Hans-Joachim Grafe

75As NMR studies of superconducting LaFeAsO0.9F0.1.

We have performed 75As nuclear magnetic resonance measurements on aligned powders of the new LaFeAsO0.9F0.1 superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggests that superconducting vortices contribute to the relaxation rate when the field is parallel to the c axis but not for the perpendicular direction.

A carbon-wrapped nanoscaled thermometer for temperature control in biological environments

AIMS A carbon-wrapped nanoscaled thermometer for a contactless temperature control in biological systems on the cellular level is presented. MATERIALS & METHODS The thermometer is based on multiwalled carbon nanotubes (MWCNTs) filled with materials with strongly temperature-dependent nuclear magnetic resonance (NMR) parameters. The NMR frequency shift and relaxation time were measured in cuprous-iodide-filled CNTs at different temperatures. RESULTS The experimental data indicate a pronounced temperature dependence of the NMR parameters, thereby realizing the nanoscaled thermometer. CONCLUSION This study is a proof-of-concept that the functionalized CNTs can be used as a contactless thermometer in biomedical applications.

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.

75As NMR-NQR study in superconducting LiFeAs

Abstract We report results of 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments as well as 7Li NMR on different samples of self flux grown LiFeAs and 5% Co doped LiFeAs single crystals, and a polycrystalline LiFeAs sample. We were able to distinguish the samples by their slightly different quadrupole frequencies, νQ, which is a direct measure of the electric field gradient (EFG) at the As site. Interestingly, samples with a large quadrupole frequency appear to show a different Knight shift and spin lattice relaxation in the superconducting state from those with a lower νQ, yet all the samples are clearly superconducting. For sample S1 which has the largest νQ, we find constant Knight shift

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

\mathcal{K}

Common effect of chemical and external pressures on the magnetic properties of RCoPO (R = La, Pr, Nd, Sm). II.

across Tc for a certain direction of the magnetic field and a peculiar upturn of the NQR spin lattice relaxation rate (T1T)−1 below Tc. In contrast, samples with a lower νQ exhibit the expected behavior for a singlet superconductor: a drop of