In iron-based superconductors the interactions driving the nematic order that breaks the lattice fourfold rotational symmetry in the iron plane may also facilitate the Cooper pairing, but experimental determination of these interactions is challenging because the temperatures of the nematic order and the order of other electronic phases appear to match each other or to be close to each other. Here we performed field-dependent 77 Se-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B 0 up to 16 T. The 77 Se-NMR spectra and Knight shift split when the direction of B 0 is away from the direction perpendicular to the iron planes (i.e. B 0 Pc) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the 77 Se nucleus, but these do not happen when B 0 is perpendicular to the iron planes, thus demonstrating that there is an orbital ordering. Moreover, stripe-type antiferromagnetism is absent, while giant antiferromagnetic spin fluctuations measured by the NMR spin-lattice relaxation gradually developed starting at ∼40 K, which is far below the nematic order temperature T nem =89 K. These results provide direct evidence of orbital-driven nematic order in FeSe.

Utilizing a novel method with the resonance frequency of a LC circuit, we measured the superconducting anisotropy of single crystals of an Fe-based superconductor FeSe with applied magnetic field up to 16 T. We found that the temperature dependence of the upper critical field Hc2(T) of FeSe coincides with the Werthamer-Helfand-Hohenberg (WHH) model when taking the Maki parameter α into consideration, suggesting an important role played by spin-paramagnetic effect in suppressing the superconductivity. When temperature T → 0, the values of Hc2,∥c(0) and Hc2,∥ab(0) derived from the WHH fitting are close to and fall within the range of the Pauli limit, for field H0 applied parallel to the c-axis and to the ab-plane, respectively. As compared with other typical iron-based high-Tc superconductors, lower values of Hc2(0) and higher superconducting anisotropy Γ(0) were observed in FeSe.

A bubble collapse experiment has been done on a garnet sample with the composition y 1.92 Sffio.1 Ca O . 98 Fe 4 . 02 GeO.98 0 12 both with and without an applied local microwave field. To explain the experimental results, the analysis of bubble domain waH motion by Callen and Josephs has been modified by introducing a nonlinear velocity-drive characteristic. In the case of no microwave excitation, a good fit between the theory and the experimental data is then obtained. A bubble collapse model in the presence of a microwave field which includes contributions from microwave-produced "dipolar pressure" and "intrinsic pressure" has been made. While some discrepancies exist between theory and experiment, nevertheless these results may be helpful in understanding the observations of microwave bubble generation reported by a number of research groups.

We experimentally investigate cluster structures formed by paramagnetic particles in a thin magnetorheological (MR) fluid layer, the thickness of which normalized by the particle diameter ranges from 3.8 to 31, under a dc magnetic field perpendicular to the fluid layer. We particularly focus on cluster structures formed in a dc magnetic field of high ramping rates, in which case the structures may be greatly different from those formed under equilibrium conditions. When a dc magnetic field is applied so rapidly, paramagnetic particles form chain clusters along the field direction as expected, but some chain clusters coalesce in the lateral direction to form bent-wall structures. We analyze the dependence of cluster structures on experimental parameters such as the thickness of the MR fluid layer and the intensity of the magnetic field.

The first observation of the process of magnetic bubble generation by microwave excitation has been made. A high-speed optical sampling microscope system was employed to sense the evolution of the bubble with time. The dependence of the microwave power threshold for bubble generation on experimental conditions such as microwave frequency, microwave pulse width, and dc bias field are reported. Magnetic garnet films with the composition Y1.92Sm0.1Ca0.98Fe4.02Ge0.98O12 were used as our experimental samples. The experimental results are not consistent with the theoretical predictions of the Seagle coherent rotation model for bubble generation. We find that the magnetization in the area driven by the strong microwave field responds rather noncoherently. Nucleation centers, which may be too small to be seen optically, are formed first. Under appropriate conditions, these centers may enlarge to a full size bubble. As examples, with a bias field setting of 65 Oe and the microwave frequency set at 550 MHz, about 25.5 dBm of power are required to generate a bubble from a 60-ns-long pulse; only 20.5 dBm are required if a 12-μs pulse is applied.