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

TL;DR

This study investigates the anisotropic properties of MgB₂ superconductors, revealing two distinct anisotropy parameters that vary with temperature, providing insights into its two-band superconducting nature.

Contribution

It provides experimental evidence of two different anisotropy parameters in MgB₂, supporting theoretical models of two-band superconductivity.

Findings

MgB₂ exhibits two anisotropy parameters, Γ_λ and Γ_ξ.

These parameters differ at low temperatures and converge at T_c.

Results align with two-band superconductor theories.

Abstract

The temperature dependence of the London penetration depth ($\lambda$) and coherence length ($\xi$) 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 ($\Gamma_\lambda = \lambda_c/\lambda_{ab}$ and $\Gamma_\xi = \xi_{ab}/\xi_c$) 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).