Study On The Preparation Of Transition-metal Chalcogenides And The Quantum States Modulation

In strongly correlated electron systems,materials are prone to undergo spontaneous symmetry breaking,due to the coupling of electronic,lattice,orbital and spin degrees of freedom,leading to the ordering of multiple degrees of freedom and forming intrinsic collective ordered quantum states.Therefore,understanding the inherent physical mechanism of those quantum states and the relationship between those is one of the most important essential questions in condensed matter physics,which has great scientific significance from the viewpoints of basic research or application.Layered transition-metal dichalcogenides（TMDs）possess abundant elemental composition with unfilled d electron orbits,resulting in a wide range of particular optical,electronic,magnetic and thermal properties at room temperature consequently.In addition,driven by strong electron-phonon coupling and electron-electron coupling,TMDs exhibits exotic quantum states,including superconductivity,charge density wave（CDW）,exciton,as well as Mott insulator etc,offering an ideal material platform.Most importantly,in TMDs,the adjacent layer is bound by van del Waals weak interaction.This has made the researcher can utilize a variety of methods,such as substitution,intercalation,high pressure,layer thickness and interface interaction to modulate the physical properties and analyse the evolution of the related quantum states,providing significant theoretical and experimental evidence to further understand the physical origin of each quantum state,especially the new quantum state.In this dissertation,based on 1T-TiSe₂ and 1T-TaS₂ materials,we have deeply investigated the physical origin of quantum states such as superconductors,CDW,and Mott insulators as well as their inherent relationship.In normal state,the band structure of 1T-TiSe₂ is featured by two band mode with Se 4p-derived hole pockets at the Brillouin zone centerΓpoint and Ti 3d-derived electron pockets at the Brillouin zone boundary M point,related by the wave vector Q_CDW for each other.When temperature reduces to below 200 K,1T-TiSe₂ undergoes a second-order phase transition to commensurate CDW state,accompanied by the Di Salvo lattice distortion model.The physical mechanism for this CDW transition in 1T-TiSe₂ is similar with the theoretical model of exciton stable phase,which condensed matter physicist Kohn has proposed in 1967.For a low carrier concentration semi-metallic or indirect narrow band gap semiconductor,the Coulomb interaction is weakly screened,allowing therefore bound states of holes and electrons i.e.,excitons,to build up in the system.If the binding energy of the exciton is large,with decreasing temperature,these excitons are prone to Bose-Einstein condensates（BEC）,forming a new exciton stable phase.In addition,a finite electron-phonon coupling creates the corresponding periodic lattice distortion,resulting in the occurrence of CDW state.While for 1T-TaS₂ with the same crystal structure,the elliptical Fermi-surface pockets in normal state are prone to nest,which promotes the formation of an incommensurate CDW（ICCDW）considerably above room temperature.As the temperature goes down,it enters the nearly commensurate CDW（NCCDW）and commensurate CDW（CCDW）states in succession.In CCDW state,1T-TaS₂ form a long-range order√13?√13 commensurate superlattice structure through the structural unit in the form of David-of-Star clusters,along with the appearance of Mott state.Moreover,it is found that the superconductivity has been triggered in 1T-TiSe₂ and 1T-TaS₂ by substitution,intercalation and high pressure,constructing electronic phase diagram.In this article,we have synthesized a series of high quality 1T-TiSe₂,1T-TaS₂ as well as doping single crystals samples by chemical vapor transport（CVT）method with iodine as a transport agent.The electrical transport results from physical properties measurement system（PPMS）indicate single or double doping can induce superconducting transition and realize the effective modulation of the CDW.On this basis,we have deeply analysied the origin mechanism of superconductivity and the relationship with CDW through magnetic susceptibility,specific heat,Raman and X-ray absorption fine structure spectroscopy（XAFS）etc,further combined with first-principles calculations.Innovative research results in this paper are as follows:（1）Although both Ta and Ti belong to transition metal elements,the origin mechanism of CDW in 1T-TiSe₂ can be well explained by the Fermi surface nesting theory.Based on the different CDW origin mechanism between 1T-TiSe₂ and 1T-TaSe₂,we have studied the effects of Ta doping on the CDW state of 1T-TiSe₂.PPMS electrical transport properties about 1T-Ti_1-xTa_xSe₂ single crystal samples show that Ta doping would cause both resistance maximum and CDW transition be weakened and shift to low temperature.When x=0.25,superconducting state occurs with a transition temperature T_c～2.1 K.Especially,we have carried out the low temperature Raman measurements to systematically analyse the evolution of phonon spectra for the first time.The results indicate the CDW-coupled amplitude mode,namely A_1g*disappears completely in the low-temperature Raman spectrum for the superconducting sample with x=0.25,further corroborating the suppression of the CDW.Additionally,in the framework of density functional theory（DFT）,DFT+U approach show that Ta substation results in the rigid upward shift of the chemical potential,which successfully achieve the transition of 1T-TiSe₂ from CDW insulating state to a metallic state and also enhance the density of states（DOS）at Fermi level E_F i.e.,N（E_F）,boosting the occurrence of the superconductivity and suggesting the competitive mechanism between superconductivity and CDW phase.Experimentally,Ti K-edge x-ray absorption near edge structure（XANES）spectra show that compared with x=0,the absorption edge of superconducting samples x=0.25 shifts to higher photon energy by～100 me V,further confirming the rising of the chemical potential.Moreover,the Ta doping would break the 2?2?2 periodic lattice distortion related with CDW transition,forming new low-energy distortion state,further suppress the occurrence of CDW state.Actually,the competition relationship between CDW and superconductivity in 1T-Ti_1-xTa_xSe₂ system is derived from those reasons i.e.,on the one hand,the Ta doping would induce the rising of the chemical potential and enhancement of the density of states,in favor of the occurence of superconductivity;on the other hand,the doping will introduce more electron carriers which strengthen the screening of the Coulomb interactions between electron–hole pairs,impeding the formation of exciton.At the same time,the doping-induced structural disorder will break the long-range coherence of CDW state.（2）In 1T-TiSe₂,the researchers have well achieved the effective modulation about the CDW state based on the strategies about substitution(1T-Ti_1-xTa_xSe₂)and interaction（1T-Cu_xTiSe₂）,forming peculiar phase diagram similar to that of the high-T_c cuprates based superconductor.On this basis,we have combined the two strategies for the first time and systematacially investigated the combined effect about simultaneous Cu intercalation and Ta substitution on quantum state in 1T-TiSe₂.PPMS electrical transport properties about 1T-Cu_xTi_0.8Ta_0.2Se₂ single crystal samples show that with increasing Cu contents,the superconducting transition temperature T_cgradually increases and reaches a maximum of about 2.5 K for the sample with x=0.04,followed by a decreased T_c for higher Cu content and then disappearance until for the sample with x=0.12.Meanwhile,magnetic susceptibility measurements on x=0.04 exhibit strong diamagnetic signal namely the Meissner effect,confirming the occurrence of superconductivity.In addition,the Cu atom are identified to intercalate randomly in the van der Waals gap by Cu K-edge XANES spectra for the first time.Based on low-temperature specific heat C_p measurement results for superconducting sample with x=0.04,combined with theoretical calculation,we find that compared with 1T-Ti_0.8Ta_0.2Se₂,the N（E_F）has been elevated to 0.866 states/e V f.u,favourable for the formation of more Cooper pairs.Therefore,we can explain the intrinsic physical mechanism of superconducting transition in the framework of Bardeen–Cooper–Schrieffer（BCS）theory.The value ofλ_ep is obtained as 0.651,implying that the x=0.04 superconductor is in weak or intermediate coupling range.In fact,the experimental phenomenon about CDW suppression in 1T-Cu_xTi_0.8Ta_0.2Se₂ is consistent with physical mode proposed by Kohn,of which the CDW transition in 1T-TiSe₂ is derived from the Bose-Einstein condensation（BEC）of excitons.The current research results provide a new research strategy for further exploring the relationship between CDW and superconductivity in 1T-TiSe₂ system.（3）For further understanding the physical mechanism of superconducting transition in 1T-TaS_2-xSe_x system,we have performed temperature-dependent synchrotron radiation x-ray absorption fine structure（XAFS）spectra to systematically and comprehensively study the local electronic and atomic structure for the first time.The temperature-dependent Ta L₃-edge X-ray absorption near edge spectroscopy（XANES）reveal from the point of view of the local electronic structure the appearance of phase transition from NCCDW to CCDW phase in 1T-TaS₂ while vanishing in 1T-TaSSe.At the same time,the temperature-dependent Se K-edge XANES suggest that Se doping would induce an enhanced p-d hybridization between Ta 5d and Se 4p states,which suppresses the CCDW-Mott state but promotes the occurrence of superconductivity.Additionally,the temperature-dependent Ta L₃-edge extended X-ray absorption fine structure spectroscopy（EXAFS）further uncover from the point of view of the local atomic structure that the periodic lattice distortion is present in CCDW state for 1T-TaS₂ and the long-range order distribution in the form of Star-of-David clusters along the Ta atom plane in the pristine is broken by Se doping in superconducting sample with x=1,suggesting the competing mechanism between CCDW and superconductivity.Finally,high resolution spherical aberration corrected STEM measurements show that the atomic scale S/Se order in 1T-TaSSe is critical factor to induce superconductivity.In summary,we have successfully realised effective controlling about the CDW state in 1T-TiSe₂ and 1T-TaS₂ and induced superconducting transition,by the means of single or double doping.Furthermore,based on a series of experimental results including magnetic susceptibility,specific heat,Raman and XAFS etc,further combined with first-principles calculations,we have deeply elucidated the origin mechanism of superconductivity as well as the inherent competing mechanism between CCDW and superconductivity,providing a novel research strategy for the controling of quantum states in strongly correlated electron systems.