Study On One-dimensional Electronic States And Magnetic Bound States Of Transition Metal Selinide

The study of ground states and relevant properties in low dimensional systems is essential to understand exiotic emergent phenomena,which are primarily dominated by electron-phonon coupling or electronic correlation interaction,including magnetism,High-Tc superconductity,giant magnetoresistance effect,and Mott insulator,etc.In this thesis,we investigated charge density wave(CDW)in Mo₆Se₆ nanowires,low energy ground states of mirror twin boundary(MTB)in MoSe₂and Yu-Shiba-Rusinov(YSR)states of isolated Fe adatom on the surface of 2H-NbSe₂ with low temperature scanning tunneling microscope(STM).The main results are summarized as follows:(1)In theory,one-dimension(1D)system is predicted to undergo Peierls phase transition by electron-phonon coupling,and form CDW.However,most studies were conducted in quasi-1D or quasi-2D systems.In our work,we observed the CDW in a pure1D system of Mo₆Se₆ nanowire for the first time.We found there are two kinds of Mo₆Se₆nanowires,namely nanowires attached to edges of monolayered MoSe₂ islands,and the isolated nanowires that have end contacts to the monolayer MoSe₂.Their CDW order behaves differently,i.e.,the former has coherence peak,while the latter not,which were systematically studied with variable temperature STS measurements.With density functional theory calculations and modeling,we uncovered the prominent effect of phason-polaron interaction,instead of weak quantum fluctuations,on the CDW.Thus,this leads to a new mechanism to suppress CDW coherent peaks in a pure 1D system.Our work elucidates influence of phason-polaron interaction on CDW order,and provides a new understanding toward the correlated states in 1D system.(2)The ground states of MTB on MoSe₂,which is a 1D electronic system,were in debate with scenarios ranging from quantum well states caused by Moire patterns,CDW caused by electron-phonon coupling,and Tomonaga-Luttinger liquid(TLL)caused by correlation effect.In order to settle the dispute,we carefully measure the low-temperature STSs of MTB on MoSe₂,and found that electronic states of MTB are confined into quantized discrete levels which are determined by the length of MTB.There are three types of states in MTBs which are distinguished by the relative phase relation of quantized discrete levels around the Fermi energy,i.e.,an out-of phase state,an in-phase state and a zero-energy state.Among the three states,the out-of phase state and in-phase state both contain an energy gap at Fermi level,and the gap size is dependent on the length of MTB.What's more,these three states can transform into each other with voltage pulses applied from the STM tip.After excluding the possiblity of coulomb blockade effect,we discussed its origin with Holstein-Hubbard model,and concluded that MTB is a correlated system which includes quantum confinement effect,electron-phonon coupling and correlation interaction.In such framework,the out-of phase state and in-phase state correspond to double-electron occupancy and single-electron occupancy of the nearest discrete level to the Fermi level,respectively,while the zero-energy state is a critical state.Such understanding is also consistent with the observations of temperature-induced CDW phase transition and signatures of TLL(spin-charge separation).Our work clarifies the dispute concerning the ground states of MTB,and is helpful for understanding the complicated quantum states dominated by electron-phonon coupling and correlation effect in a 1D system.(3)YSR state arises when magnetic impurities interact with hosting superconductivity.The intricacy of coupling and the nature of the superconductivity determine the behavior of the YSR state,whose detailed correlations are not yet fully understood.Here we study the YSR state of single Fe adatom on the surface of 2H-NbSe2 with combined low temperature STM/STS,density functional theory calculations and tight-binding modeling.we found that the Fe adatom occupies the hollow site of the Se surface layer.The YSR state exhibits three-fold symmetry along the diagonal direction of the Se lattice.The spatial decay of the YSR state follows a behavior in three-dimensional superconductivity.This behavior contrasts with a previous study of imbedded Fe impurities,whose YSR state shows six-fold symmetry and two-dimensional long-range decay.According to our theoretical modeling,the three-fold symmetry is related to the occupation site of the Fe adatom on 2H-NbSe₂,and the short-range YSR state is dominated by dimensional property of superconductity.Our results emphersize the important role of adsorption geometries in determining the behavior of the YSR state,and may provide help for clarifying the dimensional dependence of the YSR state in layered superconductors.