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Content archived on 2022-12-23

Irradiation-induced enhancement of critical currents in high-Tc superconductors

Objective

- Identification of the elementary pinning interactions in high Tc single crystals and films with controlled irradiation-induced defects;
- Determination of the mechanisms of flux motion in the presence of point defects and certain types of extended defects;
- Determination of the conditions under which flux motion can be reduced and the current-carrying capability enhanced at high temperatures;
- Determination of the stabilising effects of (heavy-ion induced) columnar defects on vortex lines, in relation with the intrinsic material anisotropy;
- Identification of the destabilising effects of disorder on vortex lattice structure, and the role of vortex lattice dislocations in determining the bulk pinning current density;
- Determination of the optimum repartition of vortices over columnar defect sites in relation to the matching field, and the role of vortex interstitials;
- To probe the vortex response and the robustness of different vortex phases to different kinds of disorder.
- A remarkable enhancement of the current-carrying capacity of high Tc materials can be achieved by the introduction of linear columnar defects through heavy-ion irradiation.
- Strong linear defects partially undo the detrimental effects of large material anisotropy. Columnar defect induces the alignment of 2D vortex segments, whereby the vortex line tension, which in the unirradiated material is practically zero at high temperatures and fields, is re-established. The effect is present up to fields close to the matching field Bf, at which the number of columnar defects is nominally equal to the number of vortex lines.
- The temperature region in which vortices are effectively pinned in Bi2Sr2CaCu2O8 increases with progressive irradiation dose, until saturation occurs at a Bf of several kG. The latter effect has also been found in less anisotropy YBa2Cu3O7, where pinning enhancement saturates at Bf 4 T.
- The introduction of strong disorder lowers the thermodynamic magnetisation of high Tc superconductors in an amount proportional to the pinning energy, which at zero field equals 1000 K per 2D vortex segment.
- The creation of extra point defects by low temperature electron irradiation does not increase pinning under all circumstances. Rather, it destabilises the low temperature, low field crystalline vortex state and promotes the formation of very mobile vortex dislocations.
- Vortex lattice dislocations were shown to be responsible for the decrease of the current-carrying capacity of YBa2Cu3O7 single crystals at high field.


- Growth of Bi2Sr2CaCu2O8 and La2-xSrxCuO4 single crystals using the travelling-solvent floating zone technique;
- Irradiation with swift heavy ions at GANIL;
- Low-temperature electron irradiation using the van der Graaf accelerator;
- Characterisation of irradiation damage using SEM techniques;
- Measurements of magnetisation (current density) as function of temperature, field (up to 16 T), and field angle, using microscopic Hall probes, Hall arrays, and vibrating sample magnetometer;
- Measurements of magnetic torque in order to characterise material anisotropy;
- Ac transmittivity ("local susceptibility") and magnetic relaxation measurements using microscopic Hall probes. In superconductors, these are equivalent to contactless transport measurements at extremely low voltage.

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Coordinator

Commissariat à l'Energie Atomique (CEA)
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Address
Centre d'Études de Saclay
91191 Gif-sur-Yvette
France

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