Superconductivity And Structural Changes In Tantalum Disulfide Under Ultrahigh Pressure

As a major branch of two-dimensional layered materials,transition metal dichalcogenides（TMDs）have been a research hotspot due to their simple crystal structure and exotic electronic properties.Pressure,as a powerful method which can effectively tune the interlayer distance and intralayer interaction without introducing impurity,has been widely utilized in TMDs.The application of pressure on TMDs can tune insulator-metal transition,suppress charge density waves,induce superconductivity.Overall,TMDs show abundant and intriguing physical properties under high pressure.The most attractive feature of TMDs is their graphene-like layered structure.However,owing to that the applied pressure in previous researches is not very high,the changes of interlayer interaction in compressed TMDs have not been studied in depth.In addition,the influence of structural modulation on electronic properties is rarely involved in previous high-pressure researches.Based on above questions,1T,2H and Restacked TaS₂,which are structurally stable and have exotic electronic properties at atmospheric pressure,were selected as the research objects respectively,and the changes of electronic properties and crystal structure under ultra-high pressure were investigated.Our results show that these three TaS₂ exhibit different electrical properties and structural evolution under high pressure,which have great importance to explore the high-pressure behavior of other TMDs.The research contents and results are as follows:1.The structural phase transition and superconductivity variation of 1T-TaS₂under high pressure were investigated through electrical transport and synchrotron XRD measurements.A series of transitions,from the 1T structure to“distorted 1T”,then a metastable“nonlayered”structure,and finally the nonlayered tetragonal I4/mmm structure,were found.The I4/mmm TaS₂ presents superior superconductivity compared to the original layered 1T structure.The strong electron-phonon coupling in I4/mmm TaS₂ reveals a good superconductivity of the nonlayered structure.The P-T phase diagram shows a dome shape centered at～20 GPa,which is attributed to the distortion of the 1T structure.Accompanied by the transition to nonlayered structure above 45 GPa,critical temperature shows an increasing trend and reaches～7 K at the highest studied pressure.It is the first time that the pressure-induced re-enhanced superconductivity was observed in TMDs,which will stimulate a great deal of experimental and theoretical reseatches on this phenomenon.The structural evolutions in 1T-TaS₂ under high pressure also provide an important reference for studying the high-pressure behavior of other TMDs.2.The structural and superconductivity evolutions of 2H-Ta S2 under ultra-high pressure were studied through electrical transport and synchrotron XRD measurements.The superconductivity of original superconducting state（SC-I）was found re-enhanced above 100 GPa after the recession around 10 GPa,and SC-I survies to the highest pressure applied in this work.An unexpected superconducting state（SC-II）was found emerged around 86.1 GPa with initial critical temperature（T_c）of 9.6 K.As pressure increases,the T_c enhances rapidly and reaches the maximum of16.4 K at 157.4 GPa,which sets a new record for TMDs.In situ high-pressure X-ray diffraction and Hall effect measurements reveal that the occurrence of SC-II is accompanied with a structural phase transition and a concurrent enhancement of hole carrier density.The new high-T_c superconducting state emerged in 2H-TaS₂ can be attributed to the electronic states change near Fermi surface owing to the pressure-induced structural modulation.The finding of this remarkable new superconducting state in 2H-TaS₂ brings a new broad of perspective on TMDs and expands the field of pressure-modified superconductivity for layered materials.3.The structural evolution and superconductivity of Restacked-TaS₂ under high pressure were investigated through electrical transport and synchrotron XRD measurements.As pressure increases,T_c of the original superconducting state reaches its maximum at 6.4 GPa,and then begins to decrease.Upon the compression to 15.1GPa,the R-T curve of normal state begins to turn into semiconductor-like trend,which is completed at 37.8 GPa.Above 40.4 GPa,the semiconductor-like trend begins to recede,and turns into metallization trend above 150 GPa.During the process,the original superconducting state keeps the recession tendency,and eventually vanishes under high pressure.The new superconducting state emerges around 67.1 GPa,and the T_c increases rapidly with increasing pressure.As pressure increases to 150 GPa,the T_c reaches a maximum of 16.2 K and then remains constant under pressure.The high-pressure XRD spectra show that the structural symmetry of compressed Restacked-TaS₂ doesn’t change.The abundant electronic property changes in Restacked-TaS₂ under high pressure are closely related to the interlayer re-arrangement and pressure-induced layer sliding.Our results reveal that the pressure-induced changes of interlayer arrangement and interlayer interaction have a significant effect on the physical properties of TMDs.