Subrahmanyam Pilla

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.

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.

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.

Thermal hysteresis of the dielectric susceptibility of solid oxygen in the audio frequency range

High sensitivity measurements of the dielectric constant of solid oxygen are reported for 4 T 54 K. The results show unexpectedly large hysteresis effects for the temperature dependence of the dielectric constant in the and phases of oxygen on thermal cycling below 44 K. The behavior is compared to that observed for solid N2-Ar mixtures where the geometrical frustration of the molecular orientational ordering leads to pronounced memory effects. In contrast to solid N2-Ar where the effects of frustration and disorder combine to form an orientational glass, there is no disorder present for pure O2 and the hystereses are attributed to the strong frustration of the interactions.

Orientational ordering of solid CO: high sensitivity dielectric measurements

Abstract Dielectric constant measurements of solid N2 have revealed strong hysteresis effects due to geometrical frustration. To test the universality of such glassy behavior arising purely from geometrical frustration, we have made dielectric constant/loss measurements on a similar system, CO. We find that CO does not show any hysteresis effects in the entire solid phase. We suspect that this is due either to the very small window of temperature where CO exists in its orientationally disordered solid phase or to the presence of a small intrinsic dipole moment making it different from a pure quadrupolar system such as N2.