| In this thesis,the main contents include exploring the preparation method of superconducting Pr2CuO4±? thin films,systematically studying their electrical transport properties with various oxygen content,and manipulating the superconductivity in both Pr2CuO4±? and FeSe thin films via ionic liquid gating experiments.The major results obtained are as follows:1.High-quality and single-orientation superconducting Pr2CuO4±? thin films were grown on the substrates SrTiO3[001]by polymer assisted deposition method.By controlling the oxygen content of Pr2CuO4±? thin films via varying annealing conditions,samples with different superconducting transition temperatures(Tc)can be achieved,and the highst Tc is up to 25 K.2.We systematically studied the electrical transport properties of superconducting Pr2CuO4±? films with different oxygen content.For under-annealed samples,the low-temperature Hall resisitivity is negative and linear with magnetic field,indicating that carrier is electron-type.Due to the existence of short-range antiferromagnetic order,the low-temperature resistance shows an upturn behavior and magnetoresistance is negative.With increasing the annealing strength,oxygen content in the sample gradually decreases;antiferromagnetic order is gradually suppressed;Tc increases.For the optimally annealed sample(its T,is highest),Hall coefficient undergoes a sign change from negative to positive with the decrease of temperature and Hall resistivity has a nonlinear relationship with the magnetic field.This means the coexistence of electron-type and hole-type carriers in the optimally annealed samples.Correspondingly,the magnetoresistance becomes positive.As the annealing strength continues to increase,oxygen content decreases further and Tc becomes smaller.Then the Hall resistivity is positive and linear with the magnetic field,indicating that hole-type carriers dominate the electrical transport properties.The above-mentioned electrical transport properties of Pr2CuO4±? with the change of the oxygen content are highly consistent with those of Pr2-xCexCuO4 as Ce-doped concentration increases.This finding points to the fact that the doped electrons induced by the oxygen removal are responsible for the superconductivity in the copper oxides cupates RE2CuO4±?(RE=La,Pr,Nd,Sm,Gd).In addition,we set the Hall coefficient as the horizontal axis to study the evolution of Tc in both Pr2CuO4±? and Pr2-xCexCuO4.It is found that Tc always reaches its maximum near RH=0,which provides important information to further study the high-Tc mechanism.3.We carried out ionic liquid gating experiments on superconducting and non-superconducting Pr2CuO4±? films.Positive bias voltages(i.e.Pr2CuO4±? film acts as a cathode)induce elelctron doping.Thus the resistance decreases in both superconducting and non-superconducting samples.Positive bias gating can drive the insulating sample into superconducting one,and push the superconducting state of sample into a non-superconducting metallic state.Such prcocess is reversible,as the sample returns to the pristine when the gate voltage is removed.In the negative voltage gating process,the non-superconducting Pr2CuO4±? film acts as an anode.The resistance and Hall resistivity show an indiscernible change in the voltage range from 0 to-3 V.As the voltage reaches-4 V,the resistance suddently increases and meanwhile the Hall resistivity drops substantially,indicating that electron carrier density decreases.As the gate voltage-4 V is withdrawn,the non-superconducting Pr2CuO4±? film obtains superconductivity.Correspondingly,there is an abrupt sign change of Hall resistivity at 30 K from negative to positive.Obviously,-4 V causes electrochemical reaction and consequently leads to the non-volatile superconductivity.It is known that negative gate volatge would reduce electrons or dope holes.Thus electron doping from electrochemical reaction is unlikely to account for the nature of non-volatile superconductivity.Combined with high-resolution scanning transmission electron microscope(HR-STEM)and in-situ x-ray diffraction measurements,here we propose a model that the repairing of oxygen vacancies in the CuO2 plane under negative voltage gating can be a rather good candidate to understand such new phenomenon.Our findings provide a new paradigm for inducing and manipulating superconductivity in copper oxide superconductors·4.We utilized ionic liquid gating technique to control the superconductivity of FeSe films,and detailedly investigated the electrical transport properties during ionic liquid gating.The following conclusions are drawn:The Tc in FeSe thin films with 200 nm thickness can be elevated to over 40 K by ionic liquid gating;The threshold bias to enhance FeSe superconductivity is generally above 4 V;As the dwell time increases,Tc gradually increases and the superconducting transition width becomes narrower;The highest Tc max mainly depends on the disorder of the pristine film;The higher the degree of disorder is,the lower the Tc max is.In addition,we find that the gating process of FeSe can be divided into two stages:in the first stage,the normal-state properties are almost unchanged and Tc gradually increases;in the second stage,Tc almost does not change,but the normal-state properties show a dramatic transformation. |
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