P. Butaud

Interaction induced delocalization for two particles in a periodic potential

We consider two interacting particles evolving in a one-dimensional periodic structure embedded in a magnetic field. We show that the strong localization induced by the magnetic field for particular values of the flux per unit cell is destroyed as soon as the particles interact. We study the spectral and dynamical aspects of this transition.

Magnetic Field Induced Localization in a Two-Dimensional Superconducting Wire Network

We report transport measurements on superconducting wire networks which provide the first experimental evidence of a new localization phenomenon induced by magnetic field on a 2D periodic structure. In the case of a superconducting wave function this phenomenon manifests itself as a depression of the network critical current and of the superconducting transition temperature at a half magnetic flux quantum per tile. In addition, the strong broadening of the resistive transition observed at this field is consistent with enhanced phase fluctuations due to this localization mechanism.

Disorder and interactions in Aharonov-Bohm cages

We study the effect of disorder and interactions on a recently proposed magnetic field induced localization mechanism. We show that both partially destroy the extreme confinement of the excitations occuring in the pure case and give rise to unusual behavior. We also point out the role of the edge states that allows for a propagation of the electrons in these systems.

NMR study of 17O in high Tc superconducting oxides

Abstract We present the temperature dependence of the Magnetic Hyperfine Shift tensors of 17 O nuclei for the sites within the CuO 2 plane (site O(I)) and in the BaO, LaO, SrO plane (O(II) site) in the compounds YBa 2 Cu 3 O 7 , YBa 2 Cu 3 O 6.65 , La 1.85 Sr 0.15 CuO 4 and Bi 2 Sr 2 CaCu 2 O 8 . For O(I) sites, the isotropic part of the tensor reflects the polarisation of the Oxygen 2s orbitals by the neigboring Cu d-holes and the temperature dependence of the susceptibility. We show that in YBa 2 Cu 3 O 6.65 , the spins of the oxygen p-holes have their own degree of freedom above 120 K.

Vortex correlations in a fully frustrated two-dimensional superconducting network

We have investigated the vortex state in a superconducting dice network using the Bitter decoration technique at several magnetic frustrations f = /0 = 1/2 and 1/3. In contrast to other regular network geometries where the existence of a commensurate state was previouly demonstrated, no ordered state was observed in the dice network at f = 1/2 and the observed vortex-vortex correlation length is close to one lattice cell.

From underdoped to overdoped regime in YBa2Cu3O6+x, an NMR investigation of single crystals

We present17O and63Cu NMR results obtained in three single crystals of YBa2 Cu3O6+x: YBa1.92Sr0.08Cu3O7 (Tc=89 K) which is overdoped, and YBa1.93Sr0.07Cu3O6.92 (Tc=91 K) and YBa2Cu3O6.52 (Tc=59 K), both of which are underdoped. We show that Cu(2) nuclear spin-lattice relaxation rate (NSLRR), the static spin susceptibility, the NSLRR for O(2,3) site as well as its anisotropy, have completely different temperature dependence according to the regime (overdoped or underdoped). The O(2,3) NSLRR and magnetic hyperfine shift (MHS) data seem to confirm that in the overdoped regime, a Fermi liquid description is correct, while in the underdoped regime, a pseudo-gap in the magnetic excitations always opens well aboveTc. We also show that both neutron inelastic scattering results and Cu(2) and O(2,3) NSLRR results can be explained using the same χ″(q, ω).

Localization effect in a two-dimensional superconducting network without disorder

The superconducting properties of a two-dimensional superconducting wire network with a new geometry have been measured as a function of the external magnetic field. The extreme localization effect recently predicted for this periodic lattice is revealed as a suppression of the critical current when the applied magnetic field corresponds to half-a-flux quantum per unit cell. For this particular magnetic field, the observed vortex state configuration is highly disordered.

NMR study of 63,65Cu in YBa2Cu3O7to 6.65 single crystals

Abstract We report 63 Cu NMR nuclear spin-lattice relaxation rates (NSLRR) and magnetic hyperfine shift (K cc ) of copper Cu(2) in the CuO 2 planes of YBa 2 Cu 3 O 7−δ as a function of temperature and δ. The independence of K cc and NSLRR (above 120 K) on δ associated with the existence of localized Cu(3d) holes. We comment on the difference in the NSLRR between the δ = 0 and δ ≠ 0 samples which appear below 120 K. It is interpreted as an indirect consequence of the changes in the oxygen 2p electronic band caused by the introduction of oxygen vacancies in the system.

NMR and NQR study of La1.85Sr0.15CuO4 and YBa2Cu3O6+x (x=0, 0.25)

The spin lattice relaxation rate of La in La 1.85 Sr 0.15 CuO 4 has been measured in the temperature range 12–300K at 5.75 Teslas. A peak in the relaxation rate has been found 15K above the superconducting transition. NQR of 63,65 Cu in antiferromagnetic YBa 2 Cu 3 O 6+x for x=0.25 and x=0 is also presented. Pairs of lines corresponding to Cu[1] have been observed at 30.1–27.8 MHz for x = 0.25 and 29.9-27.6 MHz for x=0.

NMR Investigation of Low-Energy Excitations in YBa2Cu3O6+x Single Crystals

17O and 63Cu NMR results obtained in three single crystals of YBa2Cu3O6+x, show that the static spin susceptibility and (T1T)−1 for Cu(2) and O(2,3) have completely different temperature dependence according to the regime — overdoped for x > 0.94, underdoped for x < 0.94. In the overdoped regime a Fermi liquid description seems to be correct, while in the underdoped regime a pseudo-gap in the magnetic excitations opens well above Tc, supporting the phase diagram based on the spin-charge separation. We also show that both neutron inelastic scattering results and Cu(2) and 0(2,3) NSLRR results can be explained using the same χ″(q, ω).