Friday, 2D-09, 4:30-4:40

Pinning of the Vortex Lattice versus Current Blocking by Nano-scale Columnar Inclusions in High-T_c Films*

Jose P. Rodriguez

Department of Physics and Astronomy, California State University at Los Angeles, Los Angeles, CA
90032, Tel.: 323-343-2133, FAX: 323-343-2497, e-mail: jrodrig@calstatela.edu

The in-field critical current of a thin film of YBa₂Cu₃O_7-δ (YBCO) can be enhanced considerably by the injection of nano-rod inclusions such as BaZrO₃ (BZO) and BaSnO₃ (BSO) aligned along the c axis. We report on the results of a linear elasticity analysis that yields that such nano-rod inclusions squeeze the YBCO matrix by pure shear due to an 8% lattice mismatch.¹ The sensitivity of the superconducting critical temperature in YBCO to pure shear strain implies that the phase boundary separating the nano-rod inclusion from the superconductor acts as a collective pinning center for the vortex lattice that appears in an external magnetic field; a dominant contribution to the in-field critical current can result. Recent measurements of the in-field critical current in thick films of YBCO + BSO nano-columns are consistent with this type of collective bulk pinning.² We also find that the elastic energy of the YBCO + nano-rod inclusions can be weakly metastable as a function of the nano-rod density when the inclusions are softer than the matrix. This may well be what drives their epitaxial growth. Next, we revisit the standard calculation of the superconducting depairing current, but with insulating nanorod inclusions present. Study of the Ginzburg-Landau equations yields that it scales down linearly with the superconducting volume fraction in the limit that the coherence length is short compared to the diameter of the inclusions. A numerical solution of the Ginzburg-Landau equations finds a significantly larger reduction of the depairing current in the important regime where the diameter of the nanocolumn is comparable to the coherence length. These theoretical results are contrasted with similar reductions shown by the critical current of YBCO films in self-field as a function of the volume fraction of nano-rod inclusions.

¹J.P. Rodriguez, P.N. Barnes, C.V. Varanasi, Phys. Rev. B 78, 052505 (2008).

²C. V. Varanasi, J. Burke, H. Wang, J. H. Lee, and P. N. Barnes, Appl. Phys. Lett. 93, 092501 (2008).

*Work supported in part by the Air Force Office of Scientific Research under grant no. FA9550-06-1-0479.

**In collaboration with Paul N. Barnes and Chakrapani V. Varanasi, Air Force Research Laboratory at Wright-Patterson Air Force Base.

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