We present a study of non-equilibrium phenomena observed in the electrical conductance of insulating granular aluminium thin films. An anomalous field effect and its slow relaxation are studied in some detail. The phenomenology is very similar to the one already observed in indium oxide. The origin of the phenomena is discussed. In granular systems, the present experiments can naturally be interpreted along two different lines. One relies on a slow polarisation in the dielectric surrounding the metallic islands. The other one relies on a purely electronic mechanism: the formation of an electron Coulomb glass in the granular metal. More selective experiments and/or quantitative predictions about the Coulomb glass properties are still needed to definitely distinguish between the two scenarii.

No abstract

Heavily boron doped diamond epilayers with thicknesses ranging from 40 to less than 2 nm and buried between nominally undoped thicker layers have been grown in two different reactors. Two types of [100]-oriented single crystal diamond substrates were used after being characterized by X-ray white beam topography. The chemical composition and thickness of these so-called delta-doped structures have been studied by secondary ion mass spectrometry, transmission electron microscopy, and spectroscopic ellipsometry. Temperature-dependent Hall effect and four probe resistivity measurements have been performed on mesa-patterned Hall bars. The temperature dependence of the hole sheet carrier density and mobility has been investigated over a broad temperature range (6 K < T < 450 K). Depending on the sample, metallic or non-metallic behavior was observed. A hopping conduction mechanism with an anomalous hopping exponent was detected in the non-metallic samples. All metallic delta-doped layers exhibited the same mobility value, around 3.6 ± 0.8 cm2/Vs, independently of the layer thickness and the substrate type. Comparison with previously published data and theoretical calculations showed that scattering by ionized impurities explained only partially this low common value. None of the delta-layers showed any sign of confinement-induced mobility enhancement, even for thicknesses lower than 2 nm

No abstract

We utilize the band structure of a mesoscopic Josephson junction to construct low noise ampli- The noise temperature is estimated to be around 1 Kelvin, but it is realistic to achieve T N < 0.1 Kelvin. These devices provide quantum-electronic building blocks that will be useful in low-noise, The quantum behavior of a superconducting junction is described by the Schrödingerwhere ϕ is the phase difference of the order parameter across the junction,denotes the Coulomb energy set by the junction capacitance C, and the Josephson coupling energy E J =h 2e I C is given by the critical current I C of the junction. From this Mathieu equation, energy bands (see Fig. 1) should be formed in a similar fashion as for electrons in a periodic potential (9). In the limit of small Josephson coupling, E J /E C << 1, the width of the lowest band is nearly equal to E C , while the gap between the first and second band is given by E J . In the strongly superconducting case, E J /E C 1, the band width becomes exponentially small with exp(− 8E J /E C ) and the gap grows as Zener tunneling occurs when the junction leaves its ground state at the Brillouin zone boundary by tunneling through the forbidden energy gap without a change of charge. As the bias current increases, the probability to cross the band gap by Zener tunneling grows according to the formula (whereI C , the probability of Zener tunneling becomes substantial.Relaxation downwards from higher bands can be induced either by intrinsic mechanism at rate Γ ↓ or by external quasiparticle injection Γ ext . If Γ ↓ << Γ ext , then external, active control of the junction dynamics can be achieved. Γ ↓ is a strong function of the environmental impedance R C , and it can be made small when R C R Q and when T is small (11).In order to go beyond the regime of Coulomb blockade of Cooper pairs (12) and to have a supercurrent I flowing in the JJ, the biasing voltage has to satisfy V bias >the maximum slope of the lowest band (see point A in Fig. 1). Then the junction will propagate along the lowest level and perform Bloch oscillations in a periodic fashion at f B .When the current increases, the probability to cross the band gap by Zener tunneling grows.By selecting the ratio of E J /E C and the current I properly, one can tune the probability ratio P Zener /P Bloch so that the state of the junction will tunnel into the second band after a few Bloch oscillations, N on average.| max , the maximum slope of the second band, then the state of the Josephson junction will become stationary on the higher band after Zener tunneling.If there is no intrinsic relaxation (Γ ↓ → 0), the junction will not relax and it will remain stationary, i.e., Coulomb blockaded on the higher band (sitting, e.g., at point B in Fig. 1). 3Consequently, there will be no supercurrent in the JJ before relaxation takes place due to externally-induced quasiparticle tunneling which is indicated by the slanted arrow in Fig (Fig. 2).To estimate the current gain β of the BOT, the relaxation rate due to the base current ...

We have investigated the influence of shot noise on the IV-curves of a single mesoscopic Josephson junction. We observe a linear enhancement of zero-bias conductance of the Josephson junction with increasing shot noise power. Moreover, the IV-curves become increasingly asymmetric. Our analysis on the asymmetry shows that the Coulomb blockade of Cooper pairs is strongly influenced by the non-Gaussian character of the shot noise.

No abstract

We perform electrical field effect measurements at 4 K on insulating granular aluminium thin films. When the samples size is reduced below 100µm, reproducible and stable conductance fluctuations are seen as a function of the gate voltage. Our results suggest that these fluctuations reflect the incomplete self-averaging of largely distributed microscopic resistances. We also study the anomalous field effect (conductance dip) already known to exist in large samples and its slow conductance relaxation in the presence of the conductance fluctuations. Within our measurements accuracy, the two phenomena appear to be independent of each other, like two additive contributions to the conductance. We discuss the possible physical meaning of this independence and in particular whether or not this observation is in favor of an electron glass interpretation of slow conductance anomaly relaxations.