YbGaGe is an alloy of Ytterbium, Gallium and Germanium. It sparked interest because one group of researchers reported that it exhibits zero thermal expansion, while being conductive. Such materials have applications in space and other environments where low thermal expansion materials are required. Subsequent measurements by other groups were however not able to reproduce their results, but rather found expansion coefficients similar to copper.
Nevertheless, YbGaGe does show anomalous thermal expansion. Near zero thermal expansion was observed by Booth et al. It was found that near-zero thermal expansion (NZTE) in YbGaGe is sensitive to stoichiometry and defect concentration, however, the NZTE mechanism remains elusive 
- "Zero thermal expansion in YbGaGe due to an electronic valence transition" Letter by James R. Salvador, Fu Guo, Tim Hogan & Mercouri G. Kanatzidis in Nature 425, 702-705 (16 October 2003) | doi:10.1038/nature02011; Received 5 March 2003; Accepted 19 August 2003
- "Zero Expansion Conductor". October 20, 2003. p. 1. Retrieved May 23, 2011.
- "YbGaGe: normal thermal expansion"
- [Ytterbium divalency and lattice disorder in near-zero thermal expansion YbGaGe, Phys. Rev. B 75, 012301 – Published 3 January 2007, C. H. Booth, A. D. Christianson, J. M. Lawrence, L. D. Pham, J. C. Lashley, and F. R. Drymiotis]