Optical phonon scattering and theory of magneto-polarons in a quantum cascade laser in a strong magnetic field

We report a theoretical study of the carrier relaxation in a quantum cascade laser (QCL) subjected to a strong magnetic field. Both the alloy (GaInAs) disorder effects and the Frohlich interaction are taken into account when the electron energy differences are tuned to the longitudinal optical (LO) phonon energy. In the weak electron-phonon coupling regime, a Fermi's golden rule computation of LO phonon scattering rates shows a very fast non-radiative relaxation channel for the alloy broadened Landau levels (LL's). In the strong electron-phonon coupling regime, we use a magneto-polaron formalism and compute the electron survival probabilities in the upper LL's with including increasing numbers of LO phonon modes for a large number of alloy disorder configurations. Our results predict a nonexponential decay of the upper level population once electrons are injected in this state.Comment: 10 pages, 23 figure

Phonon modes in InAs quantum dots

Phonon modes in spherical InAs quantum dots (QDs) with up to 11 855 atoms (about 8.5 nm in diameter) are calculated by using a valence force field model, and all the vibration frequencies and vibration amplitudes of the QDs are calculated directly from the lattice-dynamic matrix. The projection operators of the irreducible representations of the group theory are employed to reduce the computational intensity, which further allows us to investigate the quantum confinement effect of phonon modes with different symmetries. It is found that the size effects of phonon modes depend on the symmetry of the modes. For zinc-blende structure, the modes with A(1) symmetry has the strongest quantum confinement effect and the T-1 mode the weakest. There could be a crossover of symmetries of the highest frequencies from A(1) to T-2 as the size of the QDs decreases. The behavior of vibration amplitudes and vibration energies of phonon modes in different symmetries are also investigated in detail. These results provide microscopic details of the phonon properties of QDs that are important to the fundamental understanding and potential applications of semiconductor QDs

Dynamic Fano Resonance of Quasienergy Excitons in Superlattices

The dynamic Fano resonance (DFR) between discrete quasienergy excitons and sidebands of their ionization continua is predicted and investigated in dc- and ac-driven semiconductor superlattices. This DFR, well controlled by the ac field, delocalizes the excitons and opens an intrinsic decay channel in nonlinear four-wave mixing signals.Comment: 4pages, 4figure

Tunable Fano effect in parallel-coupled double quantum dot system

With the help of the Green function technique and the equation of motion approach, the electronic transport through a parallel-coupled double quantum dot(DQD) is theoretically studied. Owing to the inter-dot coupling, the bonding and antibonding states of the artificial quantum-dot-molecule may constitute an appropriate basis set. Based on this picture, the Fano interference in the conductance spectra of the DQD system is readily explained. The possibility of manipulating the Fano lineshape in the tunnelling spectra of the DQD system is explored by tuning the dot-lead coupling, the inter-dot coupling, the magnetic flux threading the ring connecting dots and leads, and the flux difference between two sub-rings. It has been found that by making use of various tuning, the direction of the asymmetric tail of Fano lineshape may be flipped by external fields, and the continuous conductance spectra may be magnetically manipulated with lineshape retained. More importantly, by adjusting the magnetic flux, the function of two molecular states can be exchanged, giving rise to a swap effect, which might play a role as a qubit in the quantum computation.Comment: 9 pages, 10 figure

Direct measurement of a pure spin current by a polarized light beam

The photon helicity may be mapped to a spin-1/2, whereby we put forward an intrinsic interaction between a polarized light beam as a ``photon spin current'' and a pure spin current in a semiconductor, which arises from the spin-orbit coupling in valence bands as a pure relativity effect without involving the Rashba or the Dresselhaus effect due to inversion asymmetries. The interaction leads to circular optical birefringence, which is similar to the Faraday rotation in magneto-optics but nevertheless involve no net magnetization. The birefringence effect provide a direct, non-demolition measurement of pure spin currents.Comment: Erratum version to [Physical Review Letter 100, 086603 (2008)

Tunneling conductance of graphene ferromagnet-insulator-superconductor junctions

We study the transport properties of a graphene ferromagnet-insulator superconductor (FIS) junction within the Blonder-Tinkham-Klapwijk formalism by solving spin-polarized Dirac-Bogoliubov-de-Gennes equation. We find that the retro and specular Andreev reflections in the graphene FIS junction are drastically modified in the presence of exchange interaction and that the spin-polarization ($P_T$) of tunneling current can be tuned from the positive to negative value by bias voltage ($V$). In the thin-barrier limit, the conductance $G$ of a graphene FIS junction oscillates as a function of barrier strength $\chi$. Both the amplitude and phase of the conductance oscillation varies with the exchange energy $E_{ex}$. For $E_{ex}<E_F$ (Fermi energy), the amplitude of oscillation decreases with $E_{ex}$. For $E_{ex}^{c}>E_{ex}>E_F$, the amplitude of oscillation increases with $E_{ex}$, where $E_{ex}^{c}=2E_{F}+U_{0}$ ($U_{0}$ is the applied electrostatic potential on the superconducting segment of the junction). For $E_{ex} > E_{ex}^{c}$, the amplitude of oscillation decreases with $E_{ex}$ again. Interestingly, a universal phase difference of $\pi/2$ in $\chi$ exists between the $G-\chi$ curves for $E_{ex}>E_F$ and $E_{ex}<E_F$. Finally, we find that the transitions between retro and specular Andreev reflections occur at $eV=|E_{F}-E_{ex}|$ and $eV=E_{ex}+E_{F}$, and hence the singular behavior of the conductance near these bias voltages results from the difference in transport properties between specular and retro Andreev reflections.Comment: Accepted for publication in Physical Review