High Pressure And Doping Studies Of Structures And Properties In Typical Transition Metal Se-dichalcogenides

Two-dimensional layered materials have become a research hotspot in the fields of physics and materials because of their excellent physical and chemical properties,attracting extensive attention from scientific researchers.Among numerous layered materials,transition metal dichalcogenides（TMDs）have abundant crystal structures and unique band gap structures,exhibiting rich electronic properties including semiconductors,semimetals,superconductivity,charge density waves,and giant magnetoresistance effects.These extraordinary performances of TMDs have important application value in fields of electronics and optoelectronic devices.With the progress of science and the development of the times,people have put forward higher requirements on the performance of materials.If a single material is to achieve diversified electronic properties,it is necessary to further optimize its performance through regulation.Pressure and element doping can"cleanly"regulate the structure and physical properties of materials,thereby greatly enriching the physical properties of TMDs materials.Looking at previous research work on TMDs under high pressure,there are two key scientific issues that need to be solved in this field:the types of structural changes of different TMDs under high pressure have huge diversity,and high pressure can regulate the crystal structure of TMDs.Most current research mainly focuses on single-band superconductors,and pressure can control the superconducting properties and types of TMDs.To solve these problems,we selected 1T-HfSe₂、1T-V_0.7Re_0.3Se₂and1T′-Re_0.8V_0.2Se₂ as research objects for study.We investigated the evolution of crystal structure and electrical transport properties for three samples under high pressure by using in situ high-pressure synchrotron X-ray diffraction,high-pressure Raman spectroscopy,low-temperature electrical transport,and Hall effect characterization methods,combined with first-principles calculations.The connection between crystal structure and superconductivity of TMDs was deeply analyzed and the internal mechanism of superconducting state was revealed.The following innovative results as followed:1.The transformation from a two-dimensional semiconductor to an isotropic three-dimensional superconducting states in 1T-HfSe₂ was realized by pressure regulation.The results show that two structural phase transitions occurred during the compression,from the 1T phase to two nonlayered C2/m and I4/mmm structures.The experimental results of in situ high-pressure Raman spectroscopy give direct evidence on the change of dimensionality in HfSe₂:the enhancement of the interlayer coupling under pressure and the bonding between interlayer Se-Se atoms.Compared with the original layered structure,the new three-dimensional structure has better superconductivity.The dependence of the superconducting transition temperature T_c of HfSe₂ on pressure is complicated:T_c first elevates slightly with increasing pressure,then remains a constant value of about 5.7 K in the pressure range of 42 GPa to 69.4GPa,and shows a slowly decreasing trend when the pressure further increases.Theoretical calculations reveal that the superconductivity in the two high-pressure new phases comes from the increase in the density of states at the Fermi surface.This work provides an important platform for in-depth understanding the relationship among dimensionality,structure,and superconducting phenomena in TMDs.2.The multiband superconducting state was realized in VSe₂(namely V_0.7Re_0.3Se₂)by the synergistic effect of Re doping and pressure for the first time.According to the in situ high-pressure and low-temperature electrical transport experimental measurements,it is found that when the Kondo effect is completely suppressed,the T_cof V_0.7Re_0.3Se₂displays a dome shape with the increase of pressure,reaching a maximum value of 5.5 K at 20.9 GPa.By the application of external magnetic fields,this multiband superconducting state exhibits an anomalous temperature-upper critical field relationship and the upper critical field characteristic with an upward curvature（pressure above 12.4 GPa）.In situ high-pressure Hall effect experimental measurements show that the nonlinear change behavior of the Hall resistance R_xy（H）in the high magnetic field region,and the variation of carrier type from electrons to holes indicate the Lifshitz transition at 37.3 GPa.Combined with the first-principles calculations,it is further determined that the V-3d,Re-5d,and Se-4p orbitals of Fermi level contribute to the transition from single-band（VSe₂）to multiband(V_0.7Re_0.3Se₂)superconductivity.This work opens up a new avenue for finding more unique superconducting states in TMDs.3.The stabilization pressure of the ReSe₂ superconducting phase was reduced by doping V atoms.The high-pressure and low-temperature electrical transport experiments reveal that Re_0.8V_0.2Se₂underwent an evolution from semiconducting to metallic behavior at 10 GPa.Superconductivity is observed around 52.4 GPa,and T_cshows an upward trend with increasing pressure without any sign of saturation,reaching2.9 K at the highest experimental pressure of 89.3 GPa.The preparation pressure of the superconducting phase in Re_0.8V_0.2Se₂ was reduced by nearly half compared to the binary parent ReSe₂.It is found that the stability of the original monoclinic structure up to 80.9 GPa by high-pressure synchrotron XRD experiments.The crystal structural information and Hall effect measurements indicate that the stable pressure reduction of the superconducting phase is associated with variations in d-electrons and interlayer interaction,which further has a significant effect on the changes in the Fermi surface,accompanied by an enhanced electron carrier concentration.This work offers a new idea for designing new TMDs with superconductivity at lower pressure.