Strongly correlated Fermi systems are among the most intriguing, best experimentally studied and fundamental systems in physics, representing a field never far from applications in synthesis of novel materials for cryogenics, rare earth magnets and applied superconductivity. Strongly correlated Fermi systems are, however, in defiance of theoretical understanding. The ideas based on the concepts like Kondo lattice and involving quantum and thermal fluctuations at a quantum critical point have been used to explain the unusual physics. Alas, these approaches fail to explain the scaling behavior of heavy-fermion (HF) metals. This means a real crisis in theory suggesting that there is a hidden fundamental law of nature. It turns out that the hidden fundamental law is well forgotten old one directly related to the Landau-Migdal quasiparticles, while the basic properties and the scaling behavior of HF metals can be described within the framework of the fermion condensation quantum phase transition (FCQPT) [1]. FCQPT comprises the extended quasiparticle paradigm that allows us to explain the non-Fermi liquid behavior (NFL) observed in strongly correlated Fermi systems. In contrast to the Landau paradigm stating that the quasiparticle effective mass is a constant, the effective mass of new quasiparticles strongly depends on temperature, magnetic field, pressure, and other parameters. To illustrate the above items, we study how transforms the phase diagram of the HF metal YbRh₂Si₂ under the application of negative/positive pressure. Then, we perform comprehensive theoretical analysis of high magnetic field behavior of YbRh₂Si₂. At low magnetic fields B, YbRh₂Si₂ demonstrates a quantum critical point connected to the suppression of antiferromagnetic ordering at a critical magnetic field B ortogonal c of B=B_c0≈0.06 T. At high magnetic fields B^*≈10 T and low temperatures, the suppression of the heavy fermion state takes place. Calculating the thermodynamic properties of YbRh₂Si₂ at magnetic fields B from B_c0 to high fields B^* allowed us to straddle a possible metamagnetic transition region and probe the low-field HF liquid and high-field fully polarized one. At large fields B~B^* the corresponding quasiparticle band becomes fully polarized generating the Landau Fermi liquid state and the suppression of the HF state, while at elevating temperatures both the HF state and the corresponding NFL behavior are restored. Our observations are in good agreement with experimental facts and show that FCQPT is responsible for the observed NFL behavior and quasiparticles survive both high temperatures and high magnetic fields.
    [1]. V.R. Shaginyan, M.Ya. Amusia, A.Z. Msezane, and K.G. Popov, Physics Reports 492, 31 (2010).