Characterization Of Superconducting Films By Two-coil Mutual Inductance Technique

Perfect diamagnetism is one of the basic characteristics of superconductors.Mag-netic responses of superconductors can provide information about the homogeneity of superconductivity and important parameters such as the magnetic penetration depth.The two-coil mutual inductance（TCMI）technique has unique advantages in measuring diamagnetism of superconducting films.In this thesis,we applied the TCMI technique to magnetic measurements of ionic-liquid-gated superconducting films and determining the absolute value of magnetic penetration depth of superconducting films.The main results are as follows:First,by means of the TCMI technique,we in situ measured the magnetic responses of ionic-liquid-gated FeSe and La_1.9Ce_0.1CuO₄films.It is found that although the film thicknesses are of several hundred nanometers,the gating process influences the entire film thickness.This bulk effect suggests that the electrochemical process plays a pri-mary role;On this basis,we studied the evolution of superconductivity in FeSe films with electron doping.It is observed that the critical temperature increases continuously and shows a positive correlation with the coefficient of the linear-in-temperature resis-tivity,pointing to a short-range antiferromagnetic exchange interaction mediated pairing mechanism.Second,we systematically investigated the evolution of electrical transport prop-erties in insulating FeSe films with electron doping.It is found that for the strong insulators with variable-range-hopping behavior,superconductivity does not develop in our measuring temperature range;On the contrary,superconducting fluctuations are shown to arise as soon as the resistance at low temperatures varies as ln（1/）,indicating a minimal condition on the normal-state conductivity for superconductivity to develop.Through microstructure characterizations,TCMI measurements and fluctuation analy-ses,we attributed this ln（1/）behavior to granular metallicity.This tends to establish that the development of superconductivity requires at least an insulating state containing metallic granules.Third,we built a TCMI measurement system based on a liquid-helium-free opti-cal cryostat with a delicate design.Measurement results on Nb N films show that the system could accurately measure the absolute value of magnetic penetration depth;Fur-thermore,we first applied the fast wavelet collocation method to the theory of TCMI technique,which allows to extract the absolute value of magnetic penetration depth of small-sized superconducting films.Our work gets more insight into the TCMI technique and ionic liquid gating tech-nique,laying the groundwork for efficiently extracting the scaling laws in superconductors.