We present magnetic measurements in a single crystal of the newly discovered superconducting ironpnictide Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2. The magnetization loops exhibit a second magnetization peak ͑SMP͒ similar to that observed in most high-temperature superconductors ͑HTSs͒. Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, S = d ln M / d ln t, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2 the onset and peak fields, as well as the minimum point in S, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in Ba͑Fe 0.925 Co 0.075 ͒ 2 As 2 is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of S. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.

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Magnetization measurements as a function of temperature are reported for YBa 2 Cu 3 O 6.5 crystal (T c ϭ45.2 K) for fields between 0.2 and 3.5 T. All isochamps for HϾ1 T intersect at T 2D Ã Ӎ42.8 K, implying a fluctuation contribution to the magnetization. These curves collapse into a single curve when magnetization and temperature are scaled according to the predicted ''two-dimensional ͑2D͒ scaling'' in the fluctuation regime. Surprisingly, the low-field curves also intersect, at T 3D Ã Ӎ43.4 K, and they obey a 3D scaling. We provide a theoretical picture of the magnetization in the fluctuation regime based on the Lawrence-Doniach model. Within this model we calculate the field and temperature dependence of the magnetization. The two intersection points and the 2D→3D crossover are consistent with the experimental observation.

The influence of recently discovered topological transition between type I and type II Weyl semimetals on superconductivity is considered. A set of Gorkov equations for weak superconductivity in Weyl semi-metal under topological phase transition is derived and solved. The critical temperature and superconducting gap both have spike in the point the transition point as function of the tilt parameter of the Dirac cone determined in turn by the material parameters like pressure. The spectrum of superconducting excitations is different in two phases: the sharp cone pinnacle is characteristic for a type I, while two parallel almost flat bands, are formed in type II. Spectral density is calculated on both sides of transition demonstrate different weight of the bands. The superconductivity thus can be used as a clear indicator for the topological transformation. Results are discussed in the light of recent experiments.

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