Whispering gallery mode dielectric spectroscopy of SrLaAlO4 at milliKelvin temperatures

Abstract

In this work, we excited microwave Whispering Gallery modes in single crystal SrLaAlO4 to characterise the dielectric properties from room to milliKelvin temperatures. Whispering Gallery modes behave as sensitive probes for monitoring anisotropic lattice behaviour by selective coupling to specific mode families of varying polarization. Measurements of Q-factor while cooling reveal important information on the temperature dependance of electromagnetic losses within the crystal, improving by two orders of magnitude from room temperature to 4 K to values of order 106. Quasi-transverse-electric modes show higher Q-factors than quasi-transverse-magnetic modes due to the biaxial anisotropy. The perpendicular and parallel relative permittivities are determined at room temperature to be ϵ⊥ = 16.843 ± 0.005 and ϵ∥=19.853±0.006 respectively, and 16.730 ± 0.005 and 19.602 ± 0.006 at 4 K, limited mainly by the accuracy of the dimensions of the crystal. Saturation states are observed with respect to microwave power below 4 K, which implies impurity ion electron spin interactions. Q-factors are degraded by an order of magnitude at mK temperatures and at very low input powers. At high magnetic fields between 3 to 7 T, the losses due to the spin impurities are reduced as a result of the high energy of spin polarization, eliminating unfilled two-level magnetic systems in their ground state, and improving the Q-factors back to values on the order of 106.