Dielectric Properties of Single Crystal Calcium Tungstate

Abstract

This investigation employed microwave whispering gallery mode (WGM) analysis to characterize the dielectric properties of a cylindrical, single-crystal sample of calcium tungstate (CaWO${}_4$). Through investigation of quasi-transverse\hyp{}magnetic and quasi-transverse\hyp{}electric mode families, we can assess loss mechanisms and relative permittivity from room temperature down to cryogenic conditions. We report the biaxial permittivity values of ${\epsilon }_{||}=9.029\pm 0.009$ and ${\epsilon }_{\perp }=10.761\pm 0.01$ at $295$ K, and ${\epsilon }_{||}=8.794\pm 0.009$ and ${\epsilon }_{\perp }=10.440\pm 0.01$ at $4$ K. Components are denoted with respect to the c\hyp{}axis of the crystal unit cell. The parallel component agrees well with the published literature at MHz frequencies; however, the perpendicular component is $4.8$\% lower. The WGM technique offers greater precision, with accuracy limited primarily by the uncertainty in the crystal's dimensions. WGMs also serve as sensitive probes of lattice dynamics, enabling monitoring of temperature-dependent loss mechanisms. At room temperature, the measured loss tangents were $\tan {\delta }_{||}^{295,\mathrm{K}}=(4.1\pm 1.4)\times 10^{-5}$ and $\tan {\delta }_{\perp }^{295,\mathrm{K}}=(3.64\pm 0.92)\times 10^{-5}$. Upon cooling to 4 K, the loss tangents improved by approximately two orders of magnitude, reaching $\tan {\delta }_{||}^{4,\mathrm{K}}=(1.56\pm 0.52)\times 10^{-7}$ and $\tan {\delta }_{\perp }^{4,\mathrm{K}}=(2.05\pm 0.79)\times 10^{-7}$. These cryogenic values are higher than those reported in prior studies, likely due to a magnetic loss channel associated with an unidentified paramagnetic spin ensemble. These findings have implications for the use of CaWO${}_4$ in applications such as spin-based quantum systems and cryogenic bolometry, highlighting the potential of WGMs for novel sensing applications.

arXiv commentAccepted Version

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