Nonlinear conductance fluctuations in a mesoscopic ring

We study the conductance of a single particle on a ring subject to an arbitrary dc electric field, which is generated by a linearly in time increasing magnetic flux. The full quantum mechanical time development is calculated numerically by splitting the dynamics into independent consecutive Zener tunneling transitions and free motion on the ring. The Zener transitions occur near the avoided crossings of the bandstructure which arises from the adiabatic eigenstates as a function of flux in the presence of a static scattering potential. To account for the necessary dissipation the particle is coupled to an appropriate oscillator bath which is adjusted to give a strictly linear current-voltage characteristic for arbitrary voltage and temperature in the absence of scattering. Taking a single δ-function scatterer we find that the dissipative coupling eliminates the localization in energy space found previously and leads to a well defined resistive steady state. The scattering introduces reproducible fluctuations around the average Ohmic behavior which are caused by coherent backscattering. Their magnitude depends on the strength of the scattering potential and decays slowly for large voltages. The associated correlation energy is determined by the uncertainty of the eigenstates due to the dissipative bath coupling. Thermal averaging leads to a decrease of the conductance fluctuations proportional to T^?1.     

Josephson-junction networks and roughening problems

We discuss the phase diagram of regular networks of quantum mechanical Josephson junctions in one and two dimensions for different choices of the Coulomb interaction between pairs. In a particular case this is equivalent to a quantum interface with lateral tunneling along the boundary. Using a functional integral approach the partition function is transformed into that of classical roughening or Coulomb gas problems. It is shown, in particular, that the structure of the phase diagram depends crucially on the form of the Coulomb interaction and that with dissipative interactions both globally and locally superconducting phases are possible. The relation of our results to recent experiments on granular superconducting films and ideal Josephson junction arrays is discussed briefly.Dedicated to Professor W. Brenig on the occasion of his 60th birthday     

Atomic quantum dots coupled to a reservoir of a superfluid Bose-Einstein condensate

We study the dynamics of an atomic quantum dot, i.e., a single atom in a tight optical trap which is coupled to a superfluid reservoir via laser transitions. Quantum interference between the collisional interactions and the laser induced coupling results in a tunable dot-bath coupling, allowing an essentially complete decoupling from the environment. Quantum dots embedded in a 1D Luttinger liquid of cold bosonic atoms realize a spin-boson model with Ohmic coupling, which exhibits a dissipative phase transition and allows us to directly measure atomic Luttinger parameters.     

Crossover from Coulomb-blockade to ohmic conduction in small tunnel junctions

We discuss the suppression of Coulomb effects in the low temperature conductanceg(T) of small tunnel junctions with increasing dissipation or bare conductanceg. The conductance is expressed in terms of the spin correlation fuction of a classical one dimensionalXY-model with ferromagnetic nearest neighbor plus inverse square interaction. It is shown that at low temperatures the conductance vanishes asymptotically likeT ² instead of exponentially. A Coulomb gap in the sense of a thermally activated contribution tog(T) is present only for bare conductances smaller thang _c 1. A simple model for the spin correlation functions is compared with experiments.     

The density dependence of the diffusion constant in interacting lattice gases: Application to surface diffusion of 0 onW(110)

The influence of interactions between adsorbed particles on their diffusion constant is investigated by kinetic Ising models with independent nearest neighbour hops. This leads to expressions for the ratio of the diffusion constant at arbitrary coverage to its value at =0 as a function of the interaction energies relative to temperature. It is shown that under certain conditions this quantity obeys a particle-hole symmetry. Exact results in the whole range of densities are given in one dimension for nearest neighbour interaction. They already yield a qualitative agreement with experimental results and are also compared to corresponding numerical simulations. The introduction of a next nearest neighbour interaction is shown to produce drastic changes in the density dependence of the diffusion constant in some of the cases. A generalized quasichemical approximation and a virial expansion are made in two dimensions, leading to a better agreement with the measurements.     

Über die Einwirkung von Brom auf die isomeren Cinchoninbasen

Ohne Zusammenfassung     

Dissipative tunneling at finite temperature

A simple theory is presented for the influence of a weakly coupled interaction system on the tunneling of a particle out of a metastable well. It is based on the standard model of momentum and energy transfer to an infinite set of oscillators and is applied to the case of phase tunneling in a Josephson contact. The distribution of the energy transfer and in particular the Debye-Waller factor for elastic processes is determined by the imaginary part of the dielectric function. For small damping γ the main influence of dissipation on the total tunneling probability is contained in a factor exp —AMγ(Δq)². The numerical coefficientA and the distance Δq under the barrier depend on the considered tunneling state andA(T) vanishes at a temperatureT ^* above which classical activation prevails. The tunneling probability of any level is therefore predicted to increase with temperature. In additional general expressions are derived for the correlation functions of a damped quantum oscillator in terms of the classical response of the interaction system.     

Die Einwirkung von Chloralammoniak auf Dinatriummalonester

Ohne Zusammenfassung