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. We further reveal that the bands shift with temperature, changing significantly the number of carriers they contain (up to 50%). We explain this evolution by thermal excitations of carriers among the narrow bands, possibly combined with a temperature evolution of the k-dependent 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. The very narrow NMR line of the SC phase evidences the absence of Fe vacancies and any trace of AF order. The Rb content of the SC phase is deduced from intensity measurements identifying Rb(0.3(1))Fe2Se2 as the actual composition of the SC fraction. The resulting estimate of 0.15 electrons/Fe brings this class of superconductors 245 family closer to the other Fe-based superconductor families.

Dynamics of the local moment induced by nonmagnetic defects in cuprates

We present a study of the spin dynamics of magnetic defects induced by Li substitution of the plane Cu in the normal state of YBa2Cu3O6+x. The fluctuations of the coupled Cu magnetic moments in the vicinity of Li are probed by near-neighbor 89Y and 7Li NMR spin lattice relaxation. The data indicate that the magnetic perturbation fluctuates as a single entity with a correlation time tau which scales with the local static susceptibility. This behavior is reminiscent of the low T Kondo state of magnetic impurities in conventional metals. Surprisingly it extends well above the "Kondo" temperature for the underdoped pseudogapped case.

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. In all the samples, the χ(T) at low temperature increases with doping content. The Zn-induced susceptibility is smaller than that due to effective S = 1/2 moments, possibly due to frustrating next-nearest-neighbor interactions along the leg. For Zn content x>0.01, χ(T) deviates from the Curie law at low temperatures. The magnetic specific heat data C(m)(T) for the Zn-doped samples show weak anomalies at low temperature, in agreement with the χ(T) behavior. The anomalies are suggestive of spin freezing at low- T. In contrast, prominent effects are observed in χ(T) and C(m)(T) on Ni-doped samples. The zero-field-cooled (ZFC) and field-cooled (FC) χ(T) data are different from each other at low temperature, unlike that for Zn-doped samples, clearly indicating a transition to a spin-glass-like phase. No anomalies were found in Ca- or Pb-doped samples.

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. This allows us to propose a simple common framework to explain the generic low-temperature impurity-induced freezings observed in low-dimensional spin-gapped materials.

NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2: a nesting scenario.

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na(0.5)CoO(2). Above T(N) = 86 K, both sites display a similar T dependence of the spin shift, suggesting that there is no charge segregation into Co(3+) and Co(4+) sites. Below T(N), the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition at T(MIT) = 51 K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive spin-density wave induced by two nestings of the Fermi surface specific to the x = 0.5 Na ordering.

Impurity-induced local magnetism and density of states in the superconducting state of YBa2Cu3O7.

17O NMR is used to probe the local influence of nonmagnetic Zn and magnetic Ni impurities in the superconducting state of optimally doped high TC YBa2Cu3O7. Zn and Ni induce a staggered paramagnetic polarization, similar to that evidenced above TC, with a typical extension xi=3 cell units for Zn and xi>or=3 for Ni. In addition, Zn is observed to induce a local density of states near the Fermi energy in its neighborhood, which also decays over about 3 cell units. Its magnitude decreases sharply with increasing temperature. This allows direct comparison with the STM observations done in BiSCCO.

Cascade of bulk magnetic phase transitions in NaxCoO2 as studied by muon spin rotation.

Using muon spin rotation, well-defined bulk approximately 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x=0.85 with the highest transition temperature ever observed for x>or=0.75. This stresses the diversity of x>or=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x=0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.

Persistence of Li induced Kondo moments in the superconducting state of cuprates.

Using 7Li NMR shift data, the anomalous local moment induced by spinless Li impurities persists below T(c) in YBa 2Cu 3O6+y. In the underdoped regime, the moments retain their Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (TK = 135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on first near neighbor Cu) is not drastically modified below Tc, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.

Absence of static phase separation in the high T(c) cuprate YBa(2)Cu(3)O(6+y).

We use 89Y NMR in YBa(2)Cu(3)O(6+y) in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Deltap=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates.