Recently discovered alongside its sister compounds ${\mathrm{KV}}_3{\mathrm{Sb}}_5$ and ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$, ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ crystallizes with an ideal kagome network of vanadium and antimonene layers separated by alkali metal ions. This work presents the electronic properties of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, demonstrating bulk superconductivity in single crystals with a $T_c=2.5\mathrm{K}$. The normal state electronic structure is studied via angle-resolved photoemission spectroscopy and density-functional theory, which categorize ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ as a ${\mathbb{Z}}_2$ topological metal. Multiple protected Dirac crossings are predicted in close proximity to the Fermi level ($E_F$), and signatures of normal state correlation effects are also suggested by a high-temperature charge density wavelike instability. The implications for the formation of unconventional superconductivity in this material are discussed.

The response of a material to external stimuli depends on its low-energy excitations. In conventional metals, these excitations are electrons on the Fermi surface—a contour in momentum (k) space that encloses all of the occupied states for non-interacting electrons. The pseudogap phase in the copper oxide superconductors, however, is a most unusual state of matter¹. It is metallic, but part of its Fermi surface is ‘gapped out’ (refs 2, 3); low-energy electronic excitations occupy disconnected segments known as Fermi arcs⁴. Two main interpretations of its origin have been proposed: either the pseudogap is a precursor to superconductivity⁵, or it arises from another order competing with superconductivity⁶. Using angle-resolved photoemission spectroscopy, we show that the anisotropy of the pseudogap in k-space and the resulting arcs depend only on the ratio T/T^*(x), where T^*(x) is the temperature below which the pseudogap first develops at a given hole doping x. The arcs collapse linearly with T/T^*(x) and extrapolate to zero extent as T→0. This suggests that the T=0 pseudogap state is a nodal liquid—a strange metallic state whose gapless excitations exist only at points in k-space, just as in a d-wave superconducting state.

The modifications of the crystallographic and magnetic structures of PrNi${\mathrm{O}}_3$ associated with the decrease of the metal-insulator transition temperature, $T_{\mathrm{MI}}$, with external pressure ($P_{\mathrm{ext}}\approx 5$ kbar) have been investigated by means of neutron powder diffraction. Our results indicate that the symmetry of the unit cell (orthorhombic $\mathrm{Pbnm}$ at ambient pressure), is preserved in the pressure range studied. The changes observed in the Ni-O distance and the Ni-O-Ni superexchange angle are, however, similar in sign but very different in magnitude from those obtained by applying an equivalent "internal" pressure. The antiferromagnetic ordering observed at ambient pressure for $T<\mathrm{}{T}_{\mathrm{MI}}$ remains unchanged for $P_{\mathrm{ext}}<5\mathrm{}$ kbar. This finding is in strong contrast with the results of high-pressure electrical resistance measurements, where a reentrant metallic behavior was observed for $P>\mathrm{}4\mathrm{}$ kbar and $T<\mathrm{}65\mathrm{}$ K.

Pyrochlore magnets are candidates for what Harris et al. [Phys. Rev. Lett. 79, 2554 (1997)] call “spin-ice” behavior. We present theoretical simulations of relevance for the pyrochlore family $R_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ ( $R=\mathrm{rare}\mathrm{earth}$) supported by magnetothermal measurements on selected systems. By considering long-ranged dipole-dipole as well as short-ranged superexchange interactions, we get three distinct behaviors: (i) an ordered doubly degenerate state, (ii) a highly disordered state with a broad transition to paramagnetism, and (iii) a partially ordered state with a sharp transition to paramagnetism. Closely corresponding behavior is seen in the real compounds.