Polarized Raman-scattering spectra of superconducting, single-crystalline FeSe evidence pronounced phonon anomalies with temperature reduction. A large (∼ 6.5%) hardening of the B1g(Fe) phonon mode is attributed to the suppression of local fluctuations of the iron spin state with the gradual decrease of the iron paramagnetic moment. The ab-initio lattice dynamic calculations support this conclusion. The enhancement of the low-frequency spectral weight above the structural phase transition temperature Ts and its change below Ts is discussed in relation with the opening of an energy gap between low (S = 0) and higher spin states which prevents magnetic order in FeSe. The very narrow phonon line widths compared to observations in FeTe suggests the absence of intermediate spin states in the fluctuating spin state manifold in FeSe.

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered

The magnetic and superconducting properties of a series of underdoped Ba 1−x Na x Fe 2 As 2 (BNFA) single crystals with 0.19 x 0.34 have been investigated with the complementary muon-spin-rotation (μSR) and infrared spectroscopy techniques. The focus has been on the different antiferromagnetic states in the underdoped regime and their competition with superconductivity, especially for the ones with a tetragonal crystal structure and a so-called double-Q magnetic order. Besides the collinear state with a spatially inhomogeneous spincharge-density wave (i-SCDW) order at x = 0.24 and 0.26, that was previously identified in BNFA, we obtained evidence for an orthomagnetic state with a "hedgehog"-type spin vortex crystal (SVC) structure at x = 0.32 and 0.34. Whereas in the former i-SCDW state the infrared spectra show no sign of a superconducting response down to the lowest measured temperature of about 10 K, in the SVC state there is a strong superconducting response similar to the one at optimum doping. The magnetic order is strongly suppressed here in the superconducting state and at x = 0.34 there is even a partial reentrance into a paramagnetic state at T T c .

The magnetic response of CaK(Fe 0.949 Ni 0.051) 4 As 4 was investigated by means of the muon spin rotation/relaxation. The long-range commensurate magnetic order sets in below the Néel temperature T N = 50.0(5) K. The density-functional theory calculations have identified three possible muon stopping sites. The experimental data were found to be consistent with only one type of magnetic structure, namely, the long-range magnetic spin-vortex-crystal order with the hedgehog motif within the ab plane and the antiferromagnetic stacking along the c direction. The value of the ordered magnetic moment at T ≈ 3 K was estimated to be m Fe = 0.38(11)μ B (μ B is the Bohr magneton). A microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature T c 9 K is observed. Comparison with 11, 122, and 1144 families of Fe-based pnictides points to existence of correlation between the reduction of the magnetic order parameter at T → 0 and the ratio of the transition temperatures T c /T N. Such correlations were found to be described by Machida's model for coexistence of itinerant spin-density-wave magnetism and superconductivity [K.

The band structure of a carbon-nitrogen nanotube of “zigzag” type (8, 0) with nitrogen atom concentrations of 6.25%, 12.5%, and 25% and “armchair” (5, 5) carbon-nitrogen nanotube with nitrogen atom concentrations of 10% and 20% is calculated within the density functional theory approach. The calculations are carried out taking into account optimizations of both the longitudinal and transverse nanotube structural parameters. The structure is distorted significantly both lengthwise and across with increasing nitrogen concentrations. In particular, their cross-sectional profile ceases to be a circle. Dependence of the energies of interband electronic transitions on the concentration of nitrogen can be observed in the form of a peak shift in the optical absorption spectrum.