| What is the normal state from which superconductivity emerges?It is one of the fundamental questions for the study of superconductivity.Understanding the normal state is of great importance to unveil the mechanism of superconductivity.FeSe,the structural-simplest iron-based superconductor,provides an ideal platform for studying the high-temperature superconductivity,since its superconducting critical temperature（Tc）can be greatly promoted through electron doping.In this thesis,we implement continuous electron doping in FeSe by ionic liquid gating technique,and study the evolution of normal state with the development and enhancement of superconductivity.Combined with theoretical calculations,the relationship between superconductivity and the normal state is explored from different aspects,aiming to find the key factors that determine superconductivity from the perspective of normal state.The main results are as follows:（1）We introduce superconductivity in FeSe insulating films by ionic liquid gating,study the evolution of normal-state resistance during the superconductor-insulator transition process,and eventually give the normal-state critical condition for superconductivity to develop.We find that,superconductivity cannot emerge when the resistance exhibits variable-range hopping behavior;but once the resistance behaves as a logarithmic dependence on temperature（i.e.R∝ln（1/T））,superconductivity appears at low temperatures.This observation suggests that the ln（1/T）behavior of resistance is the minimal conducting condition in normal state for superconductivity to develop.We further demonstrate that this ln（1/T）behavior originates from granular metallicity,which means that the emergence of superconductivity requires at least there are metallic granules embedded in insulating background.（2）We realize continuous enhancement of Tc in FeSe superconducting films by ionic liquid gating（from～10 K to～45 K）,and study the evolution of normal-state resistance with enhanced superconductivity.We obtain for the first time the evidence for strange metallicity in FeSe,and give a quantitative relationship between strange metal and superconductivity.With superconductivity suppressed by high magnetic field,we observe typical strange-metal behaviors:the normal-state resistivity under high magnetic field varies as a linear function of temperature（T-linear resistivity）;the normal-state resistivity at low temperatures varies as a linear function of magnetic field（H-linear resistivity）;and the magnetoresistance displays a special scaling relation between temperature and magnetic field.On this base,we carefully tune Tc using ionic liquid gating technique,and obtain systematic data of Tc and the coefficient of T-linear resistivity A1 over a wide doping range.Eventually,we give the quantitative relationship between Tc and A1:Tc∝A10.5.This quadratic relation is universal for cuprates,Bechgaard salts and iron-based superconductors,suggesting there is a unified picture of the interplay between strange metallicity and unconventional superconductivity.（3）We study the evolution of electron correlations and Fermi surface topology in the normal state and superconducting pairing with Coulomb interaction and electron doping through theoretical calculations.We find out,the change of Fermi surface topology only makes marginal influence on superconductivity,while the enhancement of electron correlation strength can prominently enhance superconductivity.For those heavily electron-doped FeSe-based superconductors with high Tc’s,the electron correlations should be greatly enhanced.This is consistent with experimental results. |
