We report on the study of the electronic anisotropy of the ${\mathrm{MgB}}_2$ superconductor using the in-plane resistivity measurements in the magnetic field applied perpendicular and parallel to Mg and B planes of ${\mathrm{MgB}}_2$ single crystals. The results show the temperature dependent anisotropy of the upper critical field with the anisotropy ratio $\gamma {=H}_{c2\mathrm{}\parallel }{/H}_{c2\mathrm{}\perp }$ increasing from 2.2 close to $T_c$ up to about 3 below 30 K. Our estimation of the in-plane and out-of-plane coherence length of about ${\xi }_{\mathrm{ab}}\mathrm{}\left(0\right)=68\mathrm{}\AA$ and ${\xi }_c\mathrm{}\left(0\right)=23\mathrm{}\AA$ and the electronic mean-free path $l_{\mathrm{ab}}=240\mathrm{}\AA$ and $l_c=60\mathrm{}\AA ,$ respectively, indicates ${\mathrm{MgB}}_2$ single crystal approaches the clean limit type-II superconductor.

• Received 18 October 2001

DOI:https://doi.org/10.1103/PhysRevB.65.140501