Study On Topological Exciton States In Single-layer Transition Metal Sulfides

In condensed matter physics,searching for topological non-trivial quantum states in solid state systems is always an intriguing topic.For fermions such as electrons,the topological nontrivial edge states have been extensively studied in theory and their existence has been observed in the experiment.The key to realize the topological nontrivial edge state is the spin orbit coupling,especially for those coupling of Dirac particles.Another key requirement is to achieve the band invert,which largely depends on the time reversal symmetry.After discovering the topological nontrivial state in the fermion system,it is very straightforward to study how to achieve the same topology non-nontrivial quantum state in other systems.For boson systems such as coupled resonator cavities,photonic crystals,spin orbit coupling can be obtained by the equivalent two components of different bosons,so that the bosonic topological state is obtained.Based on this,we ask that whether one can achieve the topological quantum state in some other systems except for the basic particle system(boson,fermion).In this article,we solve part of this problem through achieving topological nontrivial quantum states in the exciton system,which is formed by electrons and holes.We propose that one can achieve this topological state in the two-dimensional transition metal dichalcogenides(TMDS).Since the monolayer TMDs has a direct bandgap in the light cone and the valley degrees of freedom.Therefore,there are two different types of valley exciton in this system.These two kinds of exciton,each of which is composed of charged particles through Coulomb interaction have a spin-2 spin orbit coupling.When the time inversion symmetry is broken by the magnetic field,the topological exciton can be realized near the domain wall of the magnetic field.We study the change of the binding energy and the shape of the wave function with varying external control parameters as well as the response to the light field,even when the strain is applied along a direction.Since the strain will split the Dirac cone with curl-2 into two Dirac cone with curl-1,one can be expected that the shape of energy spectra of the topological exciton will move as the Dirac cone moves,which has been confirmed by us numerically.To summarize,we can use light to excite topological exciton,use magnetic domain walls to modify the energy spectra and wave functions of topological exciton.Then on can use strain to control the transport properties of topological exciton.In our study,we find the topological nontrivial quantum state of the composite particle and a new type of physical carrier for quantum information processing,which may has potential applications in new types of optoelectronic devices based on topological exciton.So our study may which may increases the possibility of the realization of topological quantum computation.