5,769 research outputs found

    On the Seasonal and Day to Day Variations in the F2 Region

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    Thermoelectric scatter measurements on seasonal and daily variations in F region electron concentratio

    89Y NMR Probe of Zn Induced Local Magnetism in YBa2(Cu(1-y)Zn(y))3O(6+x)

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    We present detailed data and analysis of the effects of Zn substitution on the planar Cu site in YBa2_2Cu3_3O6+x_{6+x} (YBCO6+x_{6+x}) as evidenced from our 89^{89}Y NMR measurements on oriented powders. For x<<1x<<1 we find additional NMR lines which are associated with the Zn substitution. From our data on the intensities and temperature dependence of the shift, width, and spin-lattice relaxation rate of these resonances, we conclude that the spinless Zn 3dd10^{10} state induces local moments on the near-neighbour (% nn) Cu atoms. Additionally, we conjecture that the local moments actually extend to the farther Cu atoms with the magnetization alternating in sign at subsequent nnnn sites. We show that this analysis is compatible with ESR data taken on dilute Gd doped (on the Y site) and on neutron scattering data reported recently on Zn substituted YBCO6+x_{6 + x}. For optimally doped compounds 89^{89}Y nnnn resonances are not detected, but a large TT% -dependent contribution to the 89^{89}Y NMR linewidth is evidenced and is also attributed to the occurence of a weak induced local moment near the Zn. These results are compatible with macroscopic magnetic measurements performed on YBCO6+x_{6 + x} samples prepared specifically in order to minimize the content of impurity phases. We find significant differences between the present results on the underdoped YBCO6+x_{6 + x} samples and % ^{27}Al NMR data taken on Al3+^{3+} substituted on the Cu site in optimally doped La2_2CuO4_4. Further experimental work is needed to clarify the detailed evolution of the impurity induced magnetism with hole content in the cuprates.Comment: To be published in EPJB 15 pages of text and figures in eps forma

    Probing large distance higher dimensional gravity from lensing data

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    The modifications induced in the standard weak-lensing formula if Newtonian gravity differs from inverse square law at large distances are studied. The possibility of putting bounds on the mass of gravitons from lensing data is explored. A bound on graviton mass, esitmated to be about 100 Mpc−1^{-1} is obtained from analysis of some recent data on gravitational lensing.Comment: 6 pages, 1 figure, added reference

    Microscopic origin of local moments in a zinc-doped high-TcT_{c} superconductor

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    The formation of a local moment around a zinc impurity in the high-TcT_{c} cuprate superconductors is studied within the framework of the bosonic resonating-valence-bond (RVB) description of the t−Jt-J model. A topological origin of the local moment has been shown based on the phase string effect in the bosonic RVB theory. It is found that such an S=1/2S=1/2 moment distributes near the zinc in a form of staggered magnetic moments at the copper sites. The corresponding magnetic properties, including NMR spin relaxation rate, uniform spin susceptibility, and dynamic spin susceptibility, etc., calculated based on the theory, are consistent with the experimental measurements. Our work suggests that the zinc substitution in the cuprates provide an important experimental evidence for the RVB nature of local physics in the original (zinc free) state.Comment: The topological reason of local moment formation is given. One figure is adde
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