First--principles Studies On Phonon-phonon Coupling And Electron--phonon Coupling

Phonon-phonon coupling and electron-phonon coupling have always been the fo-cus of research in condensed matter physics.They play an important role in under-standing the basic physical properties of materials and in finding and designing new materials.The study of phonon-phonon coupling helps people to understand the phys-ical properties of materials,such as equilibrium state properties,transport properties,dynamic properties,etc..And they also have important implications for the searching and designing of the functional materials,such as thermal insulation materials,thermal conductive materials,multiferroics,and shape memory alloys.The study of phonon-electron coupling is very important for understanding the electrical and optical prop-erties of materials,such as semiconductor carrier mobility,photoelectric conversion and electrical transport properties.Especially for superconductivity,the strength of electron-phonon coupling directly determines the superconducting transition tempera-ture of the conventional superconductors.The first-principles calculations based on density function theory have broad ap-plication on condensed matters and material science.Under this background,we use first-principles calculations to study the phonon-phonon coupling and electron-phonon coupling,and use these two kinds of interactions to explain the structure phase transi-tion and superconducting phase transition in several systems.1)Firstly,as for phonon-phonon coupling,we will introduce the important role of anharmonic effect in phonon renormalization,structural phase transition and topo-logical phase transition via lead dioxide system.The crystal structures and electronic structures of lead dioxide are investigated with ab initio calculations.It is found that the previously known tetragonal?-PbO₂(P4₂/mnm)is dynamically unstable under the harmonic approximation,and?-PbO₂becomes stable at finite temperature about 200K attributed to the enhanced anharmonic effect.Under the guidance of the vibra-tional soft mode,we find an orthorhombic structure that is a candidate of the ground state of PbO₂(Pnnm)at low temperature.The further electronic structure and sur-face states calculations suggest the low temperature orthorhombic phase is a trivial insulator,while the tetragonal high temperature phase is a topological Dirac nodal line semimetal.Therefore,there is a topological phase transition from a trivial insulator to a topological semimetal in PbO₂,strongly coupled with the stabilized soft mode and thus anharmonicity-driven structural phase transition.This work demonstrates the impor-tant role of phonon-phonon coupling in describing the structural phase transition and exploits the impact of anharmonicity to the topological property of the system.2)Next,we study the Kohn anomaly,phonon self energy correction,supercon-ducting transition and other physical properties caused by electron-phonon interaction via first-principles calculations.We focus on superconductors under the framework of Bardeen-Cooper-Schrieffer(BCS)theory,which includes several systems,as follows:2.1)As one of the few element two-dimensional materials,two-dimensional boron has attracted great attention and has been predicted to exhibit various structural poly-morphs and interesting physics,including superconductivity,ideal strength,negative Poisson ratio,and a higher thermal conductivity than graphene,etc.However,the difficulty in experimental synthesis of two-dimensional boron has further reduced its exploration and applications.We have used a crystal structure search method and first-principles calculations.We have proposed a scheme of“synthesize under high pres-sure and exfoliate at ambient conditions”to first predict a bulk phase of KB₄,and then exfoliate free standing two-dimensional boron from it.Based on electron-phonon cou-pling calculations,we estimated that two-dimensional boron is an intrinsic supercon-ductor with superconducting transition temperature T_c=17.9 K and its superconductiv-ity possesses a quasi-one-dimensional feature.Our proposed scheme for the synthesis of free standing two-dimensional materials will stimulate the further synthesis and ex-ploration of the new low-dimensional materials.2.2)Intercalation compounds,such as graphite intercalation compounds,attracted extensive attention especially for energy storage,anode materials and superconduc-tors.We predict a ternary intercalation compound Li B₆C₂can be synthesized with high pressure technique and quenched to ambient condition.Moreover,a two-dimensional B₃Clayer can be exfoliated from Li B₆C₂by using the electrochemical method.The electron-phonon coupling calculations show that both the three-dimensional Li B₆C₂ and the two dimensional B₃Clayer are intrinsic conventional superconductors with T_c=30.7K and 13.9 K,respectively.2.3)Finally,we introduce a pressure induced superconductivity in Pd₃Pd₂Se₂,a shandite compound with a Kagome lattice.It will turn into superconducting state at25 GPa with T_c=2.2 K,and the T_chardly changes with the increase of the pressure.The increase of carrier concentration and Fermi surfaces are observed near the tran-sition pressure.At about the critical pressure,there is a Van Hove singularity of the density of states near the Fermi energy,which may be the reason for the appearance of superconductivity.By using first-principles calculations and crystal structure search methods,we sys-tematically study the phonon-phonon coupling and electron-phonon coupling in sev-eral systems as well as some physical properties induced by them,such as anharmonic effect,structural phase transition,topology and superconductivity.In particular,we further demonstrates a new method of synthesizing new materials by using high pres-sure method and exfoliating two-dimensional materials at atmospheric pressure.This method is expected to play an important role in exploring new two-dimensional mate-rials which are difficult to synthesize by conventional methods.