^{75}As, ^{87}Rb, and ^{85}Rb nuclear quadrupole resonance (NQR) and ^{87}Rb nuclear magnetic resonance measurements in a RbFe_{2}As_{2} iron-based superconductor are presented. We observe a marked broadening of the ^{75}As NQR spectrum below T_{0}≃140  K which is associated with the onset of a charge order in the FeAs planes. Below T_{0} we observe a power-law decrease in the ^{75}As nuclear spin-lattice relaxation rate down to T^{*}≃20  K. Below T^{*} the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in light of an orbital selective behavior expected for the electronic correlations.

The ground-state magnetic structure of EuNi2As2 was investigated by single-crystal neutron diffraction. At base temperature, the Eu 2+ moments are found to form an incommensurate antiferromagnetic spiral-like structure with a magnetic propagation vector of k = (0, 0, 0.92). They align ferromagnetically in the ab plane with the moment size of 6.75(6) µB, but rotate spirally by 165.6(1)°around the c axis from layer to layer. The magnetic order parameter in the critical region close to the ordering temperature, TN = 15 K, shows critical behavior with a critical exponent of βEu = 0.34(1), consistent with the three-dimensional Heisenberg model. Moreover, within the experimental uncertainty, our neutron data is consistent with a model in which the Ni sublattice is not magnetically ordered.

PACS 72.15.Jf, 74.25.fg, 74.70.Xa In-plane longitudinal and transverse thermoelectric phenomena in two parent compounds of iron-based superconductors are studied. Namely, the Seebeck (S) and Nernst (ν) coefficients were measured in the temperature range 10 -300 K for BaFe2As2 and CaFe2As2 single crystals that were detwinned in-situ. The thermoelectric response shows sizeable anisotropy in the spin density wave state (SDW) for both compounds, while some dissimilarities in the vicinity of the SDW transition can be attributed to the different nature of the phase change in BaFe2As2 and CaFe2As2. Temperature dependences of S and ν can be described within a two-band model that contains a contribution from highly mobile, probably Dirac, electrons. The Dirac band seems to be rather isotropic, whereas most of the anisotropy in the transport phenomena could be attributed to "regular" hole-like charge carriers. We also observe that the off-diagonal element of the Peltier tensor αxy is not the same for the a and b orthorhombic axes, which indicates that the widely used Mott formula is not applicable to the SDW state of iron-based superconductors.

Studies of the copper-based superconductors demonstrate how their phasediagram becomes more complex as experimental probes improve, able to distinguish among subtly different electronic phases. One of those phases, nematicity, has become the matter of great interest also in the iron-based superconductors, where it is detected deep in the tetragonal state. Here we present the evolution of the in-plane Nernst effect anisotropy in the strain detwinned Ca(Fe1-xCox)2As2 single crystals. We interpret the observed behaviour using an approach developed to describe the nematic order parameter in liquid crystals. We also apply the same model to data from other superconductors: Ba(Fe1-xCox)2As2 and YBa2Cu3Oy. We conclude the observed broken rotational symmetry of the electronic system is a consequence of the emerging thermodynamic electronic nematic order at a temperature much higher than onset of the magnetic and structural transitions.

The simultaneous presence of a Fe-related spin-density wave and antiferromagnetic order of Eu 2+ moments ranks EuFe 2 As 2 among the most interesting parent compounds of iron-based pnictide superconductors. Here we explore the consequences of the dilution of Eu 2+ magnetic lattice through on-site Ca substitution. By employing macro-and microscopic techniques, including electrical transport and magnetometry, as well as muon-spin spectroscopy, we study the evolution of Eu magnetic order in both the weak and strong dilution regimes, achieved for Ca concentration x(Ca) = 0.12 and 0.43, respectively. We demonstrate the localized character of the Eu antiferromagnetism mediated via RKKY interactions, in contrast with the largely itinerant nature of Fe magnetic interactions. Our results suggest a weak coupling between the Fe and Eu magnetic sublattices and a rapid decrease of the Eu magnetic interaction strength upon Ca substitution. The latter is confirmed both by the depression of the ordering temperature of the Eu 2+ moments, T N , and the decrease of magnetic volume fraction with increasing x(Ca). We establish that, similarly to the EuFe 2 As 2 parent compound, the investigated Ca-doped compounds have a twinned structure and undergo a permanent detwinning upon applying an external magnetic field. arXiv:1805.03896v2 [cond-mat.mtrl-sci]