Heavy-fermion systems

TLDR

Heavy‑fermion systems, first identified in CeCu₂Si₂ with electrons of ~200 mₑ, comprise eight known compounds that include superconductors, magnets, and non‑ordered materials. This review aims to summarize current experimental findings on heavy‑fermion materials and to outline directions for future research. The authors compile and analyze experimental data from the eight known heavy‑fermion compounds. Heavy‑fermion compounds exhibit exceptionally large specific heat, magnetic susceptibility, and resistivity maxima, with correlations such as a low Wilson ratio indicating a tendency toward superconductivity and suggesting possible p‑wave pairing.

Abstract

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.