Shubnikov-de Haas Effect measured on single crystals of CeOs$_4$Sb$_{12}$ and NdOs$_4$Sb$_{12}$ along the high symmetry directions

The filled skutterudite compounds CeOs$_4$Sb$_{12}$, PrOs$_4$Sb$_{12}$, and NdOs$_4$Sb$_{12}$ are respectively a 1~K antiferromagnetic (AFM) Kondo insulator, a 1.85~K unconventional superconductor, and a 1~K mean-field type ferromagnet (FM), suggesting that superconductivity in PrOs$_4$Sb$_{12}$ may result from proximity to AFM and FM quantum-critical points. Fermi-surface measurements of NdOs$_4$Sb$_{12}$ and CeOs$_4$Sb$_{12}$ could therefore give insights into the pairing mechanism. We have used a MHz skin-depth technique to observe Shubnikov-de Haas oscillations (SdHos) in single crystals of these materials at fields of up to 60~T. In CeOs$_4$Sb$_{12}$ for {\bf H} // [001], a previously-unobserved semimetal-to-metal transition was detected at $\approx 25$~T; above this, a series of SdHos with a frequency of 1700~T and $ m_{\rm CR} \approx 3.6 m_{\rm e}$ emerge. For {\bf H} // [011] in NdOs$_4$Sb$_{12}$, a single series of SdHos, frequency $\approx 874$~T, was found. These may correspond to the $\beta$ band in PrOs$_4$Sb$_{12}$, but with a much smaller $m_{\rm CR} \approx 1.5 m_{\rm e}$. Research at CSU-Fresno is supported by RC CCSA $\#$7669 and the start-up fund; at NHMFL by DOE, NSF, and FL.; at UCSD by NSF$\#$0802478 and US DOE DE FG02-04ER46105; at Hokkaido U by MEXT, Japan.