Light-emitting current of electrically driven single-photon sources

The time-dependent tunnelling current arising from the electron-hole recombination of exciton state is theoretically studied using the nonequilibrium Green's function technique and the Anderson model with two energy levels. The charge conservation and gauge invariance are satisfied in the tunnelling current. Apart from the classical capacitive charging and discharging behavior, interesting oscillations superimpose on the tunnelling current for the applied rectangular pulse voltage.Comment: 14 pages, 5 figure

Modulation of the dephasing time for a magnetoplasma in a quantum well

We investigate the femtosecond kinetics of optically excited 2D magneto-plasma. We calculate the femtosecond dephasing and relaxation kinetics of the laser pulse excited magneto-plasma due to bare Coulomb potential scattering, because screening is under these conditions of minor importance. By taking into account four Landau subbands in both the conduction band and the valence band, we are now able to extend our earlier study [Phys. Rev. B {\bf 58}, 1998,in print (see also cond-mat/9808073] to lower magnetic fields. We can also fix the magnetic field and change the detuning to further investigate the carrier density-dependence of the dephasing time. For both cases, we predict strong modulation in the dephasing time.Comment: RevTex, 3 figures, to be published in Solid. Stat. Commu

Estimating the COGARCH(1,1) model - a first go

We suggest moment estimators for the parameters of a continuous time GARCH(1,1) process based on equally spaced observations. Using the fact that the increments of the COGARCH(1,1) process are ergodic, the resulting estimators are consistent. We investigate the quality of our estimators in a simulation study based on the compound Poisson driven COGARCH model. The estimated volatility with corresponding residual analysis is also presented

Non-invasive detection of molecular bonds in quantum dots

We performed charge detection on a lateral triple quantum dot with star-like geometry. The setup allows us to interpret the results in terms of two double dots with one common dot. One double dot features weak tunnel coupling and can be understood with atom-like electronic states, the other one is strongly coupled forming molecule-like states. In nonlinear measurements we identified patterns that can be analyzed in terms of the symmetry of tunneling rates. Those patterns strongly depend on the strength of interdot tunnel coupling and are completely different for atomic- or molecule-like coupled quantum dots allowing the non-invasive detection of molecular bonds.Comment: 4 pages, 4 figure

Optical response of graphene under intense terahertz fields

Optical responses of graphene in the presence of intense circularly and linearly polarized terahertz fields are investigated based on the Floquet theory. We examine the energy spectrum and density of states. It is found that gaps open in the quasi-energy spectrum due to the single-photon/multi-photon resonances. These quasi-energy gaps are pronounced at small momentum, but decrease dramatically with the increase of momentum and finally tend to be closed when the momentum is large enough. Due to the contribution from the states at large momentum, the gaps in the density of states are effectively closed, in contrast to the prediction in the previous work by Oka and Aoki [Phys. Rev. B {\bf 79}, 081406(R) (2009)]. We also investigate the optical conductivity for different field strengths and Fermi energies, and show the main features of the dynamical Franz-Keldysh effect in graphene. It is discovered that the optical conductivity exhibits a multi-step-like structure due to the sideband-modulated optical transition. It is also shown that dips appear at frequencies being the integer numbers of the applied terahertz field frequency in the case of low Fermi energy, originating from the quasi-energy gaps at small momentums. Moreover, under a circularly polarized terahertz field, we predict peaks in the middle of the "steps" and peaks induced by the contribution from the states around zero momentum in the optical conductivity.Comment: 15 pages, 10 figure

Interaction-Induced Spin Polarization in Quantum Dots

The electronic states of lateral many electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb blockade peaks are measured. A zig-zag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by including the interaction effects within spin-density-functional theory a good agreement between experiment and theory is obtained. The absence of bimodality on Landau level 1 is found to be due to strong spin polarization.Comment: 4 pages, 5 figure

Probing a Kondo correlated quantum dot with spin spectroscopy

We investigate Kondo effect and spin blockade observed on a many-electron quantum dot and study the magnetic field dependence. At lower fields a pronounced Kondo effect is found which is replaced by spin blockade at higher fields. In an intermediate regime both effects are visible. We make use of this combined effect to gain information about the internal spin configuration of our quantum dot. We find that the data cannot be explained assuming regular filling of electronic orbitals. Instead spin polarized filling seems to be probable.Comment: 4 pages, 5 figure

Channel Blockade in a Two-Path Triple-Quantum-Dot System

Electronic transport through a two-path triple-quantum-dot system with two source leads and one drain is studied. By separating the conductance of the two double dot paths, we are able to observe double dot and triple dot physics in transport and study the interaction between the paths. We observe channel blockade as a result of inter-channel Coulomb interaction. The experimental results are understood with the help of a theoretical model which calculates the parameters of the system, the stability regions of each state and the full dynamical transport in the triple dot resonances.Comment: 6 pages, 6 figure

Dynamics of photoexcited carriers in graphene

The nonequilibrium dynamics of carriers and phonons in graphene is investigated by solving the microscopic kinetic equations with the carrier-phonon and carrier-carrier Coulomb scatterings explicitly included. The Fermi distribution of hot carriers are found to be established within 100 fs and the temperatures of electrons in the conduction and valence bands are very close to each other, even when the excitation density and the equilibrium density are comparable, thanks to the strong inter-band Coulomb scattering. Moreover, the temporal evolutions of the differential transmission obtained from our calculations agree with the experiments by Wang et al. [Appl. Phys. Lett. 96, 081917 (2010)] and Hale et al. [Phys. Rev. B 83, 121404 (2011)] very well, with two distinct differential transmission relaxations presented. We show that the fast relaxation is due to the rapid carrier-phonon thermalization and the slow one is mainly because of the slow decay of hot phonons. In addition, it is found that the temperatures of the hot phonons in different branches are different and the temperature of hot carriers can be even lower than that of the hottest phonons. Finally, we show that the slow relaxation rate exhibits a mild valley in the excitation density dependence and is linearly dependent on the probe-photon energy.Comment: 9 pages, 4 figure