Research On Tuning Physical Properties Of Superconducting (Li,Fe)OHFeSe Films

Emergent electronic phenomena in unconventional superconductors are at the forefront of research in condensed matter physics.So far,only two kinds of materials,cuprates and iron pnictides/chalcogenides,have been identified as the high-Tc family.Iron-based superconductors(IBSCs),especially in the format of films,are important in the research of applications and mechanisms.For instance,IBSCs are promising for applications using high magnetic fields.And the Majorana zero mode has been observed in the vortex cores of several different compounds of IBCS,bringing a new possibility to construct the topological qubit.At the same time,IBSC has become a new paradigm for superconductivity because its complicated and exotic physical properties bring the new research area,concepts,and approaches.Among all IBSCs,the intercalated FeSe superconductor(Li,Fe)OHFeSe has the highly tunable physical properties.What is more,our team has the long-term experience and advantages in the soft-chemical synthesizing and tuning technique of(Li,Fe)OHFeSe single crystals and epitaxial films.Therefore,my dissertation focuses on the high quality(Li,Fe)OHFe1-xSe single-crystalline films and studies the responses of their physical properties for different variables by using various techniques.We constantly unravel the existing puzzles and find the unexpected phenomena in this process,and our main achievements are below:1.We have significantly improved the crystalline quality of(Li,Fe)OHFe1-xSe epitaxial films after a steady optimization of the hydrothermal synthesis method that we developed.What is more,we successfully tuned their physical properties by the three different methods,including the chemical composition,the degree of disorder,and the concentration of iron vacancies x.2.Through the chemical substitution,we studied the effects of transition metals(TM)doping on the(Li,Fe)OHFe1-xSe single-crystalline films systematically and optimized their flux pinning performance by TM doping successfully.Remarkably,the critical current density of Mn-doped(Li,Fe)OHFeSe films has reached 0.32 MA cm-2 under μ0H=33 T along c axis and T=5 K,achieving the highest record so far among all IBSCs.This work confirms the potential of nontoxic iron chalcogenides for high-field applications.3.Utilizing the precise adjustment of the disorder of samples,we identified an emergent dissipative vortex phase between the common vortex glass and vortex liquid states in the mixed state of the(Li,Fe)OHFeSe films with the extremely weak disorder,while its feature fades away in the samples with the enhanced disorder.This work has a novel understanding of the vortex phase diagram and offers the corresponding experimental evidence.4.By tuning the iron vacancy concentration x of Fe1-xSe layers continuously,we could control the Tc of(Li,Fe)OHFe1-xSe.We found a superconductor-insulator transition and a pseudogap regime in its phase diagram using the electrical transport combined with ARPES measurements.This work is conducive to understanding such exotic phenomena and the key ingredients for high-Tc superconductivity in the heavily electron-doped FeSe superconductors.