Electronic structure and optical properties of Ge-and F-doped α-Ga2O3:First-principles investigations

The prospect ofα-Ga2O3 in optical and electrical devices application is fascinating.In order to obtain better performance,Ge and F elements with similar electronegativity and atomic size are selected as dopants.Based on density functional theory(DFT),we systematically research the electronic structure and optical properties of dopedα-Ga2O3 by GGA+U calculation method.The results show that Ge atoms and F atoms are effective n-type dopants.For Ge-dopedα-Ga2O3,it is probably obtained under O-poor conditions.However,for F-dopedα-Ga2O3,it is probably obtained under O-rich conditions.The doping system of F element is more stable due to the lower formation energy.In this investigation,it is found that two kinds of doping can reduce theα-Ga2O3 band gap and improve the conductivity.What is more,it is observed that the absorption edge after doping has a blue shift and causes certain absorption effect on the visible region.Through the whole scale of comparison,Ge doping is more suitable for the application of transmittance materials,yet F doping is more appropriate for the application of deep ultraviolet devices.We expect that our research can provide guidance and reference for preparation ofα-Ga2O3 thin films and photoelectric devices.

Proposal for valleytronic materials:Ferrovalley metal and valley gapless semiconductor

Valleytronic materials can provide new degrees of freedom to future electronic devices.In this work,the concepts of the ferrovalley metal(FVM)and valley gapless semiconductor(VGS)are proposed,which can be achieved in valleytronic bilayer systems by electric field engineering.In valleytronic bilayer systems,the interaction between out-of-plane ferroelectricity and A-type antiferromagnetism can induce layer-polarized anomalous valley Hall(LP-AVH)effect.The K and−K valleys of FVM are both metallic,and electron and hole carriers simultaneously exist.In the extreme case,the FVM can become VGS by analogizing spin gapless semiconductor(SGS).Moreover,it is proposed that the valley splitting enhancement and valley polarization reversal can be achieved by electric field engineering in valleytronic bilayer systems.Taking the bilayer RuBr_(2)as an example,our proposal is confirmed by the first-principle calculations.The FVM and VGS can be achieved in bilayer RuBr_(2)by applying electric field.With appropriate electric field range,increasing electric field can enhance valley splitting,and the valley polarization can be reversed by flipping electric field direction.To effectively tune valley properties by electric field in bilayer systems,the parent monolayer should possess out-of-plane magnetization,and have large valley splitting.Our results shed light on the possible role of electric field in tuning valleytronic bilayer systems,and provide a way to design the ferrovalley-related material by electric field.

Correlation-driven threefold topological phase transition in monolayer OsBr_(2)

Spin–orbit coupling(SOC)combined with electronic correlation can induce topological phase transition,producing novel electronic states.Here,we investigate the impact of SOC combined with correlation effects on physical properties of monolayer OsBr_(2),based on first-principles calculations with generalized gradient approximation plus U(GGA+U)approach.With intrinsic out-of-plane magnetic anisotropy,OsBr_(2)undergoes threefold topological phase transition with increasing U,and valleypolarized quantum anomalous Hall insulator(VQAHI)to half-valley-metal dxy dx2−y2 dz2¯6m2(HVM)to ferrovalley insulator(FVI)to HVM to VQAHI to HVM to FVI transitions can be induced.These topological phase transitions are connected with sign-reversible Berry curvature and band inversion between/and orbitals.Due to symmetry,piezoelectric polarization of OsBr_(2)is confined along the in-plane armchair direction,and only one d11 is independent.For a given material,the correlation strength should be fixed,and OsBr_(2)may be a piezoelectric VQAHI(PVQAHI),piezoelectric HVM(PHVM)or piezoelectric FVI(PFVI).The valley polarization can be flipped by reversing the magnetization of Os atoms,and the ferrovalley(FV)and nontrivial topological properties will be suppressed by manipulating out-of-plane magnetization to in-plane one.In considered reasonable U range,the estimated Curie temperatures all are higher than room temperature.Our findings provide a comprehensive understanding on possible electronic states of OsBr_(2),and confirm that strong SOC combined with electronic correlation can induce multiple quantum phase transition.

Electronic and optical properties of single-layer MoS2

The electronic structures of a MoS2 monolayer are investigated with the all-electron first principle calculations based on the density functional theory （DFT） and the spin-orbital couplings （SOCs）. Ore＂ results show that the monolayer MoS2 is a direct band gap semiconductor with a band gap of 1.8 eV. The SOCs and d-electrons in Mo play a very significant role in deciding its electronic and optical properties. Moreover, electronic elementary excitations are studied theoretically within the diagram- matic self-consistent field theory. Under random phase approximation, it shows that two branches of plasmon modes can be achieved via the conduction-band transitions due to the SOCs, which are different from the plasmons in a two-dimensional electron gas and graphene owing to the quasi-linear energy dispersion in single-layer MoS2. Moreover, the strong optical absorption up to 105 cm-1 and two optical absorption edges I and II can be observed. This study is relevant to the applications of monolayer MoS2 as an advanced photoelectronic device.