Spin-Peierls state versus Néel state in doped CuGeO3.

Electron spin resonance ͑ESR͒ of Gd 3ϩ in the normal state (TϾT c ) of R 1Ϫx Gd x Ni 2 B 2 C (RϭY,Lu) is reported. The results show that the exchange coupling between the rare-earth localized magnetic moment and the conduction electrons depends on the conduction electrons momentum transfer (͉k F in Ϫk F out ͉ϭq), i.e., J f s (q). The temperature dependence of the ESR linewidth yields a value for one of the exchange parameters, ͗J f s 2 (q)͘ E F 1/2 , which is in agreement with that estimated from the slope of the initial linear decrease of T c by the Gd 3ϩ impurities. These results indicate that the R 1Ϫx Gd x Ni 2 B 2 C (RϭY,Lu) compounds behave as conventional BCS superconductors, in agreement with previous reports.

The Electron Spin Resonance (ESR) of diluted magnetic ions (MI) of Er3+, Yb3+ and Mn2+ in Ag nanoparticles (NPs) is reported. Monodisperse samples of Ag NPs doped with these MI were synthesized by reducing silver nitrate and MI-oxides. This simple method can be extended to all rare-earths. The measurements of the g-values and hyperfine splittings indicates that the MI are located at cubic sites in the Ag:MI NPs. The ESR spectra show that there is no g-shift and Korringa-relaxation due to the exchange interaction between the MI and the conduction electrons, suggesting that the exchange interaction is absent in the Ag:MI NPs. Thus, the nature of this interaction needs to be reexamined at the nanoscale range.

This paper presents the synthesis and characterization of colloidal NaYF4 and NaYF4:20% Gd lanthanide nanocrystals. The nanoparticles were prepared by chemical route using co-thermolysis of Na(CF3COO), Y(CF3COO)3 and Gd(CF3COO)3 precursor in oleylamine surfactant/phenylether at Ts = 250 degrees C. By tuning the precursor/surfactant molar ratio during the process, it was possible to control the crystalline phase, chemical order and size of the nanocrystals. The nanocrystals were characterized by Transmission Electron Microscopy, Small Angle X-ray Scattering, powder X-ray Diffraction, dc-magnetization and Electron Spin Resonance (ESR) techniques. The ESR experiments show the so called U-spectrum for the Gd3+ ions in bulk counterpart materials, where characteristic powder spectra of cubic and lower crystal field symmetries were observed.

dc magnetization measurements on the new Tl-Ca-Ba-Cu-0 superconductor show superconductivity with an onset temperature as high as 118 K, 24 K higher than that of a high-quality Eu-Ba2Cu307sample. The resistive onsets of these new superconductors are near 140 K. The magnetic data show complete diamagnetic flux exclusion and 10-15% Meissner expulsion at 10 K.Discoveries of 30-K superconductivity in the La-Ba-Cu-0 system' and 90-K superconductivity in the Y-Ba-Cu-0 system have stimulated a worldwide race for higher-temperature superconductors.