Electromagnetic properties of terbium gallium garnet at millikelvin temperatures and single photon energy

Abstract

Electromagnetic properties of single crystal terbium gallium garnet (TGG) are characterised from room down to millikelvin temperatures using the whispering gallery mode method. Microwave spectroscopy is performed at low powers equivalent to a few photons in energy and conducted as functions of the magnetic field and temperature. A phase transition is detected close to the temperature of 3.5 K. This is observed for multiple whispering gallery modes causing an abrupt negative frequency shift and a change in transmission due to extra losses in the new phase caused by a change in complex magnetic susceptibility.