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

Tuning the charge carrier density of two-dimensional (2D) materials by incorporating dopants into the crystal lattice is a challenging task. An attractive alternative is the surface transfer doping by adsorption of molecules on 2D crystals, which can lead to ordered molecular arrays. However, such systems, demonstrated in ultra-high vacuum conditions (UHV), are often unstable in ambient conditions. Here we show that air-stable doping of epitaxial graphene on SiC—achieved by spin-coating deposition of 2,3,5,6-tetrafluoro-tetracyano-quino-dimethane (F4TCNQ) incorporated in poly(methyl-methacrylate)—proceeds via the spontaneous accumulation of dopants at the graphene-polymer interface and by the formation of a charge-transfer complex that yields low-disorder, charge-neutral, large-area graphene with carrier mobilities ~70 000 cm² V⁻¹ s⁻¹ at cryogenic temperatures. The assembly of dopants on 2D materials assisted by a polymer matrix, demonstrated by spin-coating wafer-scale substrates in ambient conditions, opens up a scalable technological route toward expanding the functionality of 2D materials.

Topological superconductivity is central to a variety of novel phenomena involving the interplay between topologically ordered phases and broken-symmetry states. The key ingredient is an unconventional order parameter, with an orbital component containing a chiral p _x + ip _y wave term. Here we present phase-sensitive measurements, based on the quantum interference in nanoscale Josephson junctions, realized by using Bi₂Te₃ topological insulator. We demonstrate that the induced superconductivity is unconventional and consistent with a sign-changing order parameter, such as a chiral p _x + ip _y component. The magnetic field pattern of the junctions shows a dip at zero externally applied magnetic field, which is an incontrovertible signature of the simultaneous existence of 0 and π coupling within the junction, inherent to a non trivial order parameter phase. The nano-textured morphology of the Bi₂Te₃ flakes, and the dramatic role played by thermal strain are the surprising key factors for the display of an unconventional induced order parameter.

We report on the growth and characterization of ultrathin ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films on MgO (110) substrates, which exhibit superconducting properties at thicknesses down to 3 nm. YBCO nanowires, with thicknesses down to 10 nm and widths down to 65 nm, have also been successfully fabricated. The nanowires protected by a Au capping layer show superconducting properties close to the as-grown films and critical current densities, which are limited by only vortex dynamics. The 10-nm-thick YBCO nanowires without the Au capping present hysteretic current-voltage characteristics, characterized by a voltage switch which drives the nanowires directly from the superconducting to the normal state. We associate such bistability to the presence of localized normal domains within the superconductor. The presence of the voltage switch in ultrathin YBCO nanostructures, characterized by high sheet resistance values and high critical current values, makes our nanowires very attractive devices to engineer single-photon detectors.

Restoring strangeness: Cuprate superconductors host a number of complex phases, and elucidating their relationship to each other may shed light on the nature of cuprate superconductivity. Two of these intriguing phases are the strange metal phase and the charge-density wave (CDW) order, which appear next to each other in the cuprate phase diagram. The hallmark of the strange metal phase, the linear dependence of electrical resistivity on temperature, disappears in the CDW phase. Wahlberg et al . strained thin films of the cuprate YBa ₂ Cu ₃ O _7–δ to suppress CDW order (see the Perspective by Le Tacon). Using resonant inelastic x-ray scattering and transport measurements, the researchers found that suppressing CDW restored the linear-in-temperature resistivity in the CDW portion of the phase diagram. —JS

We have grown and characterized 30-nm-thick ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and ${\mathrm{SrTiO}}_3$ (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature $T_{\mathrm{c}}$ and the pseudogap temperature $T^{*})$ similar to those of the as-grown films and carry critical current densities $J_{\mathrm{c}}$ close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale.