  The superconducting state of the filled skutterudite alloy series
Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$ has been systematically studied by specific
heat, zero-field muon spin relaxation ($\mu$SR), and superconducting critical
field measurements. An additional inhomogeneous local magnetic field,
indicative of broken time-reversal symmetry (TRS), is observed in the
superconducting states of the alloys. For $x \lesssim 0.5$ the broken-TRS phase
sets in below a temperature $T_m$ distinctly lower than the superconducting
transition temperature $T_c$. For $x \gtrsim 0.5$ $T_m \approx T_c$. The local
field strength decreases as $x \to 1$, where LaPt$_{4}$Ge$_{12}$ is
characterized by conventional pairing. The lower critical field $H_{c1}(T)$ of
PrPt$_{4}$Ge$_{12}$ shows the onset of a second quadratic temperature region
below $T_q \sim T_m$. Upper critical field $H_{c2}(T)$ measurements suggest
multiband superconductivity, and point gap nodes are consistent with the
specific heat data. In Pr$_{1-x}$La$_{x}$Pt$_{4}$Ge$_{12}$ only a single
specific heat discontinuity is observed at $T_c$, in contrast to the second
jump seen in PrOs$_{4}$Sb$_{12}$ below $T_c$. These results suggest that
superconductivity in PrPt$_{4}$Ge$_{12}$ is characterized by a complex order
parameter.
