Transport In Mesoscopic Materials

Electronic transport is an interesting and important area both in classical and quan-tum area.Many important theories have been developed such as Boltzmann transport equation,Landauer-B(?)ttiker formula,Kubo formula,and nonequilibrium green's func-tion transport theory.Based on the old theory we have studied the transport properties of new material,,developed new transport theory,and use our new theory as shown below.(1)We numerically study the effect of the edge states on the conductance and thermopower in zigzag phosphorene nanoribbons(ZPNRs)based on the tight-binding model and the scattering-matrix method.It is interesting to find that the band disper-sion,conductance,and thermopower can be modulated by applying a bias voltage and boundary potentials to the two layers of the ZPNRs.Under the certain bias voltage,the two-fold degenerate quasi-flat edge bands split perfectly.The conductance can be switched off,and the thermopower around zero energy increases.In addition,when only the boundary potential of the top layer or bottom layer is adjusted,only one edge band bends and merges into the bulk band.The first conductance plateau is strongly decreased to e2/h around zero energy.Particularly,when the two boundary potentials are adjusted,all the edge bands bend and fully merge into the bulk band,and the bulk energy gap is maximized.More interestingly,a pronounced conductance plateau with G = 0 is found around zero energy,which is attributable to the opening of the bulk en-ergy gap between the valence and conduction bands.Meanwhile,the thermopower can be enhanced more than twice,compared to that of the perfect ZPNRs.The large mag-nitude of thermopower is ascribed to the appearance of the bulk energy gap around zero energy.Our results show that the modulated ZPNRs are more reliable in thermoelectric application.(2)We show that by integrating out the electric field and incorporating proper boundary conditions,a semiclassical Boltzmann equation can describe electron trans-port properties,continuously from the diffusive to ballistic regimes.General analytical formulas of the conductance in D = 1,2,3 dimensions are obtained,which recover the Boltzmann-Drude formula and Landauer-B(?)ttiker formula in the diffusive and ballistic limits,respectively.This intuitive and efficient approach can be applied to investigate the interplay of system size and impurity scattering in various charge and spin transport phenomena.(3)The surface states of three-dimensional topological insulators possess the unique property of spin-momentum interlocking.This property gives rise to the in-teresting inverse Edelstein effect(IEE),in which an applied spin bias ? is converted to a measurable charge voltage difference V.We develop a semiclassical theory for the IEE of the surface states of Bi2Se3 thin films,which is applicable from the ballistic regime to diffusive regime.We find that the efficiency of the spin-charge conversion,defined as ? = V/?,exhibits a universal dependence on the ratio between sample size and electron mean free path.The efficiency increases from ? = ?/4 in the ballistic limit to ?=1 in the diffusive limit,suggesting that sufficient strength of impurity scattering is favorable for the IEE.