J. A. Hamida

Very high sensitivity ac capacitance bridge for the dielectric study of molecular solids at low temperatures

An ac capacitance bridge with excellent stability and two parts per billion resolution in the real part of the dielectric constant at temperatures ranging from 4.2 to 80 K and in the audio frequency range (0.2–20 kHz) is reported. The method uses commercially available components as well as few homemade cables and capacitors. Construction details and data on the resolution are presented.

DIELECTRIC ANOMALIES OF SOLID CO AND N2 IN THE AUDIO FREQUENCY RANGE

Abstract Audio frequency dielectric constant measurements of CO and N 2 in their solid phases are presented. Anomalous temperature dependence of the dielectric constant in the α -phases of both CO and N 2 and strong hysteresis effects above an onset temperature T h ≃42 K in the β -phase of pure N 2 have been observed. A simple mean-field theory is proposed which explains some of the observed anomalies in the α -phase.

NMR studies of the orientational ordering in dilute solid N2-Ar mixtures

We report the results of NMR studies of the orientational ordering of15N2 molecules in solid N2-Ar mixtures for N2 concentrations in the 40–50% range. For samples with 50%15N2 a new phase distinct from the quadrupolar glass phase for the hcp structure has been observed. In this new phase, the local orientational order parameters were found to have very closely the same value at each site in contrast to the very broad distribution of order parameters observed in the quadrupolar glass state for the hcp lattice at higher concentrations (57–77%).

Molecular solid glasses: new insights into frustrated systems

High sensitivity dielectric constant and heat capacity measurements are reported for solid N2-Ar mixtures as well as pure N2 in the presence of a small external ac electric field in the audio frequency range. Unexpected field-induced aging and memory effects are observed for thermal cycles of field cooled samples, above a well-defined temperature Th. The magnitude and temperature dependences of the effects exclude any simple interpretation in terms of lattice defects. The results show that the free energy surface includes a complexity that allows for metastable states and that tunnelling can occur between the states.

The Orientational Ordering in Dilute Solid N2-Ar Mixtures

NMR studies have been carried out to determine the nature of the orientational ordering of N2in dilute solid N2–Ar mixtures. The orientational ordering for cubic lattices (x(N2)≍50%) was compared to that observed for the hexagonal lattice structures for 57 < x(N2) < 75%. The NMR line shapes for the cubic structures were found to be sharply defined and could only be explained in terms of a narrow distribution of order parameters. This is in contrast to the quadrupolar glass state with a very broad order parameter distribution observed for the hexagonal structures.

A modified dual-slope method for heat capacity measurements of condensable gases

A high resolution nonadiabatic method for measuring the heat capacity (CP) of bulk samples of condensable gases in the range of 7.5–70 K is described. In this method CP is evaluated by directly comparing the heating and cooling rates of the sample temperature for two algebraically independent heat pulse sequences without explicit use of the thermal conductance between sample and thermal bath. A fully automated calorimeter for rapid measurement of CP of molecular solids utilizing this technique is presented. The technique along with the automated calorimeter with a provision to apply external electric and magnetic fields is particularly useful for the study of continuous phase transitions in molecular solids as well as field induced changes in the heat capacity.

A new orientational glass in low concentration N2−Ar solid mixtures

NMR studies of the orientational ordering of15N2 molecules in solid N2−Ar mixtures have been carried out for the first time for low N2 concentrations. We have observed a new phase, that appears to be an orientational glass, in which the molecular axes are frozen in random directions at each crystal site but for which the mean alignment with respect to each local axis is a maximum. This is to be contrasted to the case for higher N2 concentrations, 0.57<x(N2)<0.77, for which a true quadrupolar glass is observed with a broad distribution of both local molecular axes and the mean alignment with respect to those axes. The formation of the new phase is believed to occur only for the cubic lattice structure at low N2 concentrations, while the quadrupolar glass occurs for the hcp structure.

Molecular glasses: NMR and dielectric susceptibility measurements

We review the properties of simple diatomic molecular glasses as explored by nuclear magnetic resonance techniques and measurements of the dielectric susceptibility. We focus on the behavior of classical molecular rotors formed by solid N2–Ar mixtures and discuss the time dependent behavior in terms of replica symmetry breaking theories.

Interactions and Diffusion of Methane and Hydrogen in Microporous Structures: Nuclear Magnetic Resonance (NMR) Studies

Measurements of nuclear spin relaxation times over a wide temperature range have been used to determine the interaction energies and molecular dynamics of light molecular gases trapped in the cages of microporous structures. The experiments are designed so that, in the cases explored, the local excitations and the corresponding heat capacities determine the observed nuclear spin-lattice relaxation times. The results indicate well-defined excitation energies for low densities of methane and hydrogen deuteride in zeolite structures. The values obtained for methane are consistent with Monte Carlo calculations of A.V. Kumar et al. The results also confirm the high mobility and diffusivity of hydrogen deuteride in zeolite structures at low temperatures as observed by neutron scattering.

Generalized fluctuation dissipation and thermal hysteresis of dielectric susceptibility in frustrated molecular orientational glasses

The results of high precision measurements of the dielectric susceptibility (T) of solid oxygen in the low temperature α and β phases that have highly frustrated orientational interactions, are compared with the results of earlier studies of the thermal behavior of (T) in solid N2-Ar mixtures. In the latter case the combination of disorder and frustration results in orientational glass states and strong hysteresis effects are observed for thermal cycles. The onset of the hysteresis marks the boundaries between the glass states, the long range periodically ordered state and the disordered state. A similar behavior is observed for solid O2 without substitutional disorder. In the β-phase of solid O2 the magnetic moments form a frustrated helical structure which transits to a non-magnetically frustrated collinear anti-ferromagnet in the α-phase. The molecular orientations form a highly frustrated linear alignment in both phases. The behavior of the dielectric susceptibility which measures the response of orientational degrees of freedom is compared for these two solids using a generalized fluctuation dissipation approach developed by Cugliandolo and Kurchan.