Quantum control of the many-body wavefunction is a central challenge in quantum materials research, as it could yield a precise control knob to manipulate emergent phenomena. Floquet engineering, the coherent dressing of quantum states with periodic non-resonant optical fields, has become an important strategy for quantum control. Most applications to solid-state systems have targeted weakly interacting or single-ion states, leaving the manipulation of many-body wavefunctions largely unexplored. Here, we use Floquet engineering to achieve quantum control of a strongly correlated Hubbard exciton in the one-dimensional Mott insulator Sr$_2$CuO$_3$. A nonresonant midinfrared optical field coherently dresses the exciton wavefunction, driving its rotation between bright and dark states. We use resonant third-harmonic generation to quantify ultrafast $π/2$ rotations on the Bloch sphere spanned by these exciton states. Our work advances the quest towards programmable control of correlated states and exciton-based quantum sensing.

We report the observation of an emergent charge order mode in the optically excited cuprate ladder Sr_{14}Cu_{24}O_{41}. Near-infrared light in the ladder plane drives a symmetry-protected electronic metastable state together with a partial melting of the equilibrium charge order. Our time-resolved resonant inelastic x-ray scattering measurements at the upper Hubbard band reveal a collective excitation dispersing from the charge order wave vector up to 0.8 eV with a slope on the order of the quasiparticle velocity. These findings reveal a regime where correlated carriers acquire itinerant character at finite momentum, and charge order becomes dynamically fluctuating, offering a platform to explore light-induced pairing instabilities.

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of high-temperature superconductivity. However, experimental investigations have largely been limited to the infinite-layer RNiO2 (R, rare earth) nickelates. We constructed a phase diagram of multilayer square-planar Ndn+1NinO2n+2 compounds and found signatures of superconductivity for dimensionality n = 4 to 8. Upon decreasing n, the superconducting anisotropy evolves owing to 4f electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the overdoped, nonsuperconducting regime. The superconducting regime overlaps with that of chemically doped infinite-layer nickelates, demonstrating underlying commonalities as well as differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

By laser pump-probe time-resolved coherent magnetic x-ray diffraction imaging, we have measured the migration velocity of antiferromagnetic domain walls in the Mott insulator Sr2IrO4 at 100 K. Coherent diffraction patterns, recorded at the 106 pure-magnetic Bragg peak, showed fringes, which responded distinctly to the excitation laser. These patterns could not be inverted with current phasing algorithms, but could be modeled as four domains with phase shifts consistent with the antiferromagnetic structure. During the laser-induced demagnetization, we observed these domains and their domain walls moving at 3 & times; 106 m/s, significantly faster than acoustic velocities. This is understood to arise from a purely electronic spin contribution to the magnetic structure without any role for coupling to the crystal lattice.

We probe the density-wave transition of the trilayer nickelate La_{4}Ni_{3}O_{10} with polarization-resolved infrared spectroscopy. The low-energy electrodynamics is strongly anisotropic, with metallic in-plane and insulating out-of-plane character. In the ordered phase, the anisotropy grows more than an order of magnitude as the out-of-plane conductivity is sharply suppressed. We interpret this enhancement as an effective electronic decoupling of the Ni-O layers driven by a spin-density-wave-induced redistribution of Ni-d_{z^{2}} occupation within the trilayers. This electronic response is accompanied by clearly shifting and splitting out-of-plane phonons, compatible with a density-wave instability of electronic origin.

We present the first application of simulation-based inference to resonant inelastic X-ray scattering spectroscopy. Using truncated marginal neural ratio estimation to efficiently restrict the prior and conditional flow matching as the joint density estimator, we infer full posteriors with a modest simulation budget for two Ni^2+ compounds—NiPS_3 as a representative covalent case and K_2NiF_4 as a more atomic one. We demonstrate that a vision transformer encoder whose tokenization matches the physical layout of the RIXS map yields better-covered and sharper posteriors than generic image encoders. Applying the validated method to experimental NiPS_3 and K_2NiF_4 data, we recover a joint posterior that reveals parameter correlations invisible to point estimators, and a posterior predictive distribution that closely matches the observed spectrum. The amortized posterior unlocks a class of analyses not previously available to the field such as nuisance-marginalized uncertainty quantification, multi-measurement posterior fusion and active experimental design.

Intertwined charge and spin correlations are ubiquitous in a wide range of transition metal oxides and are often perceived as intimately related to unconventional superconductivity. Theoretically envisioned as driven by strong electronic correlations, the intertwined order is usually found to be strongly coupled to the lattice as signaled by pronounced phonon softening. Recently, both charge/spin density waves (CDW/SDW) and superconductivity have been discovered in several Ruddlesden-Popper (RP) nickelates, in particular trilayer nickelates RE4Ni3O10 (RE=Pr, La). The nature of the intertwined order and the role of lattice-charge coupling are at the heart of the debate about these materials. Using inelastic X-ray scattering, we mapped the phonon dispersions in RE4Ni3O10 and found no evidence of phonon softening near the CDW wavevector over a wide temperature range. Calculations of the electronic susceptibility revealed a peak at the observed SDW ordering vector but not at the CDW wavevector. The absence of phonon softening is in sharp contrast to that in canonical oxide materials, notably cuprates. Our experimental and theoretical findings highlight the crucial role of the spin degree of freedom and establish a foundation for understanding the interplay between superconductivity and density-wave transitions in RP nickelate superconductors and beyond.

Optically excited quantum materials exhibit nonequilibrium states with remarkable emergent properties, but these phenomena are usually transient, decaying on picosecond timescales and limiting practical applications. Advancing the design and control of nonequilibrium phases requires the development of targeted strategies to achieve long-lived, metastable phases. Here, we report the discovery of symmetry-protected electronic metastability in the model cuprate ladder Sr_14Cu_24O_41. Using femtosecond resonant x-ray scattering and spectroscopy, we show that this metastability is driven by a transfer of holes from chain-like charge reservoirs into the ladders. This ultrafast charge redistribution arises from the optical dressing and activation of a hopping pathway that is forbidden by symmetry at equilibrium. Relaxation back to the ground state is hence suppressed after the pump coherence dissipates. Our findings highlight how dressing materials with electromagnetic fields can dynamically activate terms in the electronic Hamiltonian, and provide a rational design strategy for nonequilibrium phases of matter.

Magnetic interactions are thought to play a key role in the properties of many unconventional superconductors, including cuprates, iron pnictides, and square-planar nickelates. Superconductivity was also recently observed in the bilayer and trilayer Ruddlesden-Popper nickelates, the electronic structure of which is expected to differ from that of cuprates and square-planar nickelates. Here we study how electronic structure and magnetic interactions evolve with the number of layers, n, in thin film Ruddlesden-Popper nickelates Ndn+1NinO3n+1 with n = 1, 3, and 5 using resonant inelastic x-ray scattering (RIXS). The RIXS spectra are consistent with a high-spin |3d8L) electronic configuration, resembling that of La2-xSrxNiO4 and the parent perovskite, NdNiO3. The magnetic excitations soften to lower energy in the structurally self-doped, higher-n films. Our observations confirm that structural tuning is an effective route for altering electronic properties, such as magnetic superexchange, in this prominent family of materials.

Many-body excitons in CrSBr are attracting intense interest in view of their highly anisotropic magneto-optical coupling and their potential for novel optical interfaces within spintronic and magnonic devices. Characterizing the orbital character and propagation of these electronic excitations is crucial for understanding and controlling their behavior; however, this information is challenging to access. High resolution resonant inelastic x-ray scattering is a momentum-resolved technique that can address these crucial questions. We present measurements collected at the Cr L_{3}-edge which show a rich spectrum of excitations with a variety of spin-orbital characters. While most of these excitations appear to be localized, the dispersion of the lowest energy dark exciton indicates that it is able to propagate along both the a and b directions within the planes of the crystal. This two-dimensional character is surprising as it contrasts with electrical conductivity and the behavior of the bright exciton, both of which are strongly one dimensional. The discovery of this propagating dark exciton highlights an unusual coexistence of one- and two-dimensional electronic behaviors in CrSBr.

Spin-flip dark excitons are optical-dipole-forbidden quasiparticles with remarkable potential in optoelectronics, especially when they are realized within cleavable van der Waals materials. Despite this potential, dark excitons have not yet been definitively identified in ferromagnetic van der Waals materials. Here, we report two dark excitons in a model ferromagnetic material CrI3 using high-resolution resonant inelastic x-ray scattering and show that they feature narrower linewidths compared to the bright excitons previously reported in this material. These excitons are shown to have spin-flip character, to disperse as a function of momentum, and to change through the ferromagnetic transition temperature. Given the versatility of van der Waals materials, these excitons hold promise for new types of magneto-optical functionality.

Strong electronic correlation can lead to insulating behavior and to the opening of large optical gaps, even in materials with partly filled valence shells. Although the non-equilibrium optical response encodes both local (quasi atomic) and collective (long range) responses, optical spectroscopy is usually more sensitive to the latter. Resonant x-ray techniques are better suited to investigate the quasi-atomic properties of correlated solids. Using time-resolved resonant inelastic x-ray scattering (RIXS), here we study the ultrafast non-equilibrium processes in NiO following photo-excitation by ultraviolet photons with energy exceeding the optical gap. We observe the creation of charge-transfer excitons that decay with a time constant of about 2 ps, while itinerant photo-doping persists for tens of picoseconds. Following our discovery, which establishes time-resolved high-resolution RIXS as a powerful tool for the study of transient phenomena in condensed matter, the possible presence of charge-transfer excitons will need to be considered when interpreting optical pump-probe experiments on correlated quantum materials.

The many-body electron-electron interaction in cuprates causes broadening of the electronic bands in k space, leading to a deviation from the standard Fermi liquid. While a k-dependent anisotropic electronic scattering (k-DAES) has been assessed by photoemission, its fingerprint in Q space has been scarcely considered. Here, we explore the Q-dependent electron dynamics in optimally doped Bi2Sr2CaCu2O8+delta through the evolution of low-energy charge excitations as measured by resonant inelastic x-ray scattering (RIXS). In the normal state, the RIXS spectra display a continuum of excitations down to 0 meV, while the superconducting state features a spectral weight suppression below 80 meV without any enhancement at higher energies. To interpret the energy and Q evolution of our data, we introduce a phenomenological expression of the charge susceptibility by including the k-DAES. We show that only the charge susceptibility with k-DAES captures the RIXS data, highlighting the importance of k-DAES when describing the Q dependence of charge excitations from 0 to a few eV scale. Furthermore, we also find that the inclusion of k-DAES is essential when quantitative parameters such as the electronic energy gap are extracted from RIXS data.

B site ordered 5d2 double perovskites (A2BB'O6, B' = 5d2) display a remarkable range of physical properties upon variation of the chosen B and B' site ions. This sensitivity to chemical substitution reflects the delicate balance and profound impact of strong electronic correlation and spin-orbit coupling in such systems. We present rhenium L2 and L3 resonant inelastic x-ray scattering (RIXS) measurements of two such physically dissimilar materials, Mott-insultating Ba2YReO6 and semiconducting Sr2CrReO6. Despite these differences, our RIXS results reveal similar energy scales of Hund's (JH) and spin-orbit coupling (zeta) in the two materials, with both systems firmly in the intermediate Hund's coupling regime where JH/zeta similar to 1. However, there are clear differences in their RIXS spectra. The conductive character of Sr2CrReO6 broadens and obfuscates the atomic transitions within an electron-hole continuum, while the insulating character of Ba2YReO6 results in sharp atomic excitations. This contrast in their RIXS spectra despite their similar energy scales reflects a difference in the itinerancy-promoting hopping integral and illustrates the impact of the local crystal environment in double perovskites. Finally, L2 and L3 edge analyses of the atomic excitations in Ba2YReO6 reveal that the ordering of the low lying excited states is inverted compared to previous reports, such that the appropriate energy scales of Hund's and spin-orbit coupling are significantly modified. We present exact diagonalization calculations of the RIXS spectra at both edges which show good agreement with our results for new energy scales of zeta = 0.290(5) eV and JH = 0.38(2) eV [JH/zeta = 1.30(5)].

Volatile resistive switching in neuromorphic computing can be tuned by external stimuli such as temperature or electric-field. However, this type of switching is generally coupled to structural changes, resulting in slower reaction speed and higher energy consumption when incorporated into an electronic device. The vanadium dioxide (VO2), which has near room temperature metal-insulator transition (MIT), is an archetypical volatile resistive switching system. Here, we demonstrate an isostructural MIT in an ultrathin VO2 film capped with a photoconductive cadmium sulfide (CdS) layer. Transmission electron microscopy, resistivity experiments, and first-principles calculations show that the hole carriers induced by CdS photovoltaic effect are driving the MIT in rutile VO2. The insulating-rutile VO2 phase has been proved and can remain stable for hours. Our finding provides a new approach to produce purely electronically driven MIT in VO2, and widens its applications in fast-response, low-energy neuromorphic devices.

We investigated the high energy spin excitations in electron-doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$, a cuprate superconductor, by resonant inelastic x-ray scattering (RIXS) measurements. Efforts were paid to disentangle the paramagnon signal from non-spin-flip spectral weight mixing in the RIXS spectrum at ${\mathbf{Q}}_{\ensuremath{\parallel}}=(0.6\ensuremath{\pi},0)$ and $(0.9\ensuremath{\pi},0)$ along the (1 0) direction. Our results show that, for doping level $x$ from 0.07 to 0.185, the variation of the paramagnon excitation energy is marginal. We discuss the implication of our results in connection with the evolution of the electron correlation strength in this system.

Magnetic van der Waals (vdW) materials have opened new frontiers for realizing novel many-body phenomena. Recently NiPS3 has received intense interest since it hosts an excitonic quasiparticle whose properties appear to be intimately linked to the magnetic state of the lattice. Despite extensive studies, the electronic character, mobility, and magnetic interactions of the exciton remain unresolved. Here we address these issues by measuring NiPS3 with ultra-high energy resolution resonant inelastic x-ray scattering (RIXS). We find that Hund's exchange interactions are primarily responsible for the energy of formation of the exciton. Measuring the dispersion of the Hund's exciton reveals that it propagates in a way that is analogous to a double-magnon. We trace this unique behavior to fundamental similarities between the NiPS3 exciton hopping and spin exchange processes, underlining the unique magnetic characteristics of this novel quasiparticle.