The cleaved surfaces of untwinned, single-crystal YBa2Cu3069 have been probed with synchrotronradiation photoemission, utilizing both high energy and angular resolution. Acute spectral structure was observed, both at the Fermi energy and at higher binding energies, particularly near the high-symmetry points of the two-dimensional Brillouin zone, I, X, Y, and S. Many band crossings of the Fermi energy were seen, with obvious and important differences between the bands near X and those near Y. A large superconducting gap was not observed: The data are consistent with a gap of less than 10 meV. The assignment of bands and Fermi-level crossings to chain and plane states will be discussed, including comparisons to the predictions of theory, particularly local-density-approximation calculations.
Thin films of Mn 4 N and related ternary metallic perovskites with perpendicular anisotropy are interesting for spintronics, but their magnetic structures differ from the triangular ferrimagnetism of the bulk. A temperature-independent anomalous Hall conductivity of −90 -1 cm -1 was found in Mn 4 N films, in addition to the normal temperature-dependent contribution. Based on known spin structures of bulk Mn 3 ZN compounds and the distance dependence of the Mn-Mn exchange for first-and second-neighbor Mn-Mn pairs, we propose a topological noncollinear spin structure for the Mn 4 N films with perpendicular anisotropy. The Letter shows how a small change in symmetry of the spin structure can influence the magnetotransport properties of frustrated ferrimagnetic films.
Low-temperature (260'C) and short-time (& 5-min) treatment of initially insulating YBa2Cu306 ) with Br2 gas converts the material into a 90-K superconductor. Bromine diffusion is found to be four orders of magnitude greater than oxygen diffusion at the same temperature. The crystal structure, phase purity, microstructure, thermal stability, and oxygen content have been studied using xray-diffraction Rietveld analysis, Raman spectroscopy, x-ray-fluorescence microprobe, and thermal gravimetric analysis. Superconducting and normal-state properties of the brominated material have been studied with diffuse infrared reflectivity and magnetization.
Temperature-dependent positron lifetime experiments have been performed from room temperature to cryogenic temperatures on Ba1−xKxBiO3. for x=0.4 and 0.5. From the temperature dependence of the positron lifetime in the normal state, we observe a clear signature of competition between separate defect populations to trap the positron. Theoretical calculations of lifetimes of free or trapped positrons have been performed on Ba1−xKxBiO3, to help identify these defects. Lifetime measurements separated by long times have been performed and evidence of aging effects in the sample defect populations is seen in these materials.
PrBa2Cu2NbO, (PrBCNO) is an insulator analogous to the PrBa2Cu30& system having a similar structure but with Nb02 planes replacing the CuO chains. Single-phase polycrystalline samples of RBCNO have been synthesized with R =La, Pr, or Nd. These materials were characterized using magnetization, thermal gravimetric analysis in different atmospheres, x-ray diffraction, and Raman spectroscopy. The PrBCNO samples show a signature in the magnetization of a magnetic ordering at 12 K. No such magnetic phase transition is observed down to 2 K in NdBa2Cu2Nb08.
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