Quantum criticality in heavy-fermion metals

Abstract

Quantum criticality describes the collective fluctuations of matter undergoing a second-order phase transition at zero temperature. Heavy fermion metals have in recent years emerged as prototypical systems to study quantum critical points.There have been considerable efforts, both experimental and theoretical, which use these magnetic systems to address problems that are central to the broad understanding of strongly correlated quantum matter. Here, we summarize some of the basic issues, including i) the extent to which the quantum criticality in heavy fermion metals goes beyond the standard theory of order-parameter fluctuations, ii) the nature of the Kondo effect in the quantum critical regime, iii) the non-Fermi liquid phenomena that accompany quantum criticality, and iv) the interplay between quantum criticality and unconventional superconductivity.In the classical world, matter in equilibrium freezes at absolute zero temperature. A macroscopic collection of microscopic particles adopt a stationary arrangement, forming an ordered pattern, to minimize the potential energy. Quantum mechanics, on the other hand, allows fluctuations even at zero temperature. The effective strength of such zero-point motion can be tuned through the variation of a non-thermal control parameter, such as applied pressure. When such quantum fluctuations become sufficiently strong, the system undergoes a quantum phase transition to a new ground state.As simple as this sounds, quantum phase transitions are not easy to achieve. Consider, for example, the case of ice. Anybody who has skated would appreciate the fact that the melting temperature of ice is reduced by pressure. If the melting temperature were forced to vanish at a sufficiently high pressure, a quantum phase transition would take place at this pressure. However, applying pressure larger than about 0.2 GPa to ice actually makes the melting temperature go up again.

Electronic and magnetic phase diagram of β-Fe1.01Se with superconductivity at 36.7 K under pressure

“…near 5.1 GPa , Fig 2(c)). A T-linear resistivity is frequently taken as a fingerprint of a "noncanonical Fermi liquid system", as known for high T c cuprates [18] or in heavy Fermion metals [19,20]. This unusual linear temperature dependency of ρ(T) was also recently observed in the iron arsenides [21].…”

Holographic antiferromagnetic quantum criticality andAdS2scaling limit

“…Usually, the control parameter could be the composition, magnetic field, or pressure, etc. In condensed matter physics, such a quantum criticality is considered to be an important mechanism for some of the most interesting phenomena, especially in itinerant electronic systems [5,6] and other phenomenona involving strongly correlated many-body systems [7,8]. However, the complete theoretical descriptions valid at all the energy (or temperature) regions are still lacking.…”

Thermoelectric power investigations of YbAgGe across the quantum critical point

“…1 In the quantum critical regime, these systems can manifest non-Fermi-liquid ͑nFL͒ behavior: the exponent of electrical resistivity, ⌬ = AT n , has n Ͻ 2 and the electronic specific heat coefficient, ␥ = C͑T͒ / T ͉ T→0 , is either singular, so the effective mass diverges in the zero temperature limit, C͑T͒ / T ϰ −log͑T͒, or has a nonanalytic dependence on temperature, so the effective mass is finite C͑T͒ / T ϰ − ͱ T. 2 Among SCES, nFL behavior near a quantum critical point ͑QCP͒ has explicitly been identified for heavy fermion ͑HF͒ metals such as CeCu 6−x Au x ͑Ref. 3͒ which becomes magnetic when the Au atom is replaced Cu site ͑x ϳ 0.1͒; CePd 2 Si 2 ͑Ref.…”