Jean-Philippe Burin

Superconductivity and disorder enhancement in YBa2Cu3OyAx (A = F, Cl) compounds

Abstract The real and imaginary parts of the susceptibility at very low fields of halogenated YBCO ceramics have distinct onsets at temperatures Ts and Tc, respectively. We show that Ts is related to the average composition while Tc is sensitive to microscopic disorder. Thus Ts of YBa2Cu3OyAx coincides with the transition temperature of YBa2Cu3Oy+Cx, where the “doping power” 2C = −1, 1, 2, 3, for A = H, F, O, Cl, respectively. We discuss a mechanism for creation of disorder in the superconducting properties through an effective repulsive electrostatic interaction between halogen ions and oxygen vacancies. This mechanism describes the observed trends in Tcs and transition widths, even when one assumes that intergranular barriers play a minor role.

Correlations between magnetic and resistive transitions in a Bi2Sr2CaCu2O8 ceramic

Abstract Resistive and inductive transitions are measured on the same Bi 2 Sr 2 CaCu 2 O 8 ceramic. The transition curves present accidents which appear to be correlated and amenable to physical interpretation in quantitative agreement with a granular description. We consider two characteristic temperatures: T s which marks the appearance of superconductivity and T c s below which long-range coherence occur. In the paracoherent region T c s the excess conductivity is in good agreement with a power law Δσ ∝ |1− T T c | −γ with γ = 2.7 while in the coherent region T c the field of first flux penetration is satisfactorily described by a power law H 1 (T) ∝ (1− T T c ) n with n = 2.7. The validity of this description is corroborated by the correct prediction, from resistivity data, of the distance d ≅ 0.3±0.1 nm between CuO 2 layers in the unit cell.

Electrical injection in GaP-based laser waveguides and active areas

This paper presents the recent advances in device engineering towards the fabrication of an electrically pumped laser on GaP substrate for photonic integration on silicon. The letter first presents the electrical properties of a GaP-based PIN diode, in particular the reduction of the characteristic resistance of the p-contact, thanks to a judicious combination of metal choice and thermal annealing. Secondly, carrier injection in the active area is investigated by use of time-resolved photoluminescence, regarding particularly the nature and composition of the barrier and quantum wells materials, with a focus on the nitrogen incorporation issues.

Low-Field Phenomenology and Characteristic Lengths of Hydrogen- and Halogen-Doped YBa2Cu3O7−δ Compounds

We measure the diamagnetic response of YBa2Cu3OyAx (A=Cl, F, H) ceramics. We obtain low-field effective penetration depths λ of a few 10 μm, first flux-penetration field H1distinguishable from the volume penetration field HC1and two characteristic temperatures Tsand Tc, Tc<Ts, defined by the onset of the real and imaginary parts of the susceptibility, respectively. H1(T) goes to zero as a power law H1(T, x)=H1(0, x) (T−T/Tc)2.7near TC. From the values of λ and H1we infer the correlation length ξ at T=4.2 K. A large range of values for ξ(4.2) suggests that chemical disorder plays an essential role in the phenomenology of cuprates. We find λ(4.2)∼H1(4.2)−1/3. This, and the power law ξ(4.2)∼H1(4.2)−2/3result from the scaling properties of a phase transition governed by disorder.

Magnetic behaviour of chemically disordered YBa2Cu3O7−δFx thin films

Abstract We measured the ac magnetic permeability of an oxygen-deficient YBa 2 Cu 3 O 7− δ thin film epitaxially grown by Pulsed Laser Deposition on a (100) SrTiO 3 substrate, before and after fluorination under diluted NF 3 , as a function of temperature T and amplitude of the applied field. Fluorine apparently occupies empty sites on a and b axes of the perovskite structure, with very different reaction rates. For each fluorine content we determined the width of the transition, the field H 1 ( T ) of first flux penetration in the sample and the peak field H p ( T ) giving maximum dissipation at temperature T . The transition width appears to be related to the difference between fluorine populations in the two axes. H p ( T ) obeys the same power law as in YBa 2 Cu 3 O 7− δ A x (A=F, Cl) ceramics, but this is not the case of H 1 ( T ). Our results are compatible with a model of chemical disorder generating a random distribution of superconducting critical temperatures T s in the sample.