Raman And Superconducting Properties Of Two-Dimensional Layered Materials By Numerical Calculations

In recent years,two-dimensional layered materials have attracted a lot of study attention in physical field,due to their rich physics and promisingly potential applica-tions,such as superconductivity,anisotropic properties of conductivity,optics,stiffness and so on.It is well known that many physical properties dependent on the structure of the material.Raman spectroscopy has been widely used to explore and study the structural and symmetric information experimentally.On theoretical hand,Raman vi-brations can be obtained by first-principles calculations combining with group theory analysis.Meanwhile,superconductivity in layered materials is a hot topic.Our paper here consists of five chapters:In chapter one,we give an introduction to the layered materials,Raman,con-ventional superconductivity,first-principal calculations related theoretical background,fundamental conceptions,related theoretical modes and methods.In chapter two,we systematically study the Raman spectra of bulk to monolayer ReS2 samples with lower symmetry.The excellent agreement between the theoretical and experimental Raman results points to such approach as a power tool to characterize vibrational and structural properties of two-dimensional layered materials with low symmetry.Meanwhile,we find that bulk ReS2 shows no low frequency(rigid-layer vibration Raman modes)Raman modes due to the one single layer structure.Tiny thickness induced frequency shifts(?1.0cm-1)and no surface reconstruction from monolayer to bulk,suggest the very weak interlayer interaction.In chapter three,we present a detailed theoretical study about the Raman spectra of few-layer phosphorene by first-principles calculations and group theory analysis.The van der Waals corrected calculation method well reproduces the experimentally observed equilibrium geometry and Raman frequencies and quite obvious thickness induced frequency shifts of low frequency rigid-layer vibration Raman modes,sug-gesting the importance of interlayer vdW interactions in this layered phosphorene.In addition,experimentally observable Ag2 mode(?460 cm-1 in bulk)changes from cou-pled in-plane and out-of-plane vibrations in single layer to pure in-plane vibrations in few layers,leading to in-plane vibration frequency quite higher than in-plane and out-of-plane vibration one and the corresponding frequencies vary as large as over 10 cm-1,which may be a powerful tool to identify the phosphorene layer numbers in experiment.In chapter four,we mainly give an analysis of Raman vibrations of the layered semimatal material WTe2 under zero pressure by group symmetry theory and identify all the Raman modes of WTe2.Our Raman calculations by first-principles match the experiment results very well.Meanwhile,as the pressure on WTe2 increases,the lattice parameters decrease and the orbital hybridization enhances and the density of states at the Fermi level increases.Further,increased pressure leads to upturn of phonon frequencies,thereby increasing the Debye temperature.These factors may introduce superconductivity in WTe2 under pressure.And our theoretical prediction is confirmed by our experiment measurement.In chapter five,we report density functional calculations of the electronic struc-ture,Fermi surface,phonon spectrum and electron-phonon coupling for the newly dis-covered superconductor LaO0.5F0.5BiSe2.It is confirmed that similar to LaO0.5F0.5BiS2,there is a strong Fermi surface nesting at(?,?,0),which does not lead to the appearance of static long-range order by the frozen phonon total energy calculations energy calcu-lations.And this compound is also a conventional electron-phonon coupling induced superconductor.In last chapter,we make a brief summary of the paper and present an outlook.