Abstract:
We calculate the average polarisability of two dimensional mesoscopic rings in the presence of an Aharonov-Bohm flux. The screening is taken into account self-consistently within a mean-field approximation. We investigate the effects of statistical ensemble, finite frequency and disorder. We emphasize geometrical effects which make the observation of field dependent polarisability much more favourable on rings than on disks or spheres of comparable radius. The ratio of the flux dependent to the flux independent part is estimated for typical GaAs rings.

Abstract:
The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above $T_c$, pre-existing but fluctuating Cooper pairs give rise to the unconventional {\it pseudogap} (PG) state, well-characterized by experiment. At the critical temperature, the pair-pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below $T_c$, both the condensation energy and the pair-pair interaction $\beta$ are proportional to the condensate density $N_{oc}(T)$, whereas the usual Fermi-level spectral gap $\Delta_p$ is independent of temperature. The new order parameter $\beta(T)$, can be followed as a function of temperature, carrier concentration and disorder - i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the {\it coupling of quasiparticles to Cooper-pair excitations}. The latter interpretation is strongly supported by the observed quasiparticle spectral function.

Abstract:
The influence of local order on the disordering scenario of small Wigner islands is discussed. A first disordering step is put in evidence by the time correlation functions and is linked to individual excitations resulting in configuration transitions, which are very sensitive to the local symmetries. This is followed by two other transitions, corresponding to orthoradial and radial diffusion, for which both individual and collective excitations play a significant role. Finally, we show that, contrary to large systems, the focus that is commonly made on collective excitations for such small systems through the Lindemann criterion has to be made carefully in order to clearly identify the relative contributions in the whole disordering process.

Abstract:
We have measured the electromagnetic response of micron-size isolated mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them to an electromagnetic micro-resonator. Both dissipative and non dissipative responses exhibit a large magnetic field dependent quantum correction, with a characteristic flux scale which corresponds to a flux quantum in a dot. The real (dissipative) magnetoconductance changes sign as a function of frequency for low enough density of electrons. The signal observed at frequency below the mean level spacing corresponds to a negative magnetoconductance, which is opposite to the weak localization seen in connected systems, and becomes positive at higher frequency. We propose an interpretation of this phenomenon in relation to fundamental properties of energy level spacing statistics in the dots.

Abstract:
We have studied the quasiparticle excitation spectrum of the superconductor Ba8Si46 by local tunneling spectroscopy. Using high energy resolution achieved in Superconductor-Superconductor junctions we observed tunneling conductance spectra of a non-conventional shape revealing two distinct energy gaps, DeltaL = 1.3meV and DeltaS = 0.9meV. The analysis of tunneling data evidenced that DeltaL is the principal superconducting gap while DeltaS, smaller and more dispersive, is induced into an intrinsically non-superconducting band of the material by the inter-band quasiparticle scattering.

Abstract:
We have measured conductance histograms of atomic point contacts made from the noble-transition metal alloys CuNi, AgPd, and AuPt for a concentration ratio of 1:1. For all alloys these histograms at low bias voltage (below 300 mV) resemble those of the noble metals whereas at high bias (above 300 mV) they resemble those of the transition metals. We interpret this effect as a change in the composition of the point contact with bias voltage. We discuss possible explanations in terms of electromigration and differential diffusion induced by current heating.

Abstract:
Sorption isotherms of methane and hydrogen on Cu_{3}(BTC)_{2} have been measured in the temperature range from 273 to 318 K and at pressures up to 15 MPa. H_{2} excess sorption capacities of the Cu_{3}(BTC)_{2} amounted to 3.9 mg/g at 14 MPa. Promising maximum CH_{4} excess sorption capacities on the same sample were reached at approximately 5 MPa. They amounted to 101, 100, 92 and 80 mg/g at 273, 278, 293 and 318 K, respectively. The sorbed phase density was essestially the same for all temperatures and amounted to ~600 kg/m^{3}. Structural changes of the Cu_{3}(BTC)_{2} samples after thermal activation and treatment with high pressure H_{2} and CH_{4} were tested. It was found that the initial micropore structure has virtually disappeared as evidenced by a decrease of the Langmuir specific surface area by a factor ~3 and CO_{2} micropore volume by a factor of ~4 for H_{2} and ~3 for CH_{4}. This is in line with an increase in the average pore diameter from initially 9.2 to 15.7 for H_{2} and 12.8 for CH_{4}.

Abstract:
A
Minkowskian solution of the equation of General Relativity (as written by
Einstein in 1915) is trivial because it simply means that both members of the
equation are equal to zero. However, if alternatively, one considers the
complete equation with a non-zero constant Λ (Einstein
1917), a Minkowskian solution is no longer trivial because it amounts to impose
a constraint on the right hand side of the equation (i.e. a non-null
stress-energy tensor). If furthermore one identifies (as usual) this tensor to
the one of a perfect fluid, one finds that this fluid has a positive energy
density and a negative pressure that depend on the three constants of the
equation (i.e. gravitational constant G,
cosmological constant Λ and velocity of light c). When doing that (§1), one has
to consider the “Minkowskian Vacuum” as a physical object of GR (an enigmatic
non-baryonic Minkowskian fluid). Can one build a model of this object on the
basis of a dynamical equilibrium between the effective gravitational attraction
due to the positive energy density versus the negative pressure repulsion? We
propose to study such a model, where the (enigmatic) fluid is assumed to exist
only in a limited sphere whose surface acts like a “test body” sensitive to the
gravitational field created by the fluid. No static equilibrium exists, but a
pseudoNewtonian “dynamical equilibrium” (§2) can be reached if the
pseudoEuclidean fluid is in state of expansion. Up to there, we have simply
constructed a model of an “abstract Universe” (i.e.the limited sphere: There is no fluid outside this sphere!) that
gives to a (purely mathematical) constant Λ a concrete physical meaning. We
discover finally that our expanding fluid has not only dynamical
(gravitational) properties (§3) but also optical properties that are connected
with Doppler Redshift (§4). Remembering that recent observations in Cosmology
indicate that the “real Universe” seems to be “Flat” and in “Accelerated
Expansion”; remembering also (after all) that the archetypal Flat Universe is
simply a Minkowskian Universe, we logically wonder if the unexpected
Minkowskian global solution, could not be also a significant cosmological model
(conclusion).

Abstract:
We consider the scattering of particles by an obstacle which tunnels coherently between two positions. We show that the obstacle mimics two classical scatterers at fixed positions when the kinetic energy epsilon of the incident particles is smaller than the tunnel splitting Delta: If the obstacles are arranged in parallel, one observes an interference pattern as in the conventional double-slit experiment. If they are arranged in series, the observations conform with a Fabry-Perot interferometer. At larger epsilon inelastic processes result in more complex interference phenomena. Interference disappears when epsilon >> Delta, but can be recovered if only the elastic scattering channel is detected. We discuss the realization of a quantum obstacle in mesoscopic systems.

Abstract:
In order to investigate the effect of electronic phase coherence on screening we have measured the flux dependent polarizability of isolated mesoscopic rings at 350 MHz. At low temperature (below 100 mK) both non-dissipative and dissipative parts of the polarizability exhibit flux oscillations with a period of half a flux quantum in a ring. The sign and amplitude of the effect are in good agreement with recent theoretical predictions. The observed positive magneto-polarizability corresponds to an enhancement of screening when time reversal symmetry is broken. The effect of electronic density and temperature are also measured.