Design And Computational Physics Investigation Of New Material Heterojunctions For Magnetic / Electronic Devices

Recent advances in creating complex oxide heterostructures（HTS）,interfaces formed between two different transition metal oxides,have heralded a new era of materials and physics research,these heterostructure systems have exhibited such varied phenomena as superconductivity,magnetism,and ferroelasticity,all of which are tunable,demonstrating their promise for fundamental discovery and technological innovation alike.The family of two-dimensional（2D）materials has grown appreciably since the discovery of graphene.At the same time,a vast class of 2D van der Waals（vd W）layered materials has been researched intensively.The 2D vd W layered materials could present exotic physical and chemical properties,which the common three-dimensional（3D）materials don’t possess.In parallel with the efforts on 2D vd W layered materials,another research field has recently emerged and has been gaining strength over the last two years.It deals with heterostructures and devices made by stacking different 2D crystals on top of each other.Such stacks are very different from the traditional 3D semiconductor heterostructures,as each layer acts simultaneously as the bulk material and the interface,reducing the amount of charge displacement within each layer.Still,the charge transfers between the layers can be very large,inducing large electric fields and offering interesting possibilities in band-structure engineering.The specific contents are as follows:1.By using first-principles calculations,here we systematically investigate the effect of GaGeTe substrate on the electronic properties of monolayer germanene.Linear dichroism of Dirac-cone like band dispersion and higher carrier mobility(9.7×10³cm²V^-1s^-1)in Ge/GaGeTe heterostructure（HTS）are found to be preserved compared to that of free-standing germanene.Remarkably,the band structure of HTS can be flexibly modulated by applying bias voltage.Especially,when the applied electric field is close to a critical value Ec=0.4 V/?,a semiconductor to metal transition occurs.A prototype data storage device FET based on Ge/GaGeTe HTS is proposed,which presents a promising high-performance platform with tunable band gap and high carrier mobility.2.The geometries,electronic band structures and effective potentials of VS₂/MoS₂ heterojunction are calculated in details.Two configurations including AA-and AB-stacked VS₂/MoS₂ are involved and the latter is found more stable according to the calculated binding energy.The band gap of MoS₂ is slight decreased due to its weak coupling with VS₂.The p-type Schottky barrier is formed at AA-and AB-stacked VS₂/MoS₂ interface with the Schottky barrier height～0.15 and 0.10 e V,respectively.The tunneling barrier exists in AA-stacked heterojunction and vanishes in AB-stacked heterojunction.The intercalation of Na and Mg atom alters the interface contact barrier from p-type to n-type,which mainly due to the charge transfer between the metal atom and the heterojunction.Further quantum transport simulations show the carrier polarity of the system is completely reversed when the Li inserted AA-stacked and Li,Be inserted AB-stacked VS₂/MoS₂heterojunctions are used as electrodes.3.We have studied the ground state magnetic structure and the formation energy of various defects for bulk Sm₂CuO₄,and the surface energies with different surface terminations.In Sm₂CuO₄,the G type antiferromagnetic order is caused by the hole-mediated superexchange through the p-d hybridization between Cu and neighboring O atoms.We find that the V_cu is energetically favorable under O-rich conditions,under O-poor condition,the formation energies of V_O are negative and much lower than other vacancies over the whole range of Fermi level,resulting in spontaneous formations.To construct its（001）surface structure,CuOO,CuO,and Cu terminated surfaces are found to be most favorable under different experimental conditions.The stable surface structures are always accompanied by significant surface atomic reconstructions and electron charge redistribution,which are intimately correlated to each other.4.The electronic structures and magnetic properties of rare earth metal Sm and Gd-doped in topological insulator Bi₂Se₃ have been studied by first-principle calculation.Our calculated formation energies have indicated that Bi substitutional sites were strongly preferred by Sm or Gd atoms at Bi-rich condition.From spin-orbit coupling calculation,it was found that Bi₂₃Sm Se₃₆showed metallic properties,while Bi₂₃Gd Se₃₆ was still a topological insulator with energy gap of 0.03 e V.For Sm³⁺the net magnetic moment was 3.65μ_B with the low-lying J=7/2 excited state,and the magnetic moment of the cations Gd³⁺was about 6.744μ_B,because the Gd³⁺orbital moment is absent and only spin magnetic moment is left.This indicates a potential application for fabricating strong magnetic TI in spintronics.The calculated magnetic coupling between dopants have suggested that Sm doped Bi₂Se₃ is likely FM state,while the Gd doped system showed PM state.