超导薄膜磁场穿透深度的双线圈互感测量

Measurement of magnetic penetration depth in superconducting films by two-coil mutual inductance technique

磁场穿透深度是联系超导体宏观电动力学与微观机制的重要物理量,其精确测量对于研究超导机理以及探索超导应用具有重要意义.在众多的磁场穿透深度测量方法中,双线圈互感法具有测量精度高、技术相对成熟、对样品没有破坏等优点,可被用于细致地研究超导薄膜的磁场穿透深度对温度、掺杂、外延应力等参量的依赖关系.本文首先简要介绍了双线圈互感法的基本原理,指出该方法的测量精度主要受系统几何参数及薄膜边缘漏磁的影响;之后对自主设计搭建的透射型双线圈互感装置进行了系统的校验,并详细说明了其测量精度:对于厚度为100 nm,穿透深度为150 nm的典型薄膜样品,穿透深度绝对值的测量误差小于10%;最后通过测量NbN超导薄膜的磁场穿透深度进一步检验了装置的精度,分析表明穿透深度的测量值与文献报道结果符合.

The magnetic penetration depth(l)of a superconductor is an important parameter which connects the macroscopic electrodynamics with the microscopic mechanism of superconductivity.High-accuracy measurement of l is of great significance for revealing the pairing mechanism of superconductivity and exploring the applications of superconductors.Among various methods used to measure l of superconducting films,the twocoil mutual inductance(MI)technique has been widely adopted due to its high precision and simplicity.In this paper,we start with introducing the principle of MI technique and pointing out that its accuracy is mainly limited by the uncertainties in the geometric parameters(e.g.the distance between two coils)and the leakage flux around the film edge.On this basis,we build a homemade transmission-type MI device with a delicate design to achieve high-accuracy.Two coils are fixed by a single-crystal sapphire block machined with high precisions to minimize the uncertainty in geometry.As a result,the reproducibility in induced voltage measured with sample remounted is better than 4%.Besides,the flux leakage around the film edge is accurately determined by measuring a thick Nb film and Nb foils.The voltage induced by leakage flux is only around 1%of that measured in the normal state.Therefore,the absolute value of l can be accurately extracted after flux leakage subtraction and normalization.It is shown that the error of the measured l is less than 10%for a typical superconducting film with a thickness of 100 nm and a penetration depth of 150 nm.Furthermore,the performance of our apparatus is tested on epitaxial NbN films with thickness of 6.5 nm.The results show that the low temperature variation of superfluid density is well described by the dirty s-wave BCS theory,and at temperatures close to Tc,the superfluid density decrease drastically,owing to the Berezinski-Kosterlitz-Thouless transition transition.Moreover,the zero-temperature magnetic penetration depth and the superconducting energy gap extracted from the fitting parameters are both consistent with the reported values.Our device provides an ideal platform for carrying out detailed studies of the dependence of l on temperature,chemical composition and epitaxial strain,etc.It could also be utilized to characterize other parameters of superconductor such as the critical current density,and when combined with the ionic liquid gating technique,our device offers an efficient route for revealing the microscopic mechanism of superconductivity.