Upping the pressure in bilayer graphene: The discovery of superconductivity and exotic insulating phases in twisted bilayer graphene has established this material as a model system of strongly correlated electrons. To achieve superconductivity, the two layers of graphene need to be at a very precise angle with respect to each other. Yankowitz et al. now show that another experimental knob, hydrostatic pressure, can be used to tune the phase diagram of twisted bilayer graphene (see the Perspective by Feldman). Applying pressure increased the coupling between the layers, which shifted the superconducting transition to higher angles and somewhat higher temperatures. Science , this issue p. 1059 ; see also p. 1035

A Dirac nodal-line semimetal phase, which represents a new quantum state of topological materials, has been experimentally realized only in a few systems, including ${\mathrm{PbTaSe}}_2$, ${\mathrm{PtSn}}_4$, and ZrSiS. In this Letter, we report evidence of nodal-line fermions in ZrSiSe and ZrSiTe probed in de Haas–van Alphen quantum oscillations. Although ZrSiSe and ZrSiTe share a similar layered structure with ZrSiS, our studies show the Fermi surface (FS) enclosing a Dirac nodal line has a 2D character in ZrSiTe, in contrast with 3D-like FS in ZrSiSe and ZrSiS. Another important property revealed in our experiment is that the nodal-line fermion density in this family of materials ($\sim {10}^{20}{\mathrm{cm}}^{-3}$) is much higher than the Dirac fermion density of other topological materials with discrete nodes. In addition, we have demonstrated ZrSiSe and ZrSiTe single crystals can be thinned down to 2D atomic thin layers through microexfoliation, which offers the first platform to explore exotic properties of topological nodal-line fermions in low dimensions.

In the quest to increase the critical temperature T_c of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field H_c2 is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect H_c2 in the cuprates YBa₂Cu₃O_y, YBa₂Cu₄O₈ and Tl₂Ba₂CuO_6+δ, allowing us to map out H_c2 across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa₂Cu₃O_y is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from H_c2, suffers a sudden 20-fold collapse. This reveals that phase competition—associated with Fermi-surface reconstruction and charge-density-wave order—is a key limiting factor in the superconductivity of cuprates.

We report on the unusual behavior of the in-plane thermal conductivity $\kappa$ and torque $\tau$ response in the Kitaev-Heisenberg material $\alpha \text{−}{\mathrm{RuCl}}_3$. $\kappa$ shows a striking enhancement with linear growth beyond $H=7\mathrm{T}$, where magnetic order disappears, while $\tau$ for both of the in-plane symmetry directions shows an anomaly at the same field. The temperature and field dependence of $\kappa$ are far more complex than conventional phonon and magnon contributions, and require us to invoke the presence of unconventional spin excitations whose properties are characteristic of a field-induced spin-liquid phase related to the enigmatic physics of the Kitaev model in an applied magnetic field.