Electronic Properties of BaFe2As2 upon Doping and Pressure: The Prominent Role of the As p Orbitals.

Using high-resolution, lifetime removed, x-ray absorption spectroscopy at the As K edge, we evidence the strong sensitivity of the As electronic structure upon electron doping with Co or pressure change in BaFe2As2, at room temperature. Our results unravel the prominent role played by As-4p orbitals in the electronic properties of the Fe pnictide superconductors. We propose a unique picture to describe the overall effect of both external parameter doping and pressure, resolving the apparent contradiction between angle-resolved photoemission spectroscopy, transport, and absorption results, with the As-p states as a key ingredient.

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.

Multiorbital effects on the transport and the superconducting fluctuations in LiFeAs.

The resistivity, Hall effect, and transverse magnetoresistance have been measured in low residual resistivity single crystals of LiFeAs. A comparison with angle resolved photoemission spectroscopy and quantum oscillation data implies that four carrier bands unevenly contribute to the transport. However the scattering rates of the carriers all display the T(2) behavior expected for a Fermi liquid. Near T(c) low field deviations of the magnetoresistance with respect to a H(2) variation permit us to extract the superconducting fluctuation contribution to the conductivity. Though below T(c) the anisotropy of superconductivity is rather small, the superconducting fluctuation displays a quasi-ideal two-dimensional behavior which persists up to 1.4 T(c). These results call for a refined theoretical understanding of the multiband behavior of superconductivity in this pnictide.

Significant reduction of electronic correlations upon isovalent Ru substitution of BaFe2As2.

We investigate Ba(Fe0.65Ru0.35)2As2, a compound in which superconductivity appears at the expense of magnetism, by transport measurements and angle resolved photoemission spectroscopy. By resolving the different Fermi surface pockets and deducing from their volumes the number of hole and electron carriers, we show that Ru induces neither hole nor electron doping. However, the Fermi surface pockets are about twice larger than in BaFe2As2. A change of sign of the Hall coefficient with decreasing temperature evidences the contribution of both carriers to the transport. Fermi velocities increase significantly with respect to BaFe2As2, suggesting a reduction of correlation effects.

Hall effect and resistivity study of the magnetic transition, carrier content, and Fermi-Liquid Behavior in Ba(Fe(1-x) Co(x))(2)As(2).

The negative Hall constant R(H) measured all over the phase diagram of Ba(Fe(1-x) Co(x))(2)As(2) allows us to show that electron carriers always dominate the transport properties. The evolution of R(H) with x at low doping (x<2%) indicates that important band structure changes happen for x<2% prior to the emergence of superconductivity. For higher x, a change with T of the electron concentration is required to explain the low T variations of R(H), while the electron scattering rate displays the T(2) law expected for a Fermi liquid. The T=0 residual scattering is affected by Co disorder in the magnetic phase, but is rather dominated by incipient disorder in the paramagnetic state.

Irradiation-induced confinement in a quasi-one-dimensional metal.

The anisotropic resistivity of PrBa(2)Cu(4)O(8) has been measured as a function of electron irradiation fluence. Localization effects are observed for extremely small amounts of disorder corresponding to electron mean free paths of order 100 unit cells. Estimates of the localization corrections suggest that this anomalous localization threshold heralds a crossover to a ground state with pronounced one-dimensional character in which conduction electrons become confined to a small cluster of chains.

Total suppression of superconductivity by high magnetic fields in YBa(2)Cu(3)O(6.6).

We have studied the variation of transverse magnetoresistance of underdoped YBCO(6.6) crystals, either pure or with reduced T(c) down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconducting fluctuation contribution to the conductivity is suppressed only above a threshold field H(c)'(T), which is found to vanish at T(c)' > T(c). In the pure YBCO(6.6) sample, H(c)' is already 50 T at T(c). We find that increasing disorder weakly depresses H(c)'(0), T(c)', and T(nu), the onset of the Nernst signal. Thus, these energy scales appear more characteristic of the 2D local pairing than the pseudogap temperature which is not modified by disorder.

Nernst effect and disorder in the normal state of high-T(c) cuprates.

We have studied the influence of disorder induced by electron irradiation on the Nernst effect in optimally and underdoped YBa2Cu3O(7-delta) single crystals. The fluctuation regime above T(c) expands significantly with disorder, indicating that the T(c) decrease is partly due to the induced loss of phase coherence. In pure crystals the temperature extension of the Nernst signal is found to be narrow whatever the hole doping, contrary to data reported in the low-T(c) cuprate families. Our results show that the presence of intrinsic disorder can explain the enhanced range of the Nernst signal found in the pseudogap phase of the latter compounds.

Influence of pair breaking and phase fluctuations on disordered high Tc cuprate superconductors.

Electron irradiation has been used to introduce point defects in a controlled way in underdoped and optimally doped YBa(2)Cu(3)O(7-delta) crystals. This technique allows us to perform very accurate measurements of T(c) and of the ab plane resistivity in a wide range of defect contents x(d) down to T(c)=0. The variation of T(c) and of the transition width with x(d) do not follow current predictions of pair-breaking theories. The data are rather compatible, at least for the highly damaged regime, with the expected influence of phase fluctuations. These results open new questions about the evolution of the defect induced T(c) depression over the phase diagram of the cuprates.

Disorder and transport in cuprates: weak localization and magnetic contributions.

We report resistivity measurements in underdoped YBa 2Cu3O6.6 and overdoped Tl 2Ba2CuO6+x single crystals in which the concentration of defects in the CuO (2) planes is controlled by electron irradiation. Low T upturns of the resistivity are observed in both cases for large defect content. In the Tl compound the decrease of conductivity scales as expected from weak localization theory. On the contrary, in YBa 2Cu3O6.6 the much larger low T contribution to the resistivity is proportional to the defect content and might then be associated with a Kondo-like spin flip scattering term. This would be consistent with the results on the magnetic properties induced by spinless defects.