Since the discovery by Steglich et al. (1979) of superconductivity in the high-effective-mass ($\sim 200m_e$) electrons in Ce${\mathrm{Cu}}_2$${\mathrm{Si}}_2$, the search for and characterization of such "heavy-fermion" systems has been a rapidly growing field of study. The eight heavy-fermion systems known to date include superconductors (Ce${\mathrm{Cu}}_2$${\mathrm{Si}}_2$, U${\mathrm{Be}}_{13}$, U${\mathrm{Pt}}_3$), magnets (Np${\mathrm{Be}}_{13}$, ${\mathrm{U}}_2$${\mathrm{Zn}}_{17}$, U${\mathrm{Cd}}_{11}$), and materials in which no ordering is observed (Ce${\mathrm{Al}}_3$, Ce${\mathrm{Cu}}_6$). These $f$-electron materials have, in comparison to normal metals, enormous specific heat $\gamma$ values (450-1600 mJ/mol ${\mathrm{K}}^2$), large values of the low-temperature magnetic susceptibility $\chi$ (8-50×${10}^{-3\mathrm{}}$ emu/mol G), maxima in the resistivity at low temperatures with large ${\rho }_{max}$ values (100-200 μΩ cm), and unusual temperature dependences of their specific heats below 10 K. The three heavy-fermion superconductors show such unusual behavior that the possibility of $p$-wave pairing of the superconducting electrons, rather than the usual BCS $s$-wave pairing, cannot be ruled out. This paper reviews the experimental results to date, to serve both as a status report and as a starting point for future research. Several correlations between properties are pointed out, including the observation that a low value of the Wilson ratio ($\sim \frac{\chi }{\gamma }$) appears to correlate with the occurrence of superconductivity.

DOI:https://doi.org/10.1103/RevModPhys.56.755