Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

Abstract

Standard models for simple metals and insulators often fail for systems based on elements with unstable d-or f -electron shells, where strong electronic correlations can generate new and unexpected states of matter. Such a scenario can often be induced when a magnetic phase transition is tuned to absolute zero temperature by an external control parameter such as chemical composition, pressure or magnetic field. At the resulting quantum critical point (QCP), emergent phenomena, such as unconventional superconductivity and novel magnetic phases are frequently observed. The temperature and energy dependences of the physical properties are also found to deviate from expectations for a simple Fermi liquid. This "non-Fermi-liquid" (NFL) behavior is commonly manifested as weak power laws and logarithmic divergences in the physical properties at low temperatures and is often found in a V-shaped region near a QCP, which has become the "classic" QCP phase diagram. However, there is also a growing number of materials where the NFL behavior either occurs far away from the QCP, within an ordered phase, or may not be associated with any putative QCP. Thus, after nearly 20 years of research, it remains unknown whether NFL physics is universal, or if a multitude of unique subclasses exist. In this article, we review research that has primarily been carried out in our laboratory on systems that exhibit NFL behavior that does not conform to the "classic" QCP scenario.

Correlated Electron State inCe1−xYbxCoIn5Stabilized by Cooperative Valence Fluctuations

“…In this scenario, NFL behavior and unconventional SC coexist over a large part of the phase diagram, in contrast with what is expected for classical QCP systems for which the NFL behavior and unconventional SC are tightly confined to a ''V-shaped'' region around the QCP. We conclude that Ce 1Àx Yb x CoIn 5 belongs to a growing class of systems in which the NFL behavior occurs in the absence of an obvious QCP [4]. These results also suggest that VFs may play a role in the unconventional SC and NFL behavior in pure CeCoIn 5 .…”

“…It is generally thought that critical fluctuations associated with a magnetic quantum critical point (QCP), where a second-order magnetic phase transition is suppressed to 0 K by an external control parameter [e.g., composition (x), pressure (P), or magnetic field (H)], can provide a mechanism for NFL behavior and unconventional SC in a narrow ''dome'' around the QCP [1] (e.g., CeIn 3 , CePd 2 Si 2 , and CeCu 6Àx Au x ) [2,3]. However, the precise nature of the relationship between these phenomena remains to be understood, particularly since many compounds have been reported where the NFL behavior persists over an extended region of the phase diagram in the absence of any identifiable QCP [4].…”

Contiguous3dand4fMagnetism: Strongly Correlated3dElectrons inKhuntia1, Peratheepan2, Strydom3 et al. 2014Phys. Rev. Lett.17071View full textAdd to dashboardCite

“…If the RKKY spin exchange succeeds in overcoming the thermal energy of the spin system conducive to a paramagnetic-to-AFM transition, the addition of a competing Kondo spin exchange with the conduction electrons achieves a curbing effect on the phase transition. Moreover, under favorable conditions such as applied pressure or magnetic field the phase transition may become confined to temperatures arbitrarily close to zero, which in turn leads to remarkable thermal scaling in the realm of quantum criticality [3][4][5][6][7]. In exceptional cases quantum criticality presents itself under ambient conditions, such as in U 2 Pt 2 In [8,9] or in the superconductor β-YbAlB 4 [10].…”

Non-Fermi-liquid behavior from dynamical effects of impurity scattering in correlated Fermi liquids

“…1,2 Most theoretical scenarios for non-Fermi-liquid (nFL) behavior require the proximity of some kind of singularity in the phase diagram, such as a quantum critical point (QCP), [3][4][5] Griffiths singularities (GS) close to magnetic instabilities, 6 or van Hove singularities. 7 Other proposals for nFL include distribution of Kondo scales 8 and multichannel Kondo effect.…”

“…1,2,10,11 For example, in a recent study of the heavy fermion material Ce 1−p La p B 6 , 12 a resistivity of the form ρ(T ) = ρ 0 + AT α was found for p 0.4, where the nonuniversal and fractional exponent α, was found to be p dependent. In the FL regime (p 0.4), the coefficient of the quadratic term of the resistivity apparently diverges at the crossover from the FL to the nFL regime.…”