Pervasive fluctuations: Among the many intertwined phases in the cuprate superconductor phase diagram is the charge density wave (CDW) order, which has been detected in all major cuprate families. It is thought that CDW competes with superconductivity, but whether it has bearing on the mechanism of superconductivity remains unclear. Arpaia et al. undertook a comprehensive study of charge density fluctuations in a cuprate family, varying doping and temperature. They found that short-range dynamic charge fluctuations were present in a large portion of the phase diagram, at temperatures considerably higher than those at which the CDW order disappears. Science , this issue p. 906

While electron-phonon coupling (EPC) is crucial for Cooper pairing in conventional superconductors, its role in high-$T_c$ superconducting cuprates is debated. Using resonant inelastic x-ray scattering at the oxygen $K$ edge, we study the EPC in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) and ${\mathrm{Nd}}_{1+x}{\mathrm{Ba}}_{2-x}{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (NBCO) at different doping levels ranging from heavily underdoped ($p=0.07$) to overdoped ($p=0.21$). We analyze the data with a localized Lang-Firsov model that allows for the coherent excitations of two phonon modes. While electronic band dispersion effects are non-negligible, we are able to perform a study of the relative values of EPC matrix elements in these cuprate families. In the case of NBCO, the choice of the excitation energy allows us to disentangle modes related to the CuO chains and the $\mathrm{Cu}{\mathrm{O}}_2$ planes. Combining the results from the two families, we find the EPC strength decreases with doping at ${\mathbf{q}}_{\parallel }=(-0.25,0)$ r.l.u., but has a nonmonotonic trend as a function of doping at smaller momenta. This behavior is attributed to the screening effect of charge carriers. We also find that the phonon intensity is enhanced in the vicinity of the charge-density-wave excitations while the extracted EPC strength appears to be less sensitive to their proximity. By performing a comparative study of two cuprate families, we are able to identify general trends in the EPC for the cuprates and provide experimental input to theories invoking a synergistic role for this interaction in $d$-wave pairing.

Polarized x-ray absorption spectroscopy (XAS) at the copper $L_{2,3}$ and oxygen $K$ edges has been used to study the superconducting-insulating (S-I) transition in ${\mathrm{Nd}}_{1.2}{\mathrm{Ba}}_{1.8}{\mathrm{Cu}}_3{\mathrm{O}}_{7+x}$ epitaxial films, realized by reducing the thickness from $9\text{to}6\text{unit}\text{cells}$. XAS shows that the main differences between the superconducting and the insulating films cannot be ascribed to changes of Cu density of states within the conducting $\mathrm{Cu}{\mathrm{O}}_2$ planes, but rather to the increase of the $3d_{3z^2-r^2}$ states of copper and to the effective reduction of the charge transfer of holes to the oxygen $2p_{xy}$ orbitals in the $\mathrm{Cu}{\mathrm{O}}_2$ planes. The experimental results show that the S-I transition is related to the hole localization at the Cu sites of the charge reservoir layer, i.e., outside the $\mathrm{Cu}{\mathrm{O}}_2$ planes, while the electronic states, lying within the conducting planes, appear to be similar in the insulating and superconducting states.

Over the last few years, research activities have seen two-dimensional (2D) materials become protagonists in the field of nanotechnology. In particular, 2D materials characterized by ferroelectric properties are extremely interesting, as they are better suited for the development of miniaturized and high-performing devices. Here, we summarize the recent advances in this field, reviewing the realization of devices based on 2D ferroelectric materials, like FeFET, FTJ, and optoelectronics. The devices are realized with a wide range of material systems, from oxide materials at low dimensions to 2D materials exhibiting van der Waals interactions. We conclude by presenting how these materials could be useful in the field of devices based on magnons or surface acoustic waves.

Epitaxial films of high critical temperature ( $T_c$ ) cuprate superconductors preserve their transport properties even when their thickness is reduced to a few nanometers. However, when approaching the single crystalline unit cell (u.c.) of thickness, $T_c$ decreases and eventually, superconductivity is lost. Strain originating from the mismatch with the substrate, electronic reconstruction at the interface and alteration of the chemical composition and of doping can be the cause of such changes. Here, we use resonant inelastic x-ray scattering at the Cu $L_3$ edge to study the crystal field and spin excitations of NdBa ${}_2$ Cu ${}_3$ O ${}_{7-x}$ ultrathin films grown on SrTiO ${}_3$ , comparing 1, 2 and 80 u.c.-thick samples. We find that even at extremely low thicknesses, the strength of the in-plane superexchange interaction is mostly preserved, with just a slight decrease in the 1 u.c. with respect to the 80 u.c.-thick sample. We also observe spectroscopic signatures for a decrease of the hole-doping at low thickness, consistent with the expansion of the c-axis lattice parameter and oxygen deficiency in the chains of the first unit cell, determined by high-resolution transmission microscopy and x-ray diffraction.