Anisotropy of superconducting properties of flexible magnesium-diboride-coated carbon nanotube yarns*

Flexible ultralight magnesium-diboride-coated carbon nanotube (MgB$_2$-CNT) yarns have critical temperatures up to 37~K, high critical currents and fields comparable with conventional superconducting wires. Superconducting yarns containing MgB$_2$-CNT nanofibers have been prepared by conformal coating of CNT sheets with boron in photothermal chemical vapor deposition, and annealing in magnesium vapors. Electrical transport measurements in a magnetic field, whose direction is varied relative to the sample orientation, showed anisotropy in superconducting properties. The critical field anisotropy ratio $H_{c2}^\parallel{}/H_{c2}^\perp{}$ reaches 1.2-1.4 over a wide temperature range below $T_c$, comparable to but slightly lower than the factor 1.4-2 of epitaxial MgB$_2$ thin films. An X-ray diffraction study confirmed the crystalline anisotropy of composite wires and showed, that the MgB$_2$ grains prefer to grow with the \textit{ab}-plane parallel to the carbon nanotube walls and the yarn axis.