Strange-metal behaviour in a pure ferromagnetic Kondo lattice.

A wide range of metals exhibit anomalous electrical and thermodynamic properties when tuned to a quantum critical point (QCP), although the origins of such strange metals have posed a long-standing mystery. The frequent association of strange metals with unconventional superconductivity and antiferromagnetic QCPs1-4 has led to the belief that they are highly entangled quantum states5. By contrast, ferromagnets are regarded as an unlikely setting for strange metals, because they are weakly entangled and their QCPs are often interrupted by competing phases or first-order phase transitions6-8. Here we provide evidence that the pure ferromagnetic Kondo lattice9,10 CeRh6Ge4 becomes a strange metal at a pressure-induced QCP. Measurements of the specific heat and resistivity under pressure demonstrate that the ferromagnetic transition is continuously suppressed to zero temperature, revealing a strange-metal behaviour around the QCP. We argue that strong magnetic anisotropy has a key role in this process, injecting entanglement in the form of triplet resonating valence bonds into the ordered ferromagnet. We show that a singular transformation in the patterns of the entanglement between local moments and conduction electrons, from triplet resonating valence bonds to Kondo-entangled singlet pairs at the QCP, causes a jump in the Fermi surface volume-a key driver of strange-metallic behaviour. Our results open up a direction for research into ferromagnetic quantum criticality and establish an alternative setting for the strange-metal phenomenon. Most importantly, strange-metal behaviour at a ferromagnetic QCP suggests that quantum entanglement-not the destruction of antiferromagnetism-is the common driver of the varied behaviours of strange metals.

Emergent Spinon Dispersion and Symmetry Breaking in Two-Channel Kondo Lattices.

Two-channel Kondo lattice serves as a model for a growing family of heavy-fermion compounds. We employ a dynamical large-N technique and go beyond the independent bath approximation to study this model both numerically and analytically using renormalization group ideas. We show that the Kondo effect induces dynamic magnetic correlations that lead to an emergent spinon dispersion. Furthermore, we develop a quantitative framework that interpolates between infinite dimension where the channel-symmetry broken results of mean-field theory are confirmed, and one-dimension where the channel symmetry is restored and a critical fractionalized mode is found.

Triplet Resonating Valence Bond State and Superconductivity in Hund's Metals.

A central idea in strongly correlated systems is that doping a Mott insulator leads to a superconductor by transforming the resonating valence bonds (RVBs) into spin-singlet Cooper pairs. Here, we argue that a spin-triplet RVB (tRVB) state, driven by spatially, or orbitally anisotropic ferromagnetic interactions can provide the parent state for triplet superconductivity. We apply this idea to the iron-based superconductors, arguing that strong on site Hund's interactions develop intra-atomic tRVBs between the t_{2g} orbitals. On doping, the presence of two iron atoms per unit cell allows these interorbital triplets to coherently delocalize onto the Fermi surface, forming a fully gapped triplet superconductor. This mechanism gives rise to a unique staggered structure of on site pair correlations, detectable as an alternating π phase shift in a scanning Josephson tunneling microscope.

Emergent Critical Charge Fluctuations at the Kondo Breakdown of Heavy Fermions.

One of the challenges in strongly correlated electron systems is to understand the anomalous electronic behavior that develops at an antiferromagnetic quantum critical point (QCP), a phenomenon that has been extensively studied in heavy-fermion materials. Current theories have focused on the critical spin fluctuations and associated breakdown of the Kondo effect. Here we argue that the abrupt change in the Fermi surface volume that accompanies heavy-fermion criticality leads to critical charge fluctuations. Using a model one-dimensional Kondo lattice, in which each moment is connected to a separate conduction bath, we show that a Kondo breakdown transition develops between a heavy Fermi liquid and a gapped spin liquid via a QCP with ω/T scaling, which features a critical charge mode directly associated with the breakup of Kondo singlets. We discuss the possible implications of this emergent charge mode for experiments.

Model for a Ferromagnetic Quantum Critical Point in a 1D Kondo Lattice.

Motivated by recent experiments, we study a quasi-one-dimensional model of a Kondo lattice with ferromagnetic coupling between the spins. Using bosonization and dynamical large-N techniques, we establish the presence of a Fermi liquid and a magnetic phase separated by a local quantum critical point, governed by the Kondo breakdown picture. Thermodynamic properties are studied and a gapless charged mode at the quantum critical point is highlighted.

Observation of a critical charge mode in a strange metal.

Understanding the strange metallic behavior that develops at the brink of localization in quantum materials requires probing the underlying electronic charge dynamics. Using synchrotron radiation-based Mössbauer spectroscopy, we studied the charge fluctuations of the strange metal phase of ß-YbAlB4 as a function of temperature and pressure. We found that the usual single absorption peak in the Fermi-liquid regime splits into two peaks upon entering the critical regime. We interpret this spectrum as a single nuclear transition, modulated by nearby electronic valence fluctuations whose long time scales are further enhanced by the formation of charged polarons. These critical charge fluctuations may prove to be a distinct signature of strange metals.