Sr 2 IrO 4 magnetic phase diagram, from resistivity

International audienceWe show that the transition to the antiferromagnetic state in zero magnetic field does show up in the transverse resistivity, for which we point out the possibility for a direct spin orientation effect. In an applied field, we propose that the transition is split into two lines, corresponding to in-plane and out-of-plane magnetic ordering. This picture is corroborated by transverse magnetization measurements. The magnetic phase diagram for Sr2IrO4 was investigated, using the angular dependence of the resistivity transverse to the IrO2 planes

Multifractal scaling of flux penetration in the Iron-based Superconductor Ba(Fe0.925_{0.925}Co0.075_{0.075})2_{2}As2_2

International audienceThe penetration of magnetic flux fronts in the optimally doped iron based superconductor Ba(Fe$_{0.925}$Co$_{0.075}$)$_{2}$As$_2$ is studied by means of high resolution magneto-optic imaging. The analysis of roughening and growth of the magnetic flux front reveals anomalous scaling properties. While higher-order spatial correlation functions reveal multifractal behavior for the roughening, the usual Kardar-Parisi-Zhang growth exponent is found. Both exponents are found to independent of temperature. We propose a scenario for vortex penetration based on 2D percolation and cluster aggregation in an inhomogeneously disordered superconductor

Origin of the different electronic structure of Rh- and Ru-doped Sr2IrO4

One way to induce insulator to metal transitions in the spin-orbit Mott insulator Sr2IrO4 is to substitute iridium with transition metals (Ru, Rh). However, this creates intriguing inhomogeneous metallic states, which cannot be described by a simple doping effect. We detail the electronic structure of the Ru-doped case with angle-resolved photoemission and show that, contrary to Rh, it cannot be connected to the undoped case by a rigid shift. We further identify bands below $E_F$ coexisting with the metallic ones that we assign to non-bonding Ir sites. We rationalize the differences between Rh and Ru by a different hybridization with oxygen, which mediates the coupling to Ir and sensitively affects the effective doping. We argue that the spin-orbit coupling does not control neither the charge transfer nor the transition threshold

Strong pinning and vortex energy distributions in single-crystalline Ba(Fe1−xCox)2As2

International audienceThe interrelation between heterogeneity and flux pinning is studied in Ba(Fe1−xCox)2As2 single crystals with widely varying Co content x. Magnetic Bitter decoration of the superconducting vortex ensemble in crystals with x=0.075 and x=0.1 reveals highly disordered vortex structures. The width of the Meissner belt observed at the edges of the crystals, and above the surface steps formed by cleaving, as well as the width of the intervortex distance distribution, indicate that the observed vortex ensemble is established at a temperature just below the critical temperature Tc. The vortex interaction energy and pinning force distributions extracted from the images strongly suggest that the vortex lattice disorder is attributable to strong pinning due to spatial fluctuations of Tc and of the superfluid density. Correlating the results with the critical current density yields a typical length scale of the relevant disorder of 40-60 nm

Vortex pinning : a probe for nanoscale disorder in iron-based superconductors

4 pagesInternational audienceThe pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density jc at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to "weak collective pinning" and a magnetic field-independent contribution jcoll. The latter allows one to estimate quasi-particle scattering rates

Electron irradiation of Co, Ni, and P-doped BaFe2As2 - type iron-based superconductors

17 pages, soumis pour publications dans les actes de M2S-2012International audienceHigh energy electron irradiation is used to controllably introduce atomic-scale point defects into single crystalline Ba(Fe_1-xCo_x)_2As_2, Ba(Fe_1-xNi_x)_2As_2, and BaFe_2(As_1-xP_x)_2. The appearance of the collective pinning contribution to the critical current density in BaFe_2(As_1-xP_x)_2, and the magnitude of its enhancement in Ba(Fe_1-xCo_x)_2As_2, conform with the hypothesis of quasi-particle scattering by Fe vacancies created by the irradiation. Whereas the insignificant modification of the temperature dependence of the superfluid density in Ba(Fe_1-xCo_x)_2As_2 and Ba(Fe_1-xNi_x)_2As_2 points to important native disorder present before the irradiation, the critical temperatures of these materials undergo a suppression equivalent to that observed in the much cleaner BaFe_2(As_1-xP_x)_2. This lends credence to the hypothesis of line nodes of the order parameter (at finite k_{z}) in the former two materials

Charge order and suppression of superconductivity in HgBa2CuO4 at high pressures

New insight into the superconducting properties of HgBa2CuO4 (Hg-1201) cuprates is provided by combined measurements of the electrical resistivity and single crystal X-ray diffraction under pressure. The changes induced by increasing pressure up to 20GPa in optimally doped single crystals were investigated. The resistivity measurements as a function of temperature show a metallic behavior up to ~10GPa that gradually passes to an insulating state, typical of charge ordering, that totally suppresses superconductivity above 13GPa. The changes in resistivity are accompanied by the apparition of sharp Bragg peaks in the X-ray diffraction patterns indicating that the charge ordering is accompanied by a 3D oxygen ordering appearing at 10GPa of wavevector [0.25, 0, L]. As pressure induces a charge transfer of about 0.02 at 10GPa, our results are the first observation of charge order competing with superconductivity that develops in the over-doped region of the phase diagram of a cuprate.Comment: 9 pages, 3 figure

Electronic Raman Scattering in copper oxide Superconductors: Understanding the Phase Diagram

Electronic Raman scattering measurements have been performed on hole doped copper oxide superconductors as a function of temperature and doping level. In the superconducting state coherent Bogoliubov quasiparticles develop preferentially over the nodal region in the underdoped regime. We can then define the fraction of coherent Fermi surface, $f_c$ around the nodes for which quasiparticles are well defined and superconductivity sets in. We find that $f_c$ is doping dependent and leads to the emergence of two energy scales. We then establish in a one single gap shem, that the critical temperature $T_{c} \propto f_{c}\Delta_{max}$ where $\Delta_{max}$ is the maximum amplitude of the d-wave superconducting gap. In the normal state, the loss of antinodal quasiparticles spectral weight detected in the superconducting state persists and the spectral weight is only restored above the pseudogap temperature $T*$. Such a dichotomy in the quasiparticles dynamics is then responsible for the emergence of the two energy scales in the superconducting state and the appearance of the pseudogap in the normal state. We propose a 3D phase diagram where both the temperature and the energy phase diagrams have been plotted together. We anticipate that the development of coherent excitations on a restricted part of the Fermi surface only is a general feature in high $T_c$ cuprate superconductors as the Mott insulating is approaching.Comment: 40 pages, 19 figures some new references and comments have been added with respect to the first version of march 30t

Phonon dispersion in 1-layer cuprate HgBa2CuO4+d

We investigate the low energy acoustical and optical modes in $\mathrm{HgBa_2CuO_{4+\delta}}$ using inelastic x-ray scattering (IXS). The experimental phonon dispersion and the dynamical structure factor are compared with an atomic shell model, and the set of the atomic potentials obtained are discussed. Our results are also compared with those obtained by Raman spectroscopy and with density-of-state data measured by inelastic neutron scattering