Phase transitions in simplified models with long-range interactions

We study the origin of phase transitions in some simplified models with long range interactions. For the ring model, we show that a possible new phase transition predicted in a recent paper by Nardini and Casetti from an energy landscape analysis does not occur. Instead of such phase transitions we observe a sharp, although without any non-analiticity, change from a core-halo to an only core configuration in the spatial distribution functions for low energies. By introducing a new class of solvable simplified models without any critical points in the potential energy, we show that a similar behaviour to the ring model is obtained, with a first order phase transition from an almost homogeneous high energy phase to a clustered phase, and the same core-halo to core configuration transition at lower energies. We discuss the origin of these features of the simplified models, and show that the first order phase transition comes from the maximization of the entropy of the system as a function of energy an an order parameter, as previously discussed by Kastner, which seems to be the main mechanism causing phase transitions in long-range interacting systems

Muon Spin Relaxation Studies of Superconductivity in a Crystalline Array of Weakly Coupled Metal Nanoparticles

We report Muon Spin Relaxation studies in weak transverse fields of the superconductivity in the metal cluster compound, Ga$\_{84}$[N(SiMe$\_{3}$)$\_{2}$]$\_{20}$-Li$\_{6}$Br$\_{2}$(thf)$\_{20}\cdot$2toluene. The temperature and field dependence of the muon spin relaxation rate and Knight shift clearly evidence type II bulk superconductivity below $T\_{\text{c}}\approx7.8$ K, with $B\_{\text{c1}}\approx 0.06$ T, $B\_{\text{c2}}\approx 0.26$ T, $\kappa\sim 2$ and weak flux pinning. The data are well described by the s-wave BCS model with weak electron-phonon coupling in the clean limit. A qualitative explanation for the conduction mechanism in this novel type of narrow band superconductor is presented.Comment: 4 figures, 5 page

Validation of a model of the GAL regulatory system via robustness analysis of its bistability characteristics

In Saccharomyces cerevisiæ, structural bistability generates a bimodal expression of the galactose uptake genes (GAL) when exposed to low and high glucose concentrations. This indicates that yeast cells can decide between using either the limited amount of glucose or growing on galactose under changing environmental conditions. A crucial requirement for any plausible mechanistic model of this system is that it reproduces the robustness of the bistable response observed in vivo against inter-individual parametric variability and fluctuating environmental conditions

Kinetic approaches to particle acceleration at cosmic ray modified shocks

Kinetic approaches provide an effective description of the process of particle acceleration at shock fronts and allow to take into account the dynamical reaction of the accelerated particles as well as the amplification of the turbulent magnetic field as due to streaming instability. The latter does in turn affect the maximum achievable momentum and thereby the acceleration process itself, in a chain of causality which is typical of non-linear systems. Here we provide a technical description of two of these kinetic approaches and show that they basically lead to the same conclusions. In particular we discuss the effects of shock modification on the spectral shape of the accelerated particles, on the maximum momentum, on the thermodynamic properties of the background fluid and on the escaping and advected fluxes of accelerated particles.Comment: 22 pages, 7 figures, accepted for publication in MNRA

Native NIR-emitting single colour centres in CVD diamond

Single-photon sources are a fundamental element for developing quantum technologies, and sources based on colour centres in diamonds are among the most promising candidates. The well-known NV centres are characterized by several limitations, thus few other defects have recently been considered. In the present work, we characterize in detail native efficient single colour centres emitting in the near infra-red in both standard IIa single-crystal and electronic-grade polycrystalline commercial CVD diamond samples. In the former case, a high-temperature annealing process in vacuum is necessary to induce the formation/activation of luminescent centres with good emission properties, while in the latter case the annealing process has marginal beneficial effects on the number and performances of native centres in commercially available samples. Although displaying significant variability in several photo physical properties (emission wavelength, emission rate instabilities, saturation behaviours), these centres generally display appealing photophysical properties for applications as single photon sources: short lifetimes, high emission rates and strongly polarized light. The native centres are tentatively attributed to impurities incorporated in the diamond crystal during the CVD growth of high-quality type IIa samples, and offer promising perspectives in diamond-based photonics.Comment: 27 pages, 10 figures. Submitted to "New Journal of Phsyics", NJP-100003.R

Effect of two gaps on the flux lattice internal field distribution: evidence of two length scales from muSR in Mg1-xAlxB2

We have measured the transverse field muon spin precession in the flux lattice (FL) state of the two gap superconductor MgB2 and of the electron doped compounds Mg1-xAlxB2 in magnetic fields up to 2.8T. We show the effect of the two gaps on the internal field distribution in the FL, from which we determine two coherence length parameters and the doping dependence of the London penetration depth. This is an independent determination of the complex vortex structure already suggested by the STM observation of large vortices in a MgB2 single crystal. Our data agrees quantitatively with STM and we thus validate a new phenomenological model for the internal fields.Comment: now in press Phys. Rev. Lett., small modifications required by the edito

Flysch deposition and preservation of coherent bedding in an accretionary complex: Detrital zircon ages from the Upper Cretaceous Valdez Group, Chugach terrane, Alaska

The Upper Cretaceous Valdez Group represents the flysch facies of the Mesozoic Chugach terrane accretionary complex in southern Alaska. The Valdez Group is dominated by litharenite sandstone and argillite deposited as coherent beds, unlike the older McHugh Complex mélange and massive sandstones. Detrital zircons from five sandstones sampled along an ~55 km transect through the Valdez Group were dated using U-Pb laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). The youngest populations from the two oldest samples, located along strike from each other, were 82-81 Ma. Three samples across strike and outboard of the others are separated by ~50 km, but each has a youngest population dated at ca. 68 Ma. All of these samples have major grain population ages that suggest erosion from the Coast Mountains Batholith, consistent with petrography and grain modes suggesting an arc source. No apparent age gap exists between the youngest McHugh Complex samples and the oldest Valdez Group samples, suggesting continuous deposition despite the different depositional and tectonic style. We propose a model in which the onset of coherently bedded flysch marks the transition from deposition in the trench or trench slope to deposition on the oceanic plate beyond the trench after it was filled at ca. 84 Ma, i.e., the time of the youngest mélange sedimentation. Preservation of coherent bedding resulted as large coherent blocks of Valdez Group rocks were imbricated into the subduction complex during continued subduction in Paleogene time. © 2011 Geological Society of America

Forage Quality and Yield of Berseem Clover and Annual Ryegrass Grown in Pure and Mixed Stands in Relation to Different N Application Rates

Grass-legume mixtures offer several advantages over monocultures in forage-animal production systems (Haynes, 1980). In fact, in grass-legume mixtures forage yield and quality are generally higher compared to grass monocultures also due to more efficient soil N-utilisation (Ta & Faris, 1987); furthermore grasses often utilise some of the N fixed by legumes (Malhi et al., 2002). Legumes do not generally require the addition of N fertiliser due to symbiotically fixed N, but the yield of the grass component in a mixture may be further improved with N application. The objective of this study was to investigate forage yield and quality of berseem and annual ryegrass grown in pure stands, and in mixture, at different N fertiliser rates