Anisotropy of superconductivity from MgB2 single crystals

Abstract

Magnesium diboride (MgB2) single crystals, with a maximum size of 0.5×0.5×0.02 mm3, were grown by the vapor transport method in a sealed molybdenum crucible. A superconducting transition with the onset temperature of 38.6 K was confirmed by both transport and magnetization measurements. The upper critical field anisotropy ratio, Hc2∥ab(0)/Hc2∥c(0), was estimated to be 2.6 from the magnetic field-temperature phase diagram for MgB2 single crystals.

Enhancement of the upper critical field by nonmagnetic impurities in dirty two-gap superconductors

“…(32) and (33), the ratio H || c2 /H ⊥ c2 is minimum at T c and increases as T decreases. This behavior has indeed been observed on many, mostly clean, M gB 2 samples 11,12,13,14,15,16,17,18,19,20,21 . By contrast, if impurities (like Al, Mg vacancies) cause the strongest intraband scattering in the "weak" π-band, (D 2 ≪ D 1 ), then the anisotropy parameter H || c2 /H ⊥ c2 would decrease as T decreases, The temperature dependence of H || c2 /H ⊥ c2 for these limiting cases is shown in Fig.…”

“…So far all attempts to increase the critical temperature T c of M gB 2 by doping have been unsuccessful 9 . On the other hand, the significant potential of M gB 2 for applications 10 is limited by comparatively low upper critical fields H ⊥ c2 (0) ≃ 3 − 5T and H || c2 (0) ≃ 15 − 19T of hexagonal M gB 2 single crystals 11,12,13,14,15,16,17,18,19,20,21 , where the indices ⊥ and || correspond to the magnetic field H perpendicular and parallel to the ab plane, respectively. As far as H c2 is concerned, it can be increased by adding nonmagnetic impurities 22,23 , which are especially effective in the dirty limit, 2πk B T c < /τ , where τ is the elastic scattering time, is the Planck constant, and k B is the Boltzmann constant.…”

Superconductivity in polycrystalline diamond thin films

“…5. The extrapolation of the T C onset gives the value of 10.12 T. Assuming that the superconductivity in diamond is the dirty limit, the upper critical field H C2 is estimated to be 7 T, which is similar to the H C2 //c of c-axis MgB 2 (11,12). From the upper critical field of 7T, the coherent length can be estimated to be ξ=6.9nm using the equation of ξ 2 =Φ 0 /2πHc 2 .…”

Thermal conductivity of single-crystallineMgB2

“…The constant residual resistivity ρ 0 is caused by defects which actually set the maximum mean free path for electrons, independent of temperature. Indeed, results by Xu et al 27 of ρ(T ) measurements on single crystalline MgB 2 at magnetic fields which presumably exceed the upper critical field of the minor phase, demonstrate that in, e.g., H = 90 kOe the electrical resistivity remains constant with decreasing temperature down to at least 2 K.…”