Anisotropy of the upper critical field and critical current in single crystalMgB2

Abstract

We report on specific heat, high magnetic field transport and ac−susceptibility measurments on magnesium diboride single crystals. The upper critical field Hc2 for magnetic fields perpendicular and parallel to the Mg and B planes is presented for the first time in the entire temperature range. A very different temperature dependence has been observed in the two directions which yields to a temperature dependent anisotropy with Γ ∼ 5 at low temperatures and about 2 near Tc. A peak effect is observed in the susceptibility measurments for µ0H ∼2 T parallel to the c−axis and the critical current density presents a sharp maximum for H parallel to the ab−plane.

Influence of the upper critical-field anisotropy on the transport properties of polycrystalline MgB2

“…It was pointed out recently [12] that this small application range was caused by the upper critical field anisotropy of MgB 2 . In the pure material, B c2 is about 14 T, if the field is applied parallel to the boron planes, while it is smaller than 3 T, if the field is applied perpendicular to the boron planes, leading to an anisotropy of close to 5 [13,14,15,16,17,18]. In polycrystalline MgB 2 , the grains are randomly oriented and the first grains become normal conducting at B ⊥ c2 , which is a few times smaller than the upper critical field of the whole sample (B || c2 ), thus reducing the effective cross section and the critical current of the conductor.…”

Upper critical field in dirty two-band superconductors: Breakdown of the anisotropic Ginzburg-Landau theory

“…Strong temperature dependence of γ c2 has been reliably confirmed by experiment. 28,29,30,31,32,33 Typically, γ c2 drops from 5-6 at low temperatures down to ∼ 2 near T c .…”

Charged Particle Irradiation Studies on Bismuth Based High Temperature Superconductors and MgB2: a comparative study

“…β was found to be ~ 1.67 for unirradiated sample and 1.78 for irradiated sample. In MgB 2 single crystal μ 0 H c2 (0) is around 3.5T along c axis and around 15 to 17 T along ab direction [47,48]. In polycrystalline sample where the grains are randomly oriented, H c2 (0) is governed by the higher value of the H c2 c and H c2 ab .…”

“…near Tc(R=0), dissipation energy due to motion of vortices by thermally activated flux creep plays an important role in pinning [31,32]. Hence, thermally activated flux creep model [48] was used to analyse the magnetoresistance of irradiated and unirradiated BSCCO samples in the temperature regime Tc(onset) to Tc(R=0). According to this model, the resistivity in this temperature regime is given as:…”