Microwave dielectric properties of single-crystal quantum paraelectrics KTaO_3 and SrTiO_3 at cryogenic temperatures

摘要

Microwave dielectric properties of single-crystal incipient quantum ferroelectrics, KTaO_3 and SrTiO_3, have been measured at cryogenic temperatures. Cylindrical specimens were used as TE_(0n1)-mode and quasi-TE_(011)-mode dielectric resonators at temperatures ranging from 4 to 300 K. Conductive losses of the measurement resonant structures were taken into account, both as a function of frequency and temperature, so that uncertainties in the evaluated dielectric losses were ±5%. The real permittivity was measured with an accuracy of ±0.5%. The evaluated real permittivities of KTaO_3 and SrTiO_3 exhibit no ferroelectric transition, and remain paraelectric down to 5 K, consistent with soft-mode stabilization. Dielectric loss tangent values of KTaO_3 at 3 GHz were 4.2 x 10~(-5) at 5.4 K, 8.9 x 10~(-5) at 77 K, and 1.4 x 10~(-4) at 300 K, while those of SrTiO_3 were 3.4 x 10~(-3) at 5.4 K, 2.4 x 10~(-4) at 77 K, and 3.8 x 10~(-4) at 300 K. Results of the complex permittivity measurements are compared with theoretical predictions from a modified Devonshire phenomenological model.