Pressure Regulation Of Several Topological Semimetal Materials

In the past decade,the topological properties and novel quantum effects of materials have been widely studied after the discovery of topological insulators.The family of topological electronic materials has gradually expanded from the original topological insulators to topological semimetals,topological superconductors,topological semiconductors and so on.Among them,the topological semimetal have some unique properties,such as high mobility,large negative magnetoresistance,chiral anomaly,anisotropic electron transport,drumhead like surface state,possible surface magnetism/superconductivity,anomalous Landau energy level spectrum and anomalous optical response.They have potential applications in many fields and have become a research hotspot in condensed matter physics.As one of the basic state parameters,pressure can effectively tune the crystal structure and electronic state without introducing other impurities,which is of great significance for discovering new phenomena,revealing new laws and exploring new materials.The pressure applied to topological semimetallic materials can induce many interesting phenomena,such as the pressure-induced structural transformation,superconductivity induced by pressure,Lifshitz transition driven by pressure,etc.In this paper,we have studied the crystal structure and transport properties of the type-?Dirac semimetal NiTe₂,nodal-line semimetal SrAs_3,and nodal-line semimetal Ta₃SiTe₆ under high pressure by means of high-pressure electrical transport,high-pressure synchrotron X-ray diffraction and high-pressure Raman spectroscopy.The main results are included as follows:(i)Pressure-driven Lifshitz transition in type-II Dirac semimetal NiTe₂.Room-temperature synchrotron X-ray diffraction and Raman scattering measurements reveal the stability of the pristine hexagonal phase up to 52.2GPa,whereas both the pressure coefficient and linewidth of Raman mode E_g exhibit anomalies at a critical pressure P_c?16GPa.Meantime,Hall resistivity measurement indicates that the hole-dominated behavior maintains up to 15.6GPa and transforms into electron-dominated behavior at higher pressures.Our findings consistently demonstrate a pressure-induced Lifshitz transition in type-II Dirac semimetal NiTe₂.(ii)Emerging superconductivity and the origin of its enhancement in pressurized topological nodal-line semimetal SrAs₃.The comprehensive high-pressure experiments,i.e.,synchrotron X-ray diffraction,electrical transport,and Raman spectroscopy,consistently reveal a critical pressure P_c?25GPa.Around P_c,a superconducting transition appears,concomitant with the formation of high-pressure cubic Pm-3m phase.Meanwhile,the superconducting transition temperature T_c and hole carrier density increase synchronously above P_c.This fnding demonstrates that the emerging superconductivity in SrAs₃ stems from the high-pressure cubic phase on the one hand,and shows that the evolution of T_c(P)is intimately correlated with the enhanced density of states near the Fermi level on the other hand.(iii)Pressure-induced superconductivity in nodal-line semimetal Ta₃SiTe₆.High-pressure electronical transport results show that Ta₃SiTe₆ undergoes a superconducting transition under pressure.Extremely low-temperature high-pressure electronical transport experimental results of Ta₃SiTe₆ reveal that the critical pressure of superconducting transition is about 1.5GPa,the superconducting transition temperature T_c is about 0.8 K,and zero resistance appears at about4.0GPa and 35.4GPa.