Spin-dependent masses and field-induced quantum critical points

We discuss spin-dependent mass enhancements associated with field-induced quantum critical points in heavy-fermion systems. We have recently observed this phenomenon on a branch of the Fermi surface of CeRu2Si2 above its metamagnetic transition, complementing earlier work. In CeCoIn5, at high fields above a field-induced quantum critical point, we see a strong spin-dependence of the effective mass on the thermodynamically dominant sheets of the Fermi surface. These observations reinforce the suggestion that 'missing mass' in some cerium-based heavy-fermion systems will be found on heavy spin-polarised branches of the Fermi surface. In all cases where this phenomenon is observed the linear coefficient of specific heat is field dependent; however, CeCoIn5 seems to be the first such heavy-fermion system in which the f-electrons are definitely contributing to the Fermi volume, which puts it beyond the existing theory intended for metamagnetic systems.