Anisotropies of the lower and upper critical fields in MgB${}_2$ single crystals

Abstract

The temperature dependence of the London penetration depth ($\hat{I}»$) and coherence length ($\hat{I}¾$) has been deduced from Hall probe magnetization measurements in high quality MgB${}_2$ single crystals in the two main crystallographic directions. We show that, in contrast to conventional superconductors, MgB${}_2$ is characterized by two different anisotropy parameters ($\hat{I}{“}_{\hat{I}}»=\hat{I}{»}_c/\hat{I}{»}_{ab}$ and $\hat{I}{“}_{\hat{I}}¾=\hat{I}{¾}_{ab}/\hat{I}{¾}_c$) which strongly differ at low temperature and merge at $T_c$. These results are in very good agreement with recent calculations in weakly coupled two bands suprerconductors (Phys. Rev. B, 66, 020509(R) (2002).

4 pages, 4 figures