The optical probe of a scanning near-field optical microscope is shown to act as a point source of surface plasmon (SP) polaritons on gold and silver films. Plasmon excitation manifests itself by emission of light in the direction of the SP resonance angle, originating from an area with the shape of a dipole radiation pattern whose extension is given by the SP decay length. Interaction with selected, individual surface inhomogeneities gives rise to characteristic modifications of the emitted radiation, which provide detailed information about SP scattering, reflection, and interference phenomena. [S0031-9007(96)01079-4]

Near-field optical (NFO) microscopes with an auxiliary gap width regulation (shear force, tunneling) may produce images that represent the path of the probe rather than optical properties of the sample. Experimental and theoretical evidence leads us to the conclusion that many NFO results reported in the past might have been affected or even dominated by the resulting artifact. The specifications derived from such results for the different types of NFO microscopes used therefore warrant reexamination. We show that the resolving power of aperture NFO microscopes, 30–50 nm, is of genuine NFO origin but can be heavily obscured by the artifact.

problem is that we have neglected the spin on the intervening oxygen. The third is that Eq. 1 describes the ground state for a single FM bond, implying that neutron scattering should be elastic, not inelastic as seen experimentally.The first difficulty has a simple resolution. At finite hole densities, the polarization clouds overlap and the isolated impurity model is inadequate. A crude model, which considers the overlaps, simply truncates the polarization clouds at neighboring impurity sites. Because FM impurity bonds are randomly distributed, the inversion symmetry characterizing isolated impurities is broken, thus allowing intensity at q ϭ . Because we know the impurity density, x, from neutron activation analysis, the only parameters in such a description are the extent of the polarization cloud, Ϫ1, which we adjusted to optimize the fit to our data. As shown by the red lines in Fig. 4, the model provides a good account of the data with Ϫ1 ϭ 8.1 Ϯ 0.2, 7.3 Ϯ 0.2, and 7.2 Ϯ 0.5 for x ϭ 0.04, 0.095, and 0.14 respectively. These values are close to the exponential decay length of 6.03 calculated for the AFM spin polarization at the end of an S ϭ 1 chain (25).The modeling described so far does not include the spins of the holes responsible for the effective FM couplings between Ni 2ϩ ions. The holes reside in oxygen orbitals of Y 2Ϫx Ca x BaNiO 5 (5) but are almost certainly not confined to single, isolated oxygens. We consequently generalized Eq. 1 to take into account the hole spins, with-for the sake of definiteness-the same net amplitude as either of the Ni 2ϩ spins next to the FM bond and distributed (with exponential decay) over ᐉ lattice sites centered on the FM bond. As long as ᐉ exceeds the modest value of 2, comparable to the localization length deduced from transport data (5), the pronounced asymmetry about that occurs when ᐉ ϭ 0 is relieved sufficiently to produce fits indistinguishable from those in Fig. 4.How do we account for the inelasticity of the incommensurate signal? One approach is to view the chain as consisting not of the original S ϭ 1 degrees of freedom but of the composite spin degrees of freedom induced around holes. The latter interact through overlapping AFM polarization clouds and hole wave functions, to produce effective couplings of random sign because the impurity spacing can be even or odd multiples of the Ni-Ni separation. With weak interchain coupling, the ground state is likely to be a spin glass, as deduced from other experiments (10) on Y 2Ϫx Ca x BaNiO 5 . The "incommensurate" nature of the excitations continues to follow from the structure factor of the spin part of the hole wave functions.In summary, we have measured the magnetic fluctuations in single crystals of a doped one-dimensional spin liquid. At energies above the spin gap, the triplet excitations of the parent compound, Y 2 BaNiO 5 , persist with doping. However, below the gap, we find new excitations with a broad spectrum and characteristic wave vectors that are displaced from the zone boundary by an amount of...

With the discovery of high-temperature superconductivity, it seemed that the vision of superconducting power cables operating at the boiling point of liquid nitrogen (77 K) was close to realization. But it was soon found that the critical current density Jc of the supercurrents that can pass through these polycrystalline materials without destroying superconductivity is remarkably small. In many materials, Jc is suppressed at grain boundaries, by phenomena such as interface charging and bending of the electronic band structure. Partial replacement ('doping') of the yttrium in YBa2Cu3O(7-delta) with calcium has been used to increase grain-boundary Jc values substantially, but only at temperatures much lower than 77 K (ref. 9). Here we show that preferentially overdoping the grain boundaries, relative to the grains themselves, yields values of Jc at 77 K that far exceed previously published values. Our results indicate that grain-boundary doping is a viable approach for producing a practical, cost-effective superconducting power cable operating at liquid-nitrogen temperatures.

True atomic resolution of conductors and insulators is now routinely obtained in vacuum by frequency modulation atomic Ž force microscopy. So far, the imaging parameters i.e., eigenfrequency, stiffness and oscillation amplitude of the cantilever, . frequency shift which result in optimal spatial resolution for a given cantilever and sample have been found empirically. Here, we calculate the optimal set of parameters from first principles as a function of the tip-sample system. The result shows that the either the acquisition rate or the signal-to-noise ratio could be increased by up to two orders of magnitude by using stiffer cantilevers and smaller amplitudes than are in use today. q

The critical current density of grain boundaries in YBa2Cu3O 7−δ films is increased beyond the hitherto established limit by overdoping the superconductor. Using Ca-doping, values for the critical current density are obtained, that exceed the highest values reported ever for the undoped material by more than a factor of seven. Further, it is found that the overdoping strongly reduces the grain boundary normal-state resistivity. This systematic improvement of the grain boundary transport properties by doping is of great importance for applications of the high-Tc cuprates and gives insight into the mechanisms controlling the grain boundary behavior. 2for the highest values reported so far (see fig. 1) [10].

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