Tuning Physical Properties In Two-dimensional Selenides

Manipulating the transport properties of materials is one of the significant research directions in condensed matter physics.The common methods to tune the transport properties of materials include chemical doping,external pressure and electrochemical intercalation.Recently,many fascinating physical properties such as quantum anomalous Hall effect,high temperature superconductivity and two-dimensional ferromagnetism have been discovered in 2D material as a result of decreasing dimensionality and controlling the carrier density by field-effect transistors.The study of superconductivity and magnetism in 2D material have triggered widespread interest for both fundamental physics researches and device applications.In this dissertation,we have used the traditionally mechanical and Al2O3-assisted exfoliation techniques to obtain the thin flakes FeSe with different thickness,and have systematically investigated the evolution of transport properties in FeSe thin flakes as a function of thickness.As the thickness of the FeSe flake is reduced,the structural transition temperature and superconducting transition temperature gradually decrease.Furthermore,the I-V characteristic curves and anisotropic magnetoresistance measurements support 2D superconductivity in few-layer FeSe.Then,we have successfully modulated electronic properties of FeSe0.5Te0.5 thin flakes using solid ion conductor field-effect transistor(SIC-FET)and realized reversible transition from superconductor to insulator.Finally,we have systematically investigated the evolution of transport properties of VSe2 thin flakes using SIC-FET device.Most notably,we firstly realize a paramagnetic-to-ferromagnetic transition in VSe2 thin flake.Thus,the FET technique provides a powerful tool to seek for exotic phases that are difficult to obtain via conventional methods.The dissertation is divided into five chapters as follows:1.IntroductionThis chapter is mainly divided into three parts to introduce the research background of this dissertation.In first part,we introduce the development of superconductivity and describe the basic properties of high-temperature superconductors.In second part,we briefly introduce the classification of FETs and focus on the application of FET technology.In third part,we briefly review the physical properties of FeSe-based superconductors and VSe2.2.Evolution of transport properties in FeSe thin flakes with thickness approaching the two-dimensional limitIn this chapter,we have used the traditionally mechanical and Al2O3-assisted exfoliation techniques to obtain the thin flakes FeSe with different thickness,and have systematically investigated the evolution of transport properties in FeSe thin flakes as a function of thickness.As the thickness of the FeSe flake is reduced,the structural transition temperature and superconducting transition temperature gradually decrease,and the B-nonlinear behavior for Hall resistance at low temperature is also gradually suppressed.The I-V characteristic curves and anisotropic magnetoresistance measurements support 2D superconductivity in few-layer FeSe.Increase of disorder scattering from boundaries of twinned domains and anisotropic strains in the nematic state,as well as reduction of dimensionality,are considered to account for the above results.Our results show that the anisotropic strains might also become a significant factor to tune the electronic states in FeSe,which impacts the nematic and superconducting transition by blocking the tetragonal-orthorhombic structural transition at low temperature.Our work provides a new way to clarify the interaction between nematic and superconducting orders.3.Tuning electronic properties of FeSe0.5Te0.5 thin flakes using a solid ion conductor field-effect transistorIn this chapter,we have successfully modulated electronic properties of FeSe0.5Te0.5 thin flakes using SIC-FET configuration.Contrary to our previous studies in FeSe thin flakes,XRD patterns indicate that Li+intercalation is not evident in our FeSe0.5Te0.5 FET devices.Our data suggests that Li+ions likely accumulate at the interface between FeSe0.5Te0.5 and solid ion conductor with the gate voltage applied,which leads to electrostatic doping and is responsible for the drastic change of electronic properties of FeSe0.5Te0.5.A systematic variation of transport properties in FeSe0.5Te0.5 reveals a change of electronic states from a superconducting state to an insulating state,which can be related to the strong electron correlation in this material.The substitution of Te for Se significantly changes the characteristics of two-dimensional FeSe layers and enhances the electronic correlation.4.Realizing ferromagnetism in a field-effect transistor based on VSe2 thin flakesIn this chapter,we have systematically investigated the evolution of transport properties of VSe2 thin flakes with electric-field-driven Li-ion intercalation by using SICFET device.Most notably,within a certain stage in the intercalation process,butterflyshaped features and a square-shaped hysteresis loop are observed at low magnetic fields in the MR and Hall resistivity,respectively.They are clear evidence for the emergence of ferromagnetism in the VSe2 thin flakes.We attribute the origin of ferromagnetism to a Stoner mechanism which is triggered by two parallel factors:(i)a 3D-to-2D crossover induced by the Li intercalation and(ii)the effective tuning of N(EF).The realization of ferromagnetic state in VSe2 thin flakes makes them promising candidates for future spintronic applications.Moreover,our results demonstrate that the SIC-FET device can be a useful tool for manipulating the electronic and magnetic properties of 2D materials.5.Summary and expectationIn this chapter,we summarize the research results of this dissertation and put foward questions which are worth further investigating based on the result in this dissertation.