Ordering Phenomena in Heavy-Fermion Compounds

Abstract

The heavy-fermion systems are characterised by anomalies in thermodynamic and transport properties at temperatures typically below about 10 K. These anomalies indicate an effective mass of the electrons at the Fermi level, which is two orders of magnitude larger than the free-electron mass. Although the large effective mass is intriguing in itself, even more astonishing is the occurrence of superconductivity in the heavy-fermion compounds as well as the extremely large specific-heat anomalies at the superconducting transition temperature, indicating that heavy-mass electrons participate in the superconductivity. These features have given rise to suggestions of an exotic type of pair bonding between the electrons. In the familiar models of superconductivity the necessary attractive interaction between two electrons is provided by electron-phonon processes. The most favourable situation occurs for a symmetrical orbital function of the electron pair with opposite spins (s-wave or singlet pairing). Moreover, the Debye temperature that represents the range of phonon energies should be much smaller than the Fermi temperature of the electrons. In normal metals this condition is well fulfilled since the Fermi temperature is of the order of 10⁴ to 10⁵ K whereas the Debye temperature is a few hundred kelvin.