Enhanced response of current-driven coupled quantum wells

We have investigated the conditions necessary to achieve stronger Cherenkov-like instability of plasma waves leading to emission in the terahertz (THz) regime for semiconductor quantum wells (QWs). The surface response function is calculated for a bilayer two-dimensional electron gas (2DEG) system in the presence of a periodic spatial modulation of the equilibrium electron density. The 2DEG layers are coupled to surface plasmons arising from excitations of free carriers in the bulk region between the layers. A current is passed through one of the layers and is characterized by a drift velocity for the driven electric charge. By means of a surface response function formalism, the plasmon dispersion equation is obtained as a function of angular frequency, the in-plane wave vector and reciprocal lattice vector of the density modulation. The dispersion equation,is solved numerically in the complex frequency plane for real wave vector. It is ascertained that the imaginary part of the angular frequency is enhanced with decreasing period of modulation, and with increasing the doping density of the free carriers in the bulk medium for fixed period of the spatial modulation

Theory for entanglement of electrons dressed with circularly polarized light in Graphene and three-dimensional topological insulators

We have formulated a theory for investigating the conditions which are required to achieve entangled states of electrons on graphene and three-dimensional (3D) topological insulators (TIs). We consider the quantum entanglement of spins by calculating the exchange energy. A gap is opened up at the Fermi level between the valence and conduction bands in the absence of doping when graphene as well as 3D TIs are irradiated with circularly-polarized light. This energy band gap is dependent on the intensity and frequency of the applied electromagnetic field. The electron-photon coupling also gives rise to a unique energy dispersion of the dressed states which is different from either graphene or the conventional two-dimensional electron gas (2DEG). In our calculations, we obtained the dynamical polarization function for imaginary frequencies which is then employed to determine the exchange energy. The polarization function is obtained with the use of both the energy eigenstates and the overlap of pseudo-spin wave functions. We have concluded that while doping has a significant influence on the exchange energy and consequently on the entanglement, the gap of the energy dispersions affects the exchange slightly, which could be used as a good technique to tune and control entanglement for quantum information purposes

Comparison of inelastic and quasi-elastic scattering effects on nonlinear electron transport in quantum wires

When impurity and phonon scattering coexist, the Boltzmann equation has been solved accurately for nonlinear electron transport in a quantum wire. Based on the calculated non-equilibrium distribution of electrons in momentum space, the scattering effects on both the non-differential (for a fixed dc field) and differential (for a fixed temperature) mobilities of electrons as functions of temperature and dc field were demonstrated. The non-differential mobility of electrons is switched from a linearly increasing function of temperature to a parabolic-like temperature dependence as the quantum wire is tuned from an impurity-dominated system to a phonon-dominated one [see T. Fang, {\em et al.}, \prb {\bf 78}, 205403 (2008)]. In addition, a maximum has been obtained in the dc-field dependence of the differential mobility of electrons. The low-field differential mobility is dominated by the impurity scattering, whereas the high-field differential mobility is limited by the phonon scattering [see M. Hauser, {\em et al.}, Semicond. Sci. Technol. {\bf 9}, 951 (1994)]. Once a quantum wire is dominated by quasi-elastic scattering, the peak of the momentum-space distribution function becomes sharpened and both tails of the equilibrium electron distribution centered at the Fermi edges are raised by the dc field after a redistribution of the electrons is fulfilled in a symmetric way in the low-field regime. If a quantum wire is dominated by inelastic scattering, on the other hand, the peak of the momentum-space distribution function is unchanged while both shoulders centered at the Fermi edges shift leftward correspondingly with increasing dc field through an asymmetric redistribution of the electrons even in low-field regime [see C. Wirner, {\em et al.}, \prl {\bf 70}, 2609 (1993)]

The Effect of a Magnetic Field on the Acoustoelectric current in a Narrow Channel

The effect of a perpendicular magnetic field on the quantized current induced by a surface acoustic wave in a quasi-1D channel is studied. The channel has been produced experimentally in a GaAs heterostructure by shallow etching techniques and by the application of a negative gate voltage to Schottky split gates. Commensurability oscillations of the quantized current in this constriction have been observed in the interval of current between quantized plateaus. The results can be understood in terms of a moving quantum dot with the electron in the dot tunneling into the adjacent two-dimensional region. The goal is to explain qualitatively the mechanism for the steplike nature of the acoustoelectric current as a function of gate voltage and the oscillations when a magnetic field is applied. A transfer Hamiltonian formalism is employed.Comment: 5 pages, 2 figure