Structure Design And Property Investigation Of Two-dimensional Transition Metal Boride

Since the discovery of graphene in 2004,the first two-dimensional material with a single atomic thickness,its excellent physical and chemical properties have aroused the study of two-dimensional materials as one of the key areas of condensed matter physics.The boron atom is adjacent to the carbon atom in the periodic table and has many properties similar to carbon.However,the stabilities of two-dimensional boron monolayers are usually low due to the electron-deficient nature of boron.Therefore,applying transition metal elements to donate electrons to boron elements has become one of the important strategy to stabilize boron-based monolayers.Herein,we report the stable configurations and excellent electronic,magnetic,and mechanical properties of several novel two-dimensional transition metal borides using crystal structure prediction combined with first-principles calculations.The main contents are listed below:(i)Motivated by a recent work reporting a novel half auxetic effect in monolayer Pd B₄ with a hypercoordinated structure,here,we extensively explore similar two-dimensional transition metal boride structures MB ₄ with M covering 3d and 4d elements MB₄(M=Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn,Y,Zr,Nb,Mo,Tc,Ru,Rh,Ag and Cd).Our investigation screens out one s table can didate,the monolayer Rh B₄.We find that monolayer Rh B₄ also shows half auxeticity.We show that this special mechanical character is intimately tied to the hypercoordinated structure with the M?B₈structural motif.Furthermore,regarding electronic properties,monolayer Rh B₄ is found to be the first example of an almost ideal two-dimensional spin-orbit Dirac point semimetal.The low-energy band structure is clean,with a pair of fourfold degenerate Dirac points robust under spin-orbit coupling located close to the Fermi level.These Dirac points are enforced by the specific crystal symmetry.The half auxeticity and the spin-orbit Dirac points will make monolayer Rh B₄ a promising platform for nanomechanics and nanoelectronics applications.(ii)We predict two low-energy Mn B₆ monolayers,namely Mn B₆-I and Mn B₆-II,with high stabilities and intriguing electronic and magnetic properties.Specifically,the phonon spectrum and ab initio molecular dynamics simulations indicate the two sheets are dynamically and thermally stable.The Mn B₆-I and Mn B₆-II layers are both metallic with ferromagnetic(FM)and antiferromagnetic(AFM)ground state,respectively.When spin-orbital coupling is considered,the type-II Dirac points and a sizable magnetocrystalline anisotropy energy(938?e V per Mn atom)can be found in the Mn B₆-I sheet.The Monte Carlo simulation based on the Heisenberg model suggests the Curie temperature for Mn B₆-I sheet is up to 470 K,which is above room temperature.The superior electronic and magn etic properties of the Mn B₆ monolayers render them promising candidates for spintronic applications.