Study On The Superconductivity Of Typical Layered Chalcogenides Under High Pressure

Two-dimensional layered materials have attracted much attention due to their unique structures and properties.Especially since the superconducting phenomena of single-layer Nb Se₂ and Mo S₂ were reported,the superconducting properties of layered chalcogenides have become a hot research topic in condensed matter physics.The superconducting properties of materials are inseparable from the crystal structure,electronic structure,and energy band structure.Pressure can reduce the distance between material atoms,enhance electron orbital coupling,and change the charge density distribution,so it has become an essential method of regulating superconducting properties.In this paper,layered chulfide compounds Hf S₂,Sn Se₂ and Si₂Te₃ are taken as research objects,and the structure and electrical transport properties of layered chulfide compounds are studied in depth and systematically by using high-pressure in-situ electrical transport,Raman scattering,ultraviolet absorption spectroscopy and synchrotron radiation X-ray diffraction measurement techniques combined with first-principles calculation,and the following innovative results are obtained:1.The superconductivity and structural phase of Hf S₂ under high pressure were studied by in-situ electrical transport measurements,ultraviolet absorption spectroscopy,Raman spectroscopy,synchrotron radiation XRD and first-principles calculations.It is found that Hf S₂ transforms into a semiconductor-metal transport type near 60.6 GPa,and becomes a superconductor at the pressure of 126.6 GPa.The critical temperature of superconductivity increases with the pressure and reaches 4.4K at 153.0GPa.Measurements of the Hall effect observes that the major carrier type of Hf S₂changes from electron to hole at 133.1 GPa.The synchrotron radiation diffraction results reveals that there are two structural phase transitions in Hf S₂at 11.0 GPa and35.5 GPa.The superconducting transition at 126.6 GPa is independent of structure and is attributed to the enhanced electron-phonon coupling due to the change in electronic structure.The band-gap and phonon vibration characteristics of Hf S₂ under pressure were measured by high pressure ultraviolet spectroscopy and Raman scattering experiments.It was found that the structural phase transition at 11.0 GPa can cause the abnormal reduction of band-gap and the sudden change of Raman vibration mode.In addition,we mapped the temperature-pressure phase of Hf S₂ and revealed the relationship between the electrical conduction mechanism,superconductivity,and crystal structure.2.The superconducting behavior and electronic structure of Sn Se₂ under high pressure were studied by in-situ measurement of high pressure electrical transport properties,Raman scattering spectroscopy and first principles calculation.The results show that the superconducting transition occurs at 22.0 GPa and the critical superconducting temperature₍（8） is 5.4 K.Moreover,₍（8） increases with increasing pressure and reaches the maximum of 6.0 K at 42.8 GPa.The superconductivity of Sn Se₂ was enhanced during decompression.₍（8） increased from 6.0 K to 7.9 K with decompressing to 14.0 GPa,and superconductivity was maintained until pressure decreased to 8.8 GPa.The increase of carrier concentration during decompression was observed by Hall effect measurement.The changes in lattice parameters and cell volume calculated from first principles indicate that Sn Se₂ undergoes an electronic topological phase transition（ETT）at about 22.0 GPa due to enhanced interlayer Se atomic interactions.The results of Raman scattering spectra confirm the ETT phase transition and the formation of the strong interlayer Se-Se interaction.The Raman spectra of the decompression process show that the strong interaction can be retained under very low pressure.Therefore,the superconductivity enhancement of the decompression process can be attributed to the fact that the new electronic structure formed by the electron topological phase transition provides a high carrier concentration during the decompression process,which in combination with the phonon softening of the lattice during the decompression process increases the superconductivity.This work provides a new idea and possibility for retaining pressure-induced superconductivity to ambient.3.The metallization,superconductivity and structural phase transition of Si₂Te₃under high pressure were studied by in-situ electrical transport measurement,ultraviolet absorption spectrum,Raman scattering spectrum and synchrotron radiation XRD.Electrical transport measurement under high pressure shows that Si₂Te₃ undergoes a metallization transition at 11 GPa and becomes a superconducting state at the critical temperature below 3.8 K.With the increasing of pressure,₍（8） increaseds continuously,and at 58.0 GPa increases to 5.8 K.The abnormal changes of Raman vibration mode in the low-pressure part were found in the measurements of ultraviolet absorption and Raman scattering spectra can be attributed to the phase transition of electronic structure which is caused by the orientation rearrangement of Si-Si dimer under pressure.And the possibility of optical bandgap type transformation of Si₂Te₃ within 0-10 GPa is discussed.The results of synchrotron radiation XRD show that the phase transition of Si₂Te₃ starts at 11.7 GPa and finishes at 19.2 GPa.The pressure will cause irreversible nanocrystalline or amorphous,and the appearance of metallization and superconductivity is related to structural phase transition.Our study is the first to identify a superconductor in a silicon-like material and to confirm the highly adjustable band gap of Si₂Te₃ in the low pressure range.