High temporal stability of supercurrents in MgB₂materials

Abstract

Fine grained polycrystalline samples of MgB 2 superconductor were synthesized from the elements to contain < 5 % of impurity phases, according to x-ray powder diffractometry. The superconductive transition was sharp with a midpoint T c = 38.5 K. The magnetization in the vortex state was studied as a function of applied field H , temperature T and time t. From the equilibrium magnetization, the London penetration depth was obtained. The supercurrent density J (T , H, t) in the vortex state (derived from the irreversible magnetization) decreases approximately linearly with T , in contrast to the quasi-exponential fall-off in high-T c superconductors. The current is highly stable in time, with normalized decay rates S = − d ln(J )/d ln(t) well below those in high-T c materials. These results are compared with those of other superconductors.

Influence of microstructures and crystalline defects on the superconductivity of MgB2

“…where λ(0) = 110-180 nm, 14,15,16,17 for MgB 2 . Therefore, the penetration length can be significantly larger near T c than the particle size (leading to transition broadening) and decreases with temperature.…”

Limits of the upper critical field in dirty two-gap superconductors

“…Given these promising results combined with weak current blocking by grain boundaries [97], the lack of electromagnetic granularity [98], and very slow thermally-activated flux creep [99,100], it is not surprising that MgB 2 is being regarded as a strong contender of traditional high-field magnet materials like NbTi and Nb 3 Sn. Despite these achievements, a detailed theory of pinning in MgB 2 is trill lacking.…”

Magnetic field dependence of vortex activation energy: A comparison between MgB2, NbSe2 and Bi2Sr2Ca2Cu3O10 superconductors

“…The magnitude of U 0 at low fields is given by U 0 ~ φ 0 2 ξ/48 π 2 λ 2 [17]. For MgB 2 using the value ξ ∼ 5 nm and λ ~ 100 nm [22] U o is estimated to be to be ~ 7400 K where as for the case of NbSe 2 with ξ ∼ 2.3 nm and λ ~ 230 nm, U o is calculated to be ~ 1000K. This explains our data very well.…”