Metals and Semimetals with Strongly Correlated Electrons

A characteristic feature of strongly correlated electrons is the appearance of low energy scales. They may result in heavy-quasiparticle excitations at low temperatures. Our understanding of these excitations has been considerably modified over the last few years. While it was originally thought that the Kondo effect is the sole source of heavy-fermion behavior, two additional mechanisms have been identified meanwhile. One is based on the Zeeman effect and occurs in Nd$\text{}_{2-x}$Ce$\text{}_{x}$CuO$\text{}_{4}$. The other is operative in the semimetal Yb$\text{}_{4}$As$\text{}_{3}$ and is based on nearly half filled one-dimensional Hubbard chains. In all three cases the low energy excitations result from spin degrees of freedom.