Anisotropy of the Energy Gap in the Insulating Phase of the U-t-t' Hubbard Model

We apply a diagrammatic expansion method around the atomic limit (U >> t) for the U-t-t' Hubbard model at half filling and finite temperature by means of a continued fraction representation of the one-particle Green's function. From the analysis of the spectral function A(\vec{k},\omega) we find an energy dispersion relation with a (cos k_x-cos k_y)^2 modulation of the energy gap in the insulating phase. This anisotropy is compared with experimental ARPES results on insulating cuprates.Comment: 4 pages Revtex, 6 embedded eps figures; Figures 5 and 6 were in error and have been replaced including the discussion of the figure

Disorder-induced enhancement of the persistent current for strongly interacting electrons in one-dimensional rings

We show that disorder increases the persistent current of a half-filled one-dimensional Hubbard-Anderson ring at strong interaction. This unexpected effect results from a perturbative expansion starting from the strongly interacting Mott insulator ground state. The analytical result is confirmed and extended by numerical calculations.Comment: 7 pages, 2 figures, LaTeX, using epl.cls (included), considerably revised final versio

Factors controlling tropospheric O3, OH, NOx, and SO2 over the tropical Pacific during PEM-Tropics B

Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)-Tropics B experiment (March-April 1999) are analyzed. Concentrations of CO and long-lived nonmethane hydrocarbons in the region are significantly enhanced due to transport of pollutants from northern industrial continents. This pollutant import also enhances moderately O3 concentrations but not NOx concentrations. It therefore tends to depress OH concentrations over the tropical Pacific. These effects contrast to the large enhancements of O3 and NOx concentrations and the moderate increase of OH concentrations due to biomass burning outflow during the PEM-Tropics A experiment (September-October 1996). Observed CH3I concentrations, as in PEM-Tropics A, indicate that convective mass outflux in the middle and upper troposphere is largely independent of altitude over the tropical Pacific. Constraining a one-dimensiohal model with CH3I observations yields a 10-day timescale for convective turnover of the free troposphere, a factor of 2 faster than during PEM-Tropics A. Model simulated HO2, CH2O, H2O2, and CH3OOH concentrations are generally in agreement with observations. However, simulated OH concentrations are lower (∼25%) than observations above 6 km. Whereas models tend to overestimate previous field measurements, simulated HNO3 concentrations during PEM-Tropics B are too low (a factor of 2-4 below 6 km) compared to observations. Budget analyses indicate that chemical production of O3 accounts for only 50% of chemical loss; significant transport of O3 into the region appears to take place within the tropics. Convective transport of CH3OOH enhances the production of HOx and O3 in the upper troposphere, but this effect is offset by HOx loss due to the scavenging of H2O2. Convective transport and scavenging of reactive nitrogen species imply a necessary source of 0.4-1 Tg yr-1 of NOx in the free troposphere (above 4 km) over the tropics. A large fraction of the source could be from marine lightning. Oxidation of DMS transported by convection from the boundary layer could explain the observed free tropospheric SO2 concentrations over the tropical Pacific. This source of DMS due to convection, however, would imply in the model free tropospheric concentrations much higher than observed. The model overestimate cannot be reconciled using recent kinetics measurements of the DMS-OH adduct reaction at low pressures and temperatures and may reflect enhanced OH oxidation of DMS during convection. Copyright 2001 by the American Geophysical Union

Dephasing in sequential tunneling through a double-dot interferometer

We analyze dephasing in a model system where electrons tunnel sequentially through a symmetric interference setup consisting of two single-level quantum dots. Depending on the phase difference between the two tunneling paths, this may result in perfect destructive interference. However, if the dots are coupled to a bath, it may act as a which-way detector, leading to partial suppression of the phase-coherence and the reappearance of a finite tunneling current. In our approach, the tunneling is treated in leading order whereas coupling to the bath is kept to all orders (using P(E) theory). We discuss the influence of different bath spectra on the visibility of the interference pattern, including the distinction between "mere renormalization effects" and "true dephasing".Comment: 18 pages, 8 figures; For a tutorial introduction to dephasing see http://iff.physik.unibas.ch/~florian/dephasing/dephasing.htm

Dephasing Effect in Photon-Assisted Resonant Tunneling through Quantum Dots

We analyze dephasing in single and double quantum dot systems. The decoherence is introduced by the B\"{u}ttiker model with current conserving fictitious voltage leads connected to the dots. By using the non-equilibrium Green function method, we investigate the dephasing effect on the tunneling current. It is shown that a finite dephasing rate leads to observable effects. The result can be used to measure dephasing rates in quantum dots.Comment: 4 pages, 3 figures, to be published in Rapid Communications of Phys. Rev.

Current Switch by Coherent Trapping of Electrons in Quantum Dots

We propose a new transport mechanism through tunnel-coupled quantum dots based on the coherent population trapping effect. Coupling to an excited level by the coherent radiation of two microwaves can lead to an extremely narrow current antiresonance. The effect can be used to determine interdot dephasing rates and is a mechanism for a very sensitive, optically controlled current switch.Comment: to appear in Phys. Rev. Let

Photon-Assisted Transport Through Ultrasmall Quantum Dots: Influence of Intradot Transitions

We study transport through one or two ultrasmall quantum dots with discrete energy levels to which a time-dependent field is applied (e.g., microwaves). The AC field causes photon-assisted tunneling and also transitions between discrete energy levels of the dot. We treat the problem by introducing a generalization of the rotating-wave approximation to arbitrarily many levels. We calculate the dc-current through one dot and find satisfactory agreement with recent experiments by Oosterkamp et al. . In addition, we propose a novel electron pump consisting of two serially coupled single-level quantum dots with a time-dependent interdot barrier.Comment: 16 pages, Revtex, 10 eps-figure

Quantum fluctuations in the mazer

Quantum fluctuations in the mazer are considered, arising either from the atomic motion or from the quantized intracavity field. Analytical results, for both the meza and the hyperbolic secant mode profile, predict for example an attenuation of tunneling resonances due to such fluctuations. The case of a Gaussian mode profile is studied numerically using a wave packet propagation approach. The method automatically takes into account fluctuations in the atomic motion and the dynamics is especially considered at or adjacent to a tunnel resonance. We find that the system evolution is greatly sensitive to the atom-field detuning, bringing about a discussion about the concept of adiabaticity in this model. Further, a novel collapse-revival phenomena is demonstrated, originating from the quantum fluctuations in the atomic motion rather from field fluctuations as is normally the case.Comment: 15 pages, 8 figures. Replaced with final versio