Screening current effects in Josephson junction arrays

The purpose of this work is to compare the dynamics of arrays of Josephson junctions in presence of magnetic field in two different frameworks: the so called XY frustrated model with no self inductance and an approach that takes into account the screening currents (considering self inductances only). We show that while for a range of parameters the simpler model is sufficiently accurate, in a region of the parameter space solutions arise that are not contained in the XY model equations.Comment: Figures available from the author

The effect of the dynamical state of clusters on gas expulsion and infant mortality

The star formation efficiency (SFE) of a star cluster is thought to be the critical factor in determining if the cluster can survive for a significant (>50 Myr) time. There is an often quoted critical SFE of ~30 per cent for a cluster to survive gas expulsion. I reiterate that the SFE is not the critical factor, rather it is the dynamical state of the stars (as measured by their virial ratio) immediately before gas expulsion that is the critical factor. If the stars in a star cluster are born in an even slightly cold dynamical state then the survivability of a cluster can be greatly increased.Comment: 6 pages, 2 figures. Review talk given at the meeting on "Young massive star clusters - Initial conditions and environments", E. Perez, R. de Grijs, R. M. Gonzalez Delgado, eds., Granada (Spain), September 2007, Springer: Dordrecht. Replacement to correct mistake in a referenc

Josephson flux-flow oscillators in nonuniform microwave fields

We present a simple theory for Josephson flux-flow oscillators in the presence of nonuniform microwave fields. In particular we derive an analytical expression for the I−V characteristic of the oscillator from which we show that satellite steps are spaced around the main flux-flow resonance by only even harmonics of the rf frequency. This result is found to be in good agreement with our numerical results and with experiments

FLUXON-BREATHER-PLASMA OSCILLATION DECAY IN LONG JOSEPHSON JUNCTIONS

Les solutions exactes de l'équation de sine-Gordon, qui décrivent les oscillations sur une jonction Josephson longue et uni-dimensionnelle avec conditions au bord de circuit ouvert, suggèrent que les tourbillons qui ralentissent jusqu'à une vitesse critique de propagation se transforment en "breathers", qui à leur tour, lorsque leur amplitude diminue jusqu'à la valeur critique, se transforment en oscillations du plasma.Exact solutions of the sine-Gordon equation describing oscillations on a long, one-dimensional Josephson junction with open circuit boundary conditions suggest that fluxons that slow to a critical propagation velocity decay into breathers, which in turn, when their amplitude diminishes to a critical value, decay into plasma oscillations

Numerical evidence for global bifurcations leading to switching phenomena in long Josephson junctions

Fluxons in long Josephson junctions are physical manifestations of travelling waves that connect rest states of the model partial differential equation (p.d.e.), which is a perturbed sine-Gordon equation. In the reduced traavelling wave ordinary differential equation (o.d.e.), fluxons correspond to heteroclinic connections between fixed points. In the absence of surface impedence effects, fluxons persist in parameter regimes until the fixed points disappear, after which the system ‘switches’ to another configuration. It is known that the presence of surface impedence produces a singular perturbation of the model equation, together with a new phenomenon: the fluxons switch in parameter regimes before the fixed points are lost. Why this occurs is unknown, and is the focus of this paper. Two disjoint possibilities are: (1) instability: fluxons still exist, but they become unstable in the p.d.e. due to surface impedance effects; (2) nonexistence: the fluxons fail to exist, even though the fixed points remain. Here, we provide compelling numerical evidence for the second scenario, characterized by a global bifurcation in the travelling wave phase space: a breakdown of heteroclinicorbits, undetected at the local linearized level. Moreover, this global o.d.e. bifurcation occurs at parameter values consistent with the p.d.e. switching phenomenon

Strategies of the beetle Oochrotus unicolor (Tenebrionidae) thriving in the waste dumps of seed‐harvesting Messor ants (Formicidae)

1. A diverse group of arthropods have adapted to the niches found inside the nests of social insects. Studies mostly focused on very specialised parasites residing in the brood chambers. However, the biology and strategies of symbionts occupying other niches, such as waste dumps, are underexplored. 2. Using a series of complementary experiments, this study demonstrated that the Mediterranean beetle Oochrotus unicolor has adapted to the waste dump niche found in the nests of Messor harvester ants. 3. Laboratory experiments confirmed field observations that the beetle preferentially resided in the refuse pits. Next, it was shown that the beetles readily consumed seeds and flour, whereas other food sources were poorly accepted and ant brood was never even eaten. The beetles did not elicit a strong aggression response in Messor ants, and they could tolerate very high densities of workers without clear costs. The beetles modestly mimicked the nest recognition cues of their Messor host. This imperfect mimicry could promote the adoption of the beetle in the ant colony, in concert with mechanical defence generated by its tank-like body. Isolation of the beetle from its host did not significantly affect the beetle's chemical cuticular profile nor did it provoke elevated ant aggression, indicating that the beetle does not acquire the chemicals passively from its host. 4. This paper discusses the fact that waste dumps in social insect nests are hotspots for arthropod symbionts. It shows that symbionts in this niche may employ behavioural, trophic and chemical strategies that are different from those found in other niches of social insect nests