Strained Superconductor: Distorting a material and observing its response can allow insight into its electronic properties. Thin films can be strained by placing them on a substrate with a different lattice constant; bulk samples present more of a challenge. Hicks et al. (p. 283 ) designed an apparatus to apply both tensile and compressive strain and used it to study the properties of the superconductor Sr ₂ RuO ₄ , which has long been hypothesized to host the unusual p-wave superconductivity. The response of the superconducting transition temperature T _c to the applied strain depended on the direction in which the strain was applied, and did not exhibit a cusp predicted to occur around zero strain. As the technique leaves a surface of the probe open to external probes, it could be adopted for a wide range of methods.

Squeezing out the oddness: The material Sr ₂ RuO ₄ has long been thought to exhibit an exotic, odd-parity kind of superconductivity, not unlike the superfluidity in ₃ He. How would perturbing this material's electronic structure affect its superconductivity? Steppke et al. put the material under large uniaxial pressure and found that the critical temperature more than doubled and then fell as a function of strain (see the Perspective by Shen). The maximum critical temperature roughly coincided with the point at which the material's Fermi surface underwent a topological change. One intriguing possibility is that squeezing changed the parity of the superconducting gap from odd to even. Science , this issue p. 10.1126/science.aaf9398 ; see also p. 133

We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about 2, due to electronic correlations. The self-energies for the $\beta$ and $\gamma$ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies and provides strong evidence for an electronic origin of the observed nonlinear frequency dependence of the self-energies, leading to “kinks” in the quasiparticle dispersion of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We present de~Haas\char21{}van~Alphen and resistivity data on single crystals of the delafossite PdCoO2. At 295~K we measure an in-plane resistivity of 2.6\,\,$\mu${}$\Omega${}\,cm, making PdCoO2 the most conductive oxide known. The low-temperature in-plane resistivity has an activated rather than the usual T5 temperature dependence, suggesting a gapping of effective scattering that is consistent with phonon drag. Below 10~K, the transport mean free path is $\sim${}20\,\,$\mu${}m, approximately 105 lattice spacings and an astoundingly high value for flux-grown crystals. We discuss the origin of these properties in light of our data.

We report a detailed high-resolution powder neutron diffraction investigation of the structural behavior of the multiferroic hexagonal polymorph of YMnO${}_3$ between room temperature and 1403 K. The study was aimed at resolving previous uncertainties regarding the nature of the paraelectric-ferroelectric transition and the possibilities of any secondary structural transitions. We observe a clear transition at $1258\pm 14$ K corresponding to a unit-cell tripling and a change in space group from centrosymmetric $P{6}_3/\mathit{mmc}$ to polar $P{6}_3\mathit{cm}$. Despite the fact that this symmetry permits ferroelectricity, our experimental data for this transition (analyzed in terms of symmetry-adapted displacement modes) clearly support previous theoretical analysis that the transition is driven primarily by the antiferrodistortive $K_3$ mode. We therefore verify previous suggestions that YMnO${}_3$ is an improper ferrielectric. Furthermore, our data confirm that the previously suggested intermediate phase with space group $P{6}_3/\mathit{mcm}$ does not occur. However, we do find evidence for an isosymmetric phase transition (i.e., $P{6}_3\mathit{cm}$ to $P{6}_3\mathit{cm}$) at $\approx 920$ K, which involves a sharp decrease in polarization. This secondary transition correlates well with several previous reports of anomalies in physical properties in this temperature region and may be related to Y-O hybridization.

We report the results of a combined study of the normal-state resistivity and superconducting transition temperature $T_c$ of the unconventional superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ under uniaxial pressure. There is strong evidence that, as well as driving $T_c$ through a maximum at $\sim 3.5\mathrm{K}$, compressive strains $ϵ$ of nearly 1% along the crystallographic [100] axis drive the $\gamma$ Fermi surface sheet through a van Hove singularity, changing the temperature dependence of the resistivity from $T^2$ above, and below the transition region to $T^{1.5}$ within it. This occurs in extremely pure single-crystals in which the impurity contribution to the resistivity is $<100\mathrm{n}\mathrm{\Omega }\mathrm{cm}$, so our study also highlights the potential of uniaxial pressure as a more general probe of this class of physics in clean systems.