Polarized and unpolarized neutron triple-axis spectrometry was used to study the dynamical magnetic susceptibility χ ′′ (q, ω) as a function of energy (hω) and wave vector (q) in a wide temperature range for the bilayer superconductor YBa2Cu3O6+x with oxygen concentrations, x, of 0. 45, 0.5, 0.6, 0.7, 0.8, 0.93, and 0.95. The most prominent features in the magnetic spectra include a spin gap in the superconducting state, a pseudogap in the normal state, the much-discussed resonance, and incommensurate spin fluctuations below the resonance. We establish the doping dependence of the spin gap in the superconducting state, the resonance energy, and the incommensurability of the spin fluctuations. The magnitude of the spin gap (Esg) up to the optimal doping is proportional to the superconducting transition temperature Tc with Esg/kBTc = 3.8. The resonance, which exists exclusively below Tc for highly doped YBa2Cu3O6+x with x = 0.93 and 0.95, appears above Tc for underdoped compounds with x ≤ 0.8. The resonance energy (Er) also scales with kBTc, but saturates at Er ≈ 40 meV for x close to 0.93. The incommensurate spin fluctuations at energies below the resonance have structures similar to that of the single-layer superconducting La2−xSrxCuO4. However, there are also important differences. While the incommensurability (δ) of the spin fluctuations in La2−xSrxCuO4 is proportional to Tc for the entire hole-doping range up to the optimal value, the incommensurability in YBa2Cu3O6+x increases with Tc for low oxygen doping and saturates to δ = 0.1 for x ≥ 0.6. In addition, the incommensurability decreases with increasing energy close to the resonance. Finally, the incommensurate spin fluctuations appear above Tc in underdoped compounds with x ≤ 0.6 but for highly doped materials they are only observed below Tc. We discuss in detail the procedure used for separating the magnetic scattering from phonon and other spurious effects. In the comparison of our experimental results with various microscopic theoretical models, particular emphasis was made to address the similarities and differences in the spin fluctuations of the two most studied superconductors. Finally, we briefly mention recent magnetic field dependent studies of the spin fluctuations and discuss their relevance in understanding the microscopic origin of the resonance.

Inelastic neutron scattering was used to study the wave vector- and frequency-dependent magnetic fluctuations in single crystals of superconducting YBa2Cu3O6+x. The spectra contain several important features, including a gap in the superconducting state, a pseudogap in the normal state, and the much-discussed resonance peak. The appearance of the pseudogap determined from transport and nuclear resonance coincides with formation of the resonance in the magnetic excitations. The exchange energy associated with the resonance has the temperature and doping dependences as well as the magnitude to describe approximately the electronic specific heat near the superconducting transition temperature (Tc).

There is increasing evidence that inhomogeneous distributions of charge and spinÐso-called`striped phases'Ðplay an important role in determining the properties of the high-temperature superconductors. For example, recent neutron-scattering measurements on the YBa 2 Cu 3 O 7-x family of materials show both spin and charge¯uctuations that are consistent with the stripedphase picture. But the¯uctuations associated with a striped phase are expected to be one-dimensional, whereas the magnetic uctuations observed to date appear to display two-dimensional symmetry. We show here that this apparent two-dimensionality results from measurements on twinned crystals, and that similar measurements on substantially detwinned crystals of YBa 2 Cu 3 O 6.6 reveal the one-dimensional character of the magnetic¯uctuations, thus greatly strengthening the striped-phase interpretation. Moreover, our results also suggest that superconductivity originates in charge stripes that extend along the b crystal axis, where the super¯uid density is found to be substantially larger than for the a direction.Striped phases are inhomogeneous distributions of charge and spin that have been suggested to account for many of the unusual properties of the high-T c copper oxide superconductors 1±7 (T c is the superconducting transition temperature). In the simplest picture, the charge and spin can be thought of as being con®ned to separate linear regions in the crystal and thus resembling stripes. We expect static striped phases for the YBa 2 Cu 3 O 7-x materials not to coexist with superconductivity, except perhaps for materials with low oxygen contents and low values of T c . However, neutronscattering measurements have shown results for¯uctuations of both spin 8,9 and charge 10 that support the existence of a dynamic striped phase in YBa 2 Cu 3 O 7-x materials that have high T c values. Nevertheless, a dif®culty for the dynamic striped-phase picture is that, for both YBa 2 Cu 3 O 7-x (refs 8, 9) and La 2-x Sr x CuO 4 (refs 11, 12) copper oxide superconductors, the magnetic¯uctuations stemming from the spins have displayed a four-fold pattern at incommensurate points around the magnetic (1/2, 1/2) reciprocal lattice position, as shown in Fig. 1A. The interpretation of these measurements has been unclear, with both striped phases 1±7 and nested Fermi surfaces 13±15 being possible explanations. Because the striped phase is expected to propagate along a single direction, only a single set of satellites around the antiferromagnetic position should be observed. Tranquada 7,16 has suggested that the four-fold symmetry might arise from stripes that alternate in direction as the planes are stacked along the c axis.The problem in interpreting the four-fold pattern stems from the fact that the crystals used in the neutron experiments are twinned, so that along a given a or b direction, domains with lattice spacing a or b exist in equal proportion. This makes it impossible to distinguish whether satellites originate from the a* or b* direction in the reciprocal ...

Pulsed-laser ablated uranium atoms were codeposited with 14N2(15N2) and excess Ar at 12 K. The Fourier transform infrared (FTIR) spectrum revealed a single product, UN2, which exhibited a ν3 absorption at 1051.0 cm−1. Ultraviolet (UV) photolysis increased the yield of UN2 by threefold and showed that electronic excitation facilitated the insertion reaction. N2 perturbed UN2 bands at 1041.3 and 1031.5 cm−1 grew sharply during matrix annealings. In 14N15N experiments the ν1 and ν3 modes of 14NU15N were observed at 987.2 and 1040.7 cm−1, respectively; FG matrix calculations were performed to determine Fr=8.27 mdyn/Å and Frr=0.12 mdyn/Å and to estimate the IR-inactive ν1 modes of U14N2 and U15N2 at 1008.3 and 985.7 cm−1, respectively. Energetic considerations suggest that the U+N2 insertion reaction has little exothermicity and that the activation energy for this reaction may be provided by hypothermal uranium atoms.

Uranium atoms from the Nd:YAG laser ablation of a uranium target were codeposited with molecular oxygen and excess argon at 12 K. Infrared spectra following the U+O2 reaction revealed a wide range of reaction products. The 776.0 cm−1 band due to UO2 was the strongest product absorption, strong UO3 bands were observed at 852.5 and 745.5 cm−1, and a weak UO absorption appeared at 819.8 cm−1. These product absorptions are in agreement with earlier work, which evaporated UO2 from a tungsten Knudsen cell at 2000 °C. The 16O2/18O2 reaction gave only U 16O2 and U 18O2, which verified an insertion mechanism. New product absorptions were observed at 952.3, 892.3, and 842.4 cm−1. The 842.4 cm−1 absorption due to the UO3–O2 complex and the 892.3 cm−1 band assigned to the charge-transfer complex (UO2+)(O2−) grew markedly at the expense of the other uranium oxides during annealing the matrix to allow diffusion and reaction of O2. With 25% 16O2, 50% 16O18O, and 25% 18O2 samples, the 952.3 cm−1 band became a sharp triplet at 952.3, 936.5, and 904.5 cm−1 and exhibited an isotopic ratio appropriate for a linear OUO species. Agreement of this band with uranyl ion spectra suggests assignment to a (UO22+) complex. Mechanisms of formation of charged species are discussed.