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The heavy-fermion compound YbRh₂Si₂ possesses a quantum critical point, at which the standard theory of electron behaviour in metals is expected to break down; such anomalous behaviour has now been observed.

Physicists have puzzled over a hidden electronic order in a uranium-based material for decades. A new theory attributes it to not just a single but a double breaking of time-reversal symmetry. See Article p.621

Strange metals exhibit anomalous electronic and thermodynamic properties near a quantum critical point. Here the authors show theoretically that entanglement quantified by quantum Fisher information is amplified in a strange metal at a quantum critical point, which is supported by analysis of experimental data.

The authors find low-energy magnetic excitations and a flat band near the Fermi level in kagome metal superconductor CsCr3Sb5 by angle-resolved photoemission and resonant inelastic X-ray scattering. They suggest that the flat band plays a role in the emergence of charge/magnetic order at low temperatures.

The controlled manipulation of the topological phases of electronic materials is a central goal of modern condensed matter research. Here, the authors demonstrate controllable switching between two distinct topological phases in a layered ferromagnet via thermal cycling.

Inelastic neutron scattering measurements show that superconductivity in UTe₂ is associated with a resonance near antiferromagnetic order that suggests an unexpected spin-singlet component to the electron pairing.

Transitions between stable quantum phases of matter typically involve going through an unstable quantum critical point, the unique properties of which have become a focus of research in the past decade or so. Extensive bulk measurements on the nickel oxypnictide system CeNiAsO uncover heavy-fermion behaviour, suggesting the family of oxipnictides may be ideal materials for examining quantum criticality more broadly.

At a zero-temperature phase transition from one ordered state to another, fluctuations between the two states lead to quantum critical behaviour that can lead to unexpected physics. Metals with ‘heavy’ electrons often harbour such weird states.

Strongly correlated topological materials are hard to identify. Now a design principle suggests a method for producing many topological metals where strong electron–electron interactions are a driving force.

The authors use inelastic neutron scattering to map out the spin excitations of FeSe dewtinned with a uniaxial-strain device. They establish a spin-interaction phase diagram and conclude that FeSe is close to a crossover region between the antiferroquadrupolar, Néel, and stripe ordering regimes.

Whether an actual Mott insulator phase exists in iron pnictides remains elusive. Here, Songet al. demonstrate an antiferromagnetic insulator phase persisting above the Néel temperature in NaFe_1−xCu_xAs, indicative of a Mott insulator, highlighting the role of electron correlations in high-T_csuperconductivity.

The strange-metal state that develops close to a quantum critical point in strongly correlated electron systems is not well understood. This Review summarizes how the notion of Kondo destruction can describe much of the experimental phenomenology.

Strong correlations may produce states of matter that do not have non-interacting counterparts, with new types of quantum criticality, superconductivity, and topological phases being recent highlights. This Review describes the physics underlying these correlated states and points to their potential for quantum applications.

Spectroscopic study on a kagome magnet thin film uncovers the local-moment nature of the magnetism in the presence of topological flat bands, as well as a strong spin- and orbital-selective electronic correlation effect.

An optical analysis reveals that the electronic correlations in the 'parent' compounds of the iron pnictide superconductors are sufficiently strong to significantly impede the mobility of the electrons.

Flat electronic bands can give rise to correlation-driven phases but for this, they need to be tuned to the Fermi level. Here the authors predict flat bands pinned at the Fermi level due to orbital-selective interactions and discuss implications for the design of topological Kondo semimetal in d-electron systems.

Observations of strong electron correlation effects have been mostly confined to compounds containing f orbital electrons. Now, the study of the 3d pyrochlore metal CuV₂S₄ reveals that similar effects can be induced by flat-band engineering.

Flat-band materials such as kagome and moiré lattices and strongly correlated electron systems including heavy-fermion compounds exhibit strikingly similar phenomena of topology and strong correlations. This Perspective article discusses Kondo physics as the underlying theme and a route to a unified understanding.

Iron pnictide and iron chalcogenide superconductors exhibit similar transition temperatures but markedly different electronic structure. Yu et al.suggest that this could be due to pairing being the strongest in the vicinity of a transition between localization and itineracy in both systems.

Manipulation of topology of the electronic structure is highly desirable for practical applications of topological materials. Here the authors demonstrate tuning and annihilation of Weyl nodes in momentum space by means of the Zeeman effect in a strongly correlated topological semimetal Ce₃Bi₄Pd₃.

The mechanism that drives nematic behaviour in iron-based superconductors is still unclear. Now, nematicity and anisotropy in spin excitations are shown to disappear at the same temperature, indicating that the transition is primarily spin-driven.

Pyrochlore iridates lie at a tuning-free magnetic quantum critical point hosting several complex exotic phenomena. Here, the authors discover an electronic phase separation in single crystalline Pr₂Ir₂O₇, where well-defined Kondo resonances are interweaved with a non-magnetic metallic phase with Kondo-destruction.

A phase of quantum-critical behaviour is observed in a kagome lattice material. This arises from the interplay of strong interactions between electrons, and the frustration that arises both from the interactions and the lattice geometry.

Iron-based superconductors display high transition temperatures. The physics behind the unconventional superconductivity of these systems can be investigated by taking into consideration the observed strong electronic correlations and bad-metal behaviour, the nature of their magnetic properties, and the presence of electronic nematicity and of quantum criticalities.

The underlying mechanism of iron-based superconductivity, the role of electron correlations, and the extent to which the behavior resembles those of the cuprates has been debated since their discovery. Here, using angle resolved photoemission spectroscopy, the authors report reconstruction of the Fermi surface for FeTe_1−xSe_x driven by orbital-dependent correlation effects in the absence of symmetry breaking and find evidence for an orbital-selective Mott transition.