| Manipulating collectively electronic states in a correlated system is at the core of condensed matter physics.Many exotic electronic phases have been discovered via var-ious methods including chemical doping and applying high pressure.Superconductivity is one of the hottest topics due to its fascinating phenomenon and promising application.Modulation of carrier density plays a critical role in the studies of superconductivity and other novel electronic phases.Controlling superconductivity and phase transition through tuning carrier density and beyond can not only provide new hints to reveal the mechanism of high-temperature superconductivity but also pave a new way to regulate physical properties in further multifunctional devices.In this dissertation,we firstly report an electric-field controlled reversible transi-tion from superconductor to ferromagnetic insulator in(Li,Fe)OHFeSe thin flakes via solid ion conductor based field-effect transistor(SIC-FET).Then,we exploit this new developed gating technique to realize various metastable phases in its sister material,(Li,Fe)OHFeS,and achieve a rich phase diagram involving superconductivity,soft fer-romagnetism,and hard ferromagnetism.In these works,structural phase transforma-tions as well as physical properties can be controlled by the electric field.These findings demonstrate the superior performance of the SIC-FET in regulating the physical prop-erties of layered materials and its potential applications for multifunctional devices.Fi-nally,we successfully synthesize new organic-ion-intercalated SnSe2 superconductors,namely(TBA)xSnSe2 and(CTA)xSnSe2,via electrochemical intercalation method and discover quasi-two-dimensional superconductivity in these materials.This work offers a new pathway to modify the interlayer coupling in van der waals materials,and sheds new light on exploring two-dimensional superconductivity in bulk layered materials.The dissertation is divided into four chapters as follows:1.IntroductionIn this chapter,we firstly review the history of superconductivity as well as the basic properties of high-temperature and two-dimensiomal superconductors.Then,the methods of field-effect transistor gating and electrochemical intercalation are briefly introduced.At last,we summarize the physical properties and research progress of(Li,Fe)OHFeSe,(Li,Fe)OHFeS and SnSe2.2.Electric-field controlled superconductor-ferromagnetic insulator transition in(Li,Fe)OHFeSe thin flakesSuperconductivity beyond electron-phonon mechanism is always twisted with magnetism.Here,we report an electric field controlled reversible transition from super-conductor to ferromagnetic insulator in(Li,Fe)OHFeSe thin flake.Using the SIC-FET device,Li ions can be driven into or extracted from the(Li,Fe)OHFeSe thin flake by electric field.A dome-shaped superconducting phase with optimal Tc of 43 K is contin-uously tuned into a ferromagnetic insulating phase.The latter is induced by Li injection into the materials by electric field.The Fe ions expelled from the(Li,Fe)OH layers by Li injection migrate to the center of Se square of FeSe layer and become ordered,which leads to a long-range ferromagnetic order.The device fabricated on solid ion conductor based field-effect transistor can reversibly manipulate collectively ordered electronic states of the materials and stabilize new metastable structures by electric field.Our work paves a way to access metastable phases and to control structural phase transfor-mation as well as physical properties by the electric field.These findings demonstrate the superior performance of the SIC-FET in regulating physical properties of layered crystals and its potential applications for multifunctional devices.3.Phase diagram and evolution of the magnetic ordering state driven by field-effect transistor in(Li,Fe)OHFeS thin flakesManipulating collectively ordered electronic and magnetic states in a correlated system is at the core of condensed matter physics.Besides tuning the carrier density of a crystal,the latest developed field effect transistor(FET)with solid ion conductor(SIC)can also drive ions into the crystal and lead to structural transformations that are difficult to access with conventional methods including conventional field effect tran-sistors.Here,we exploit the SIC-FET gating technique to realize a metastable phase in(Li,Fe)OHFeS thin flakes,and achieve a rich phase diagram involving superconduc-tivity,soft ferromagnetism and hard ferromagnetism.With the injection of Li ions into the thin flake,the superconductivity is gradually suppressed and a soft ferromagnetic phase with new crystal structure sets in,evidenced by the anomalous Hall effect and magnetoresistance measurements.Further injection of Li ion drives the soft ferromag-netic state into a hard ferromagnetic state,where different types of magnetic interac-tions dominate due to the simultaneous tuning of carrier density and magnetic Fe-ion concentration in the Li-ion injection process.Our work paves a way to control structural phase transformations as well as physical properties by the electric field.These find-ings demonstrate the superior performance of the SIC-FET in regulating the physical properties of layered crystals and its potential applications for multifunctional devices.4.Quasi-two-dimensional superconductivity in SnSe2 via organic ions intercala-LionExploring two-dimensional(2D)superconductivity in new layered materials has attracted significant research interests in condensed matter physics.In this work,new organic-ion-intercalated SnSe2 superconductors,namely(TBA)xSnSe2 and(CTA)xSnSe2,are successfully synthesized by an electrochemical intercalation method.Via organic-ion-intercalation,the c-axis parameter of(TBA)xSnSe2 and(CTA)xSnSe2 are enlarged from 6.12A of pristine SnSe2 to 18.62A and 14.74A,respectively.Mag-netic susceptibility measurement suggests both materials exhibiting strong anisotropic superconducting properties below 7K.Moreover,further electric transport measure-ments on(CTA)xSnSe2,such as anisotropic resistivity,angular dependent magnetore-sistance and V-I measurement,demonstrate quasi-two-dimensional superconductivity in this material.Furthermore,for the first time,a clear dome-like behavior in the Tc vs d phase diagram is observed in this system,which probably hints some unconven-tional mechanism behind it.Our work offers a new pathway to modify the interlayer coupling in van der waals materials,and sheds new light on exploring two-dimensional superconductivity in bulk layered materials. |
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