Heavy-fermion systems

Since the discovery by Steglich et al. (1979) of superconductivity in the high-effective-mass ($\ensuremath{\sim}200{m}_{e}$) 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 $\ensuremath{\gamma}$ values (450-1600 mJ/mol ${\mathrm{K}}^{2}$), large values of the low-temperature magnetic susceptibility $\ensuremath{\chi}$ (8-50\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ emu/mol G), maxima in the resistivity at low temperatures with large ${\ensuremath{\rho}}_{max}$ values (100-200 \ensuremath{\mu}\ensuremath{\Omega} 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 ($\ensuremath{\sim}\frac{\ensuremath{\chi}}{\ensuremath{\gamma}}$) appears to correlate with the occurrence of superconductivity.