Quantum Transparency of Anderson Insulator Junctions: Statistics of Transmission Eigenvalues, Shot Noise, and Proximity Conductance

We investigate quantum transport through strongly disordered barriers, made of a material with exceptionally high resistivity that behaves as an Anderson insulator or a ``bad metal'' in the bulk, by analyzing the distribution of Landauer transmission eigenvalues for a junction where such barrier is attached to two clean metallic leads. We find that scaling of the transmission eigenvalue distribution with the junction thickness (starting from the single interface limit) always predicts a non-zero probability to find high transmission channels even in relatively thick barriers. Using this distribution, we compute the zero frequency shot noise power (as well as its sample-to-sample fluctuations) and demonstrate how it provides a single number characterization of non-trivial transmission properties of different types of disordered barriers. The appearance of open conducting channels, whose transmission eigenvalue is close to one, and corresponding violent mesoscopic fluctuations of transport quantities explain at least some of the peculiar zero-bias anomalies in the Anderson-insulator/superconductor junctions observed in recent experiments [Phys. Rev. B {\bf 61}, 13037 (2000)]. Our findings are also relevant for the understanding of the role of defects that can undermine quality of thin tunnel barriers made of conventional band-insulators.Comment: 9 pages, 8 color EPS figures; one additional figure on mesoscopic fluctuations of Fano facto

Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells

The implementation of a wide range of novel concepts for next-generation high-efficiency solar cells is based on nanostructures with configuration-tunable optoelectronic properties. On the other hand, effective nano-optical light-trapping concepts enable the use of ultra-thin absorber architectures. In both cases, the local density of electronic and optical states deviates strongly from that in a homogeneous bulk material. At the same time, non-local and coherent phenomena like tunneling or ballistic transport become increasingly relevant. As a consequence, the semi-classical, diffusive bulk picture conventionally assumed may no longer be appropriate to describe the physical processes of generation, transport, and recombination governing the photovoltaic operation of such devices. In this review, we provide a quantum-kinetic perspective on photovoltaic device operation that reaches beyond the limits of the standard simulation models for bulk solar cells. Deviations from bulk physics are assessed in ultra-thin film and nanostructure-based solar cell architectures by comparing the predictions of the semi-classical models for key physical quantities such as absorption coefficients, emission spectra, generation and recombination rates as well as potentials, densities and currents with the corresponding properties as given by a more fundamental description based on non-equilibrium quantum statistical mechanics. This advanced approach, while paving the way to a comprehensive quantum theory of photovoltaics, bridges simulations at microscopic material and macroscopic device levels by providing the charge carrier dynamics at the mesoscale.Comment: 22 pages, 8 figures; review article based on an invited talk at the MRS Spring Meeting 2017 in Phoeni

Describing many-body bosonic waveguide scattering with the truncated Wigner method

We consider quasi-stationary scattering of interacting bosonic matter waves in one-dimensional waveguides, as they arise in guided atom lasers. We show how the truncated Wigner (tW) method, which corresponds to the semiclassical description of the bosonic many-body system on the level of the diagonal approximation, can be utilized in order to describe such many-body bosonic scattering processes. Special emphasis is put on the discretization of space at the exact quantum level, in order to properly implement the semiclassical approximation and the tW method, as well as on the discussion of the results to be obtained in the continuous limit.Comment: 9 pages, 3 figure

Transmission phase of a quantum dot and statistical fluctuations of partial-width amplitudes

Experimentally, the phase of the amplitude for electron transmission through a quantum dot (transmission phase) shows the same pattern between consecutive resonances. Such universal behavior, found for long sequences of resonances, is caused by correlations of the signs of the partial-width amplitudes of the resonances. We investigate the stability of these correlations in terms of a statistical model. For a classically chaotic dot, the resonance eigenfunctions are assumed to be Gaussian distributed. Under this hypothesis, statistical fluctuations are found to reduce the tendency towards universal phase evolution. Long sequences of resonances with universal behavior only persist in the semiclassical limit of very large electron numbers in the dot and for specific energy intervals. Numerical calculations qualitatively agree with the statistical model but quantitatively are closer to universality.Comment: 8 pages, 4 figure

Electrostatic potential profiles of molecular conductors

The electrostatic potential across a short ballistic molecular conductor depends sensitively on the geometry of its environment, and can affect its conduction significantly by influencing its energy levels and wave functions. We illustrate some of the issues involved by evaluating the potential profiles for a conducting gold wire and an aromatic phenyl dithiol molecule in various geometries. The potential profile is obtained by solving Poisson's equation with boundary conditions set by the contact electrochemical potentials and coupling the result self-consistently with a nonequilibrium Green's function (NEGF) formulation of transport. The overall shape of the potential profile (ramp vs. flat) depends on the feasibility of transverse screening of electric fields. Accordingly, the screening is better for a thick wire, a multiwalled nanotube or a close-packed self-assembled monolayer (SAM), in comparison to a thin wire, a single-walled nanotube or an isolated molecular conductor. The electrostatic potential further governs the alignment or misalignment of intramolecular levels, which can strongly influence the molecular I-V characteristic. An external gate voltage can modify the overall potential profile, changing the current-voltage (I-V) characteristic from a resonant conducting to a saturating one. The degree of saturation and gate modulation depends on the metal-induced-gap states (MIGS) and on the electrostatic gate control parameter set by the ratio of the gate oxide thickness to the channel length.Comment: to be published in Phys. Rev. B 69, No.3, 0353XX (2004

Suppression of weak-localization (and enhancement of noise) by tunnelling in semiclassical chaotic transport

We add simple tunnelling effects and ray-splitting into the recent trajectory-based semiclassical theory of quantum chaotic transport. We use this to derive the weak-localization correction to conductance and the shot-noise for a quantum chaotic cavity (billiard) coupled to $n$ leads via tunnel-barriers. We derive results for arbitrary tunnelling rates and arbitrary (positive) Ehrenfest time, $\tau_{\rm E}$. For all Ehrenfest times, we show that the shot-noise is enhanced by the tunnelling, while the weak-localization is suppressed. In the opaque barrier limit (small tunnelling rates with large lead widths, such that Drude conductance remains finite), the weak-localization goes to zero linearly with the tunnelling rate, while the Fano factor of the shot-noise remains finite but becomes independent of the Ehrenfest time. The crossover from RMT behaviour ($\tau_{\rm E}=0$) to classical behaviour ($\tau_{\rm E}=\infty$) goes exponentially with the ratio of the Ehrenfest time to the paired-paths survival time. The paired-paths survival time varies between the dwell time (in the transparent barrier limit) and half the dwell time (in the opaque barrier limit). Finally our method enables us to see the physical origin of the suppression of weak-localization; it is due to the fact that tunnel-barriers ``smear'' the coherent-backscattering peak over reflection and transmission modes.Comment: 20 pages (version3: fixed error in sect. VC - results unchanged) - Contents: Tunnelling in semiclassics (3pages), Weak-localization (5pages), Shot-noise (5pages

Simulation of Transport and Gain in Quantum Cascade Lasers

Quantum cascade lasers can be modeled within a hierarchy of different approaches: Standard rate equations for the electron densities in the levels, semiclassical Boltzmann equation for the microscopic distribution functions, and quantum kinetics including the coherent evolution between the states. Here we present a quantum transport approach based on nonequilibrium Green functions. This allows for quantitative simulations of the transport and optical gain of the device. The division of the current density in two terms shows that semiclassical transitions are likely to dominate the transport for the prototype device of Sirtori et al. but not for a recent THz-laser with only a few layers per period. The many particle effects are extremely dependent on the design of the heterostructure, and for the case considered here, inclusion of electron-electron interaction at the Hartree Fock level, provides a sizable change in absorption but imparts only a minor shift of the gain peak.Comment: 12 pages, 5 figures included, to appear in in "Advances in Solid State Physics", ed. by B. Kramer (Springer 2003