R. Schäfer

One-dimensional Josephson junction arrays: Lifting the Coulomb blockade by depinning

Physikalisches Institut,Karlsruhe Institute of Technology, D-76128 Karlsruhe, Germany(Dated: July 16, 2014)Experiments with one-dimensional Josephson junction arrays in the regime of dominating chargingenergy show that the Coulomb blockade is lifted at the threshold voltage, which is proportional tothe array’s length and depends strongly on the Josephson energy. We explain this behavior bydepinning of the charges in the array. We assume strong charge disorder and argue that physicsaround the depinning point is governed by a disordered sine-Gordon-like model. This allows us toemploy the well-known theory of charge density wave depinning. Our model is in good agreementwith the experimental data.

Josephson vortex coupled to a flux qubit

Experiments towards realizing a readout of superconducting qubits by using ballistic Josephson vortices are reported. We measured the microwave radiation induced by a fluxon moving in an annular Josephson junction. By coupling a flux qubit as a current dipole to the annular junction, we detect periodic variations of the fluxons oscillation frequency versus magnetic flux through the qubit. We found that the scattering of a fluxon on a current dipole can lead to the acceleration of a fluxon regardless of a dipole polarity. We use the perturbation theory and numerical simulations of the perturbed sine-Gordon equation to analyze our results.

Conductance of the single-electron transistor: A comparison of experimental data with Monte Carlo calculations

We report on experimental results for the conductance of metallic single-electron transistors as a function of temperature, gate voltage, and dimensionless conductance. In contrast to previous experiments our transistor layout allows for a direct measurement of the parallel conductance and no ad hoc assumptions on the symmetry of the transistors are necessary. Thus we can make a comparison between our data and theoretical predictions without any adjustable parameter. Even for rather weakly conducting transistors significant deviations from the perturbative results are noted. On the other hand, path-integral Monte Carlo calculations show remarkable agreement with experiments for the whole range of temperatures and conductances.

Thermally activated conductance in arrays of small Josephson junctions

We present measurements of the temperature-dependent conductance for series arrays of small-capacitance SQUIDs. At low bias voltages, the arrays exhibit a strong Coulomb blockade, which we study in detail as a function of temperature and Josephson energy

Coulomb blockade in two-island systems with highly conductive junctions

E_J

Single-Electron Transistors in the Regime of High Conductance

. We find that the zero-bias conductance is well described by thermally activated charge transport with the activation energy on the order of

SINGLE-ELECTRON PUMP: DEVICE CHARACTERIZATION AND LINEAR-RESPONSE MEASUREMENTS

\Lambda E_C

Linear-response conductance of the normal conducting single-electron pump

, where

Influence of the Measurement Process on the Step Width in the Coulomb Staircase

\Lambda

Aluminum hard mask technique for the fabrication of high quality submicron Nb/Al–AlOx/Nb Josephson junctions

is the charge screening length in the array and