Studies On Structure And Electrical Transport Properties Of Layered Topological Nodal-Line Semimetals ZrSiSe,ZrSiTe And Ferromagnetic Material Cr₂Ge₂Te₆ Under High Pressure

High pressure,as a kind of extreme physical condition,can effectively regulate the layered interaction,and even transform their bonding styles.Thus,the electronic structure and crystallographic structure of materials are modulated and materials show rich phases and excellent physical properties,which can not be obtained under ambient condition.This is of great importance to understand the physical mechanism of pressure induced metallization,superconductivity,topological phase transition and other phenomena.As a potential application prospects material,layered materials,such as graphene,MoS₂,born nitride,etc.,which possess electrical,optical,mechanical and other properties,and have broad application prospects in many fields.Therefore,the study of layered materials under pressure is a hot topic in condensed matter physics.Topological nodal-line semimetal ZrSiSe,ZrSiTe and ferromagnetic material Cr₂Ge₂Te₆are two kinds of representative layered materials.Exploring their structure and physical properties under pressure which will further deepen the understanding of layered materials.In this article,we focus on the novel physical properties of the layered materials under high pressure and the effect of high pressure on the structures and properties of the layered materials.Several in situ high pressure experimental measurements have been combined with theoretical calculations to systematically study the evolvement of structure and electron transport properties of layered topological nodal-line semimetal ZrSiSe,ZrSiTe and ferromagnetic material Cr₂Ge₂Te₆.Meanwhile,the corresponding mechanisms in these processes are also discussed.This study has a vital significance to extending the research dimensions and enriching the knowledge of pressure-modulated structures and properties of layered materials.1.We have investigated the evolution of structure and properties of topological nodal-line semimetal ZrSiSe with experimental measurements including synchrotron-based X-ray diffraction,Raman,Infrared reflectance and electric transport measurements.ZrSiSe undergoes an isostructural electronic transition and a structural transition from tetragonal phase to orthorhombic phase under pressure.At?6 GPa,an isostructural electronic transition is observed which is indicated by the pressure-induced anomalies of the lattice parameters and Raman shift without breaking of crystal symmetry.In this proess,the enhanced of the interlayer interaction plays an important role.Upon further compression,pressure-induced the lattice distortion and stacking faults could result in a structural phase transition from the original tetragonal phase to an orthorhombic phase,and two phases coexist until the maximal experimental pressure.The IR reflectivity and resistance measurements represent abnormal behaviors at similar pressure range and it further corroborates the electronic and structural phase transitions.Theoretical calculations indicate pressure can alter the conduction band and valence band near Fermi level along the path from?to X point,which leads to the isostructural phase.In addition,the carrier density increases first and then decreases with pressure,which is related to the structural phase transition.These results provide an important basis for studying the high-pressure behavior of topological nodal line semimetals.2.To investigate the structural evolution and superconductivity of topological nodal line ZrSiTe,electrical transport and synchrotron XRD measurements are employed.It is found that a superconducting transition takes place at 6.9 GPa with a critical temperature(Tc)of 2.9 K,and then Tc monotonously increases with pressure.High pressure X-ray diffraction study and DFT calculation demonstrate that there is no structural phase transition in the whole pressure range.However,ZrSiTe undergoes two Lifshitz transitions at 4 and 7 GPa with the drastic changes of Fermi surface.Hall measurement reveals that the emergence of Lifshitz transitions is accompanied with the carrier-type inversion from the hole-type dominated to electron-type dominated and variation of carrier density,which are propitious to the superconducting transition.The appearance of superconductivity without breaking crystal symmetry implies the superconducting state is induced by proximity effect in the surface state may be topologically nontrivial,and thus high-pressure state of ZrSiTe could serve as a candidate for topological superconductor.Our results provide a research platform for exploring topological superconductors and expand the knowledge of pressure-induced behaviors of topological nodal semimetals.3.High pressure study on the structure and properties of layered ferromagnetic Cr₂Ge₂Te₆find that Cr₂Ge₂Te₆ undergoes an isostructural electronic transition from layered to non-layered structure due to the enhanced interlayer interaction under pressure.The results of IR and electric transport measurements present abnormal behaviors at similar pressure range which reveal that the isostructural transition is accompanied with a semiconductor to metal transition.Upon further compression,pressure-induced lattice distortion results in a metastable mixed structure,and it finally transforms into the amorphous state at higher pressure.The density functional theory shows that the closure of band gap happens under high pressure and it indicates that pressure-induced metallization is attributed to changes of band gap near Fermi lever.These results indicate that pressure has an important influence on the modulation of the structure and properties of layered materials.Our study provides experimental basis for understanding the structures and properties of layered ferromagnetic materials.