First-principles Study Of Electronic Correlations In Quantum Materials

Quantum materials have emerged as a very relevant frontier field in the condensed matter physics due to their unique quantum behaviors and fascinating physical features,which originates from strong electronic correlations of d or f electrons in transition metal elements.On the other hand,exploring the structure and performance of two-dimensional(2D)quantum materials,as well as systemically studying their electronic correlations in theory,is one of the ultimate problems of science and industry.To obtain a better understanding of essential physical and functional characteristics,we perform the first-principles calculations based on the density functional theory(DFT)to describe the electronic structures and corresponding properties of three materials using its two-dimensional structures.such as FeSe,WSe2 and BiVO4.The contents are listed below:First,we focus on the thickness dependence of superconductivity of FeSe films(from monolayer to five-layer).Monolayer FeSe is known to exhibit the highest superconducting critical temperature among iron-based superconductors.However,its Cooper pairing mechanism is still under debate.In this work,we obtain the eigenvalues of system from DFT calculation,and then consider spin fluctuationmediated scenario of superconducting pairing generated by the spin-fluctuation part of the Hamiltonian that can be derived by using the Hubbard model.Tc～40K corresponds to U～2eV for monolayer FeSe film,in agreement with the experimental data.Similarly,the results for the 5L system(～20K)and the bulk system(～8K)agree with the experimental results.Our results clearly indicate that spin fluctuations indeed trigger superconducting state which leads to the formation of Cooper pairs for obtaining high superconducting transition temperature and describe the superconductivity with dependence of thick.Second,we investigate electronic correlations of bright and dark excitons in monolayer WSe2 using time-dependent density functional theory(TDDFT)calculations.WSe2 possesses two time-reversal electronic valleys(K and K’)with a direct band gap in the visible light region of the Brillouin zone.Owing to strong spin-orbital coupling,the valence and conduction bands split into two sub-bands with opposite spins.The energy splitting of the top of the valence band is 500 meV.while that of the bottom of the conduction band is 38 meV approximately.The optical properties of monolayer WSe2 are governed by intra-and intervalley exciton.The obtained results include properties of intra-and intervalley excitons,and hybridization of excitonic states.The distinctive consequences between intra-and intervalley are explained by inconsistent electronic correlations.The lifetime of excitons is considered by the electron-phonon scattering effect.Our finding explores that consequence of electron correlation effects on material properties in 2D limit.Third,we take vanadate as a specimen and investigate the effect of W/Mo codoping on electronic structure of monoclinic scheelite BiVO4.Generally,the structure is controlled by doping in the lab,which is expected to obtain desirable performance.The adoption of DFT+U method is used to properly describe the electronic properties of doping transition metal atoms(W/Mo)with multiple doping concentrations that enhances photocatalytic properties of BiVO4.Fourth,we discover the surface-properties relationships in multiple phase BiVO4,and the reason causing distinct photocatalysis with similar band structure in its bulk crystals.BiVO4 is often used for photocatalytic applications with two crystalline phases,which are monoclinic scheelite(ms-)and tetragonal scheelite(ts-).These two phases exhibit a similar electronic structure but inconsistent photocatalytic performance.We study the structural morphology and electronic correlation states of different low-index surfaces,which are derived from these two crystalline phases.The surfaces include(001)/{001},(011)/{011},(101)/{101}.We find that the(011)and {011} facets present distinct surface properties,leading to distinct electronic properties.Specifically,mid-band gap states appear at surface{011} caused by the isolated O 2p states.For the other four surfaces,surface(001)shares geometric and electronic structure characteristics with {001},and surface(101)possesses identical features with {101}.Our results provide the corresponding relationship between the 2D surface configurations,and the functional properties derived from electron correlation states.