Low-Temperature Anomalies in the Microwave Dielectric Properties of Na<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>β</mml:mi></mml:math>-Alumina

Abstract

At low temperatures we have observed a strong microwave absorption in Na $\ensuremath{\beta}$-alumina which increases with decreasing temperature below 4 K. At electromagnetic intensities $I\underset{\ensuremath{\simeq}}{&lt;}0.1$ mW/${\mathrm{cm}}^{2}$ saturation occurs. This absorption is ascribed to low-energy tunneling states which are associated with the Na ions.