arXiv subjects

Baigeng Wang

Publications and source records attributed to Baigeng Wang.

106 records · Page 6Linked to original sources

arXiv2002

Non-adiabatic charge pump: an exact solution

We derived a general and exact expression of current for quantum parametric charge pumps in the non-adiabatic regime at finite pumping frequency and finite driving amplitude. The non-perturbative theory predicts a remarkable plateau structure in the pumped current due to multi-photon assisted processes in a double-barrier quantum well pump involving only a {\it single} pumping potential. It also predicts a current reversal as the resonant level of the pump crosses the Fermi energy of the leads.

cond-mat↗

arXiv2002

Parametric quantum spin pump

We investigate a non-adiabatic parametric quantum pump consists of a nonmagnetic scattering region connected by two ferromagnetic leads. The presence of ferromagnetic leads allows electrons with different spins to experience different potential landscape. Using this effect we propose a quantum spin pump that drives spin-up electrons to flow in one direction and spin-down electrons to flow in opposite direction. As a result, the spin pump can deliver a spin current with vanishing charge current.

cond-mat↗

arXiv2002

Quantum spin field effect transistor

We propose, theoretically, a new type of quantum field effect transistor that operates purely on the flow of spin current in the absence of charge current. This spin field effect transistor (SFET) is constructed without any magnetic material, but with the help of spin flip mechanism provided by a rotating external magnetic field of uniform strength. The SFET generates a constant instantaneous spin current that is sensitively controllable by a gate voltage as well as by the frequency and strength of the rotating field. The characteristics of a Carbon nanotube based SFET is provided as an example.

cond-mat↗

arXiv2002

Optimal quantum pump in the presence of a superconducting lead

We investigate the parametric pumping of a hybrid structure consisting of a normal quantum dot, a normal lead and a superconducting lead. Using the time dependent scattering matrix theory, we have derived a general expression for the pumped electric current and heat current. We have also derived the relationship among the instantaneous pumped heat current, electric current, and shot noise. This gives a lower bound for the pumped heat current in the hybrid system similar to that of the normal case obtained by Avron et al.

cond-mat↗

arXiv2002

Statistical correlation for the composite Boson

It is well known that the particles in a beam of Boson obeying Bose-Einstein statistics tend to cluster (bunching effect), while the particles in a degenerate beam of Fermion obeying Fermi-Dirac statistics expel each other (anti-bunching effect). Here we investigate, for the first time, the statistical correlation effect for the composite Boson, which is formed from a spin singlet entangled electron pair. By using nonequilibrium Green's function technique, we obtain a positive cross correlation for this kind of the composite Boson when the external voltage is smaller than the gap energy, which demonstrates that a spin singlet entangled electron pair looks like a composite Boson. In the larger voltage limit, the cross correlation becomes negative due to the contribution of the quasiparticles. At large voltages, the oscillation between Fermionic and Bosonic behavior of cross correlation is also observed in the strong coupling regime as one changes the position of the resonant levels. Our result can be easily tested in a three-terminal normal-superconductor-superconductor (N-S-S) hybrid mesoscopic system.

cond-mat↗

arXiv2002

Spin polarized parametric pumping

We have developed a general theory for a parametric pump consisting of a nonmagnetic system with two ferromagnetic leads whose magnetic moments orient at an angle $θ$ with respect to each other. In this theory, the leads can maintain at different chemical potential. As a result, the current is driven due to both the external bias and the pumping potentials. When both $θ$ and external bias are zero, our theory recovers the known theory. In particular, two cases are considered: (a). in the adiabatic regime, we have derived the pumped current for arbitrary pumping amplitude and external bias. (b). at finite frequency, the system is away from equilibrium, we have derived the pumped current up to quadratic order in pumping amplitude. Numerical results show interesting spin valve effects for pumped current.

cond-mat↗

arXiv2002

Parametric electron pumping through a quantum dot in the Kondo regime

We report a theoretical analysis of parametric electron pump through a quantum dot in the Kondo regime. In the adiabatic regime, we have derived the expression for pumped current in the Kondo regime using non-equilibrium Green's function. The pumped current versus different system parameters such as gate voltage, pumping amplitude, as well as the phase difference between two pumping forces are calculated and interesting physics are revealed.

cond-mat↗

arXiv2002

Heat current in parametric quantum pump

We investigate the heat flow in the parametric quantum pump. Using the time dependent scattering matrix theory, we have developed a general theory for the pumped heat current at finite pumping amplitude and frequency. We have applied our theory to a double barrier structure and studied pumped heat current in both the weak and strong pumping regimes as different system parameters vary. By comparing the pumped heat current and the power of Joule heat generated in the system, we found that the double barrier structure can function as an optimal pump in the strong pumping regime.

cond-mat↗

arXiv2002

Quantization of adiabatic pumped charge in the presence of superconducting lead

We investigate the parametric electron pumping of a double barrier structure in the presence of a superconducting lead. The parametric pumping is facilitated by cyclic variation of the barrier heights $x_1$ and $x_2$ of the barriers. In the weak coupling regime, there exists a resonance line in the parameter space $(x_1,x_2)$ so that the energy of the quasi-bound state is in line with the incoming Fermi energy. Levinson et al found recently that the pumped charge for each pumping cycle is quantized with $Q=2e$ for normal structure when the pumping contour encircles the resonance line. In the presence of a superconducting lead, we find that the pumped charge is quantized with the value $2e$.

cond-mat↗

arXiv2001

Enhancement of parametric pumping due to Andreev reflection

We report properties of parametric electron pumping in the presence of a superconducting lead. Due to a constructive interference between the direct reflection and the multiple Andreev reflection, the pumped current is greatly enhanced. For both quantum point contacts and double barrier structures at resonance, we obtain exact solutions in the weak pumping regime showing that $I_p^{NS} = 4 I_p^N$, which should be compared with the result of conductance $G_{NS} = 2G_N$. Numerical results are also provided for the strong pumping regime showing interesting Andreev assisted pumping behaviour.

cond-mat↗

arXiv2001

Parametric pumping at finite frequency

We report on a first principles theory for analyzing the parametric electron pump at a finite frequency. The pump is controlled by two pumping parameters with phase difference $ϕ$. In the zero frequency limit, our theory predicts the well known result that the pumped current is proportional to $\sinϕ$. For the more general situation of a finite frequency, our theory predicts a non-vanishing pumped current even when the two driving forces are in phase, in agreement with the recent experimental results. We present the physical mechanism behind the nonzero pumped current at $ϕ=0$, which we found to be due to photon-assisted processes.

cond-mat↗

arXiv1999

Nonlinear Spin Polarized Transport Through a Quantum Dot

We present a theoretical analysis of the nonlinear bias and temperature dependence of current-voltage characteristics of a spin-valve device which is formed by connecting a quantum dot to two ferromagnetic electrodes whose magnetic moments orient at an angle $θ$ with respect to each other. The theory is based on nonequilibrium Green's function approach and focused on current perpendicular to plane geometry. Coulomb interaction has been taken into account explicitly at the Hartree level. We derive a formula in closed form for current flowing through the device in general terms of bias and temperature. In the wideband limit we report exact results for the TMR junction nonlinear I-V curve as a function of $θ$. We also report the conductance slope at zero bias as a function of temperature for which experimental results reported an anomalous behavior.

cond-mat↗

arXiv1999

Nonlinear Quantum Capacitance

We analyze the nonlinear voltage dependence of elelctrochemical capacitance for nano-scale conductors. This voltage dependence is due to finite density of states of the conductors. We derive an exact expression for the electrochemical capacitance-voltage curve for a parallel plate system. The result suggests a quantum scanning capacitance microscopy at the nano-scale: by inverting the capacitance-voltage expression one is able to deduce the local spectral function of the nano-scale conductor.

cond-mat↗

arXiv1999

Nonlinear I-V Characteristics of a Mesoscopic Conductor

We present a general theoretical formulation, based on nonequilibrium Green's functions, for nonlinear DC transport in multi-probe mesoscopic conductors. The theory is gauge invariant and is useful for the predictions of current-voltage characteristics and the nonequilibrium charge pile-ups inside the conductor. We have provided a detailed comparison between the gauge invariant scattering matrix theory and our theory. We have also given several examples where the I-V curve can be obtained analytically. The effects of exchange and correlation have been considered explicitly.

cond-mat↗

arXiv1999

Nonlinear voltage dependence of shot noise

The current noise in a multi-probe mesoscopic conductor can have a nonlinear dependence on the strength of driving bias voltage. This paper presents a theoretical formulation for the nonlinear noise spectra. We pay special attention to maintain gauge invariance at the nonlinear level. At small but finite voltages, explicit expressions for nonlinear noise spectra, expanded order by order in the bias, have been derived. In the wideband limit, a closed form solution of the noise spectra for finite voltages is obtained.

cond-mat↗

arXiv1998

Current partition: Nonequilibrium Green's function Approach

We present a solution to the problem of AC current partition in a multi-probe mesoscopic conductor within the nonequilibrium Green's function formalism. This allows the derivation of dynamic conductance which is appropriate for nonequilibrium situations and which satisfies the current conservation and gauge invariance requirements. This formalism presents a significant generalization to previous theory: (i) there is no limit in the frequency, and (ii) it allows detailed treatments of interactions in the mesoscopic region. The formalism is applied to calculate dynamic conductance of tunneling structures with and without assuming wideband limit.

cond-mat↗