Vortex Softening: Origin of the second peak effect in Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta}

Transverse ac permeability measurements in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta }$ single crystals at low fields and temperatures in a vortex configuration free of external forces show that the decrease of the critical current as measured by magnetization loops at the second peak effect is an artifact due to creep. On the other hand, the increase of critical current at the second peak is due to a genuine softening of the tilting elastic properties of vortices in the individual pinning regime that precedes the transition to a disorder state.Comment: 4 pages, 5 figures, RevTex, two column versio

Remarkable magnetostructural coupling around the magnetic transition in CeCo0.85_{0.85}Fe0.15_{0.15}Si

We report a detailed study of the magnetic properties of CeCo$_{0.85}$Fe$_{0.15}$Si under high magnetic fields (up to 16 Tesla) measuring different physical properties such as specific heat, magnetization, electrical resistivity, thermal expansion and magnetostriction. CeCo$_{0.85}$Fe$_{0.15}$Si becomes antiferromagnetic at $T_N \approx$ 6.7 K. However, a broad tail (onset at $T_X \approx$ 13 K) in the specific heat precedes that second order transition. This tail is also observed in the temperature derivative of the resistivity. However, it is particularly noticeable in the thermal expansion coefficient where it takes the form of a large bump centered at $T_X$. A high magnetic field practically washes out that tail in the resistivity. But surprisingly, the bump in the thermal expansion becomes a well pronounced peak fully split from the magnetic transition at $T_N$. Concurrently, the magnetoresistance also switches from negative to positive just below $T_X$. The magnetostriction is considerable and irreversible at low temperature ($\frac {\Delta L}{L} \left(16 T\right) \sim$ 4$\times$10$^{-4}$ at 2 K) when the magnetic interactions dominate. A broad jump in the field dependence of the magnetostriction observed at low $T$ may be the signature of a weak ongoing metamagnetic transition. Taking altogether, the results indicate the importance of the lattice effects in the development of the magnetic order in these alloys.Comment: 5 pages, 6 figure

Extinction time in growth models subject to binomial catastrophes

Populations are often subject to catastrophes that cause mass removal of individuals. Many stochastic growth models have been considered to explain such dynamics. Among the results reported, it has been considered whether dispersion strategies, at times of catastrophes, increase the survival probability of the population. In this paper, we contrast dispersion strategies comparing mean extinction times of the population when extinction occurs almost surely. In particular, we consider populations subject to binomial catastrophes, that is, the population size is reduced according to a binomial law when a catastrophe occurs. Our results show which is the best strategy (dispersion or non-dispersion) depending on model parameter values.Comment: 15 pages, 2 figures. arXiv admin note: substantial text overlap with arXiv:2109.1099

Nonlinear Supersymmetry as a Hidden Symmetry

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Enhanced weak superconductivity in trigonal γ\gamma-PtBi2_2

Electrical transport experiments show superconductivity in a high-quality single crystal of trigonal $\gamma$-PtBi$_2$. The critical temperature shows a large dependence on the electrical current and in the limit of very low currents, a $T_c$ = 1.1 K is observed, while a zero temperature critical field $B_c$(0) $\approx$ 1.5 Tesla is estimated. These are the highest superconducting parameters reported (at ambient pressure) in a stoichiometric $\gamma$-PtBi$_2$ bulk sample so far. Under a magnetic field a strict zero resistance state is no longer observed even though an incipient superconducting transition is seen. Such a behavior is most probably associated with very low critical currents and is reminiscent of filamentary superconductivity. The superconducting state is elusive to magnetization measurements discarding a bulk phase down to $T =$ 0.3 K.Comment: 5 pages, 5 figure