non-Fermi Liquid Phenomena Research Articles

Gifts from anomalies: Exact results for Landau phase transitions in metals

Non-Fermi liquid phenomena arise naturally near critical points of Landau ordering transitions in metallic systems, where strong fluctuations of a bosonic order parameter destroy coherent quasiparticles. Despite progress in developing controlled perturbative techniques, much of the low energy physics of such metallic quantum critical points remains poorly understood. We demonstrate that exact, non-perburbative results can be obtained for both optical transport and static susceptibilities in “Hertz-Millis” theories of Fermi surfaces coupled to critical bosons. Such models possess a large emergent symmetry and anomaly structure, which we leverage to fix these quantities. In particular, we show that in the infrared limit, the boson self energy at zero wave vector, \mathbf{q}=0𝐪=0, is a constant independent of frequency, and the real part of the optical conductivity, \sigma(\omega)σ(ω), is purely a delta function Drude peak with no other corrections. Therefore, further frequency dependence in the boson self energy or optical conductivity can only come from irrelevant operators in a clean system. Exact relations between Fermi liquid parameters as the critical point is approached from the disordered phase are also obtained. The absence of a universal, power law frequency dependence in the boson self energy contrasts with previous perturbative calculations, and we explain the origin of this difference.

Disorder versus two transport lifetimes in a strongly correlated electron liquid

We report on angle-dependent measurements of the sheet resistances and Hall coefficients of electron liquids in SmTiO3/SrTiO3/SmTiO3 quantum well structures, which were grown by molecular beam epitaxy on (001) DyScO3. We compare their transport properties with those of similar structures grown on LSAT [(La0.3Sr0.7)(Al0.65Ta0.35)O3]. On DyScO3, planar defects normal to the quantum wells lead to a strong in-plane anisotropy in the transport properties. This allows for quantifying the role of defects in transport. In particular, we investigate differences in the longitudinal and Hall scattering rates, which is a non-Fermi liquid phenomenon known as lifetime separation. The residuals in both the longitudinal resistance and Hall angle were found to depend on the relative orientations of the transport direction to the planar defects. The Hall angle exhibited a robust T2 temperature dependence along all directions, whereas no simple power law could describe the temperature dependence of the longitudinal resistances. Remarkably, the degree of the carrier lifetime separation, as manifested in the distinctly different temperature dependences and diverging residuals near a critical quantum well thickness, was completely insensitive to disorder. The results allow for a clear distinction between disorder-induced contributions to the transport and intrinsic, non-Fermi liquid phenomena, which includes the lifetime separation.

HEAVY FERMIONS. Strange metal without magnetic criticality.

A fundamental challenge to our current understanding of metals is the observation of qualitative departures from Fermi liquid behavior. The standard view attributes such non-Fermi liquid phenomena to the scattering of electrons off quantum critical fluctuations of an underlying order parameter. Although the possibility of non-Fermi liquid behavior isolated from the border of magnetism has long been speculated, no experimental confirmation has been made. Here, we report on the observation of a strange metal region away from a magnetic instability in an ultrapure single crystal. In particular, we show that the heavy-fermion superconductor β-YbAlB4 forms a possible phase with strange metallic behavior across an extensive pressure regime, distinctly separated from a high-pressure magnetic quantum phase transition by a Fermi liquid phase.

Non-Fermi liquid phenomena and intermediate valence in Yb-based compounds located close to the quantum critical point

In this paper, we focus on the mechanism of non-Fermi liquid phenomena in the vicinity of the magnetic field-tuned quantum critical point in Yb-based heavy fermion compounds YbAuCu 4, YbRh 2Si 2 and YbFe 4P 12. A comparative study of YbAuCu 4 and YbPdCu 4 with differently magnetically ordered ground states of antiferromagnetic (AFM) and ferromagnetic (FM) indicates that the competition between the field-destabilized AFM and field-stabilized FM correlations plays an important role on the field-tuned quantum critical behavior. The non-integer valence of these compounds leads to a consideration that the experimental susceptibility is dominated by the weakly broadened 4f hole level above the Fermi sea that extends to the cf hybridized band located at the Fermi level. Whilst, the monotonic increase of T 0 (an estimate of Kondo temperature) with field, derived from the transport and spin-fluctuation measurements, is ascribed to the variation of conduction bands from a weakly cf hybridized band toward the originally underlying conduction bands. The important role of the crossover transition upon cooling from the mixed valence localized f hole state to an itinerant electron state with the intermediate valence is also suggested.

On non-Fermi liquid quantum critical points in heavy fermion metals

Heavy electron metals on the verge of a quantum phase transition to magnetism show a number of unusual non-Fermi liquid properties which are poorly understood. This article discusses in a general way various theoretical aspects of this phase transition with an eye toward understanding the non-Fermi liquid phenomena. We suggest that the non-Fermi liquid quantum critical state may have a sharp Fermi surface with power law quasiparticles but with a volume not set by the usual Luttinger rule. We also discuss the possibility that the electronic structure change associated with the possible Fermi surface reconstruction may diverge at a different time/length scale from that associated with magnetic phenomena.

The local quantum critical point and non-Fermi liquid properties

Quantum criticality provides a means to understand the apparent non-Fermiliquid phenomena in correlated electron systems. How to properly describequantum critical points (QCPs) in electronic systems has however been poorlyunderstood. The issues have become particularly well defined due to recentexperiments on heavy-fermion metals, in which QCPs have been explicitlyidentified. In this paper, I summarize some recent theoretical work on the subject,with an emphasis on the notion of ‘local quantum criticality’. I describe themicroscopic work based on an extended dynamical mean-field theory,as well as Ginzburg–Landau arguments for the robustness of the localQCP beyond the microscopics. I also present the consequences of thispicture for inelastic neutron scattering, NMR, Fermi surface properties, andthe Hall coefficient, and compare them with the available experiments.Some analogies with the Mott transition phenomena are also noted.