We have performed ${}^{75}\mathrm{As}$ nuclear magnetic resonance measurements on aligned powders of the new $\mathrm{LaFeAs}{\mathrm{O}}_{0.9}{\mathrm{F}}_{0.1}$ 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.

Microwave absorption measurements in magnetic fields from 0 up to 16 T were used to determine the temperature range of superconducting fluctuations above the superconducting critical temperature Tc in YBa2Cu3O7-$\delta${}. Measurements were performed on deeply underdoped, slightly underdoped, and overdoped single crystals. The temperature range of the superconducting fluctuations above Tc is determined by an experimental method which is free from arbitrary assumptions about subtracting the nonsuperconducting contributions to the total measured signal and/or theoretical models to extract the unknown parameters. The superconducting fluctuations are detected in the ab plane, and c-axis conductivity, by identifying the onset temperature T$'${}. Within the sensitivity of the method, this fluctuation regime is found only within a fairly narrow region above Tc. Its width increases from 7 K in the overdoped sample (Tc=89$\hskip0.3em${}K) to, at most, 23 K in the deeply underdoped sample (Tc=57$\hskip0.3em${}K), so that T$'${} falls well below the pseudogap temperature T*. Implications of these findings are discussed in the context of other experimental probes of superconducting fluctuations in the cuprates.

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.

The microwave response of high quality niobium films in a perpendicular static magnetic field has been investigated. The complex frequency shift was measured up to the upper critical fields. The data have been analyzed by the effective conductivity model for the type-II superconductors in the mixed state. This model is found to yield consistent results for the coherence lengths in high-$\kappa$ superconducting samples, and can be used with high-temperature superconductors even at temperatures much below $T_c$. It is shown that for samples with high values of depinning frequency, one should measure both components of the complex frequency shift in order to determine the flow resistivity. The thick Nb film $\left(160\mathrm{nm}\right)$ has low resistivity at $10\mathrm{K}$, comparable to the best single crystals, and low $\kappa$ value. In contrast, the thinnest $\left(10\mathrm{nm}\right)$ film has $\kappa \approx 9.5$ and exhibits a high depinning frequency $(\approx 20\mathrm{GHz})$. The upper critical field determined from microwave measurements is related to the radius of nonoverlaping vortices, and appears to be larger than the one determined by the transition to the normal state.

Cavity perturbation by superconducting films is treated in an unified way for the sample positions in both magnetic and electric microwave fields. The role of demagnetizing and depolarizing effects in the boundary conditions of the fields is analyzed. The general solutions for the complex frequency shift are specified for the samples having slab geometry and the field being parallel to the plane of the sample. For electromagnetically thick samples, the shifts for samples placed in the magnetic and electric fields are found to have the same magnitude and temperature dependence, while for thin films dramatic differences are obtained. The magnitude of the shift is reduced in the magnetic and increased by orders of magnitude in the electric field. A remarkable feature in the temperature dependence of the real frequency shift in the electric field is obtained. Experiments are performed on an ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin film, and all the predictions of the theory are confirmed. It is also shown that microwave cavity perturbation and ac susceptibility measurements in a dc magnetic field can be covered by the same theory. Their profoundly different temperature dependence can be accounted for by their different frequencies.