Heavy-fermion systems

Since the discovery by Steglich et al. (1979) of superconductivity in the high-effective-mass (~200mₑ) electrons in CeCu₂{Si}₂$, the search for and characterization of such "heavy-fermion" systems has been a rapidly growing field of study. The eight heavy-fermion systems known to date include superconductors (Ce${Cu}₂Si₂, UBe₁₃, UPt₃), magnets (NpBe₁₃, U₂{Zn}₁₇$, U${Cd}₁₁$), and materials in which no ordering is observed (Ce${Al}₃$, Ce${Cu}₆$). These $f$-electron materials have, in comparison to normal metals, enormous specific heat ${γ}$ values (450-1600 mJ/mol ${K}²$), large values of the low-temperature magnetic susceptibility ${χ}(8-50×{10}^{{-}3}$ emu/mol G), maxima in the resistivity at low temperatures with large ${{ρ}}ₘₐₓ$ values (100-200 {μ}{Ω} 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 (~χγ) appears to correlate with the occurrence of superconductivity.