How Do Heavy Fermions Get Polarized And Die?

In paramagnetic heavy fermion systems the f-spins dissolve into Kondo singlets and reappear within the Fermi volume, producing a ``large" Fermi surface populated by heavy quasiparticles. According to theory, when a very large magnetic field is applied to such a system the Kondo singlets are broken and the fully polarized bare f-spins vanish from the Fermi volume, leaving behind a ``small" Fermi surface populated by light quasiparticles. How the system passes from the low-field to the high-field limit is not clear. This talk will discuss recent transport and de Haas van Alphen studies of the archetypal heavy fermion systems $\rm CeRu_2Si_2$ [1] and $\rm YbRh_2Si_2$ [2], which are interpreted as showing that the f-electron disappears from the Fermi volume via two successive Lifshitz transitions: in the first transition a majority spin band sinks below the Fermi level, while in the second a new minority spin band appears at the Fermi level. While this interpretation is in accord with recent theoretical work of Kusminskiy et al. [3], it could be criticized on the grounds that only the first of the two postulated Lifshitz transitions have so far been observed.\\[4pt] References:\\[0pt] [1] R. Daou, C. Bergemann and S.R. Julian, ``Continuous evolution of the Fermi surface of $\rm CeRu_2Si_2$ across the metamagnetic transition,'' Physical Review Letters {\bf 96} (2006) 026401.\\[0pt] [2] P.M.C. Rourke, A. McCollam, G. Lapertot, G. Knebel, J. Flouquet and S.R. Julian, ``Magnetic field dependence of the $\rm YbRh_2Si_2$ Fermi surface,'' arXiv:0807.3970; accepted, Physical Review Letters.\\[0pt] [3] S. Viola-Kusminskiy, K.S.D. Beach, A.H. Castro Neto and D.K. Campbell, ``Mean-field study of the heavy fermion metamagnetic transition,'' Physical Review B {\bf 77} (2008) 094419.