标题： 观察残余铌杂质引起的KTaO$_3$相变 显示英文标题

标题： Observation of a phase transition in KTaO$_3$ induced by residual niobium impurities

摘要： 我们报告观察到KTaO$_3$晶体中的相变，对应于铁电-顺电相变。该晶体被放置在铜腔中以形成介质加载微波腔，并且观察到相变发生在约134 K附近。随着腔体冷却，横电磁和横电谐振模式的频率下降（对应于介电常数的增加）。 模式频率在相变温度（约134 K）处汇聚，并且在此点以下，它们的调谐方向反转，随着温度的降低频率增加。 这种行为对应于介电常数的减少，对于纯KTaO$_3$来说这是不寻常的。 为了进一步研究，我们使用激光烧蚀电感耦合质谱法（LA-ICPMS）进行了杂质分析，结果显示晶体中铌（Nb）的浓度显著（$\sim$ 7%）。 这表明观察到的相变是由残留的Nb杂质驱动的，这些杂质在原本顺电的基质中诱导出铁电性。 具有较低浓度（$<$ 2%）的类似晶体没有发生相变，但在该温度下表现出损耗峰。 这些发现对可调器件的设计具有实际意义，例如基于谐振器的暗物质探测器，其中低损耗材料的相稳定性和可调性至关重要。 显示英文摘要

摘要： We report the observation of a phase transition in a KTaO$_3$ crystal, corresponding to a paraelectric-to-ferroelectric transition. The crystal was placed inside a copper cavity to form a dielectric-loaded microwave cavity, and the transition was observed to occur near 134 K. As the cavity was cooled, the frequencies of both transverse electric and transverse magnetic resonant modes decreased (corresponding to an increase in permittivity). The mode frequencies converge at the transition temperature (near 134 K) and, below this point, reverse their tuning direction, increasing their frequency with decreasing temperature. This behaviour corresponds to a decrease in dielectric permittivity and is atypical for pure KTaO$_3$. To investigate further, we conducted impurity analysis using Laser Ablation inductively coupled mass spectrometry (LA-ICPMS), revealing a significant concentration ($\sim$ 7\%) of niobium (Nb) in the crystal. This suggests that the observed phase transition is driven by residual Nb impurities, which induce ferroelectricity in an otherwise paraelectric host. Similar crystals with a lower concentration ($<$ 2\%) did not undergo a phase transition but exhibited a loss peak at this temperature. These findings have practical implications for the design of tunable devices, for example, resonator-based dark matter detectors, where low-loss material phase stability and tunability are crucial.