Key words Heavy fermions, quantum critical point, spin fluctuations, YbRh2Si2.
This article is dedicated to Dieter Vollhardt on the occasion of his 60th birthday.Landau quasiparticle (qp) theory may be extended into the non-Fermi liquid regime of metallic compounds near a quantum critical point (QCP), provided the width of the quasiparticle states is less than their energy. Sufficiently far from the QCP, when the critical fluctuations are non-interacting, their influence on the qp properties may be calculated in a controlled way. A prominent example is the 'marginal Fermi liquid,' characterized by a logarithmic variation of the specific heat coefficient and a linear variation of the resistivity with temperature. The latter is induced by the interaction of qps with three-dimensional ferromagnetic (FM) or, in a disordered system, with quasi-two-dimensional antiferromagnetic (AFM) spin fluctuations. It is argued that both types of fluctuations are present in YbRh2Si2. It is suggested that the experimentally observed crossover line at T * marks the changeover from AFM to FM fluctuations with increasing magnetic field. The non-Fermi liquid behavior found in the electron spin resonance data on YbRh2Si2 may be completely interpreted using critical quasiparticle theory.