Nature of the quantum critical point as
disclosed by extraordinary behavior of magnetotransport and the
Lorentz number in the heavy-fermion metal YbRh$_2$Si$_2$
Abstract:
Physicists are engaged in vigorous debate on
the nature of the quantum critical points (QCP) governing the
low-temperature properties of heavy-fermion (HF) metals.
Recent experimental observations of the much-studied compound
YbRh$_2$Si$_2$ in the regime of vanishing temperature
incisively probe the nature of its magnetic-field-tuned
QCP. The jumps revealed both in the residual resistivity $\rho_0$
and the Hall resistivity $R_{\mathrm H}$, along with violation of the
Wiedemann–Franz law, provide vital clues to the origin of such
non-Fermi-liquid behavior. The empirical facts point unambiguously
to association of the observed QCP with a fermion-condensation phase
transition. Based on this insight, the resistivities $\rho_0$ and $R_{\mathrm H}$
are predicted to show jumps at the crossing of the QCP produced
by application of a magnetic field, with attendant violation of the
Wiedemann–Franz law. It is further demonstrated that experimentally
identifiable multiple energy scales are related to the scaling
behavior of the effective mass of the quasiparticles responsible
for the low-temperature properties of such HF metals.