Experimental Manifestations of Fermion Condensation in Strongly Correlated Fermi Systems

Many strongly correlated Fermi systems including heavy-fermion (HF) metals and high-Tc superconductors belong to that class of quantum many-body systems for which the Landau-Fermi liquid theory fails. Instead, these systems exhibit non-Fermi-liquid properties that arise from violation of time-reversal (T) and particle-hole (C) invariance. Here we consider two most recent experimental puzzles, which cannot be explained neither within the Landau-Fermi liquid picture nor can they be made intelligible by the approaches like the Hubbard model and/or the Kondo effect, which are commonly used to spell out the typical non-Fermi-liquid behavior. The first experimental puzzle is the asymmetric (with respect to bias voltage V) tunneling conductance (more specifically differential conductivity dI=dV, where I is the current) of HF metals like CeCoIn5 and YbRh2Si2 and Leggett theorem violation in overdoped copper HTSC oxides. The second puzzle is strange properties of geometrically frustrated 2D magnets like herbertsmithite ZnCu3(OH)6Cl(2), of which unusual properties are related to the emergence of so-called quantum spin liquid formed from fermionic spinons - the quasiparticles which substitute ordinary bosonic magnons in geometrically frustrated substances. It turns out that in both above classes of compounds, the background Fermi liquid (quantum spin liquid in geometrically frustrated magnets and electron liquid in other substances) is considered to undergo a transformation that renders a portion of its excitation spectrum dispersionless, giving rise to so-called flat bands. The presence of a flat band indicates that the system is close to a special quantum critical point, namely a topological fermion-condensation quantum phase transition. An essential aspect of the behavior of a system hosting a flat band is that application of a magnetic field restores its normal Fermi-liquid properties, including T- and C-invariance, thus removing above non-Fermi-liquid anomalies.