Zhizhou Yu

Full-counting statistics of transient energy current in mesoscopic systems

We investigate the full-counting statistics (FCS) of energy flow carried by electrons in the transient regime. Based on two measurement scheme we formulate a non-equilibrium Keldysh Greens function theory to compute the generating function for FCS of energy transport. Specifically, we express the generating function using the path integral along Keldysh contour and obtain exact solution of the generating function using the Grassmann algebra. With this formalism, we calculate the transient energy current and higher order cumulants for both single and double quantum dot (QD) systems in the transient regime. To examine finite bandwidth effect of leads to FCS of energy transport, we have used an exact solvable model with a Lorentizian linewidth where all non-equilibrium Greens functions can be solved exactly in the time domain. It is found that the transient energy current exhibits damped oscillatory behavior. For the single quantum dot system the frequency of oscillation is independent of bandwidth of the leads while the decay rate of the oscillation amplitude is determined by the lifetime of resonant state which increases as the bandwidth decreases. At short times, a universal scaling of maximum amplitude of normalized cumulants is identified for the single QD system. For the double QD system, the damped oscillation of energy current is dominated by Rabi oscillation with frequency approximately proportional to the coupling constant between two quantum dots. In general, the transient energy current increases when the coupling between two QDs is stronger. However, when the interdot coupling is larger than half of the external bias the transient energy current is suppressed significantly. All these results can be understood analytically.

Full-counting statistics of energy transport of molecular junctions in the polaronic regime

We investigate the full-counting statistics (FCS) of energy transport carried by electrons in molecular junctions for the Anderson-Holstein model in the polaronic regime. Using two-time quantum measurement scheme, generating function (GF) for the energy transport is derived and expressed as a Fredholm determinant in terms of Keldysh nonequilibrium Greens function in the time domain. Dressed tunneling approximation is used in decoupling the phonon cloud operator in the polaronic regime. This formalism enables us to analyze the time evolution of energy transport dynamics after a sudden switch-on of the coupling between the dot and the leads towards the stationary state. The steady state energy current cumulant GF in the long time limit is obtained in the energy domain as well. Universal relations for steady state energy current FCS are derived under finite temperature gradient with zero bias and this enables us to express the equilibrium energy current cumulant by a linear combination of lower order cumulants. Behaviors of energy current cumulants in steady state under temperature gradient and external bias are numerically studied and explained. Transient dynamics of energy current cumulants is numerically calculated and analyzed. The universal scaling of normalized transient energy cumulants is found under both temperature gradient and external bias.

Nonequilibrium spin injection in monolayer black phosphorus

Monolayer black phosphorus (MBP) is an interesting emerging electronic material with a direct band gap and relatively high carrier mobility. In this work we report a theoretical investigation of nonequilibrium spin injection and spin-polarized quantum transport in MBP from ferromagnetic Ni contacts, in two-dimensional magnetic tunneling structures. We investigate physical properties such as the spin injection efficiency, the tunnel magnetoresistance ratio, spin-polarized currents, charge currents and transmission coefficients as a function of external bias voltage, for two different device contact structures where MBP is contacted by Ni(111) and by Ni(100). While both structures are predicted to give respectable spin-polarized quantum transport, the Ni(100)/MBP/Ni(100) trilayer has the superior properties where the spin injection and magnetoresistance ratio maintains almost a constant value against the bias voltage. The nonequilibrium quantum transport phenomenon is understood by analyzing the transmission spectrum at nonequilibrium.

Direct tunneling through high-κ amorphous HfO2: Effects of chemical modification

We report first principles modeling of quantum tunneling through amorphous HfO

Spin-polarized quantum transport properties through flexible phosphorene

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First-principles investigation of quantum transport through an endohedral N@C60 in the Coulomb blockade regime

dielectric layer of metal-oxide-semiconductor (MOS) nanostructures in the form of n-Si/HfO

A novel electrically controllable volatile memory device based on few-layer black phosphorus

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Fast algorithm for transient current through open quantum systems

/Al. In particular we predict that chemically modifying the amorphous HfO

Valley Seebeck effect in gate tunable zigzag graphene nanoribbons

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Transmission spectra and valley processing of graphene and carbon nanotube superlattices with inter-valley coupling

barrier by doping N and Al atoms in the middle region - far from the two interfaces of the MOS structure, can reduce the gate-to-channel tunnel leakage by more than one order of magnitude. Several other types of modification are found to enhance tunneling or induce substantial band bending in the Si, both are not desired from leakage point of view. By analyzing transmission coefficients and projected density of states, the microscopic physics of electron traversing the tunnel barrier with or without impurity atoms in the high-