The correlations between stripe order, superconductivity, and crystal structure in La2−x Bax CuO4 single crystals have been studied by means of x-ray and neutron diffraction as well as static magnetization measurements. The derived phase diagram shows that charge stripe order (CO) coexists with bulk superconductivity in a broad range of doping around x = 1/8, although the CO order parameter falls off quickly for x = 1/8. Except for x = 0.155, the onset of CO always coincides with the transition between the orthorhombic and the tetragonal low temperature structures. The CO transition evolves from a sharp drop at low x to a more gradual transition at higher x, eventually falling below the structural phase boundary for optimum doping. With respect to the interlayer CO correlations, we find no qualitative change of the stripe stacking order as a function of doping, and in-plane and out-of-plane correlations disappear simultaneously at the transition. Similarly to the CO, the spin stripe order (SO) is also most pronounced at x = 1/8. Truly static SO sets in below the CO and coincides with the first appearance of in-plane superconducting correlations at temperatures significantly above the bulk transition to superconductivity (SC). Indications that bulk SC causes a reduction of the spin or charge stripe order could not be identified. We argue that CO is the dominant order that is compatible with SC pairing but competes with SC phase coherence. Comparing our results with data from the literature, we find good agreement if all results are plotted as a function of x ′ instead of the nominal x, where x ′ represents an estimate of the actual Ba content, extracted from the doping dependence of the structural transition between the orthorhombic phase and the tetragonal high-temperature phase.

Neutron scattering experiments continue to improve our knowledge of spin fluctuations in layered cuprates, excitations that are symptomatic of the electronic correlations underlying high-temperature superconductivity. Time-of-flight spectrometers, together with new and varied single crystal samples, have provided a more complete characterization of the magnetic energy spectrum and its variation with carrier concentration. While the spin excitations appear anomalous in comparison with simple model systems, there is clear consistency among a variety of cuprate families. Focusing initially on hole-doped systems, we review the nature of the magnetic spectrum, and variations in magnetic spectral weight with doping. We consider connections with the phenomena of charge and spin stripe order, and the potential generality of such correlations as suggested by studies of magnetic-field and impurity induced order. We contrast the behavior of the hole-doped systems with the trends found in the electron-doped superconductors. Returning to hole-doped cuprates, studies of translation-symmetry-preserving magnetic order are discussed, along with efforts to explore new systems. We conclude with a discussion of future challenges.

We present new x-ray and neutron-scattering measurements of stripe order in La 1.875 Ba 0.125 CuO 4 , along with low-field susceptibility, thermal conductivity, and specific-heat data. We compare these with previously reported results for resistivity and thermopower. Temperature-dependent features indicating transitions ͑or cross-overs͒ are correlated among the various experimental quantities. Taking into account recent spectroscopic studies, we argue that the most likely interpretation of the complete collection of results is that an unusual form of two-dimensional superconducting correlations appears together with the onset of spin-stripe order. Recent theoretical proposals for a sinusoidally modulated superconducting state compatible with stripe order provide an intriguing explanation of our results and motivate further experimental tests. We also discuss evidence for one-dimensional pairing correlations that appear together with the charge order. With regard to the overall phenomenology, we consider the degree to which similar behavior may have been observed in other cuprates and describe possible connections to various puzzling phenomena in cuprate superconductors.

We report a neutron scattering study of the spin- 1/2 alternating bond antiferromagnet Cu(NO3)(2);2.5D(2)O for 0.06

Much of what we know about the electronic states of hightemperature superconductors is due to photoemission 1-3 and scanning tunnelling spectroscopy 4,5 studies of the compound Bi 2 Sr 2 CaCu 2 O 8+δ. The demonstration of well-defined quasiparticles in the superconducting state has encouraged many theorists to apply the conventional theory of metals, Fermiliquid theory, to the cuprates 6-9. In particular, the spin excitations observed by neutron scattering at energies below twice the superconducting gap energy are commonly believed to correspond to an excitonic state involving itinerant electrons 10-14. Here, we present the first measurements of the magnetic spectral weight of optimally doped Bi 2 Sr 2 CaCu 2 O 8+δ in absolute units. The lack of temperature dependence of the local spin susceptibility across the superconducting transition temperature, T c , is incompatible with the itinerant calculations. Alternatively, the magnetic excitations could be due to local moments, as the magnetic spectrum is similar to that in La 1.875 Ba 0.125 CuO 4 (ref. 15), where quasiparticles 16 and local moments 17 coexist. Bi 2 Sr 2 CaCu 2 O 8+δ has been the cuprate system of choice for surface-sensitive techniques such as angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling spectroscopy (STS) because it cleaves easily, thus enabling simple preparation of fresh surfaces. ARPES (refs 1-3) and STS (ref. 4) studies of Bi 2 Sr 2 CaCu 2 O 8+δ have convincingly demonstrated the existence of coherent electronic excitations (Bogoliubov quasiparticles) in the superconducting state. The excitation gap for quasiparticles has d-wave symmetry, going to zero at four nodal points along the nominal Fermi surface in the two-dimensional reciprocal space for a CuO 2 plane 1-3. On warming into the normal state, coherent electronic states are observed, at most, only over finite arcs about the nodal points; in the 'antinodal' regions, there is a so-called pseudogap and an absence of quasiparticles 1,2. The ARPES results on Bi 2 Sr 2 CaCu 2 O 8+δ have been used as a basis for predicting the magnetic excitation spectrum in the superconducting state, measurable by inelastic neutron scattering 10,14. To detect the magnetic signal with neutrons, however, one needs crystals of large volume to compensate for limited neutron source strength and weak scattering cross-section, and such crystals have been difficult to grow. The magnetic spectral weight is typically presented as the imaginary part of the dynamical spin susceptibility, χ (Q,ω); here Q is the wave vector of a magnetic excitation and E =hω is its energy, withh being Planck's constant divided by 2π and ω the angular frequency. Previous neutron scattering studies 11-14 of Bi 2 Sr 2 CaCu 2 O 8+δ , working with crystals of limited size, focused on the change in χ (Q,ω) on cooling through the superconducting transition temperature, T c .