F. Hammerath

Nanoscale electronic order in iron pnictides.

The charge distribution in RFeAsO1-xFx (R=La,Sm) iron pnictides is probed using As nuclear quadrupole resonance. Whereas undoped and optimally doped or overdoped compounds feature a single charge environment, two charge environments are detected in the underdoped region. Spin-lattice relaxation measurements show their coexistence at the nanoscale. Together with the quantitative variations of the spectra with doping, they point to a local electronic order in the iron layers, where low- and high-doping-like regions would coexist. Implications for the interplay of static magnetism and superconductivity are discussed.

Lithium dynamics in carbon-rich polymer-derived SiCN ceramics probed by nuclear magnetic resonance

Abstract We report 7Li, 29Si, and 13C NMR studies of two different carbon-rich SiCN ceramics SiCN-1 and SiCN-3 derived from the preceramic polymers polyphenylvinylsilylcarbodiimide and polyphenylvinylsilazane, respectively. From the spectral analysis of the three nuclei, we find that only the 13C spectrum is strongly influenced by Li insertion/extraction, suggesting that carbon phases are the major electrochemically active sites for Li storage. Temperature (T) and Larmor frequency ( ω L ) dependences of the 7Li linewidth and spin-lattice relaxation rates T 1 − 1 are described by an activated law with the activation energy E A of 0.31 eV and the correlation time τ 0 in the high temperature limit of 1.3 ps. The 3 / 2 power law dependence of T 1 − 1 on ω L which deviates from the standard Bloembergen, Purcell, and Pound (BPP) model implies that the Li motion on the μs timescale is governed by continuum diffusion mechanism rather than jump diffusion. On the other hand, the rotating frame relaxation rate T 1 ρ − 1 results suggest that the slow motion of Li on the ms timescale may be affected by complex diffusion and/or non-diffusion processes.

Electronic properties of LaO1−xFxFeAs in the normal state probed by NMR/NQR

We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO1?xFxFeAs superconductor for x=0 and 0.1 at temperatures up to 480?K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x=0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for all nuclei also scale with one another, despite the fact that the form factors for each site sample different regions of q-space. This result suggests a lack of any q-space structure in the dynamical spin susceptibility that might be expected in the presence of antiferromagnetic correlations. Rather, our results are more compatible with simple quasi-particle scattering. Furthermore, we find that the increase in the electric field gradient at the As cannot be accounted for by LDA calculations, suggesting that structural changes, in particular the position of the As in the unit cell, dominate the NQR response.

Poisoning effect of Mn in LaFe1-xMnxAsO0.89F0.11: Unveiling a quantum critical point in the phase diagram of iron-based superconductors

A superconducting-to-magnetic transition is reported for LaFe

As vacancies, local moments, and Pauli limiting in LaFeAs1−δO0.9F0.1superconductors

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Spin gap in the zigzag spin-1/2 chain cuprate Sr(0.9)Ca(0.1)CuO(2).

Mn

Diluted paramagnetic impurities in nonmagnetic Ba2YIrO6

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Progressive slowing down of spin fluctuations in underdoped LaFeAsO 1 − x F x

AsO

Effect of different in-chain impurities on the magnetic properties of the spin chain compound SrCuO2 probed by NMR

_{0.89}

Effects of Quantum Spin-1/2 Impurities on the Magnetic Properties of Zigzag Spin Chains

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