Evolution of superconductivity in Pr${}_{1-x}$La${}_x$Os${}_4$Sb${}_{12}$: Upper critical field measurements

Measurements of the upper critical field $H_{c2}$ near $T_c$ of Pr${1- x}$La${x}$Os${4}$Sb${12}$ were performed by specific heat. A positive curvature in $H_{c2}$ versus $T$ was observed in samples and concentrations exhibiting two superconducting transitions. These results argue against this curvature being due to two-band superconductivity. The critical field slope - $d{H}_{c2}\mathrm{/}dT$ suggests the existence of a crossover concentration $x_{cr} \approx 0.25$, below which there is a rapid suppression of effective electron mass with La-alloying. This crossover concentration was previously detected in the measurement of the discontinuity of $C\mathrm{/}T$ at $T_c$.