Low temperature normal and superconducting state properties of lightly doped PrOs$_{4}$Sb$_{12}$

The compound PrOs$_{4}$Sb$_{12}$ is a heavy fermion superconductor, the first one based on Pr, with a superconducting transition temperature $T_{c} = 1.85$ K. It has many interesting properties at low temperatures: multiple superconducting phases, apparently with nodes in the energy gap; possible time reversal symmetry breaking; and a High Field Ordered Phase (HFOP), believed to be due to antiferroquadrupolar ordering, in the normal state above $4.5$ T. When Ru is substituted for Os to form Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$, the multiple superconducting transitions observed in the specific heat seem to disappear, the gap structure may change, and the HFOP is drastically reduced. However, these previous results only included a minimum Ru substitution of $x = 0.05$. In this talk, we will present new measurements of the specific heat, magnetic susceptibility, and electrical resistivity of Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$ for $x \leq 0.05$ at low temperatures and high magnetic fields. This research was supported by the U.S. DOE (DE-FG02-04ER-46105) and the NSF (DMR-03-35173).