Lower Critical Field at Odds with A S-Wave Superconductivity in The New Superconductor $Mg{B}_2$

Abstract

The lower critical field $H_{c1}$ has been carefully measured on a well shaped cylindrical sample of the new superconductor $Mg{B}_2$ fabricated by high pressure synthesis. The penetration depth $\lambda$ is calculated from the $H_{c1}$ data. It is found that a linear relation of $H_{c1}(T)$ appears in whole temperature region below $T_c$. Furthermore a finite slope of $d{H}_{c1}/dT$ and $d\lambda (T)/dT$ remains down to the lowest temperature (2 K). These are inconsistent with the expectation for a widely thought s-wave superconductivity in $Mg{B}_2$.

4 pages, 4 figures