Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses

TL;DR

This paper explains the temperature-independent plateau in dielectric constant at ultralow temperatures in glasses by considering nuclear quadrupole interactions, predicting magnetic field effects that restore temperature dependence.

Contribution

It introduces a model incorporating nuclear quadrupole interactions to explain the plateau effect in dielectric properties of glasses at ultralow temperatures.

Findings

Nuclear quadrupole interactions reduce effective tunneling amplitudes at ultralow temperatures.

The plateau in dielectric constant occurs below a characteristic temperature related to quadrupole energy.

Applying a strong magnetic field suppresses quadrupole effects, restoring the logarithmic temperature dependence.

Abstract

The standard tunneling model describes quite satisfactorily the thermal properties of amorphous solids at temperatures $T<1K$ in terms of an ensemble of two-level systems possessing logarithmically uniform distribution over their tunneling amplitudes and uniform distribution over their asymmetry energies. In particular, this distribution explains the observable logarithmic temperature dependence of the dielectric constant. Yet, experiments have shown that at ultralow temperatures $T<5mK$ such a temperature behavior breaks down and the dielectric constant becomes temperature independent (plateau effect). In this letter we suggest an explanation of this behavior exploiting the effect of the nuclear quadrupole interaction on tunneling. We show that below a temperature corresponding to the characteristic energy of the nuclear quadrupole interaction the effective tunneling amplitude is reduced by a small overlap factor of the nuclear quadrupole ground states in the left and right potential wells of the tunneling system. It is just this reduction that explains the plateau effect . We predict that the application of a sufficiently large magnetic field $B>10T$ should restore the logarithmic dependence because of the suppression of the nuclear quadrupole interaction.