First-princi Les Study On The Physical Properties Of Molybdenum-based Compound Materials

Molybdenum has many different valence states,so it can form a rich family of compounds.Among these compounds,two-dimensional layered materials composed of molybdenum and chalcogenide have received much attention in the frontier research field,due to their graphene-like geometrical structure and novel topological properties.In this dissertation,we focus mainly on the three-dimensional bulk material MoP and two-dimensional single layered Td-MoTe2,studying their geometrical structures,elec-tronic properties and lattice dynamics etc.by numerical simulations.Both MoP and Td-MoTe2 are molybdenum-based topological materials,and the detailed experimental and theoretical researches of them is a hot key subject in the condensed matter physics and the materials science in recent years.The dissertation begins with Chapter 1,reviewing research background and current research status of molybdenum-based materials,and the basic concepts and classifica-tion of topological materials up to now.The content of Chapter 2 is the theoretical foundation of this dissertation.We introduce the Bonn-Oppenheimer approximation,the Hartley-Fock method,the density functional theory,the lattice dynamics theory,thermal expansion theory and electron-phonon coupling.The two research works in the Chapter 3 and 4 are based on these theories and calculation methods.In Chapter 3,we investigate the crystal structure,electronic structure,lattice dy-namics and superconductivity of MoP under high pressure through first-principles cal-culation method.MoP has triple degenerate fermions around the Fermi Level,and the superconducting transition occurs under high pressure.We found that the lattice con-stant of MoP decreases and the electronic structure remains almost unchanged as the pressure increases.The optical branches of the phonon spectrum presents an overall hardening.Our calculations indicate that the logarithmic mean value of the phonon vibration frequency ωlog and the electron-phonon coupling parameter λ increase with the increasing of pressure,resulting that the superconducting transition temperature in-creases from the zero at ambient pressure to 0.16 K at 30 GPa,and finally to 1.21 K at 50 GPa.The result of theoretical calculations is qualitatively consistent with the ex-perimental measurements,indicating that the superconducting transition of MoP under high pressure is in line with the Bardeen-Cooper-Schrieffer(BCS)theoretical frame-work,which provides some theoretical support for understanding the phenomenon of coexistence of topology and superconductivity observed by experiments.Finally,in Chapter 4,we study the electronic properties,phonon spectrum and the negative thermal expansion effect of the single-layered Td-MoTe2.Since the dis-covery of two-dimensional layered graphene,a large number of two-dimensional ma-terials with novel properties have emerged.Transition metal chalcogenides MX2 have attracted extensive research interests due to their unique physical properties.Gener-ally,MX2 materials have four different structural phases:2H,1T,1T’ and Td,and MX2 with different structure and thickness presents very different physical properties.The observed magnetoresistive switching(turn-on)phenomenon in Td-MoTe2 may be re-lated to the change of electronic structure caused by temperature,which has aroused widespread research interest.At the same time,the experimental measurements also show that Td-MoTe2 exhibits negative thermal expansion.At low temperature,the scan-ning tunneling microscope(STM)experiments show a significant abnormal change in the lattice constant from 70 K to 7 K.As potential electronic materials,the performance of devices based on MoTe2 will change significantly due to the internal stress exerted by anisotropic thermal expansion coefficients,so it is necessary to study its phonon vi-bration and thermal expansion properties.Theoretical calculations without spin-orbital coupling show that the single layered Td-MoTe2 is a typical Dirac semi-metal at 7 K and 70 K,and the Dirac cone tilts as the temperature increases and changes from type Ⅰto type Ⅱ.The introducing of spin-orbital coupling makes energy band open a small en-ergy interval at the Dirac point,and single-layered MoTe2 undergoes a semiconductor-to-trival-semimetal transition from 7 K to 70 K.The Gruneisen parameter calculations indicate that system has really a negative thermal expansion property at low tempera-tures,which is confirmed by the further SCF-QHA calculation and consistent with the experimental observation.Meanwhile,the negative thermal expansion exhibits strong anisotropy,i.e.the variation of lattice constant b is relatively bigger than that of lattice constant a.