Correlated enhancement of<i>H</i>_c2and<i>J</i>_cin carbon nanotube doped MgB₂

Abstract

The use of MgB2 in superconducting applications still awaits for the\ndevelopment of a MgB2-based material where both current-carrying performance\nand critical magnetic field are optimized simultaneously. We achieved this by\ndoping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in\npolycrystalline samples. The optimum nominal DWCNT content for increasing the\ncritical current density, Jc is in the range 2.5-10%at depending on field and\ntemperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with\nextrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT\ncontent. The measured Hc2 vs T in all samples are successfully described using\na theoretical model for a two-gap superconductor in the dirty limit first\nproposed by Gurevich et al.\n