Heavy-quasiparticle phenomena in metals and semimetals

Heavy-quasiparticle excitations, a hallmark of heavy-fermion or heavy-electron systems require the existence of low-energy scales. As regards the origin of these scales, our understanding has considerably improved over the last few years. For a long time it was believed that the Kondo effect is the sole route to heavy-electron excitations. But more recently two others were discovered: One applies to Nd2-xCexCuO4 and is based on the Zeeman effect; the other-one applies to the semimetal Yb,As,. Here one is dealing with nearly one-dimensional Hubbard chains close to half filling. The system is a good example for an almost perfect decoupling between spin and charge degrees of freedom. It is shown that it is essentially a neutral heavy-Fermi liquid in distinction to, e.g., CeAl3 or CeCu2Si2 where the heavy quasiparticles carry a charge.