The Design Of Pb-based Two-dimensional Materials And The Investigation On Quantum Nature

Band topology and Rashba spin splitting?RSS?are two extensively explored yet exotic properties in condensed matter physics.Nonetheless,their coexistence has rare been achieved in a simple stoichiometric two-dimensional?2D?ultrathin films so far.And,spin orbital coupling?SOC?in topological insulators?TIs?has recently attracted great interests due to its potential in spintronics.Pb?Plumbum?and Bi?Bismuth?are known for their stronger SOC which can drive nontrivially topological states.So here we through first-principles calculations to predict a new inversion-asymmetric BiPbH monolayer,allows for the simultaneous presence of nontrivially topological order and large RSS,the nontrivial quantum state originated from p_x,y–p_z band inversion is confirmed by Z₂ index and helical edge states.Interestingly,the coexistence of the topological band gaps and RSS in this system is robust and stable over a wide range of strain?-6-6%?,with the maximum gap at?point being enhanced to 0.40 eV and Rashba energy as large as 53.1 meV under achievable strains,respectively,which makes them viable for practical realization of quantum spin hall?QSH?state at room-temperature.Then,in order to study the influence cause by halogen elements,based on the previous structure,we apply first-principles calculations to design a family of 2D large-gap TIs composed of hexagonal Bi and PbX?X=F,Cl,Br and I?dimers.The nontrivial topology,induced via p_x,y–p_z band inversion,is also confirmed by Z₂ index and helical edge states.Noticeably,the RSS energy in these films reaches 81 meV,which is further tunable over a wide range of strains?-2-14%?.Additionally,the h-BN semiconductor is an ideal substrate for experimental realization of BiPbX,without destroying its nontrivial topology.Considering the robustness of band topology under h-BN substrate,our works open a new route for designing topological spintronics devices based on 2D inversion-asymmetric films.On the other hand,Topological phases,especially topological crystalline insulators?TCIs?,have been intensively explored observed experimentally in three-dimensional?3D?materials.However,the 2D films are explored much less than 3D TCI,and even 2D topological insulators.Based on ab initio calculations,here we investigate the electronic and topological properties of2D PbTe?001?few-layers.The monolayer and trilayer PbTe are both intrinsic 2D TCIs with a large band gap reaching 0.27 eV,indicating a high possibility for room-temperature observation of quantized conductance.The origin of TCI phase can be attributed to the p_x,y–p_z band inversion,which is determined by the competitions of orbital hybridization and quantum confinement effect.We also observe a semimetal-TCI-normal insulator transition under biaxial strains,whereas a uniaxial strains lead to Z₂ nontrivial states.Especially,the TCI phase of PbTe monolayer remains when epitaxial grow on NaI semiconductor substrate.Our findings on the controllable quantum states with sizable band gaps present an ideal platform for realizing future topological quantum devices with ultralow dissipation.