Quasiparticles beyond the Fermi liquid and heavy fermion criticality

We give a self-consistent theory of the scale-dependent effective mass enhancement m*/m of quasiparticles by three-dimensional (3D) antiferromagnetic (AFM) spin fluctuations in the presence of disorder at an AFM quantum critical point. The coupling of fermionic and bosonic degrees of freedom in the critical regime is described in terms of a critical quasiparticle theory. Using the fact that even in the non-Fermi liquid regime the quasiparticle width does not exceed the quasiparticle energy, we adopt relations from Fermi liquid theory to determine the dependence of the spin fluctuation spectrum on m*/m, from which the self-energy and hence m*/m may be calculated. The self-consistent equation for m*/m has a strong coupling solution provided the initial value is sufficiently large. We argue that in YbRh2Si2, quasi-2D AFM and/or 3D ferromagnetic Gaussian fluctuations existing over a wide range drive the system into the 3D strongly coupled fluctuation regime. We find critical exponents of the temperature dependence of the specific heat coefficient gamma proportional to T-1/4 and of the resistivity rho(T) = rho(0) + AT(3/4) in good agreement with experiments on YbRh2Si2 in the temperature range T < 0.3 K.