| Chemical doping is a conventional way to introduce charge carriers into solids.This approach has led to the finding of high-T_c superconductivity in cuprates and iron-based superconductors by suppressing the antiferromagnetism or spin density wave.However,the element replacement and the variation of carrier density cannot practically cover a large regime and leave many phases unexplored.In recent years,the applica-tion of field effect transistors(FET)in two-dimensional systems is an effective way to control electronic properties via reversible changes of charge carrier density.Such an electrostatic doping is desirable to study novel phases that cannot be achieved by ma-terial synthetic methods.The FET devices have been widely applied in the exploration of new superconductors,the preparation for new devices as well as many applications in semiconductor industry.In this dissertation,we have synthesized high quality(Li,Fe)OHFeSe single crystals,and observed a first-order transition from superconductor to AFM insulator with a strong charge doping induced by ionic gating in the thin flakes of single crys-tal(Li,Fe)OHFeSe.The superconducting transition temperature(T_c)is continuously enhanced with electron doping by ionic gating up to a maximum T_c of 43 K,and a striking superconductor-insulator transition occurs just at the verge of optimal doping.We have also grew high quality FeSe single crystals,and investigated the evolution of superconductivity in the FeSe thin flake with systematically regulated carrier concen-trations by the liquid-gating technique.With electron doping tuned by gate voltage,high-temperature superconductivity with an onset transition at 48 K could be achieved in FeSe thin flake with T_c less than 10 K,and accompanied by a Lifshitz transition at a certain carrier concentration.Also,we have developed a novel field-effect transistor(FET)device using solid ion conductor(SIC)as the gate dielectric,which can tune the carrier density of FeSe by driving lithium ions into or out of the FeSe thin flakes and consequently control the carrier density of FeSe thin flakes.With increasing Li con-tent,we observed a dome-shaped superconducting phase diagram with T_c?46.6 K for the optimal doping,and an insulating phase was reached at the extremely overdoped regime.The dissertation is divided into four chapters as follows:1.IntroductionIn this chapter,we briefly review the historical development of FET gating tech-nique,maily introducing the traditional electric-double-layer FET and newly developed solid ion conductors based FET.In addition,we also briefly summarize the types of gate dielectric and possible applications,and their advantages and disadvantages,focusing on the tunable superconductivity by gate voltage.2.Gate-tuned Superconductor-Insulator transition in(Li,Fe)OHFeSeThe antiferromagnetic(AFM)insulator-superconductor transition has been always a center of interest in the underlying physics of unconventional superconductors.How-ever,in the family of iron-based high-T_c superconductors,no intrinsic superconductor-insulator transition has been confirmed so far.In this chapter,we report a first-order transition from superconductor to AFM insulator with a strong charge doping induced by ionic gating in the thin flakes of single crystal(Li,Fe)OHFeSe.The superconduct-ing transition temperature(T_c)is continuously enhanced with electron doping by ionic gating up to a maximum T_c of 43 K,and a striking superconductor-insulator transition occurs just at the verge of optimal doping with highest T_c.A novel phase diagram of temperature-gating voltage with the superconductor-insulator transition is mapped out,indicating that the superconductor-insulator transition is a common feature for uncon-ventional superconductivity.These results help to uncover the underlying physics of iron-based superconductivity as well as the universal mechanism of high-T_c supercon-ductivity.Our finding also suggests that the gate-controlled strong charge doping makes it possible to explore novel states of matter in a way beyond traditional methods.3.Evolution of high-temperature superconductivity from low-T_c phase tuned by carrier concentration in FeSe thin flakesIn this chapter,we report the evolution of superconductivity in the FeSe thin flake with systematically regulated carrier concentrations by the liquid-gating technique.With electron doping tuned by gate voltage,high-temperature superconductivity with an on?set at 48 K can be achieved in FeSe thin flake with T_c less than 10 K.This is the first time to achieve such a high temperature superconductivity in FeSe without either epitaxial interface or external pressure,and it definitely proves that the simple electron-doping process is able to induce high-temperature superconductivity with T_conset as high as 48 K in bulk FeSe.Intriguingly,our data also indicates that the superconductivity is sud-denly changed from low-T_c phase to high-T_c phase with a Lifshitz transition at a cer-tain carrier concentration.These results help to build a unified picture to understand the high-temperature superconductivity among FeSe-derived superconductors and shed light on further pursuit of higher T_c in these materials.4.Tuning phase transitions of FeSe thin flakes by field effect transistor with solid ion conductor as gate dielectricIn this chapter,we have developed a novel field-effect transistor(FET)device us-ing solid ion conductor(SIC)as the gate dielectric,which can tune the carrier density of FeSe by driving lithium ions into or out of the FeSe thin flakes,and consequently control the physical properties and phase transitions.A dome-shaped superconducting phase diagram was mapped out with increasing Li content,and T_c?46.6 K is obtained for the optimal doping,and an insulating phase was reached at the extremely overdoped regime.Our study suggests that,using solid ion conductor as the gate dielectric,the SIC-FET device is able to induce much higher carrier doping in the bulk,and is suit-able for many surface sensitive experimental probes,and stabilize the novel metastable structural phases that are inaccessible in ordinary conditions. |
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