Low Temperature Susceptibility of the Noncentrosymmetric Superconductor Ce$_{1-x}$La$_{x}$Pt$_{3}$Si

We report ac susceptibility measurements of polycrystalline Ce$_{1-x}$La$_x$Pt$_3$Si down to 60 mK and in applied fields up to 9 T. In zero applied field, a full Meissner state emerges for pure CePt$_3$Si at temperatures $T/T_{c}$$<$ 0.3, where $T_{c}$ = 0.65 K is the onset transition temperature. Though transport measurements show a relatively high upper critical field $B_{c2}\sim$ 4 - 5 T, the low temperature susceptibility, $\chi \prime$, is quit fragile to applied field, with $\chi \prime$ diminishing rapidly in fields of a few kG. Interestingly, the field dependence of $\chi \prime$ is well described by the power law, 4$\pi$$\chi \prime$ + 1 = $(B/B_{c})^{1/2}$, where $B_{c}$ is the field at which the onset of resistance is observed in transport measurements. The effects of La doping on the superconductivity will also be presented.