  Several observed transport and thermodynamic properties of the heavy-fermion
compound YbRh2Si2 in the quantum critical regime are unusual and suggest that
the fermionic quasiparticles are critical, characterized by a scale-dependent
diverging effective mass. A theory based on the concept of critical
quasiparticles (CQP) scattering off antiferromagnetic spin fluctuations in a
strong-coupling regime has been shown to successfully explain the unusual
existing data and to predict a number of so far unobserved properties. In this
paper, we point out a new feature of a magnetic field-tuned quantum critical
point of a heavy-fermion metal: anomalies in the transport and thermodynamic
properties caused by the freezing out of spin-flip scattering of critical
quasiparticles and the scattering off collective spin excitations. We show that
a step-like behavior as a function of magnetic field of e.g. the Hall
coefficient and magnetoresistivity results, which accounts quantitatively for
the observed behavior of these quantities. That behavior has been described as
a crossover line T*(H) in the T - H phase diagram of YbRh2Si2. Whereas some
authors have interpreted this observation as signaling the breakdown of Kondo
screening and an associated abrupt change of the Fermi surface, our results
suggest that the T* line may be quantitatively understood within the picture of
robust critical quasiparticles.
