低温宽带微波反射计测量的基于样品的校准
Sample-based calibration for cryogenic broadband microwave reflectometry measurements
摘要：
系统的特征频率提供了关于支配其物理性质的现象的重要信息。在这个框架内，最近有新的兴趣在低温微波表征凝聚态系统，因为它允许探测几个$μ$ eV的顺序的能量尺度。然而，在该频率范围内对样品响应的绝对值进行宽带测量对其环境极其敏感，需要仔细校准。在本文中，我们提出了一个\textit{in situ}校准方法的低温宽带微波反射测量实验，既简单的实现，通过它的样品的电磁环境的影响可以最小化。校准参考在这里由样品本身提供，在三个参考温度下，其阻抗被假定或测量，而不是像通常那样由外部标准提供。我们比较了工作在很低温度（0.02--4 K）下的a-Nb $_{15}$ Si $_{85}$超导薄膜的基于样品的校准（SBC）和通过开路-短路-负载标准校准（SC）获得的频率依赖的复阻抗（0.1--2 GHz），并表明SBC允许我们获得样品的绝对响应。该方法使校准平面尽可能靠近样品，以便环境电动响应不会影响测量，前提是它与温度无关。对于给定的实验设置，这导致灵敏度提高。
英文摘要：
The characteristic frequencies of a system provide important information on the phenomena that govern its physical properties. In this framework, there has recently been renewed interest in cryogenic microwave characterization for condensed matter systems since it allows to probe energy scales of the order of a few $μ$eV. However, broadband measurements of the absolute value of a sample response in this frequency range are extremely sensitive to its environment and require a careful calibration. In this paper, we present an \textit{in situ} calibration method for cryogenic broadband microwave reflectometry experiments that is both simple to implement and through which the effect of the sample electromagnetic environment can be minimized. The calibration references are here provided by the sample itself, at three reference temperatures where its impedance is assumed or measured, and not by external standards as is usual. We compare the frequency-dependent complex impedance (0.1--2 GHz) of an a-Nb$_{15}$Si$_{85}$ superconducting thin film obtained through this Sample-Based Calibration (SBC) and through an Open-Short-Load Standard Calibration (SC) when working at very low temperature (0.02--4 K) and show that the SBC allows us to obtain the absolute response of the sample. This method brings the calibration planes as close as possible to the sample, so that the environment electrodynamic response does not affect the measurement, provided it is temperature independent. This results in a heightened sensitivity, for a given experimental set--up.
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