A State of High T _c Superconductivity: There are strong indications that high-temperature superconductivity in the cuprates is formed amid competing orders, but only two have been observed unambiguously. The so-called stripe order has been observed in a Lanthanum-based cuprate family and consists of coexisting charge-and-spin modulations and occurs at a characteristic dopant concentration in which the critical temperature T _c has a dip. Now, Ghiringhelli et al. (p. 821 , published online 12 July; see the Perspective by Tranquada ) have used resonant inelastic x-ray scattering to uncover a related but apparently two-dimensional charge order in the much cleaner YBCO cuprate family. The charge fluctuations were not commensurate with the lattice and did not originate in the characteristic oxygen chains of YBCO. The order appeared only in a narrow interval of dopant concentrations and competed with superconductivity, which provides a natural explanation for a plateau in T _c observed in the same range.

The high-$T_c$ superconductor ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{CuO}}_{6+\delta }$ is studied by angle-resolved photoemission spectroscopy. For a very overdoped $T_c=30\mathrm{K}$ sample, the Fermi surface consists of a single large hole pocket centered at $(\pi ,\pi )$ and is approaching a topological transition. Although a superconducting gap with $d_{x^2-y^2}$ symmetry is tentatively identified, the quasiparticle evolution with momentum and binding energy exhibits a marked departure from the behavior observed in under and optimally doped cuprates. The relevance of these findings to scattering, many-body, and quantum-critical phenomena is discussed.

We have investigated the spin-fluctuations at energy transfers up to $\sim 110\mathrm{meV}$, well above the resonance energy $\left(33\mathrm{meV}\right)$ in the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ ortho-II superconductor using neutron time-of-flight and triple-axis techniques. The spectrum at high energies differs from the low-energy incommensurate modulations previously reported where the incommensurate wave vector is largely independent of energy. Well above the resonance the peak of the spin response lies at wave vectors that increase with energy. Within error the excitations at all energies above the resonance are best described by a ring around the $(\pi ,\pi )$ position. The isotropic wave-vector pattern differs from a recently reported square pattern in different but related systems. The spin spectral weight at high energies is similar to that in the insulator but the characteristic velocity is $\sim 40\%$ lower. We introduce a method of extracting the acoustic and optic weights at all energies from time-of-flight data. We find that the optic spectral weight extends to surprisingly low energies of $\sim 25\mathrm{meV}$, and infer that the bilayer spin correlations weaken with increase in hole doping. When the low-energy optic excitations are taken into account we measure the total integrated weight around $(\pi ,\pi )$, for energies below $120\mathrm{meV}$, to agree with that expected from the insulator. As a qualitative guide, we compare spin-wave calculations based on models with ordered and disordered stripes and describe the inadequacy of these and other stripe models for the high-energy fluctuations.

We describe the relation between spin fluctuations and superconductivity in a highly ordered sample of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ using both polarized and unpolarized neutron inelastic scattering. The spin susceptibility in the superconducting phase exhibits one-dimensional incommensurate modulations at low energies, consistent with hydrodynamic stripes. With increasing energy the susceptibility curves upward to a commensurate, intense, well-defined, and asymmetric resonance at 33 meV with a precipitous high-energy cutoff. In the normal phase, which we show is gapless, the resonance remains surprisingly strong and persists clearly in Q scans and energy scans. Its similar asymmetric spectral form above $T_c=59\mathrm{}\mathrm{K}$ suggests that incoherent superconducting pairing fluctuations are present in the normal state. On cooling, the resonance and the stripe modulations grow in well above $T_c$ below a temperature that is comparable to the pseudogap temperature where suppression occurs in local and low-momentum properties. The spectral weight that accrues to the resonance is largely acquired by transfer from suppressed low-energy fluctuations. We find the resonance to be isotropically polarized, consistent with a triplet carrying $\sim 2.6\%$ of the total spectral weight of the Cu spins in the planes.

In non-magnetic bulk materials, inversion symmetry protects the spin degeneracy. If the bulk crystal structure lacks a centre of inversion, however, spin–orbit interactions lift the spin degeneracy, leading to a Rashba metal whose Fermi surfaces exhibit an intricate spin texture. In superconducting Rashba metals a pairing wavefunction constructed from these complex spin structures will generally contain both singlet and triplet character. Here we examine the possible triplet components of the order parameter in noncentrosymmetric BiPd, combining for the first time in a noncentrosymmetric superconductor macroscopic characterization, atomic-scale ultra-low-temperature scanning tunnelling spectroscopy, and relativistic first-principles calculations. While the superconducting state of BiPd appears topologically trivial, consistent with Bardeen–Cooper–Schrieffer theory with an order parameter governed by a single isotropic s-wave gap, we show that the material exhibits Dirac-cone surface states with a helical spin polarization.

Using resonant inelastic x-ray scattering (RIXS) at the Cu $L$-absorption edge, we have observed intense, dispersive spin excitations in highly overdoped Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$ (superconducting $T_c=6$ K), a model compound whose normal-state charge transport and thermodynamic properties have been shown to exhibit canonical Fermi-liquid behavior. Complementary RIXS experiments on slightly overdoped Tl${}_2$Ba${}_2$CuO${}_{6+\delta }$ ($T_c=89$ K) and on Y${}_{1-x}$Ca${}_x$Ba${}_2$Cu${}_3$O${}_{6+\delta }$ compounds spanning a wide range of doping levels indicate that these excitations exhibit energies and energy-integrated spectral weights closely similar to those of antiferromagnetic magnons in undoped cuprates, indicating the persistence of substantial antiferromagnetic spin correlations over a wide doping range. The surprising coexistence of such correlations with Fermi-liquid-like charge excitations in highly overdoped cuprates poses a challenge to current theoretical models of correlated-electron metals.