D. Colson

Two energy scales and two distinct quasiparticle dynamics in the superconducting state of underdoped cuprates

The superconducting temperature Tc of hole-doped high-temperature superconductors has a dome-like shape as a function of hole concentration, with a maximum Tc at ‘optimal’ doping. On the underdoped side, the superconducting state is often described in terms of one energy scale, associated with the maximum of the d-wave gap (at the antinodes), which increases as the doping decreases. Here, we report electronic Raman scattering experiments that show a second energy scale in the gap function: the slope of the gap at the nodes, which decreases with decreasing doping. Our measurements also reveal two distinct quasiparticle dynamics; electronic coherence persists down to low doping levels at the nodes, whereas antinodal quasiparticles become incoherent. Using a sum-rule, we find that the low-frequency Raman response and the temperature dependence of the superfluid density, both controlled by nodal excitations, behave in a qualitatively similar manner with doping variation.

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.

Infrared phonon dynamics of a multiferroic BiFeO3 single crystal

R. P. S. M. Lobo,1,* R. L. Moreira,2 D. Lebeugle,3 and D. Colson3 1Laboratoire Photons et Matiere (CNRS UPR 5), ESPCI, Universite Pierre et Marie Curie, 10 rue Vauquelin, 75231 Paris Cedex 5, France 2Departamento de Fisica, ICEx, Universidade Federal de Minas Gerais, Caixa Postale 702, Belo Horizonte, Minas Gerais, 31270-901, Brazil 3Service de Physique de l’Etat Condense (CNRS URA 2464), DSM/DRECAM/SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France Received 17 October 2007; published 28 November 2007

Observation of incipient charge nematicity in Ba(Fe(1-x)Co(x))2As2.

Using electronic Raman spectroscopy, we report direct measurements of charge nematic fluctuations in the tetragonal phase of strain-free Ba(Fe(1-x)Co(x))2As2 single crystals. The strong enhancement of the Raman response at low temperatures unveils an underlying charge nematic state that extends to superconducting compositions and which has hitherto remained unnoticed. Comparison between the extracted charge nematic susceptibility and the elastic modulus allows us to disentangle the charge contribution to the nematic instability, and to show that charge nematic fluctuations are weakly coupled to the lattice.

Atomic coexistence of superconductivity and incommensurate magnetic order in the pnictide Ba(Fe1-xCox)2As2

75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6%. Nuclear Magnetic Resonance (NMR) spectra and relaxation rates allow to show that all Fe sites experience an incommensurate magnetic ordering below T=31K. Comparison with undoped compound allows to estimate a typical moment of 0.05 muB. Anisotropy of the NMR widths can be interpreted using a model of incommensurability with a wavevector (1/2-eps,0,l) with eps of the order of 0.04. Below TC=21.8K, a full volume superconductivity develops as shown by susceptibility and relaxation rate, and magnetic order remains unaffected, demonstrating coexistence of both states on each Fe site.

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.

Doping dependence of the lattice dynamics in Ba(Fe1-xCox)2As2 studied by Raman spectroscopy

We report Raman scattering spectra of iron-pnictide superconductor Ba(Fe

Hole and electron contributions to the transport properties of Ba ( Fe 1 − x Ru x ) 2 As 2 single crystals

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Nesting between hole and electron pockets in Ba ( Fe 1 − x Co x ) 2 As 2 ( x = 0 – 0.3 ) observed with angle-resolved photoemission

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Collapse of the normal-state pseudogap at a Lifshitz transition in the Bi(2)Sr(2)CaCu(2)O(8+δ) cuprate superconductor.

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