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The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of ∼3.2 lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba 2 Cu 3 O 6+x , with hole concentrations of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature (T c ); further cooling below T c abruptly reverses the divergence of the charge correlations. In combination with earlier observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge density wave instability that competes with superconductivity.

One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional \"striped\" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La₁.₆₇₅Eu₀.₂Sr₀.₁₂₅CuO₄, into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

The temperature (T) dependence of the electrical resistivity offers clues about the behavior of electrical carriers. One of the more puzzling observations is the T-linear resistivity found in systems known or suspected to exhibit quantum criticality, such as cuprate and organic superconductors, and heavy fermion materials; the origin of this behavior remains elusive. Bruin et al. (p. 804 ) find that the ruthenate Sr 3 Ru 2 O 7 also exhibits T-linear resistivity in the vicinity of its quantum critical point, and that its scattering rate per kelvin is approximately given by the inverse of a characteristic time made up of the Planck and Boltzmann constants. A comprehensive analysis of other systems with T-linear resistivity, including ordinary metals at high temperatures, indicates that their scattering rates are similarly close to the characteristic rate. That the rates are similar across a wide range of materials with diverse microscopic scattering mechanisms may indicate universal behavior. Transport measurements show little variation across metals with resistivity that scales linearly with temperature. Many exotic compounds, such as cuprate superconductors and heavy fermion materials, exhibit a linear in temperature ( T ) resistivity, the origin of which is not well understood. We found that the resistivity of the quantum critical metal Sr 3 Ru 2 O 7 is also T- linear at the critical magnetic field of 7.9 T. Using the precise existing data for the Fermi surface topography and quasiparticle velocities of Sr 3 Ru 2 O 7 , we show that in the region of the T- linear resistivity, the scattering rate per kelvin is well approximated by the ratio of the Boltzmann constant to the Planck constant divided by 2π. Extending the analysis to a number of other materials reveals similar results in the T- linear region, in spite of large differences in the microscopic origins of the scattering.

The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T c ), entangled in an energy-momentum—dependent manner with the preexisting pseudogap features, ushering in a ground state with coexisting orders.

A dome-shaped superconducting region appears in the phase diagrams of many unconventional superconductors. In doped band insulators, however, reaching optimal superconductivity by the fine-tuning of carriers has seldom been seen. We report the observation of a superconducting dome in the temperature—carrier density phase diagram of MoS₂, an archetypal band insulator. By quasi-continuous electrostatic carrier doping achieved through a combination of liquid and solid gating, we revealed a large enhancement in the transition temperature T c occurring at optimal doping in the chemically inaccessible low-carrier density regime. This observation indicates that the superconducting dome may anse even in doped band insulators.

In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We used a femtosecond laser pump pulse to perturb superconducting Bi₂Sr₂CaCu₂08+δ and studied subsequent dynamics using time and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics revealed marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap was sensitive to the pump intensity, and Cooper pairs recombined slowly. Far from the nodes, pumping affected the gap only weakly, and recombination processes were faster. These results demonstrate a new window into the dynamical processes that govern quasiparticle recombination and gap formation in cuprates.

The superconducting state is characterized by a pairing of electrons with a superconducting gap on the Fermi surface. In iron-based superconductors, an unconventional pairing state has been argued for theoretically. We used scanning tunneling microscopy on Fe(Se,Te) single crystals to image the quasi-particle scattering interference patterns in the superconducting state. By applying a magnetic field to break the time-reversal symmetry, the relative sign of the superconducting gap can be determined from the magnetic-field dependence of quasi-particle scattering amplitudes. Our results indicate that the sign is reversed between the hole and the electron Fermi-surface pockets (s±-wave), favoring the unconventional pairing mechanism associated with spin fluctuations.

High-temperature superconductivity often emerges in the proximity of a symmetry-breaking ground state. For superconducting iron arsenides, in addition to the antiferromagnetic ground state, a small structural distortion breaks the crystal's C₄ rotational symmetry in the underdoped part of the phase diagram. We reveal that the representative iron arsenide Ba(Fe₁₋xCox)₂As₂ develops a large electronic anisotropy at this transition via measurements of the in-plane resistivity of detwinned single crystals, with the resistivity along the shorter b axis ρb being greater than ρa. The anisotropy reaches a maximum value of approximately 2 for compositions in the neighborhood of the beginning of the superconducting dome. For temperatures well above the structural transition, uniaxial stress induces a resistivity anisotropy, indicating a substantial nematic susceptibility.

Charge-stripe order and superconductivity Nuclear magnetic resonance measurements of the model high-temperature copper oxide superconductor YBa 2 Cu 3 O y demonstrate that high magnetic fields induce charge order, without spin order, within the material's CuO 2 planes. The observed charge order has characteristics similar to those of stripe-ordered copper oxides, in which electronic charges spontaneously organize themselves into 'stripes'. The charge order develops only when superconductivity fades away. This work suggests that stripes are more common objects in the cuprates than was thought. They seem to compete with superconductivity, although the tendency to form stripes may be a necessary ingredient of high temperature superconductivity. Electronic charges introduced in copper-oxide (CuO 2 ) planes generate high-transition-temperature ( T c ) superconductivity but, under special circumstances, they can also order into filaments called stripes 1 . Whether an underlying tendency towards charge order is present in all copper oxides and whether this has any relationship with superconductivity are, however, two highly controversial issues 2 , 3 . To uncover underlying electronic order, magnetic fields strong enough to destabilize superconductivity can be used. Such experiments, including quantum oscillations 4 , 5 , 6 in YBa 2 Cu 3 O y (an extremely clean copper oxide in which charge order has not until now been observed) have suggested that superconductivity competes with spin, rather than charge, order 7 , 8 , 9 . Here we report nuclear magnetic resonance measurements showing that high magnetic fields actually induce charge order, without spin order, in the CuO 2 planes of YBa 2 Cu 3 O y . The observed static, unidirectional, modulation of the charge density breaks translational symmetry, thus explaining quantum oscillation results, and we argue that it is most probably the same 4 a -periodic modulation as in stripe-ordered copper oxides 1 . That it develops only when superconductivity fades away and near the same 1/8 hole doping as in La 2− x Ba x CuO 4 (ref.  1 ) suggests that charge order, although visibly pinned by CuO chains in YBa 2 Cu 3 O y , is an intrinsic propensity of the superconducting planes of high- T c copper oxides.

Spin fluctuation scattering in organic superconductors The relative importance of phenomena such as spin and charge (stripe) order and a pseudogap phase are still a matter of great debate with regard to the high-transition-temperature copper oxides. In electron-doped materials, the absence of a pseudogap phase in the underdoped region of the phase diagram and weaker electron correlations suggest that antiferromagnetic spin fluctuations are the dominant feature. Jin et al . report a study of magnetotransport in thin films of the electron-doped copper oxide La 2 − x Ce x CuO 4 (LCCO). They show that a linear-temperature (T-linear) scattering rate is correlated with the electron pairing. Comparison with similar behaviour found in organic superconductors strongly suggests that the T-linear resistivity is caused by spin-fluctuation scattering. Although it is generally accepted that superconductivity is unconventional in the high-transition-temperature copper oxides, the relative importance of phenomena such as spin and charge (stripe) order, superconductivity fluctuations, proximity to a Mott insulator, a pseudogap phase and quantum criticality are still a matter of debate 1 . In electron-doped copper oxides, the absence of an anomalous pseudogap phase in the underdoped region of the phase diagram 2 and weaker electron correlations 3 , 4 suggest that Mott physics and other unidentified competing orders are less relevant and that antiferromagnetic spin fluctuations are the dominant feature. Here we report a study of magnetotransport in thin films of the electron-doped copper oxide La 2 −  x Ce x CuO 4 . We show that a scattering rate that is linearly dependent on temperature—a key feature of the anomalous normal state properties of the copper oxides—is correlated with the electron pairing. We also show that an envelope of such scattering surrounds the superconducting phase, surviving to zero temperature when superconductivity is suppressed by magnetic fields. Comparison with similar behaviour found in organic superconductors 5 strongly suggests that the linear dependence on temperature of the resistivity in the electron-doped copper oxides is caused by spin-fluctuation scattering.