Speaker
Description
High-quality microwave resonators are a key enabling technology for axion haloscopes and other wave-like dark matter experiments, where detector sensitivity depends critically on the stability and microwave loss of dielectric materials at cryogenic temperatures. Potassium tantalate (KTaO₃) is a promising quantum paraelectric dielectric owing to its high relative permittivity and intrinsically low microwave loss. We present cryogenic microwave dielectric spectroscopy measurements of a nominally pure KTaO₃ single crystal that reveal an unexpected paraelectric-to-ferroelectric phase transition at approximately 134 K. Measurements of multiple resonant modes show a pronounced anomaly in the temperature dependence of the dielectric response, including a reversal in the resonance frequency shift below the transition temperature. To determine the origin of this behaviour, laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was performed, revealing a significantly higher residual niobium concentration than in crystals exhibiting the expected quantum paraelectric behaviour. These results indicate that unintentional Nb incorporation effectively forms KTa₁₋ₓNbₓO₃ (KTN), driving the observed ferroelectric transition. The study demonstrates that trace impurity levels can substantially modify the microwave properties of dielectric resonator materials and highlights the importance of rigorous materials characterisation for precision cryogenic experiments. These findings provide practical guidance for the selection, qualification, and modelling of dielectric materials used in high-Q tunable microwave resonators for future axion and other wave-like dark matter searches.
| Primary Abstract Topic | Experiment: Axions and Wave-Like-DM |
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