Fermi-liquid instability at magnetic–nonmagnetic quantum phase transitions

In metals with strong electronic correlations such as heavy-fermion systems or itinerant-electron magnets it is possible to change from a magnetically ordered to a nonmagnetic groundstate by variation of an external parameter such as composition or pressure. In principle a transition between these groundstates can occur at zero temperature. In case of a continuous transition quantum fluctuations take the role of thermal fluctuations in finite-temperature transitions. The abundance of low-lying magnetic excitations leads in the vicinity of the quantum critical point to unusual behavior of thermodynamic and transport properties at low temperatures T not envisioned by the classical Fermi-liquid behavior that is observed even in strongly correlated electron systems away from the quantum phase transition. We discuss in detail a few examples of this `non-Fermi-liquid behavior', viz., CeCu 6− x Au x , Ce 1− x La x Ru 2Si 2, Ce 7Ni 3, CeCu 2Si 2 and CeCu 2Ge 2, CePd 2Si 2, and UCu 1− x Pd x . In CeCu 6− x Au x the very unusual low- T behavior of the linear specific-heat coefficient C/ T∼−ln( T/ T 0) and of the resistivity Δ ρ∼ T can be attributed to quasi-two-dimensional fluctuations as determined from inelastic neutron scattering. The systems CeCu 2Ge 2 and CePd 2Si 2 are particuarly interesting since here the magnetic order which is suppressed under hydrostatic pressure gives way to superconductivity, suggesting that spin fluctuations mediate the formation of Cooper pairs at least in the latter system.