Two-soliton collisions in a near-integrable lattice system

We examine collisions between identical solitons in a weakly perturbed Ablowitz-Ladik (AL) model, augmented by either onsite cubic nonlinearity (which corresponds to the Salerno model, and may be realized as an array of strongly overlapping nonlinear optical waveguides), or a quintic perturbation, or both. Complex dependences of the outcomes of the collisions on the initial phase difference between the solitons and location of the collision point are observed. Large changes of amplitudes and velocities of the colliding solitons are generated by weak perturbations, showing that the elasticity of soliton collisions in the AL model is fragile (for instance, the Salerno's perturbation with the relative strength of 0.08 can give rise to a change of the solitons' amplitudes by a factor exceeding 2). Exact and approximate conservation laws in the perturbed system are examined, with a conclusion that the small perturbations very weakly affect the norm and energy conservation, but completely destroy the conservation of the lattice momentum, which is explained by the absence of the translational symmetry in generic nonintegrable lattice models. Data collected for a very large number of collisions correlate with this conclusion. Asymmetry of the collisions (which is explained by the dependence on the location of the central point of the collision relative to the lattice, and on the phase difference between the solitons) is investigated too, showing that the nonintegrability-induced effects grow almost linearly with the perturbation strength. Different perturbations (cubic and quintic ones) produce virtually identical collision-induced effects, which makes it possible to compensate them, thus finding a special perturbed system with almost elastic soliton collisions.Comment: Phys. Rev. E, in pres

Andreev reflection spectroscopy of the new Fe-based superconductor EuAsFeO₀.₈₅F₀.₁₅: evidence for the strong order parameter anisotropy

Andreev reflection spectra have been measured in a new superconductor EuAsFeO₀.₈₅F₀.₁₅ having an unexpectedly low superconducting transition temperature Tc≈11.3 K among related FeAs compounds on a base Sm and Gd surrounding Eu in the series of lanthanides. The nearly fivefold lower Tc, as against the expected value, is attributed to the divalent properties of Eu ions when in the compound investigated along with the weakly magnetic Eu³⁺ ions may be present and the strongly magnetic Eu²⁺ ones that is a strong destructive factor for superconductivity. Most of the spectra measured showed features that corresponds to two energy gaps whose values varied from contact to contact within 2Δ s/kTc = 2.2–4.7 and 2Δ1/kTc = 5.1–11.7 for small and large gap, respectively. The corresponding variations for single-gap spectra are 2Δ/kTc = 2.6–6.4. The relatively large size of crystallites (no less than ~25 µm) and the large number of contacts measured (several tens) suggest with a high degree of probability that the spectra obtained account quite fully for the gap distribution practically in all crystallographic directions. The data obtained and the absence of zero gaps in the measured spectra evidence in favor of the anisotropic s- or s±-symmetry of the order parameter in EuAsFeO₀.₈₅F₀.₁₅ that was revealed in other similar compounds with higher Tc. Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tc pnictides

Superconducting and magnetic properties of a new EuAsFeO₀,₈₅F₀,₁₅ superconductor

Polycrystalline samples of a new superconducting EuAsFeO₀,₈₅F₀,₁₅ compound with critical temperature Tc = 11 K were prepared by solid state synthesis. Its electric and magnetic properties have been investigated in magnetic fields from 0.1 to 140000 Oe. Critical magnetic fields Hc1, and Hc2 were measured and hence the magnetic penetration depths l and the coherence length x have been estimated. The temperature dependence Hc2(T) exhibits clear hyperbolic – type behavior starting with the lowest fields. The data derived were used to estimate probable high Tc and Hc2 in compounds doped with rare-earths having small atomic radii