arXiv subjects

F. Pistolesi

Publications and source records attributed to F. Pistolesi.

36 records · Page 2Linked to original sources

arXiv2005

Energy dependence of current noise in double-barrier normal-superconducting structures

We study theoretically the current-noise energy dependence for a N-N'-S structure, where N and S stand for bulk normal metal and superconductor, respectively, and N' for a short diffusive normal metal. Using quasiclassical theory of current fluctuations we obtain explicit expressions for the noise valid for arbitrary distributions of channel transparencies on both junctions. The differential Fano factor turns out to depend on both junction transparencies and the ratio of the two conductances. We conclude that measurement of differential conductance and noise can be used to probe the channel distribution of the interfaces.

cond-mat.mes-hall↗

arXiv2004

Large energy dependence of current noise in superconducting/normal metal junctions

Interference of electronic waves undergoing Andreev reflection in diffusive conductors determines the energy profile of the conductance on the scale of the Thouless energy. A similar dependence exists in the current noise, but its behavior is known only in few limiting cases. We consider a metallic diffusive wire connected to a superconducting reservoir through an interface characterized by an arbitrary distribution of channel transparencies. Within the quasiclassical theory for current fluctuations we provide a general expression for the energy dependence of the current noise. We derive closed analytical expressions for large energy.

cond-mat.mes-hall↗

arXiv2004

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.

cond-mat.mes-hall↗

arXiv2004

Subgap noise of a superconductor-normal-metal tunnel interface

It is well established that the subgap conductivity through a normal-metal-insulator-superconductor (NIS) tunnel junction is strongly affected by interference of electron waves scattered by impurities. In this paper we investigate how the same phenomenon affects the low frequency current noise, $S$, for voltages $V$ and temperatures $T$ much smaller than the superconducting gap. If the normal metal is at equilibrium we find that the simple relation $S(V,T) = 4e {\rm coth} (eV/T) I(V,T) $ holds quite generally even for non-linear $I$-$V$ characteristics. Only when the normal metal is out of equilibrium, noise and current become independent. Their ratio, the Fano factor, depends then on the details of the layout.

cond-mat.mes-hall↗

arXiv2004

Full Counting Statistics of a charge shuttle

We study the charge transfer in a small grain oscillating between two leads. Coulomb blockade restricts the charge fluctuations in such a way that only zero or one additional electrons can sit on the grain. The system thus acts as a charge shuttle. We obtain the full counting statistics of charge transfer and discuss its behavior. For large oscillation amplitude the probability of transferring $\tilde n$ electrons per cycle is strongly peaked around one. The peak is asymmetric since its form is controlled by different parameters for $\tilde n>1$ and $\tilde n < 1$. Under certain conditions the systems behaves as if the effective charge is 1/2 of the elementary one. Knowledge of the counting statistics gives a new insight on the mechanism of charge transfer.

cond-mat.mes-hall↗

arXiv2003

Energy dependence of current noise in superconducting/normal metal junctions

cond-mat.mes-hall↗

arXiv2003

Current-current correlations in hybrid superconducting and normal metal multiterminal structures

We consider a hybrid system consisting of two normal metal leads weakly connected to a superconductor. Current-current correlations of the normal leads are studied in the tunneling limit at subgap voltages and temperatures. We find that only two processes contribute to the cross-correlation: the crossed Andreev reflection (emission of electrons in different leads) and the elastic cotunneling. Both processes are possible due to the finite size of the Cooper pair. Noise measurements can thus be used to probe directly the superconducting wave function without the drawbacks appearing in average current measurements where the current is dominated by direct Andreev reflection processes. By tuning the voltages it is possible to change the sign of the cross correlation. Quantitative predictions are presented both in the diffusive and ballistic regimes.

cond-mat.mes-hall↗

arXiv2003

Renormalization Group Approach to the Infrared Behavior of a Zero-Temperature Bose System

We exploit the renormalization-group approach to establish the {\em exact} infrared behavior of an interacting Bose system at zero temperature. The local-gauge symmetry in the broken-symmetry phase is implemented through the associated Ward identities, which reduce the number of independent running couplings to a single one. For this coupling the $ε$-expansion can be controlled to all orders in $ε$ ($=3-d$). For spatial dimensions $1 < d \leq 3$ the Bogoliubov fixed point is unstable towards a different fixed point characterized by the divergence of the longitudinal correlation function. The Bogoliubov linear spectrum, however, is found to be independent from the critical behavior of this correlation function, being exactly constrained by Ward identities. The new fixed point properly gives a finite value of the coupling among transverse fluctuations, but due to virtual intermediate longitudinal fluctuations the effective coupling affecting the transverse correlation function flows to zero. As a result, no transverse anomalous dimension is present. This treatment allows us to recover known results for the quantum Bose gas in the context of a unifying framework and also to reveal the non-trivial skeleton structure of its perturbation theory.

cond-mat.stat-mech↗

arXiv2003

Influence of thermal fluctuations on an underdamped Josephson tunnel junction

Inspired by a recent experiment, we study the influence of thermal fluctuations on the $I$-$V$ characteristics of a Josephson junction, coupled to a strongly resistive environment. We obtain analytical results in the limit where the Josephson energy is larger than the charging energy and quasiparticles are absent.

cond-mat.mes-hall↗

arXiv2002

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.

cond-mat.supr-con↗

arXiv2002

Observation of off-diagonal geometric phase in polarized neutron interferometer experiments

Off-diagonal geometric phases acquired in the evolution of a spin-1/2 system have been investigated by means of a polarized neutron interferometer. Final counts with and without polarization analysis enable us to observe simultaneously the off-diagonal and diagonal geometric phases in two detectors. We have quantitatively measured the off-diagonal geometric phase for noncyclic evolutions, confirming the theoretical predictions. We discuss the significance of our experiment in terms of geometric phases (both diagonal and off-diagonal) and in terms of the quantum erasing phenomenon.

quant-ph↗

arXiv2001

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 $μ$ is small compared to their surface tension $γ$, more precisely $ μa < γ$ where $a$ 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 $d$ apart, which is a $1/d^2$ 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.

cond-mat.stat-mech↗

arXiv2000

Geometric Phases and Multiple Degeneracies in Harmonic Resonators

In a recent experiment Lauber et al. have deformed cyclically a microwave resonator and have measured the adiabatic normal-mode wavefunctions for each shape along the path of deformation. The nontrivial observed cyclic phases around a 3-fold degeneracy were accounted for by Manolopoulos and Child within an approximate theory. However, open-path geometrical phases disagree with experiment. By solving exactly the problem, we find unsuspected extra degeneracies around the multiple one that account for the measured phase changes throughout the path. It turns out that proliferation of additional degeneracies around a multiple one is a common feature of quantum mechanics.

cond-mat↗

arXiv1999

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.

cond-mat.supr-con↗

arXiv1998

Theory and Data Analysis for the High Momentum End of $^4$He spectrum

The hybridization of the single-excitation branch with the two-excitation continuum is reconsidered from the theoretical point of view by including the effect of the interference term between one and two excitations. The phenomenological theory presented is used to reproduce the experimental data over a wide region of momentum (2.3-3.6 A^{-1}) and energy (0-12 meV). It is thus possible to extract the final part of $^4$He spectrum with higher accuracy. It is found that data agree with a dispersion relation always below twice the roton energy. This is consistent with the negative value of the roton-roton interaction found.

cond-mat↗

arXiv1998

Theory and data analysis for excitations in liquid $^4$He beyond the roton minimum

The hybridization of the single-excitation branch with the two-excitation continuum in the momentum region beyond the roton minimum is reconsidered by including the effect of the interference term between one and two excitations. Fits to the latest experimental data with our model allow us to extract with improved accuracy the high momentum end of the $^4$He dispersion relation. In contrast with previous results we find that the undamped excitations below two times the roton energy survive up to Q=3.6 A^{-1} due to the attractive interaction between rotons.

cond-mat↗

arXiv1997

Evolution from BCS Superconductivity to Bose Condensation: Analytic Results for the crossover in three dimensions

We provide an analytic solution for the mean-field equations and for the relevant physical quantities at the Gaussian level, in terms of the complete elliptic integrals of the first and second kinds, for the crossover problem from BCS superconductivity to Bose-Einstein condensation of a three-dimensional system of free fermions interacting via an attractive contact potential at zero temperature. This analytic solution enables us to follow the evolution between the two limits in a particularly simple and transparent way, as well as to verify the absence of singularities during the evolution.

cond-mat.supr-con↗

arXiv1997

Infrared Behavior of Interacting Bosons at Zero Temperature

We exploit the symmetries associated with the stability of the superfluid phase to solve the long-standing problem of interacting bosons in the presence of a condensate at zero temperature. Implementation of these symmetries poses strong conditions on the renormalizations that heal the singularities of perturbation theory. The renormalized theory gives: For d>3 the Bogoliubov quasiparticles as an exact result; for 1<d<=3 a nontrivial solution with the exact exponent for the singular longitudinal correlation function, with phonons again as low-lying excitations.

cond-mat↗