[en] Desorption of O₂ in certain high-T_c superconductors was observed from scraping the surfaces of superconductors by Rosenberg and Wen. Their conclusion agrees with the results from the observation of O₂ trapping by a laser ablation of superconductors. However, the local heating due to the scraping process can possibly raise the local surface temperature significantly higher than the temperature of the bulk

[en] A recent theoretical prediction of the magnetic order to be expected in an erbium barium copper oxide high-temperature superconductor is shown to be based on questionable grounds

[en] By comparing data of the angular dependence of the remanent magnetization for a ceramic sample and for a single crystal of YBa₂Cu₃O₇ with similar demagnetization factors, we conclude that intrinsic anisotropy contributes significantly to anisotropic flux trapping in the crystal

[en] We present an alternate interpretation to the comments by P. Thalmeier [preceding paper, Phys. Rev. B 44, 7120 (1991)] based on the assumption that the antiferromagnetic transition is a secondary effect of some other phase transition at T_N [C. M. Varma (unpublished); see also Palstra et al., Phys. Rev. Lett. 55, 2727 (1985)]. This view is motivated by the observation that the ordered moment in URu₂Si₂ is very small (∼0.02μ_B) and similar to the ordered moment in UPt₃, another heavy-fermion system, and yet the specific-heat anomaly is at least 2 orders of magnitude larger in URu₂Si₂ than in UPt₃

[en] The choice of Pb as a counterelectrode is defended in high-T/sub c/ materials as well as the subsequent analysis of the NbN data

[en] It is pointed out that Stewart (Phys. Rev. B, to be published) is concerned with a series of approximations which are frequently made in practice, with good results for real materials

[en] Inelastic-scattering cross sections of 300--10 000-eV electrons in Si, Ti, Fe, and Pd and of 300--2000-eV electrons in Cu, Ag, and Au have been investigated theoretically and experimentally. The product of the inelastic mean free path and the cross section were determined experimentally through an analysis, based on a recent formula, of reflection electron-energy-loss spectra. To study energies above 2000 eV a special experimental setup was developed. Theoretical cross sections were determined through a dielectric-response description of the solid-electron interaction using Drude-Lindhard model dielectric functions. At high energies, good agreement is observed between theoretically and experimentally determined cross sections. At lower energies differences are observed. These are discussed partly in terms of a breakdown of the dielectric model used, and partly surface excitations

[en] Within the framework of the BCS pairing approximation, a theory of nonlinear electromagnetic rectification in a Cooper-paired jellium is established. It is pointed out that the electromagnetic rectification phenomenon, inevitably present as the many-body system's reaction in a second-harmonic generation process, is of special interest in a superconductor where induced dc currents flow without resistance. Based on a density-matrix approach the nonlinear response tensor is derived and discussed. The importance of nonlocality for the rectification process in centrosymmetric media is stressed. A simple expression for the response tensor is derived in the limit in which the nonlocal pairing effects are included to lowest order. It is demonstrated that electromagnetic rectification occurs only when the nonlinear response has a diamagnetic part. By considering the electromagnetic-rectification phenomenon as a nonlinear-dynamical Meissner effect, the relation to the linear Meissner effect is emphasized. A qualitative discussion of the rectification process at a superconductor-vacuum interface is undertaken. An integrodifferential equation describing, on the basis of the semiclassical infinite-barrier model, the diamagnetic screening of the nonlinear dc magnetic field is set up and discussed

[en] We analyze the possible existence of incommensurate correlations in the t-J model as a function of doping and in the J₁-J₂-J₃ spin-1/2 Heisenberg model as a function of frustration using exact diagonalization techniques on small lattices. For the t-J model we found that when holes are introduced the spin structure factor S(q) shifts the position of its maximum away from (π,π). However, we do not observe indications of the existence of long-range incommensurate order. The frustrated Heisenberg model is analyzed using spin-wave techniques. We present the phase diagram including 1/S corrections. It has special regions where there is no magnetic order that are natural candidates for new nonclassical ground states. Using a Lanczos method we study the line J₃=J₂/2, which is completely disordered in the spin-wave analysis for J₂/J₁>1/4. S(q) behaves as in the t-J model replacing doping by frustration. We also study the square of order parameters related with Neel, twisted, dimer, and chiral order. After the Neel order is destabilized by frustration, the twisted and dimer states are enhanced. On the other hand, there are no indications that the uniform or staggered chiral state can be the ground state of the frustrated Heisenberg model

[en] Icosahedral and amorphous structures have been produced in multilayered Al-V by ion-beam mixing. In contrast to the possibilities with the Al-Mn alloy system, we find that the formation of ''single-phase'' icosahedral Al-V at higher temperatures is inhibited by a coexisting, as yet unreported, bcc phase (a = 3.82 A). When the mixing is performed with 500-keV Xe⁺, transmission-electron-microscopy (TEM) --amorphous structures have been obtained at liquid-nitrogen temperature in an extended concentration regime (15--26 at. % V). Electrical resistivity data for all the alloys studied show a negative temperature dependence which cannot be explained quantitatively in the framework of current theories. The Hall coefficient in glassy Al-V is found to change sign and become positive above 16 at. % V. This fact is discussed in the context of existing theories for the sign of the Hall coefficient. As far as we know, this is the first time such a change of sign has been observed for a nonmagnetic glassy metallic system which is not of the ''early-late'' transition-metal type