804 research outputs found
From semiconductors to superconductors: a simple model for pseudogaps
We consider a two dimensional semiconductor with a local attraction among the
carriers. We study the ground state of this system as a function of the
semiconductor gap. We find a direct transition from a superconducting to an
insulating phase for no doping at a critical value, the single particle
excitations being always gapped. For finite doping we find a smooth crossover.
We calculate the critical temperature due to both the particle excitations and
the Berezinkii-Kosterlitz-Thouless transition.Comment: 14 pages. Accepted for publication on Eur. Phys. Jour.
Steps and facets at the surface of soft crystals
We consider the shape of crystals which are soft in the sense that their
elastic modulus is small compared to their surface tension , more
precisely where is the lattice spacing. We show that
their surface steps penetrate inside the crystal as edge dislocations. As a
consequence, these steps are broad with a small energy which we calculate. We
also calculate the elastic interaction between steps a distance apart,
which is a repulsion. We finally calculate the roughening temperatures
of successive facets in order to compare with the remarkable shapes of
lyotropic crystals recently observed by P. Pieranski et al. Good agreement is
found.Comment: 8 Pages, 1 Figure. To appear on Eur. Phys. Journal.
Charge noise at Cooper-pair resonances
We analyze the charge dynamics of a superconducting single-electron
transistor (SSET) in the regime where charge transport occurs via Cooper-pair
resonances. Using an approximate description of the system Hamiltonian, in
terms of a series of resonant doublets, we derive a Born-Markov master equation
describing the dynamics of the SSET. The average current displays sharp peaks
at the Cooper-pair resonances and we find that the charge noise spectrum has a
characteristic structure which consists of a series of asymmetric triplets of
peaks. The strongest feature in the charge noise spectrum is the triplet of
peaks centered at zero frequency which has a peak spacing equal to the level
separation within the doublets and is similar to the triplet in the spectrum of
a driven, damped, two-level system. We also explore the back-action that the
SSET charge noise would have on an oscillator coupled to the island charge,
measurement of which provides a way of probing the charge noise spectrum.Comment: 14 pages, 7 figure
The charge shuttle as a nanomechanical ratchet
We consider the charge shuttle proposed by Gorelik {\em et al.} driven by a
time-dependent voltage bias. In the case of asymmetric setup, the system
behaves as a rachet. For pure AC drive, the rectified current shows a complex
frequency dependent response characterized by frequency locking at fracional
values of the external frequency. Due to the non-linear dynamics of the
shuttle, the rachet effect is present also for very low frequencies.Comment: 4 pages, 4 figure
Large current noise in nanoelectromechanical systems close to continuous mechanical instabilities
We investigate the current noise of nanoelectromechanical systems close to a
continuous mechanical instability. In the vicinity of the latter, the
vibrational frequency of the nanomechanical system vanishes, rendering the
system very sensitive to charge fluctuations and, hence, resulting in very
large (super-Poissonian) current noise. Specifically, we consider a suspended
single-electron transistor close to the Euler buckling instability. We show
that such a system exhibits an exponential enhancement of the current noise
when approaching the Euler instability which we explain in terms of telegraph
noise.Comment: 11 pages, 12 figures; v2: minor changes, published versio
Popov approximation for composite bosons in the BCS-BEC crossover
Theoretical treatments of the BCS-BEC crossover need to provide as accurate
as possible descriptions of the two regimes where the diluteness condition
applies, either in terms of the constituent fermions (BCS limit) or of the
composite bosons which form as bound-fermion pairs (BEC limit). This has to
occur via a single fermionic theory that bridges across these two limiting
representations. In this paper, we set up successive improvements of the
fermionic theory, that result into composite bosons described at the level of
either the Bogoliubov or the Popov approximations for point-like bosons. This
work bears on the recent experimental advances on the BCS-BEC crossover with
trapped Fermi atoms, which show the need for accurate theoretical descriptions
of BEC side of the crossover.Comment: 13 pages, 4 figure
Discontinuous Euler instability in nanoelectromechanical systems
We investigate nanoelectromechanical systems near mechanical instabilities.
We show that quite generally, the interaction between the electronic and the
vibronic degrees of freedom can be accounted for essentially exactly when the
instability is continuous. We apply our general framework to the Euler buckling
instability and find that the interaction between electronic and vibronic
degrees of freedom qualitatively affects the mechanical instability, turning it
into a discontinuous one in close analogy with tricritical points in the Landau
theory of phase transitions.Comment: 4+ pages, 3 figures, published versio
Superconductivity with hard-core repulsion: BCS-Bose crossover and s-/d-wave competition
We consider fermions on a 2D lattice interacting repulsively on the same site
and attractively on the nearest neighbor sites. The model is relevant, for
instance, to study the competition between antiferromagnetism and
superconductivity in a Kondo lattice. We first solve the two-body problem to
show that in the dilute and strong coupling limit the s-wave Bose condensed
state is always the ground state. We then consider the many-body problem and
treat it at mean-field level by solving exactly the usual gap equation. This
guarantees that the superconducting wave-function correctly vanishes when the
two fermions (with antiparallel spin) sit on the same site. This fact has
important consequences on the superconducting state that are somewhat unusual.
In particular this implies a radial node-line for the gap function. When a next
neighbor hopping t' is present we find that the s-wave state may develop nodes
on the Fermi surface.Comment: 10 pages, 9 fig
Physical and Aerodynamic Characterization of Particle Clusters at Sakurajima Volcano (Japan)
The process of particle aggregation significantly affects ash settling dynamics associated with volcanic explosive eruptions. Several experiments have been carried out to investigate the physics of ash aggregation and dedicated numerical schemes have been developed to produce more accurate forecasting of ash dispersal and sedimentation. However, numerical description of particle aggregation is complicated by the lack of complete datasets on natural samples required for model validation and calibration. Here we present a first comprehensive dataset for the internal structure, aerodynamical properties (e.g., size, density, terminal velocity) and grain size of constituting particles of a variety of aggregate types collected in the natural laboratory of Sakurajima Volcano (Japan). Even though the described particle clusters represent the most common types of aggregates associated with ash-rich fallouts, they are of difficult characterization due to the very low potential of preservation in tephra-fallout deposits. Properties were, therefore, derived based on a combination of high-resolution-high-speed videos of tephra fallout, scanning electron microscope analysis of aggregates collected on adhesive paper and analysis of tephra samples collected in dedicated trays. Three main types of particle clusters were recognized and quantitively characterized: cored clusters (PC3), coated particles (PC2), and ash clusters (PC1) (in order of abundance). A wide range of terminal velocities (0.5–4 m/s) has been observed for these aggregates, with most values varying between 1 and 2 m/s, while aggregate size varies between 200 and 1,200 µm. PC1, PC2, and PC3 have densities between 250 and 500, 1,500 and 2,000, and 500 and 1,500 kg/m3, respectively. The size of the aggregate core, where present, varies between 200 and 750 µm and increases with aggregate size. Grain size of tephra samples was deconvoluted into a fine and a coarse Gaussian subpopulation, well correlated with the grain size of shells and of the internal cores of aggregates, respectively. This aspect, together with the revealed abundance of PC3 aggregates, reconciles the presence of a large amount of fine ash (aggregate shells) with coarse ash (aggregate cores) and better explains the grain size distribution bimodality, the high settling velocity with respect to typical PC1 velocities and the low settling velocities of large aggregates with respect to typical PC2 velocity. Furthermore, ash forming the aggregates was shown to be always finer than 45 µm, confirming the key role played by aggregation processes in fine ash deposition at Sakurajima
i rexfo life an innovative business model to reduce food waste
Abstract Every year the food produced and wasted consumes a volume of water equal to 250 km3, requires around 30% of the world agricultural land, and it is responsible for the emission of 3,3 billion tons of greenhouse gases. The direct economic consequences of food waste are ranging around 750 billion dollars per year (FAO source). i-REXFO designs an innovative business model with the objective of reducing significantly the amount of landfilled food waste. The actions are economically sustained by public incentives, tax reductions and private revenues from energy valorization of residual food waste. Uptaking the good practices from other EU countries (Denmark) the project will develop a tool to design the integrated model, optimize it from a technical, economic and environmental point of view and transfer it to other EU regions. i-REXFO will increase consumer awareness on food waste reduction in retail malls and HORECA while facilitating the sale and donation to charities and food banks of close to expiration and aesthetically not adequate food; it will also remove the barriers that hamper the use of food residues in biogas plants. The actions are economically sustained from energy valorization of food waste in biogas plant that use the digestate as fertilizer, closing the cycle. I-REXFO will achieve an overall reduction of 17000 tons/year of food waste landfilled during the project duration and in the after life phase. This will correspond to an overall reduction of 41000 tons of CO2 equivalent emissions
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