Study On Superconductivity Of Spinel Oxide And Development Of A New-generation High-throughput Facility

Chemical doping is one of the most effective methods to manipulate the unconven-tional superconductivity.Characterizing superconductors with different doping levels is helpful to extract key parameters dominating superconductivity,which is essential to understand the mechanism of unconventional superconductivity.In this thesis,we tune the composition of the spinel oxide superconductor Li Ti₂O₄by adjusting sample growth parameters and performing ionic liquid gating,and investigate its physical prop-erties via electrical transport and point-contact tunneling spectroscopy.In addition,we develop a combined system of laser molecular beam epitaxy and scanning tunneling mi-croscopy,which aims at fabricating combinatorial thin films with gradient composition and characterizing their electronic states in situ.The main results are as follows:(i)High-field electrical transport and point-contact tunneling spectroscopy are used to investigate superconducting properties of spinel oxide Li Ti₂O_4-?films with various oxygen contents.It is striking that although the superconducting transition temperature and energy gap are almost unchanged,an isotropic upper critical field B_c2up to 26.0T is observed in the oxygen-rich sample,which is more than twice the B_c2of 11.3 T in the anoxic one.The change of the dominating pair-breaking mechanism from the orbital effect to the spin flip at B_c2is achieved by tuning oxygen contents,which can be explained by the appearance of small Fermi pockets due to extra oxygen.(ii)Based on ionic liquid gating,we successfully manipulate the superconductivity of spinel oxide Li_1±xTi₂O_4-?.A dome-shaped superconducting phase diagram is estab-lished,where two insulating phases are disclosed both in heavily electron-doping and hole-doping regions.The superconductor-insulator transition(SIT)in the hole-doping region can be attributed to the loss of Ti valence electrons.In the electron-doping re-gion,Li_1±xTi₂O_4-?exhibits an unexpected SIT instead of a metallic behavior despite an increase in carrier density.Furthermore,a thermal hysteresis is observed in the nor-mal state resistance curve,suggesting a first-order phase transition.We speculate that the SIT and the thermal hysteresis stem from the enhanced 3d electron correlations and the formation of orbital ordering by comparing the transport and structural results of Li_1±xTi₂O_4-?with the other spinel oxide superconductor Mg Ti₂O₄,as well as analysing the electronic structure by first-principles calculations.(iii)We develop a facility comprised of a combinatorial laser molecular beam epi-taxy(LMBE)system and in situ scanning tunneling microscopy(STM).The LMBE chamber is equipped with a homemade rotary-mask,which enables the growth of com-binatorial thin films with gradient composition or thickness.By the specially designed delivery device,the film can be transferred in situ to the STM chamber.The STM is optimized for the characterization of combinatorial thin films in an XY coarse motion range of 10×10mm²with sub-micrometer location precision.The performance of the facility is demonstrated by synthesizing high-quality superconducting Fe Se thin films with gradient thickness,imaging surfaces of Au(111),Bi₂Sr₂Ca Cu₂O_8+?,Fe Se_0.5Te_0.5and Fe Se.In addition,we have carried out experiments on superconducting Ca KFe₄As₄,and studied its reconstructed surfaces as well as superconducting energy gaps.