interaction U_c and order parameter b are calculated. The system displays anomalous behaviour: away from half-filling particle-density fluctuations due to weak disorder and/or low temperatures favour spontaneous symmetry breaking. The authors analyse and explain this unconventional phenomenon.

The scattering of a one-dimensional disordered wire consisting of elastic, time-reversal-invariant scatterers, is specified completely by the transmission intensity T, reflection phase phi _r and transmission phase phi _t. The problem of extracting the joint distribution p_{1 to Lz}(T, phi _r, phi _t) of these variables for a system of large length Lz, given the distribution p₁(T, phi _r, phi _t) of the individual scatterers is examined without recourse to the random-phase assumption and without restriction to weak disorder. The method adopted is to expand the distribution in terms of irreducible representations of SU(1,1), the group topologically formed by the k-space transfer matrices which describe the multiple scattering. Both the bulk of the distribution, and the resonance tail are examined. The method is applied to the case of the Anderson model over a range of disorders.

X-ray photoemission, X-ray absorption and reflection electron-energy-loss spectra of SmNbS₃ have been measured for the first time. The results give no clear evidence for valence fluctuation or the occurrence of mixed-valence states for Sm atoms. The Sm 4d and 3d X-ray photoemission spectra suggest that Sm exists mainly as a trivalent ion like the pressure-induced metallic phase of SmS, in agreement with a recent lattice parameter measurement. The author has estimated that the number of Sm²⁺ ions is less than a few per cent of the number of Sm³⁺ ions, if they exist. Charge transfer from SmS layers to the incompletely filled Nb d(z²) band of NbS₂ layers has an important effect on the valence state of Sm and the disappearance of a shoulder near the Nb L₂ X-ray absorption edge. The Sm N_4,5 inner-shell-electron-energy-loss spectrum exhibits non-optical transitions below the threshold, as has already been observed for the elemental metal Sm. The valence and conduction bands are also discussed.

Substitutions for Cu with Al in YbCu₅ stabilize the hexagonal CaCu₅ structure and cause a crossover from a divalent behaviour of the Yb ion in YbCu₅ to a stable integer 3+ behaviour of Yb in YbCu₃Al₂. Additionally, this substitution is responsible for the appearance of long-range magnetic order of a certain antiferromagnetic type in the compound YbCu₃Al₂ below 1.9 K.

In haematite the magnetic phase change from a weak ferromagnetic state to an antiferromagnetic state that occurs as the temperature is decreased through the critical value is known as the Morin transition. The different magnetic states give rise to distinct Mossbauer spectra and this technique was used to study the phase changes in samples of haematite that had been exposed to different levels of neutron irradiation. The magnetic phase transitions occurring in all the irradiated samples were studied and the critical temperatures measured. The mechanism and conditions required for the normal and reverse Morin phase transitions are discussed in terms of a model of competing single-ion and magnetic dipole anisotropy energy.