(Li,Fe)OHFeSe Films And Single Crystals:Syntheses And Novel Physical Properties

Iron-based superconductors have attracted extensive attention with regard to the high critical temperature?T_c?mechanism and potential applications.Much progress has been made experimentally and theoretically,thus promoted the understanding of the iron-based high-temperature superconductivity.In particular,FeSe-based superconductors are emerging as a hot topic due to its highly tunable T_c and novel physical phenomena.Understanding the evolution of high-T_c from a low-T_c phase in FeSe bulk material,will not only shed new light on its mechanism,but also provide important clues to the application.This thesis focuses on?Li,Fe?OHFeSe high-T_c superconductor.The main conclusions are as follows:?1?Through optimizing the hydrothermal ion-exchange method,we synthesized a series of?Li,Fe?OHFeSe single crystals with a variable T_c.A metal-insulator transition exists in the low-T_c samples,and the non-superconducting crystal sample shows an insulating behavior via the electrical transport measurements.?2?Our team develops a new soft chemical approach of the matrix-assisted hydrothermal epitaxial growth technique–MAHEG.Via the MAHEG technique,we have successfully grown the?Li,Fe?OHFeSe single-crystalline film of high quality and high critical parameters on a LaAlO₃ substrate.Its bulk superconducting transition temperature_c of 42.4 K is characterized by both zero electrical resistance and diamagnetization measurements.The upper critical field_c2 is estimated to be 79.5 T and 443 T for the magnetic field perpendicular and parallel to the(6(7 plane,respectively.Moreover,a large critical current density_c of a value over 0.5 MA/cm² is achieved at?20 K.Such a?Li,Fe?OHFeSe film is therefore not only important to the fundamental research for understanding the high-T_c mechanism,but also promising in the field of high-T_c superconductivity application,especially in high-performance electronic devices and large scientific facilities such as superconducting accelerators.?3?Through the MAHEG technique,we have succeeded in growing a series of?Li,Fe?OHFeSe films covering the whole superconducting regime,with the superconducting transition temperature T_c from 4 K up to 42 K.This film technique opens up a new way for fabricating other complex functional materials as well.Furthermore,our systematic transport investigation on the film samples indicates that both the electron and hole carriers contribute to the charge transport,with the scattering rates deviating from the Fermi liquid.We find that the superconductivity occurs upon the electron and hole mobility becoming divergent.And in the high T_c samples,the electron carriers are found much more mobile than the holes,a distinct feature from the low T_c samples.Hence,the transport results provide key insights into the underlying physics for iron-based high-T_c superconductivity.