Study On Electronic Structures And Transport Properties Of Topological Nodal Line Semimetals

In recent years,with the discovery of topological insulators,topological electronics and related topological materials have become one of the hot topics in condensed matter physics.Taking topological insulators as an example,this kind of materials can realize the non-dissipative boundary states(edge states)only by the intrinsic spin-orbit coupling(SOC)effects without magnetic field,which makes it have a broad application prospect in the field of low-dissipation,high-performance electronic devices and quantum computing.Based on the development of topological insulators,the concept of topology can be further extended from insulators to semimetals or metals.The corresponding topological materials are generally called topological semimetals.For example,Dirac semimetals,Weyl semimetals,and nodal line semimetals.Among these topological semimetals,nodal line semimetals are not only compatible with other non-trivial fermions,but also have many exotic properties,such as high-temperature surface superconductivity,nondispersive Landau energy level,and specific long-range Coulomb interactions.Therefore,it is of great significance to find new topological nodal line semimetals and find their potentially excellent physical properties.In this paper,using density functional theory and tight-binding models,we theoretically predict several nodal line semimetals and reveal their potentially excellent physical properties.The specific research contents and results are as follows:1.Electric field/magnetic proximity effects induced topological phase transition in nodal-line semimetal,?-boropheneRecently,the experimentally successful observation of the Dirac fermion states in?12-and ?3-boron sheets has sparked intensive interest to explore the two-dimensional(2D)topological semimetals(TSMs).Interestingly,it has also been predicted by several works that nodal-line fermions may exist in various planar phases of boron sheets,such as 6B:P6mmm boron bilayers,P6-boron bilayer,and striped boron sheet.However,the complex layer structures or a mixture of different planar phases severely hinders the experiment synthesis and observation of the nodal-line fermions in these proposed boron sheets.Based on density functional theory and tight-binding models,we discover a coexistence of open and closed nodal lines produced by three-band touching in the electronic structure of recently synthesized ?-boron sheet.In addition,there are three Dirac cones nearby the Fermi level with an ultrahigh Fermi velocity(1.33 × 106 m/s).The symmetry analysis reveals that the nodal lines are protected by both time-reversal symmetry(TRS)and mirror-reflection symmetry(MRS).When electric field induced Rashba SOC is turned on to break the MRS,spin-up and-down states are shifted toward opposite momentum directions,leading to the emergence of two new nodal loops around the ? point.Furthermore,when TRS is broken e.g.,via introducing magnetic proximity effect,the original spin-degenerate nodal line(loop)decays into fully spin-polarized nodal line(loop).Our findings not only reveal a new form of nodal line in the ?-boron sheet,but also offer an alternative approach to realize spin-polarized nodal line semimetals with promising applications in spintronic devices.Please see Chapter three for detailed contents.2.Study of spin Hall effects in nodal line semimetals MH2(M=Ti,Zr,Hf,Th)The spin Hall effect(SHE)is attracting intensive attention due to its great potential in spintronic applications.Recently,several topological matters,e.g.,topological insulators or nodal-point semimetals(NPSMs),have been proposed to exhibit large intrinisc spin Hall conductivity(SHC).However,their practice applications have been strongly hindered by their unstable SHE due to either bulk or external doping problems.Therefore,it is necessary to find SHE materials with giant spin Hall conductivities/spin Hall ratios that are robust against large charge doping.Our work find the existence of nodal-net electronic states in transitional metal hydrides MH2(M=Hf,Th)and spin Hall conductance of up to 1000(h/e)(? cm)-1 near the Fermi level that are robust against large charge doping.The further analysis reveals that the strong SOC-induced band splitting of the nodal-net structure contributes to the large SBC distribution in the entire BZ and resultant giant SHC.Meanwhile,due to the rather small intrinsic electronic conductivities,large spin Hall angles(SHA)can be achieved in MH2 for practice applications.In the end,we also find that the the origin of large SHC in the fcc Pt and?-W is actually from the symmetry-protected nodal lines near the Fermi level.Please see Chapter four for detailed contents.3.Study of spin Hall effects in nodal line semimetals M2C(M=V,Nb,Ta)At present,the theoretical and experimental studies of spin Hall effects mainly focus on the bulk materials such as 5d transition metals and topological materials,and the researches on their two-dimensional phases are relatively less,which limits its applications in low-dimensional nanodevices to a certain extent.Based on the idea of second work,we find a class of layered materials M2C(M=V Nb,Ta)with giant spin Hall conductivities/spin Hall ratios that are robust against large charge doping.Both bulk and monolayer phases host symmetry protected nodal lines near the Fermi level.With the presence of the SOC,the degeneracy of the nodal lines is removed,and a large band gap is generated,resulting in large spin Berry curvature around the nodal lines.Finally,we obtain large spin hall conductivities of up to-1167 and 191(h/e)(? cm)-1 for the bulk and monolayer phases of Ta2C,respectively.Meanwhile,due to the rather small intrinsic electronic conductivities,large spin Hall angles of 0.22 and 0.05 can be achieved in the bulk and monolayer phases for practice applications,respectively.Please see Chapter five for detailed contents.The thesis includes the following six chapters:The first chapter mainly introduces the research background of the family of Hall effects,topological insulators,progress in experimental and theoretical studies of topological semimetals,progress in experimental and theoretical studies of topological materials and spin Hall effects.The second chapter is the theoretical method part.We briefly introduce the method of density funnctional theory,Wannier funnctions,tight-binding model,and k.p methods.In chapter three,we study the electronic structures of ?-borophene and reveal the nodal line semimetal states near the Fermi level.Moreover,we also study the possibility of topological phase transition by introducing electric field induced Rashba SOC and magnetic proximity effects.In chapter four,we study the nodal-net semimetal states in transitional metal dihydrides MH2(M-Hf,Th)and reveal its giant spin Hall conductivities/spin Hall angles.Furthermore,we also find the origin of large spin Hall conductivies in transition metals.In the following chapter,we study the electronic structures and its related spin Hall effects in the bulk and monolayer phases of layered materials M2C(M=V,Nb,Ta).The last chapter draws a conclusion and gives a prospect for the whole work of this thesis.