M. Onellion

Anisotropic magnetoresistance in tetragonal La1-xCaxMnOδ thin films

We have fabricated thin films of La1−xCaxMnOδ with tetragonal symmetry. For low temperatures and magnetic fields the measured magnetoresistance is anisotropic: initially positive for applied magnetic field perpendicular to the film plane and negative for field applied parallel to the film plane. At high temperatures the magnetoresistance is negative for all fields and field orientations. We also observe an in‐plane magnetoresistance anisotropy with an angular dependence corresponding to that observed in transition metal ferromagnets. We suggest an interpretation requiring a substantial spin‐orbit interaction in the material.

Temperature Dependence of the Superconducting Gap Anisotropy in Bi2Sr2CaCu2O8+x

Detailed data on the momentum-resolved temperature dependence of the superconducting gap of Bi2Sr2CaCu2O8+x are presented, complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the Γ-Mand Γ-X directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors, such as lead. Specifically, the size of the superconducting gap along the Γ-X direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the Γ-M direction. These data rule out the simplest type of d-wave order parameter.

Observation of a van Hove singularity in Bi2Sr2CaCu2O8+x with angle-resolved photoemission.

Keywords: HIGH-TC SUPERCONDUCTORS ; DENSITY-OF-STATES ; FERMI-SURFACE ; ELECTRONIC-STRUCTURE ; TEMPERATURE Note: Univ wisconsin,ctr appl superconduct,madison,wi 53706. ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland. Ma, j, univ wisconsin,dept phys,1150 univ ave,madison,wi 53706.ISI Document Delivery No.: QH079 Reference LSE-ARTICLE-1995-019View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12

CHROMIUM OXIDE FILMS FABRICATED BY CR(CO)6 CHEMICAL VAPOR DEPOSITION

Abstract Cr2O3 and ferromagnetic chromium dioxide have been fabricated by ultraviolet photolytic and plasma-assisted decomposition of Cr(CO)6 in oxygen carrier gas. The photolysis process is surface mediated and proceeds by the sequential removal of (CO) groups. It yields different surface morphologies, and control of the substrate temperature is essential for optimum results. The plasma-assisted deposited films exhibit long-range ferromagnetic order and sufficient flatness to be utilized as a magneto-optic recording medium. The thermodynamic cycle of the plasma process was determined. Consequences for using such films are discussed.

Advances in single-crystal Bi2Sr2CaCu2O8+δ superconductors

Abstract Susceptibility, resistivity and X-ray diffraction measurements have been performed on single crystals of Bi2Sr2CaCu2O8+δ. Very sharp (0.5 K width) superconducting transition temperatures as low as 55 K have been reached in the overdoped part of the phase diagram. Anneals in low oxygen pressures have allowed us to access the entire undordoped region of the phase diagram. The samples have been reversibly changed from a high-temperature superconductor to an insulator.

Doping-induced change of superconducting gap anisotropy in Bi2Sr2Ca1Cu2O8+δ

High-resolution angle-resolved photoemission measurements were performed on single crystals of Bi2Sr2Ca1Cu2O8+δ with different oxygen stoichiometries. The data establish that the gap anisotropy (ratio of the gap along Γ-M to the gap along Γ-X) can be reversibly changed from ∼20:1 (optimal or underdoped) to ∼2:1 (overdoped). Differences in sample doping explain the conflicting reports on gap anisotropy in the literature. Possible effects of this change in gap anisotropy on the symmetry of the order parameter are discussed. There remains some ambiguity as to the relation between the order parameter and doping.

Superfluid and normal fluid density in high-Tc superconductors

The electronic properties of the cuprate superconductors have been studied by measuring the reflectance over the frequency range from the far-infrared to the near-ultraviolet(roughly, 10 meV-5 eV). There is an interesting behavior in both the normal state and the superconducting state. In the normal state, there is the well-known non-Drude distribution of the doping-induced spectral weight. In the superconducting state, the spectral weight of the superconducting condensate correlates with Tc in a variety of materials. Remarkably, in optimally doped superconductors, only about 20% of the doping-induced carriers joins the superfluid; the rest of the spectral weight remains at finite frequencies.

Pb0.4bi1.6sr2cacu2o8+X and Oxygen Stoichiometry - Structure, Resistivity, Fermi-Surface Topology, and Normal-State Properties

Keywords: ANGLE-RESOLVED-PHOTOEMISSION ; CU-O SUPERCONDUCTORS ; ELECTRONIC-STRUCTURE ; MODULATED STRUCTURE ; CONVERGENT-BEAM ; BAND-STRUCTURE ; BI2SR2CACU2O8 ; PB ; YBA2CU3O7-DELTA ; YBA2CU3O6.9 Note: Univ wisconsin,ctr appl superconduct,madison,wi 53706. ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland. Ma, ja, univ wisconsin,dept phys,1150 univ ave,madison,wi 53706.ISI Document Delivery No.: QT246 Reference LSE-ARTICLE-1995-022View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12

High-Temperature Superconductor Ba2ycu3o7-X - Plasmon and Ultraviolet Optical-Transition Studies

Abstract We present the first electron energy loss data on a high-temperature superconductor. A detailed comparison with photoemission energy distribution curves and partial-yield spectra identifies the nature of the corresponding features. This analysis shows that the bulk plasmon energy is 24.6 eV, and that there are two peaks in the density of empty states, 2.3–2.5 eV and 4.3–4.6 eV above the Fermi level.

Fabrication of ferromagnetic and antiferromagnetic chromium oxides by organometallic chemical vapor deposition

We have been able to fabricate CrO2 and Cr2O3 from ultraviolet photolytic and plasma‐assisted decomposition of Cr(CO)6 in an oxygen carrier gas. The composition of these films is uniform in composition and contains little contamination. Using magneto‐optic Kerr effect measurements, we have been able to demonstrate that the CrO2 films are ferromagnetic.