First-principles Study Of Electronic Structure Modulations In Novel Low-dimensional Materials

In condensed-matter physics,many properties of materials are based on their symmetries in space and time,while in low-dimensional materials,symmetries have more significant effects on the properties due to the enhanced quantum confinement.The two-dimensional materials in Janus structure exhibit Rashba effect,piezoelectric effect and excellent catalytic performance owing to the inversion symmetry breaking.Among them,the 1H phased Janus transition metal dichalcogenides are explored mostly.A large enhancement of the magnetic moment is demonstrated in their nanoribbons compared to the traditional transition metal dichalcogenides counterpart.In addition,there are spin non-degenerate Dirac states in the Brillouin zone of 1T'phased Janus transition metal dichalcogenides.Under the certain strain,the band gap of the Dirac cone tends to zero,thus resulting in the massless Dirac dispersion relation against spin-orbit coupling,which is expected to achieve low power consumption and ultra-high-speed transmission.On the other hand,in heterostructures of topological insulators and antiferromagnets,the magnetic proximity-effect can break the time-reversal symmetry of topological insulators and thus regulate the topological surface states.It has been reported that the anomalous Hall effect with a high Curie temperature has been observed in such structures.This thesis mainly studies the respective work of the above three systems through the first-principles calculations,and the main contents and results are as follows:1.The increased magnetic moment in zigzag MoSSe nanoribbons arises from the hybridized(9_z2 and d_yz orbitals of edge Mo atom.Due to the mirror symmetry breaking,the tilted(9_z2 orbital largely overlaps the d_yz orbital.As a result,a new spin-up band emerged and holds more electrons,leading to a large spin polarization near the Fermi level.2.1T'phased transition metal dichalcogenides monolayers are spin-Hall insulators with wide band gaps,which have intrinsic band inversion between the p orbital of the sulfur group elements and the d orbital of the metal elements.1T'phased Janus transition metal dichalcogenides monolayers are two-dimensional spin-valley coupled Dirac semi-metal under a certain strain,with the massless Dirac dispersion relation against spin-orbit coupling.3.In Sb₂Te₃/CrSb heterostructure,the antiferromagnetic CrSb layer magnetized the atoms of Sb₂Te₃ near the interface,and a band gap was introduced into the Dirac cone at the interface.The induced surface magnetic moment and the size of the band gap could be adjusted by the interfacial spacing.The analysis of the Hamiltonian model shows that the bandgap size of the surface states is mainly related to the energy difference between the top and bottom surface states.