Fuming Xu

Electronics and optoelectronics of lateral heterostructures within monolayer indium monochalcogenides

Lateral heterostructures have attracted a great deal of attention due to their advanced properties, which may open up unforeseen opportunities in materials science and device physics. Here, we demonstrate a novel type of lateral heterostructure within monolayer indium monochalcogenides. The thermal stability of the structure is obtained based on the ab initio molecular dynamics calculations. Our results reveal that the proposed lateral heterostructures have direct bandgaps, tunable electronic properties, and type-II band alignment. In addition, the predicted carrier mobilities exceed 103 cm2 (V s)−1, which are 1–2 orders of magnitude higher compared to those of transition metal chalcogenide (TMD) materials. For the first time, the photoresponse and photovoltaic performance of such lateral heterostructures are evaluated based on the first-principles calculations. Upon illumination, the photoinduced current is generated throughout the heterojunction, with an external quantum efficiency up to 7.1%. These results make indium monochalcogenide lateral heterostructures promising candidates for next-generation of electronic and optoelectronic devices.

Waiting time distribution of quantum electronic transport in the transient regime

Waiting time is an important transport quantity that is complementary to average current and its fluctuation. So far all the studies of waiting time distribution (WTD) are limited to steady state transport (either dc or ac). In this work, we present a theory to calculate WTD for coherent electronic systems in transient regime. We express the generating function of full counting statistics using Keldysh non-equilibrium Greens functions formalism. Our analysis goes beyond the wideband approximation and is suitable for first principles calculation on realistic systems. Analytic solution has been obtained for short and long time behaviors of waiting times. At short times, the WTD shows a linear dependence on the waiting time while in the long time limit, WTD follows Poisson distribution. We have applied this theory to a quantum dot connected by two leads and calculated cumulants of transferred charge as well as WTD in the transient regime. We have demonstrated how to relate WTD to experimental measured data.

Enhancement of shot noise due to the fluctuation of Coulomb interaction

We have developed a theoretical formalism to investigate the contribution of fluctuation of Coulomb interaction to the shot noise based on Keldysh non-equilibrium Greens function method. We have applied our theory to study the behavior of dc shot noise of atomic junctions using the method of nonequilibrium Greens function combined with the density functional theory (NEGF-DFT). In particular, for atomic carbon wire consisting 4 carbon atoms in contact with two Al(100) electrodes, first principles calculation within NEGF-DFT formalism shows a negative differential resistance (NDR) region in I-V curve at finite bias due to the effective band bottom of the Al lead. We have calculated the shot noise spectrum using the conventional gauge invariant transport theory with Coulomb interaction considered explicitly on the Hartree level along with exchange and correlation effect. Although the Fano factor is enhanced from 0.6 to 0.8 in the NDR region, the expected super-Poissonian behavior in the NDR regionis not observed. When the fluctuation of Coulomb interaction is included in the shot noise, our numerical results show that the Fano factor is greater than one in the NDR region indicating a super-Poissonian behavior.

Emittance fluctuation of mesoscopic conductors in the presence of disorders

We report an investigation of the dynamic conductance fluctuation of disordered mesoscopic conductors including one-dimensional, two-dimensional, and quantum dot systems. Our numerical results show that in the quasi-ballistic regime the average emittance is negative, indicating the expected inductive-like behavior. However, in the diffusive and localized regime, the average emittance is still negative. This disagrees qualitatively with the result obtained from random matrix theory. Our analysis suggests that this counterintuitive result is due to the appearance of non-diffusive elements in the system, the necklace states (or the precursor of necklace states in the diffusive regime) whose existence has been confirmed experimentally in an optical system.

Numerical study of parametric pumping current in mesoscopic systems in the presence of a magnetic field

We numerically study the parametric pumped current when magnetic field is applied both in the adiabatic and non-adiabatic regimes. In particular, we investigate the nature of pumped current for systems with resonance as well as anti-resonance. It is found that in the adiabatic regime, the pumped current changes sign across the sharp resonance with long lifetime while the non-adiabatic pumped current at finite frequency does not. When the lifetime of resonant level is short, the behaviors of adiabatic and non-adiabatic pumped current are similar with sign changes. Our results show that at the energy where complete transmission occurs the adiabatic pumped current is zero while non-adiabatic pumped current is non-zero. Different from the resonant case, both adiabatic and non-adiabatic pumped current are zero at anti-resonance with complete reflection. We also investigate the pumped current when the other system parameters such as magnetic field, pumped frequency, and pumping potentials. Interesting behaviors are revealed. Finally, we study the symmetry relation of pumped current for several systems with different spatial symmetry upon reversal of magnetic field. Different from the previous theoretical prediction, we find that a system with general inversion symmetry can pump out a finite current in the adiabatic regime. At small magnetic field, the pumped current has an approximate relation I(B) \approx I(-B) both in adiabatic and non-adiabatic regimes.

Statistics of Wigner delay time in Anderson disordered systems

We numerically investigate the statistical properties of Wigner delay time in Anderson disordered 1D, 2D and quantum dot (QD) systems. The distribution of proper delay time for each conducting channel is found to be universal in 2D and QD systems for all Dysons symmetry classes and shows a piece-wise power law behavior in the strong localized regime. Two power law behaviors were identified with asymptotical scaling

Tunable current partition at zero-line intersection of quantum anomalous Hall topologies

{\tau^{-1.5}}

Dynamic response of silicon nanostructures at finite frequency: An orbital-free density functional theory and non-equilibrium Green's function study

and

Influence of magnetic disorders on quantum anomalous Hall effect in magnetic topological insulator films beyond the two-dimensional limit

{\tau^{-2}}

Geometric decoherence of valley excitons in monolayer transition metal dichalcogenides

, respectively that are independent of the number of conducting channels and Dysons symmetry class. Two power-law regimes are separated by the relevant time scale