We show that the quasi-skutterudite superconductor ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ undergoes a structural transition from a simple cubic parent structure, the $I$ phase, to a superlattice variant, the $I^{\prime }$ phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature $T^{*}$ can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature (T_c). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving non-magnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe have considerable overlap, how each order affects superconductivity remains perplexing. Here we construct the three-dimensional electronic phase diagram, temperature (T) against pressure (P) and isovalent S-substitution (x), for FeSe_1−x S _x . By simultaneously tuning chemical and physical pressures, against which the chalcogen height shows a contrasting variation, we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended non-magnetic tetragonal phase emerges, where T_c shows a striking enhancement. The completed phase diagram uncovers that high-T_c superconductivity lies near both ends of the dome-shaped antiferromagnetic phase, whereas T_c remains low near the nematic critical point.

Diamond-based sensors: Material properties can change dramatically under pressure. Typically, to achieve high-pressure conditions, researchers place their samples in diamond anvil cells (DACs). However, monitoring the properties of the sample inside a DAC is tricky (see the Perspective by Hamlin and Zhou). Hsieh et al. , Lesik et al. , and Yip et al. developed monitoring techniques based on nitrogen-vacancy (NV) centers in diamond. The NV centers can act as sensors because their energy levels and the associated spectra are sensitive to strain and magnetic fields. This enabled optical readout of a spatially resolved signal. Science , this issue p. 1349 , p. 1359 , p. 1355 ; see also p. 1312

The quasiskutterudite superconductor ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ features a pronounced anomaly in electrical resistivity at $T^{*}\sim 138\mathrm{K}$. We show that the anomaly is caused by a second-order structural transition, which can be tuned to 0 K by applying physical pressure and chemical pressure via the substitution of Ca for Sr. A broad superconducting dome is centered around the structural quantum critical point. Detailed analysis of the tuning parameter dependence of $T^{*}$ as well as insights from lattice dynamics calculations strongly support the existence of a structural quantum critical point at ambient pressure when the fraction of Ca is 0.9 (i.e., $x_c=0.9$). This establishes the ${({\mathrm{Ca}}_x{\mathrm{Sr}}_{1-x})}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ series as an important system for exploring the physics of structural quantum criticality without the need of applying high pressures.

The kagome metal CsV₃Sb₅ features an unusual competition between the charge-density-wave (CDW) order and superconductivity. Evidence for time reversal symmetry breaking (TRSB) inside the CDW phase has been accumulating. Hence, the superconductivity in CsV₃Sb₅ emerges from a TRSB normal state, potentially resulting in an exotic superconducting state. To reveal the pairing symmetry, we first investigate the effect of nonmagnetic impurity. Our results show that the superconducting critical temperature is insensitive to disorder, pointing to conventional s-wave superconductivity. Moreover, our measurements of the self-field critical current (I_c,sf), which is related to the London penetration depth, also confirm conventional s-wave superconductivity with strong coupling. Finally, we measure I_c,sf where the CDW order is removed by pressure and superconductivity emerges from the pristine normal state. Our results show that s-wave gap symmetry is retained, providing strong evidence for the presence of conventional s-wave superconductivity in CsV₃Sb₅ irrespective of the presence of the TRSB.

We report a highly unusual angular variation of the upper critical field ($H_{c2}$) in epitaxial superlattices ${\mathrm{CeCoIn}}_5(n)/{\mathrm{YbCoIn}}_5(5)$, formed by alternating layers of $n$ and a 5 unit-cell thick heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$ with a strong Pauli effect and normal metal ${\mathrm{YbCoIn}}_5$, respectively. For the $n=3$ superlattice, $H_{c2}(\theta )$ changes smoothly as a function of the field angle $\theta$. However, close to the superconducting transition temperature, $H_{c2}(\theta )$ exhibits a cusp near the parallel field ($\theta =0°$). This cusp behavior disappears for $n=4$ and 5 superlattices. This sudden disappearance suggests the relative dominance of the orbital depairing effect in the $n=3$ superlattice, which may be due to the suppression of the Pauli effect in a system with local inversion symmetry breaking. Taking into account the temperature dependence of $H_{c2}(\theta )$ as well, our results suggest that some exotic superconducting states, including a helical superconducting state, might be realized at high magnetic fields.