Superconductivity and charge density waves (CDWs) are competitive, yet coexisting, orders in cuprate superconductors. To understand their microscopic interdependence, a probe capable of discerning their interaction on its natural length and time scale is necessary. We use ultrafast resonant soft x-ray scattering to track the transient evolution of CDW correlations in YBa ₂ Cu ₃ O _{6+ x} after the quench of superconductivity by an infrared laser pulse. We observe a nonthermal response of the CDW order characterized by a near doubling of the correlation length within ≈1 picosecond of the superconducting quench. Our results are consistent with a model in which the interaction between superconductivity and CDWs manifests inhomogeneously through disruption of spatial coherence, with superconductivity playing the dominant role in stabilizing CDW topological defects, such as discommensurations.

We explore magnetic order in the quantum spin chain compound SrCo₂V₂O₈ up to 14.9 T and down to 50 mK, using single-crystal neutron diffraction. Upon cooling in zero-field, commensurate antiferromagnetic (C-AFM) order with modulation vector k C = (0, 0, 1) develops below T _N ≃ 5.0 K. Applying an external magnetic field (H∥c axis) destabilizes this C-AFM order, leading to an order-disorder transition between T _N and ∼1.5 K. Below 1.5 K, a commensurate to incommensurate (IC-AFM) transition occurs at 3.9 T, above which the magnetic reflections can be indexed by k IC = (0, 0, 1 ± δl). The incommensurability δl scales monotonically with H until the IC-AFM order disappears around 7.0 T. Magnetic reflections modulated by k C emerge again at higher fields. While the characters of the C-AFM, IC-AFM and the emergent AFM order in SrCo₂V₂O₈ appear to fit the descriptions of the Néel, longitudinal spin density wave and transverse AFM order observed in the related compound BaCo₂V₂O₈, our results also reveal several unique signatures that are not present in the latter, highlighting the inadequacy of mean-field theory in addressing the complex magnetic order in systems of this class.

Van der Waals (vdW) magnetic materials are comprised of layers of atomically thin sheets, making them ideal platforms for studying magnetism at the two-dimensional (2D) limit. These materials are at the center of a host of novel types of experiments, however, there are notably few pathways to directly probe their magnetic structure. We confirm the magnetic order within a single crystal of NiPS₃ and show it can be accessed with resonant elastic X-ray diffraction along the edge of the vdW planes in a carefully grown crystal by detecting structurally forbidden resonant magnetic X-ray scattering. We find the magnetic order parameter has a critical exponent of β ~ 0.36, indicating that the magnetism of these vdW crystals is more adequately characterized by the three-dimensional (3D) Heisenberg universality class. We verify these findings with first-principles density functional theory, Monte-Carlo simulations, and density matrix renormalization group calculations.

In the cuprate superconductors, the spatial coherence of the charge density wave (CDW) state grows below a temperature $T_{\mathrm{CDW}}$, the origin of which is debated. Using x-ray photon correlation spectroscopy, we have studied the temporal atomic relaxation dynamics in ${\mathrm{La}}_{1.88}{\mathrm{Sr}}_{0.12}{\mathrm{CuO}}_4$ to shed light on this question. Cooling within an emergent structurally distorted phase, which favors the CDW modulation in symmetry and develops in two stages between 180 and 120 K, we observe a crossover from cooperativelike to incoherentlike relaxation dynamics at $T_{\mathrm{CDW}}=75\left(10\right) \mathrm{K}$ . We argue that, if the CDW is hosted by this distortion, the concomitant relaxational crossover and enhancement of CDW spatial coherence supports the interplay between relaxational atomic fluctuations and CDWs in materials of this class on quasistatic times cales.

In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced $d^1{d}^0\stackrel{}{\to }{d}^0{d}^1$ excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods—both femtosecond resonant x-ray diffraction and optical reflectivity—to demonstrate that the orbital response in the layered manganite ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_{1+x}{\mathrm{MnO}}_4\left(x=2/3\right)$ does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical $d^1{d}^0\stackrel{}{\to }{d}^0{d}^1$ transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the $d^0\stackrel{}{\to }{d}^{1}L$, where $\mathit{L}$ denotes a hole in the oxygen band. This finding may pave a different avenue for selectively manipulating specific types of order in complex materials of this class.

In unconventional superconductors, coupled charge and lattice degrees of freedom can manifest in ordered phases of matter that are intertwined. In the cuprate family, fluctuating short-range charge correlations can coalesce into a longer-range charge density wave (CDW) order which is thought to intertwine with superconductivity, yet the nature of the interaction is still poorly understood. Here, by measuring subtle lattice fluctuations in underdoped YBa ₂ Cu ₃ O _{6+ y} on quasi-static timescales (thousands of seconds) through X-ray photon correlation spectroscopy, we report sensitivity to both superconductivity and CDW. The atomic lattice shows remarkably faster relaxational dynamics upon approaching the superconducting transition at T _c ≈ 65 K. By tracking the momentum dependence, we show that the intermediate scattering function almost monotonically scales with the relaxation distance of atoms away from their average positions above T _c and in the presence of the CDW state, while this peculiar trend is reversed for other temperatures. These observations are consistent with an incipient CDW stabilized by local strain. This work provides insights into the crucial role of relaxational atomic fluctuations for understanding the electronic physics cuprates, which are inherently disordered due to carrier doping.