Tunnelling current and emission spectrum of a single electron transistor under optical pumping

Theoretical studies of the tunnelling current and emission spectrum of a single electron transistor (SET) under optical pumping are presented. The calculation is performed via Keldysh Green's function method within the Anderson model with two energy levels. It is found that holes in the quantum dot (QD) created by optical pumping lead to new channels for the electron tunnelling from emitter to collector. As a consequence, an electron can tunnel through the QD via additional channels, characterized by the exciton, trion and biexciton states. It is found that the tunnelling current as a function of the gate voltage displays a series of sharp peaks and the spacing between these peaks can be used to determine the exciton binding energy as well as the electron-electron Coulomb repulsion energy. In addition, we show that the single-photon emission associated with the electron-hole recombination in the exciton complexes formed in the QD can be controlled both electrically and optically.Comment: 24 pages, 10 figure

Efficient grid-based method in nonequilibrium Green's function calculations. Application to model atoms and molecules

We propose and apply the finite-element discrete variable representation to express the nonequilibrium Green's function for strongly inhomogeneous quantum systems. This method is highly favorable against a general basis approach with regard to numerical complexity, memory resources, and computation time. Its flexibility also allows for an accurate representation of spatially extended hamiltonians, and thus opens the way towards a direct solution of the two-time Schwinger/Keldysh/Kadanoff-Baym equations on spatial grids, including e.g. the description of highly excited states in atoms. As first benchmarks, we compute and characterize, in Hartree-Fock and second Born approximation, the ground states of the He atom, the H$_2$ molecule and the LiH molecule in one spatial dimension. Thereby, the ground-state/binding energies, densities and bond-lengths are compared with the direct solution of the time-dependent Schr\"odinger equation.Comment: 11 pages, 5 figures, submitted to Physical Review

Nonequilibrium transport and optical properties of model metal--Mott-insulator--metal heterostructures

Electronic properties of heterostructures in which a finite number of Mott-insulator layers are sandwiched by semi-infinite metallic leads are investigated by using the dynamical-mean-field method combined with the Keldysh Green's function technique to account for the finite bias voltage between the leads. Current across the junction is computed as a function of bias voltage. Electron spectral functions in the interacting region are shown to evolve by an applied bias voltage. This effect is measurable by photoemission spectroscopy and scanning tunneling microscopy. Further predictions are made for the optical conductivity under a bias voltage as a possible tool to detect a deformed density of states. A general discussion of correlated-electron based heterostructures and future prospect is given.Comment: 11 pages, 11 figures, published versio

The effects of non-abelian statistics on two-terminal shot noise in a quantum Hall liquid in the Pfaffian state

We study non-equilibrium noise in the tunnelling current between the edges of a quantum Hall liquid in the Pfaffian state, which is a strong candidate for the plateau at $\nu=5/2$. To first non-vanishing order in perturbation theory (in the tunneling amplitude) we find that one can extract the value of the fractional charge of the tunnelling quasiparticles. We note however that no direct information about non-abelian statistics can be retrieved at this level. If we go to higher-order in the perturbative calculation of the non-equilibrium shot noise, we find effects due to non-Abelian statistics. They are subtle, but eventually may have an experimental signature on the frequency dependent shot noise. We suggest how multi-terminal noise measurements might yield a more dramatic signature of non-Abelian statistics and develop some of the relevant formalism.Comment: 13 pages, 8 figures, a few change

Competing order and nature of the pairing state in the iron pnictides

We show that the competition between magnetism and superconductivity can be used to determine the pairing state in the iron arsenides. To this end we demonstrate that the itinerant antiferromagnetic phase (AFM) and the unconventional $s^{+-}$ sign-changing superconducting state (SC) are near the borderline of microscopic coexistence and macroscopic phase separation, explaining the experimentally observed competition of both ordered states. In contrast, conventional $s^{++}$ pairing is not able to coexist with magnetism. Expanding the microscopic free energy of the system with competing orders around the multicritical point, we find that static magnetism plays the role of an intrinsic interband Josephson coupling, making the phase diagram sensitive to the symmetry of the Cooper pair wavefunction. We relate this result to the quasiparticle excitation spectrum and to the emergent SO$(5)$ symmetry of systems with particle-hole symmetry. Our results rely on the assumption that the same electrons that form the ordered moment contribute to the superconducting condensate and that the system is close to particle-hole symmetry. We also compare the suppression of SC in different regions of the FeAs phase diagram, showing that while in the underdoped side it is due to the competition with AFM, in the overdoped side it is related to the disappearance of pockets from the Fermi surface.Comment: 24 pages, 13 figures; revised versio

Low-Energy Structures in Strong Field Ionization Revealed by Quantum Orbits

Experiments on atoms in intense laser pulses and the corresponding exact ab initio solutions of the time-dependent Schr\"odinger equation (TDSE) yield photoelectron spectra with low-energy features that are not reproduced by the otherwise successful work horse of strong field laser physics: the "strong field approximation" (SFA). In the semi-classical limit, the SFA possesses an appealing interpretation in terms of interfering quantum trajectories. It is shown that a conceptually simple extension towards the inclusion of Coulomb effects yields very good agreement with exact TDSE results. Moreover, the Coulomb quantum orbits allow for a physically intuitive interpretation and detailed analysis of all low-energy features in the semi-classical regime, in particular the recently discovered "low-energy structure" [C.I. Blaga et al., Nature Physics 5, 335 (2009) and W. Quan et al., Phys. Rev. Lett. 103, 093001 (2009)].Comment: 4 pages, 3 figures, REVTe

Emission and absorption noise in the fractional quantum Hall effect

We compute the high-frequency emission and absorption noise in a fractional quantum Hall effect (FQHE) sample at arbitrary temperature. We model the edges of the FQHE as chiral Luttinger liquids (LL) and we use the non-equilibrium perturbative Keldysh formalism. We find that the non-symmetrized high frequency noise contains important signatures of the electron-electron interactions that can be used to test the Luttinger liquid physics, not only in FQHE edge states, but possibly also in other one-dimensional systems such as carbon nanotubes. In particular we find that the emission and absorption components of the excess noise (defined as the difference between the noise at finite voltage and at zero voltage) are different in an interacting system, as opposed to the non-interacting case when they are identical. We study the resonance features which appear in the noise at the Josephson frequency (proportional to the applied voltage), and we also analyze the effect of the distance between the measurement point and the backscattering site. Most of our analysis is performed in the weak backscattering limit, but we also compute and discuss briefly the high-frequency noise in the tunneling regime.Comment: 26 pages, 11 figure

Bose--Hubbard Models Coupled to Cavity Light Fields

Recent experiments on strongly coupled cavity quantum electrodynamics present new directions in "matter-light" systems. Following on from our previous work [Phys. Rev. Lett. 102, 135301 (2009)] we investigate Bose-Hubbard models coupled to a cavity light field. We discuss the emergence of photoexcitations or "polaritons" within the Mott phase, and obtain the complete variational phase diagram. Exploiting connections to the super-radiance transition in the Dicke model we discuss the nature of polariton condensation within this novel state. Incorporating the effects of carrier superfluidity, we identify a first-order transition between the superradiant Mott phase and the single component atomic superfluid. The overall predictions of mean field theory are in excellent agreement with exact diagonalization and we provide details of superfluid fractions, density fluctuations, and finite size effects. We highlight connections to recent work on coupled cavity arrays.Comment: 16 pages, 17 figure

Bias-induced insulator-metal transition in organic electronics

We investigate the bias-induced insulator-metal transition in organic electronics devices, on the basis of the Su-Schrieffer-Heeger model combined with the non-equilibrium Green's function formalism. The insulator-metal transition is explained with the energy levels crossover that eliminates the Peierls phase and delocalizes the electron states near the threshold voltage. This may account for the experimental observations on the devices that exhibit intrinsic bistable conductance switching with large on-off ratio.Comment: 6 pages, 3 figures. To appear in Applied Physics Letter

Qualitatively Different Theoretical Predictions for Strong-Field Photoionization Rates

We give examples showing that two well-known versions of the S-matrix theory, which describes a nonresonant multiphoton ionization of atoms and ions in intense laser fields, lead to qualitatively different results. The latter refer not only to total ionization rates, but also to energy distributions of photoelectrons, for instance in a polarization plane of the laser field. It should be possible to make an experiment testing predictions of both theories in the near future.Comment: 12 pages, 5 figures; submitted to Physical Revie