Yu He

Lead-free perovskites have emerged as environmentally benign alternatives to lead halide counterparts for photovoltaic and optoelectronic applications. Among them, the double perovskite Cs ₂ AgInCl ₆ family, with proper composition engineering, exhibits remarkable white-light emission characteristics enabled by strong electron-phonon coupling and the formation of self-trapped excitons (STEs). Despite these advantages, the fundamental photophysics and structural dynamics governing their excited-state behavior remain poorly understood. Here, we report a long-lived metastable phase in the Cs ₂ AgInCl ₆ double perovskite family and unravel this process and the concomitant electronic and structural evolution using a suite of tools including transient optical spectroscopy, time-resolved x-ray diffraction (TR-XRD), time-resolved x-ray absorption spectroscopy (TR-XAS), and inelastic x-ray scattering (IXS). We show that the photoinduced, transient metastable phase is associated with B-site [silver (Ag)–indium (In)] disorder, which induces a markedly reduced optical bandgap. Supported by TR-XRD and first-principles calculations, the Ag-In disorder drives the formation of Ag-rich and In-rich domains with millisecond lifetimes, with lifetimes increasing at lower temperatures. TR-XAS further reveals that the photogenerated STEs oxidize Ag ⁺ to Ag ²⁺ , which facilitates this highly temporally asymmetric order-disorder transition. Our findings demonstrate a previously unidentified mechanism, mediated by hole-localized STE formation, that enables prolongation of transient light-induced states to the multimillisecond regime in halide double perovskites, opening possibilities to harvesting the functional properties of metastable phases of these material systems.

Among the family of intercalated transition-metal dichalcogenides (TMDs), Fe_{x}NbS_{2} is found to possess unique current-induced resistive switching behaviors, tunable antiferromagnetic states, and a commensurate charge order, all of which are tied to a critical Fe doping of x_{c}=1/3. However, the electronic origin of such extreme stoichiometry sensitivities remains unclear. Combining angle-resolved photoemission spectroscopy (ARPES) with density functional theory (DFT) calculations, we identify and characterize a dramatic eV-scale electronic restructuring that occurs across the x_{c}. Moment-carrying Fe 3d_{z^{2}} electrons manifest as narrow bands within 200 meV of the Fermi level, distinct from other transition metal intercalated TMD magnets. These states strongly hybridize with itinerant electrons in the TMD layer and rapidly lose coherence above x_{c} due to correlation-driven effects. This sudden quasiparticle decoherence collapses the Fe-Nb hybridization, which explicitly suppresses the out-of-plane effective Fe-Fe exchange interaction, driving the transformation of the magnetic ground state from an antiferromagnetic stripe phase to a zigzag phase. These observations resemble the exceptional electronic and magnetic sensitivity of strongly correlated systems, and demonstrate that quantifying orbital-specific hybridization via ARPES offers an alternative pathway to evaluate effective magnetic exchange in metallic magnets, complementing inelastic neutron and resonant x-ray scattering probes.

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa ₂ Cu ₃ O _{7− δ} (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y ³⁺ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO’s superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+ U calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher–Rice (FR) and Liechtenstein–Mazin (LM) models, the low-energy electronic structure contains no signature of any f -electron hybridization or additional f -state Fermi surface sheets. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO ₂ bilayer decoupling and enhanced hopping along the CuO chain, implying indirect electron-release pathways beyond simple 4 f state ionization. Our results challenge the long-standing FR/LM mechanism, and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D–2D systems.

The authors challenge a long-standing view of Ta${}_{2}$NiSe${}_{5}$ as an excitonic insulator by revealing a nonexcitonic origin for its phase transitions. Using systematic density functional theory calculations across the Ta${}_{2}$Ni(Se,S)${}_{5}$ series, they show that structural distortions and charge doping account for the observed metal-insulator transitions. The calculations explain measured angle-resolved photoemission spectra with excellent accuracy, reconciling conflicting experimental observations and establishing a lattice-driven mechanism for the transitions in this system.

Dimensionality and electronic correlations are crucial elements of many quantum material properties. An example is the change of the electronic structure accompanied by the loss of quasiparticles when a metal is reduced from three dimensions to a lower dimension, where the Coulomb interaction between carriers becomes poorly screened. Here, using angle-resolved photoemission spectroscopy, we report an orbital-selective decoherence of spectral density in the perovskite nickelate LaNi O 3 toward the monolayer limit. The spectral weight of the d z 2 band vanishes much faster than that of the d x 2 − y 2 band as the thickness of the LaNi O 3 layer is decreased to a single unit cell, indicating a stronger correlation effect for the former upon dimensional confinement. Dynamical mean-field theory calculations show an orbital-selective Mott transition largely due to the localization of d z 2 electrons along the c axis in the monolayer limit. This orbital-selective correlation effect underpins many macroscopic properties of nickelates, such as metal-to-insulator transition and superconductivity, where most theories are built upon a d x 2 − y 2 – d z 2 two-band model.