Plasticity in current-driven vortex lattices

We present a theoretical analysis of recent experiments on current-driven vortex dynamics in the Corbino disk geometry. This geometry introduces controlled spatial gradients in the driving force and allows the study of the onset of plasticity and tearing in clean vortex lattices. We describe plastic slip in terms of the stress-driven unbinding of dislocation pairs, which in turn contribute to the relaxation of the shear, yielding a nonlinear response. The steady state density of free dislocations induced by the applied stress is calculated as a function of the applied current and temperature. A criterion for the onset of plasticity at a radial location $r$ in the disk yields a temperature-dependent critical current that is in qualitative agreement with experiments.Comment: 11 pages, 4 figure

Contact process under renewals I

Motivated by questions regarding long range percolation, we investigate a non-Markovian analogue of the Harris contact process in $\mathbb{Z}^d$: an individual is attached to each site $x \in \mathbb{Z}^d$, and it can be infected or healthy; the infection propagates to healthy neighbors just as in the usual contact process, according to independent exponential times with a fixed rate $\lambda$; nevertheless, the possible recovery times for an individual are given by the points of a renewal process with heavy tail; the renewal processes are assumed to be independent for different sites. We show that the resulting processes have a critical value equal to zero.Comment: 13 page

Non-precious metal carbamates as catalysts for the aziridine/CO2coupling reaction under mild conditions

The catalytic potential of a large series of easily available metal carbamates (based on thirteen different non-precious metal elements) was explored for the first time in the coupling reaction between 2-aryl-aziridines and carbon dioxide, working under solventless and ambient conditions and using tetraalkylammonium halides as co-catalysts. The straightforward synthesis of novel [NbCl3(O2CNEt2)2],NbCl, and [NbBr3(O2CNEt2)2],NbBr, is reported. The niobium complexNbCl, in combination with NBu4I, emerged as the best catalyst of the overall series to convert aziridines with smallN-alkyl substituents into the corresponding 5-aryl-oxazolidin-2-ones

Non-equilibrium Berezinskii-Kosterlitz-Thouless Transition in a Driven Open Quantum System

The Berezinskii-Kosterlitz-Thouless mechanism, in which a phase transition is mediated by the proliferation of topological defects, governs the critical behaviour of a wide range of equilibrium two-dimensional systems with a continuous symmetry, ranging from superconducting thin films to two-dimensional Bose fluids, such as liquid helium and ultracold atoms. We show here that this phenomenon is not restricted to thermal equilibrium, rather it survives more generally in a dissipative highly non-equilibrium system driven into a steady-state. By considering a light-matter superfluid of polaritons, in the so-called optical parametric oscillator regime, we demonstrate that it indeed undergoes a vortex binding-unbinding phase transition. Yet, the exponent of the power-law decay of the first order correlation function in the (algebraically) ordered phase can exceed the equilibrium upper limit -- a surprising occurrence, which has also been observed in a recent experiment. Thus we demonstrate that the ordered phase is somehow more robust against the quantum fluctuations of driven systems than thermal ones in equilibrium.Comment: 11 pages, 9 figure

Critical Hysteresis in Random Field XY and Heisenberg Models

We study zero-temperature hysteresis in random-field XY and Heisenberg models in the zero-frequency limit of a cyclic driving field. We consider three distributions of the random field and present exact solutions in the mean field limit. The results show a strong effect of the form of disorder on critical hysteresis as well as the shape of hysteresis loops. A discrepancy with an earlier study based on the renormalization group is resolved.Comment: 10 pages, 6 figures; this is published version (added some text and references

Models of plastic depinning of driven disordered systems

Two classes of models of driven disordered systems that exhibit history-dependent dynamics are discussed. The first class incorporates local inertia in the dynamics via nonmonotonic stress transfer between adjacent degrees of freedom. The second class allows for proliferation of topological defects due to the interplay of strong disorder and drive. In mean field theory both models exhibit a tricritical point as a function of disorder strength. At weak disorder depinning is continuous and the sliding state is unique. At strong disorder depinning is discontinuous and hysteretic.Comment: 3 figures, invited talk at StatPhys 2

Hysteresis in Random Field XY and Heisenberg Models: Mean Field Theory and Simulations at Zero Temperature

We examine zero temperature hysteresis in random field XY and Heisenberg models in the zero frequency limit of a cyclic driving field. Exact expressions for hysteresis loops are obtained in the mean field approximation. These show rather unusual features. We also perform simulations of the two models on a simple cubic lattice and compare them with the predictions of the mean field theory.Comment: replaced by the published versio

In-Plane Conductivity Anisotropy in Underdoped Cuprates in the Spin-Charge Gauge Approach

Applying the recently developed spin-charge gauge theory for the pseudogap phase in cuprates, we propose a self-consistent explanation of several peculiar features of the far-infrared in-plane AC conductivity, including a broad peak as a function of frequency and significant anisotropy at low temperatures, along with a similar temperature-dependent in-plane anisotropy of DC conductivity in lightly doped cuprates. The anisotropy of the metal-insulator crossover scale is considered to be responsible for these phenomena. The obtained results are in good agreement with experiments. An explicit proposal is made to further check the theory.Comment: 5 pages, 3 figures, to appear in Phys. Rev.

Degradation of the surfaces exposed to the space environment

The presence of several atomic species in the LEO (Low Earth Orbits) could be considered one of the reasons for the degradation of the surfaces exposed to the Space Environment. At an average height of 400 Km (the altitude of International Space Station), the concentration of the main atomic species during the high sun activity are: 1.5 x 10(9) cm(-3) for atomic oxygen (AO), 1.6 x 10(8) cm(-3) for molecular nitrogen (N-2) and 1.4 x 10(8) m(-3) for atomic nitrogen (N). The energy with which the atoms collide with the surface of orbiting vehicle depends on the relative speed of the vehicle itself. For instance, the atoms colliding the International Space Station (ISS) (orbit average height: 400 Km; relative speed: 7.5 Km/s) have an energy of 8 eV for N-2, 5 eV for OA and 4 eV for N. The atomic oxygen is the most abundant species presents in LEO and it is considered the main responsible of the thermal, optical and mechanical alteration of the surfaces exposed to the Space Environment. Different hypothesis are reported in literature in order to explain the physical/chemical mechanisms that govern the material degradation in the Space, but no conclusion has been reached. In the energy range of few of eV, the main mechanism with which colliding atoms transfer its energy to the atoms of the surface is by phonons. In this paper the effect of an oxygen ion beam produced in the space environment simulator on materials for Space applications is studied in the frame of the thermal spike theory. Comparison between the measured erosion and the calculated one will be reported. The erosion mechanism will be modelled in order to understand the main thermodynamic parameters that govern the interaction between the atomic oxygen and the surface of the tested materials. (c) 2006 Elsevier Ltd. All rights reserved

Bypassing the Inertness of Aziridine/CO2 Systems to Access 5-Aryl-2-Oxazolidinones: Catalyst-Free Synthesis Under Ambient Conditions

The development of sustainable synthetic routes to access valuable oxazolidinones via CO2 fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction features a high activation barrier and thus requires a catalytic system, and may present some other critical issues. Here, the straightforward gram-scale synthesis of a series of 5-aryl-2-oxazolidinones was developed at ambient temperature and atmospheric CO2 pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO2 adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO2. The reaction mechanism was investigated by NMR spectroscopy and DFT calculations applied to model cases