Entropy excess in strongly correlated Fermi systems near a quantum critical point

Abstract

A system of interacting, identical fermions described by standard Landau Fermi-liquid (FL) theory can experience a rearrangement of its Fermi surface if the correlations grow sufficiently strong, as occurs at a quantum critical point where the effective mass diverges. As yet, this phenomenon defies full understanding, but salient aspects of the non-Fermi-liquid (NFL) behavior observed beyond the quantum critical point are still accessible within the general framework of the Landau quasiparticle picture. Self-consistent solutions of the coupled Landau equations for the quasiparticle momentum distribution n(p) and quasiparticle energy spectrum ǫ(p) are shown to exist in two distinct classes, depending on coupling strength and on whether the quasiparticle interaction is regular or singular at zero momentum transfer. One class of solutions maintains the idempotency condition n 2 (p) = n(p) of standard FL theory at zero temperature T while adding pockets to the Paradoxically, this extraordinary solution carries the burden of a large temperature-dependent excess entropy down to very low temperatures, threatening violation of the Nernst Theorem. It is argued that certain low-temperature phase transitions, notably those involving Cooper-pair formation, offer effective mechanisms for shedding the entropy excess. Available measurements in heavy-fermion compounds provide concrete support for such a scenario.

Thermodynamic, Dynamic, and Transport Properties of Quantum Spin Liquid in Herbertsmithite from an Experimental and Theoretical Point of View

“…The shaded area reports the region, where the NFL state transits to weakly polarized LFL one. The temperature T * (B) of the transition is defined by the expression [45], and the NFL features arise from "traces" of the FC state manifested at finite temperatures. Also, at low but finite temperatures, the magnetic field acts to suppress NFL behavior (i.e., the "FC traces") and, on growing sufficiently strong, restores the LFL phase.…”

Topological basis for understanding the behavior of the heavy-fermion metalβ−YbAlB4under application of magnetic field and pressure

“…This degeneracy stimulates the onset of certain phase transitions and is thereby lifted before reaching T = 0, as required by the Nernst theorem. 27,36 With rising pressure (indicated by arrows in Fig. 6), the system enters the region P/P c > 1, where it is situated prior to the onset of the FCQPT and demonstrates LFL behavior at sufficiently low temperatures (shaded area in the figure).…”

Theory of Fermion Condensation as an Analog of the Liquid-Drop Theory of Atomic Nuclei

“…We propose that the Lifshitz topological phase transition associated with the formation of additional pockets of the Fermi surface is the precursor of fermion condensation.A theory of strongly correlated Fermi systems, called theory of fermion condensation, was created more than 20 years ago [1-3], (for recent articles, see e.g. [4][5][6][7][8][9][10][11]). A key feature of the phenomenon of fermion condensation is related to swelling the Fermi surface: in momentum space, there exists a domain Ω, in which the T = 0 single-particle spectrum ǫ(p) becomes completely flat:…”