L. Fruchter

Magnetic study of oxygen-deficient YBaCuO7-δ

Abstract The anisotropy YBaCuO 7-δ (0≤δ≤0.5) was studied as a function of the oxygen deficiency. The mass ratio is found to increase rapidly as the oxygen concentration is reduced, due to an exponential increase of the transverse effective mass for carriers. This indicates that oxygen-deficient samples have a more pronounced 2D-like behaviour. For the fully oxidised compound, the numerical value for the in-plane penetration depth is consistent with a complete condensation of the in-plane normal-state carriers.

Contributions of pinning and anisotropy to torque in YBaCuO

Abstract Torque signals for Y 2 Ba 2 Cu 3 O 7 single crystals were decomposed into a reversible and an irreversible component between 85 K and 8 K. The reversible component below 40 K cannot be modelled by the standard anisotropic Ginzburg-Landau theory. In the low temperature regime, an increase of the signal close to the ab direction is observed. We suggest that this could be the signature of a crossover between 3D and 2D behaviour of the flux lines. The irreversible component provides evidence for pinning by twin planes, 2D structure of the material and some isotropic pinning sensitive to the defects in the material.

Magnetization and anisotropy of a HgBa2Ca2Ca2Cu3O8+δ single crystal

Abstract From torque measurements as a function of the angle and the magnetic field, we extract the magnetization curves and the anisotropy factor for a HgBa 2 Ca 2 Cu 3 O 8+δ single crystal. The anisotropy is found γ = 52 ± 3; it is similar to the one found for the HgBa 2 Ca 3 Cu 4 O 10+δ compound by Zech et al. The analysis of the magnetization close to H c2 ( T ) within the fluctuations theory yields the critical fluctuations parameter τ f = (3.9 ± 0.4) 10 −2 and the in-plane penetration depth λ(0) = 1500 ± 100 A . The superconducting planes separation is found of the order of the unit cell length. The upper critical field is estimaated ( d H c 2 d T ) T c 0 ≈ 14 kOe K −1 .

Anisotropy of the critical current density in bulk Y1Ba2Cu3O7-σ. measured by torque magnetometry for various oxygen deficiencies

Abstract The irreversibility of torque signals is used to measure the anisotropy of the critical current in aligned mono-crystalline grains of YBCO, with various oxygen contents. The fully oxidized compound behaves essentially as a 3D anisotropic material. In particular, the scaling with the transverse field component, expected in the 2D limit, is limited to fields close to the direction of the planes. The oxygen deficient samples show characteristics expected in the 2D case: no angular resolved contribution of the twin planes, and extended angular range for scaling.

Lower critical fields and pinning in Y1Ba2Cu3O7−δ

Abstract We have used the irreversibility of torque signals on a YBaCuO single crystal as a criterion for flux penetration at the lower critical fields H c1 . We estimate in this way H c1 ( H ∥ AB )=240 G , H c1 ( H ∥ C )=1100 G at 4.2 K, and by combining with published data we estimate the isotropic thermodynamic critical field H c to be 10 kG. We also show that the flux line pinning is highly anisotropic.

Parallel and perpendicular magnetization on YBaCuO single crystals in the irreversible regime

Abstract The magnetic behaviour of YBaCuO oriented crystals is studied in the irreversible regime using combined results of torque and magnetometry. Strong transverse magnetization components are observed; the data show that the irreversible magnetization is essentially parallel to the c direction whatever the field orientation except for angles very close to the ab plane.

Magnetic torque investigation of YBaCuO single crystals

Abstract We present new torque data recorded on YBaCuO single crystals following different measuring protocols. We observed a strong pinning (characterized by an uniaxial anisotropy) along the c axis. No field penetration was detected below 80–100 G.

Nonlinear excess conductivity of Bi2Sr2Can-1CunO2n+4+x (n=1,2) thin films

The suppression of excess conductivity with electric field is studied for Bi

Flux line pinning in the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br

_2

Anisotropy of the in-plane angular magnetoresistance of electron-dopedSr1−xLaxCuO2thin films

Sr