Recently, another consequence of the size of the Caions has been discovered. Iyo et al. 15 have found that a family of ordered CaAFe 4 As 4 (1144) compounds can be formed for A = K, Rb, Cs where the key to the formation is the difference in ionic size between the Ca and the A ion. This family is not a (Ca 1−x A x )Fe 2 As 2 solid-solution, where the Ca and A ions randomly occupy a single crystallographic site, 16 but rather is a distinct, quaternary, line compound in which the Ca and A sites form alternating planes along the crystallographic c-axis, separated by FeAs slabs 15 . In essence, the CaAFe 4 As 4 structure is identical to the CaFe 2 As 2 structure, just with layer by layer segregation of the Ca and A ions. The 1144 structure was also found for SrAFe 4 As 4 (A = Rb, Cs). Solid-solutions of Ca (Sr) 122 structures were found for arXiv:1605.05617v2 [cond-mat.supr-con]

The in-plane resistivity anisotropy is studied in strain-detwinned single crystals of FeSe. In contrast to other iron-based superconductors, FeSe does not develop long-range magnetic order below the nematic/structural transition at Ts ≈90 K. This allows for the disentanglement of the contributions to the resistivity anisotropy due to nematic and magnetic orders. Comparing direct transport and elastoresistivity measurements, we extract the intrinsic resistivity anisotropy of strainfree samples. The anisotropy peaks slightly below Ts and decreases to nearly zero on cooling down to the superconducting transition. This behavior is consistent with a scenario in which the in-plane resistivity anisotropy in FeSe is dominated by inelastic scattering by anisotropic spin fluctuations.PACS numbers: 74.70. Xa, 74.25.Ld Electronic nematicity has emerged as a key concept in iron-based superconductors since the observation of inplane resistivity anisotropy in stress-detwinned crystals of Co-doped BaFe 2 As 2 [1, 2]. The fact that the resistivity anisotropy is much larger than what is expected from the small lattice distortion led to the proposal that the tetragonal-to-orthorhombic transition in the iron pnictides is driven not by phonons, but by an electronic nematic phase. Subsequent experiments revealed an intricate dependence of the resistivity anisotropy on doping (a sign change between electron-and hole-doped materials [2-6]), and disorder [7,8], sparking hot debates about its microscopic origins (see Refs. [9 and 10] for reviews).Electronic contributions involved in the in-plane resistivity anisotropy [10] can be separated into the Drude weight and/or of the scattering rate anisotropies. Fermisurface anisotropies arising, for instance, from the ferroorbital order triggered at the nematic transition, affect mostly the Drude weight [11][12][13]. Anisotropic scattering, can be due to elastic processes, such as the development of local magnetic order around an impurity [14,15], or inelastic processes, such as the scattering of electrons by anisotropic magnetic fluctuations [16,17] known to exist below T s [18]. Recent stress-dependent optical reflectivity studies in Co-doped BaFe 2 As 2 point to a dominant effect of the Drude weight [19,20]. However, stripe magnetic order appearing at the magnetic transition severely complicates the analysis. This is because the magnetic state breaks tetragonal symmetry leading to an anisotropic reconstruction of the Fermi surface [7,21] and to the appearance of "Dirac cones" [22], which may dramatically alter the resistivity anisotropy [23]. Disentangling these contributions is fundamental to reveal the origin of the resistivity anisotropy and, consequently, of the nematic state.In this context, the stoichiometric FeSe [24] is an ideal system. It is rather clean (residual resistivity ratios as high as 50 [25]) and its orthorhombic/nematic phase transition at T s ≈ 90 K is not accompanied by a longrange magnetic order [26] eliminating effects of Fermi surface folding.In this Letter we rep...

Using angle-resolved photoemission spectroscopy (ARPES), it is revealed that the low-energy electronic excitation spectra of highly underdoped superconducting and nonsuperconducting La(2-x)Sr(x)CuO(4) cuprates are gapped along the entire underlying Fermi surface at low temperatures. We show how the gap function evolves to a d(x(2)-y(2)) form with increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates. Our results provide essential information for uncovering the symmetry of the order parameter(s) in strongly underdoped cuprates, which is a prerequisite for understanding the pairing mechanism and how superconductivity emerges from a Mott insulator.