The low-field phase in mesoscopic vortex matter

Y. FASANO

Bath

Workshop; Charla invitada, International Workshop on Mesoscopic Superconductivity and Vortex Imaging; 2011

ESF Research Networking Programme: Nanoscience and Engineering in Superconductivity

Weak disorder in the low-field and temperature phase of vortex matter competes with the elasticity of the structure resulting in the establishment of two different pinning regimes. At short distances vortices are individually pinned, the so-called Larkin pinning regime. For distances larger than the crossover Larkin lengths vortices collectively pin in a random manifold regime [1]. The crossover length $r_{c}$ is achieved when vortex displacements with respect to a perfect lattice are larger than the pinning force range. For the case of the high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 +y}$ the in-plane crossover length is in the range of microns and for a given field depends on temperature. Therefore, sufficiently reducing the sample size results in a it{mesoscopic Larkin vortex phase} that is not evident in macroscopic systems. In this work we engineered micron-ranged disks from bulk Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 +y}$ samples by combining optical lithographic and etching techniques [2]. We use the magnetic decoration technique in order to take snapshots of the mesoscopic vortex structure, with single vortex resolution, at the irreversibility temperature [3]. The dependence of the positional order on the sample size is discussed in connection to the variation of the Larkin length with temperature.  [1] T. Giamarchi and P. L. Doussal, Phys. Rev. Lett. 72, 1530 (1994). T. Giamarchi and P. L. Doussal, Phys. Rev. B. 52, 1242 (1995). [2] M. I. Dolz, A. B. Kolton and H. Pastoriza, Phy. Rev. B 81, 092502 (2010). [3] Y. Fasano and M. Menghini, Supercond. Sci. and Tech. 21, 023001 (2008).