Theoretical Study Of The Electronic Transport Properties Of Several Two-dimensional Metals And Topological Semimetals

It is well known that microelectronics is an important engine to promote the progress of contemporary human civilization,and most electronic devices are designed based on the conductive characteristics of certain materials in principle.Therefore,fully understanding and regulating the conductive properties of various materials or structures can not only enrich physical knowledge at the basic level,but also have great application prospects,which has always been an important research topic in condensed matter physics and related fields.The quantitative understanding of the conductivity of materials by human beings begins with the establishment of Ohm’s law of macroscopic materials.Materials can then be divided into metals,semiconductors and insulators according to the quantitative results of the conductivity of materials.Since then,the transport theory for mesoscopic systems can correctly describe the quantum characteristics of electron transport in nanostructures,which opens the door for quantum devices to move from prototype design to practical application.In this field,miniaturization and low power consumption of the devices are two main goals.The discovery of the quantum Hall effect and the continuous in-depth understanding of its physical nature ultimately lead to the emergence of the new physical concept of topological materials.At the same time,a variety of topological materials are emerging to provide rich material candidates for realizing the functions of low-energy dissipation electronic devices.In addition,the successful preparation of graphene samples marks that two-dimensional materials begin to become a real existence.Subsequently,a variety of two-dimensional materials continue to emerge,which provides a material basis for the realization of ultra-thin or flexible electronic devices.In this context,it is necessary to understand the electron transport behaviour of topological materials and two-dimensional materials.Therefore,in the research works involved in this dissertation,we firstly obtained the quantitative results of some key physical quantities by using first-principles calculations for several commonly used two-dimensional metals in experiments,and then studied the conductive characteristics of these materials at room temperature limited by electron-phonon(e-ph)scattering.Secondly,for topological semi-metals,we study the influence of inter-valley and intravalley scatterings on their quantum transport properties under external magnetic fields by using the Kubo formula and the self-consistent Born approximation.Specifically,we have carried out the following research works:Firstly,we study the effect of the mirror symmetry of the ABA-stacked trilayer graphene on the e-ph coupling.By comparing the conductivities of the ABA-stacked trilayer graphene and monolayer graphene,we find that the conductivity of monolayer graphene is higher.At first sight,this result contradicts the one inferred by the band structures of the two materials.In other words,the band structure of trilayer graphene consists of a pair of linear bands,which is almost the replica of monolayer graphene,and a pair of parabolic bands passing the Fermi level.The parabolic bands provide more Fermi surface branches and hence contribute more to the conductivity compared with monolayer graphene.Consequently,ABA-stacked trilayer graphene is supposed to have higher conductivity than monolayer graphene.By analyzing the symmetry of ABA-stacked trilayer graphene,we propose the selection rule for the e-ph coupling and we find that the out-of-plane acoustic phonons(phonons in ZA mode)lead to strong inter-band scattering,and hence the significant lowering of conductivity.However,there are only linear bands in monolayer graphene and hence no inter-band scattering exists.Therefore,the phonons in ZA mode are decoupled with the electrons.Through this symmetry analysis,and considering that the conductivity of metal materials at room temperature mainly depends on e-ph scattering,our work clarifies the physical reason why ABA-stacked trilayer graphene has lower room-temperature conductivity than monolayer graphene.Nonetheless,our numerical calculation shows that the room-temperature conductivity of ABA-stacked trilayer graphene is only slightly lower than that of monolayer graphene.Therefore,ABA-stacked trilayer graphene is still a two-dimensional metal with excellent conductivity.Secondly,by first-principles calculations,we study quantitatively the role of the nearly free electronic states(NFESs)in determining the room-temperature conductivity of the hydroxyl-terminated 2D MXene Hf2C(OH)2.(1)We calculated the roomtemperature conductivity of Hf2C(OH)2 and find that Hf2C(OH)2 is a good 2D conductor with high conductivity.(2)The Fermi surface of Hf2C(OH)2 consists of two branches,namely the one from the NFESs and the other one from the ordinary electronic states(OESs).The NFESs manifest themselves by floating over the atomic planes,so they should suffer from weaker e-ph scattering.Our numerical results agree with the exception that the e-ph couplings for NFESs are weaker.But their band velocities are much lower than those of the OESs.Therefore,its high conductivity is determined by the OESs instead of the NFESs.(3)Based on the above numerical results,we can draw the following conclusions.In order to take fully the advantages of NFESs to obtain ultra-high conductivity at room temperature,these special electronic states need to have relatively high electron band velocities and sufficiently large Fermi surface to accommodate enough electrons to participate in the transport process in addition to the weak e-ph coupling.By doing further numerical calculations,we find that this result is applicable to other similar hydroxyl-terminated two-dimensional MXene materials,such as Zr2C(OH)2,that is,the NFES is not the main contributor to its intrinsic conductivity.Thirdly,we study the influence of the inter-and intra-valley impurity scatterings on the electronic transport properties of Weyl semimetals.(1)We find that the influences of high-order Feynman diagrams on the contributions of the inter-and intra-valley impurity scatterings to the transverse conductivities are different.The transverse conductivity is suppressed for inter-valley impurity scattering but is enhanced for intra-valley impurity scattering due to high-order Feynman diagrams.(2)Increasing the scattering strength will destroy the linear behaviour of the transverse conductivity and enhance its magnitude.(3)The intra-valley scattering will be invalid for the longitudinal conductivity if all the Feynman diagrams are under consideration,then the longitudinal resistivity will disappear.More importantly,the contributions of the high-order Feynman diagram can not be ignored even at the weak scattering limitation.(4)The peaks of the transverse conductivity always meet the valleys of the longitudinal conductivity by comparing the curves of the two kinds of conductivities with respect to the Fermi energy.Fourthly,we study the influence of the inter-and intra-valley scatterings on the Hall conductivity of Dirac semimetals.(1)When there is only inter-valley or intravalley scattering with the same strength in the system,the density of states spectra given by the two kinds of scattering are almost identical.(2)The Hall conductivity can be divided into three terms,namely,σxy=σxyⅠ+σxyⅡ(1)+σxyⅡ(2),where σxyⅠ is the normal term determined by only the states around the Fermi surface,and σxyⅡ(1)andσxyⅡ(2)are the anomalous terms contributed by all the states below the Fermi energy.When the Fermi energy is near the Dirac point,the Hall conductivities for the interand intra-valley scatterings behave similarly no matter how strong the scatterings are.When the Fermi energy is far from the Dirac point,the Hall conductivities for the inter-and intra-valley scatterings with strong strengths are similar to each other.But the conductivity for the intra-valley scattering is smaller than that for the inter-valley scattering when the scattering strength decreases.So it is not reliable to ignore the inter-valley scatterings for most cases.(3)Our study points out that the tilt of the Dirac cone can lead to further enhancement of Hall conductivity.Finally,we intend to study the intrinsic conductivity of several typical topological semi-metals and their thin film structures at room temperature at the first principle calculation level,which is of great significance for evaluating the practical application value of topological materials and their low dimensional structures and interpreting the relevant experimental results.We have established the theoretical framework based on the Boltzmann transport equation and the Wannier interpolation technique.In practice,however,the computational load is too heavy to be performed.At present,we are optimizing the algorithm and improving the efficiency of the interface program,and strive to carry out some more practical theoretical research in this area in the near future.