Large temperature dependence of the number of carriers in co-doped BaFe(2)As(2).

Using angle-resolved photoemission spectroscopy, we study the evolution of the number of carriers in Ba(Fe(1-x)Co(x))(2)As(2) as a function of Co content and temperature. We show that there is a k-dependent energy shift compared to density functional calculations, which is large below 100 K at low Co contents and reduces the volume of hole and electron pockets by a factor 2. This k shift becomes negligible at high Co content and could be due to interband charge or spin fluctuations.

NMR study in the iron-selenide Rb0.74Fe1.6Se2: determination of the superconducting phase as iron vacancy-free Rb0.3Fe2Se2.

77Se and 87Rb nuclear magnetic resonance (NMR) experiments on Rb0.74Fe1.6Se2 reveal clearly distinct spectra originating from a majority antiferromagnetic (AF) and a minority metallic-superconducting (SC) phase.

Doping effects in the coupled, two-leg spin ladder BiCu(2)PO(6).

We report preparation, x-ray diffraction, magnetic susceptibility χ(T) and heat capacity C(p)(T) measurements on undoped samples as well as samples with Zn-doped (S = 0) at the Cu site BiCu(2(1-x))Zn(2x)PO(6), Ni-doped (S = 1) at the Cu site BiCu(2(1-y))Ni(2y)PO(6), and Ca doped (holes) at the Bi site Bi(1-z)Ca(z)Cu(2)PO(6) in the coupled two-leg spin ladder system BiCu(2)PO(6). While, Zn shows complete solid solubility, Ni could be doped to about 20% and Ca to about 15%. Magnetization and heat capacity data in the undoped compound point towards the existence of frustration effects.

Impurity-induced magnetic order in low-dimensional spin-gapped materials.

We have studied the effect of nonmagnetic Zn impurities in the coupled spin ladder Bi(Cu_{1-x}Zn_{x})_{2}PO_{6} using ;{31}P NMR, muon spin resonance (microSR), and quantum Monte Carlo simulations. Our results show that the impurities induce in their vicinity antiferromagnetic polarizations, extending over a few unit cells. At low temperature, these extended moments freeze in a process which is found universal among various other spin-gapped compounds: isolated ladders, Haldane, or spin-Peierls chains.