Enhancement of penetration field due to surface Andreev bound states in vortex nanocrystals of Bi2Sr2CaCu2O8−d

Y. FASANO; M. I. DOLTZ; N. R. CEJAS BOLECEK; H. PASTORIZA; J. GUIMPEL; G. NIEVA; M. KONCZYKOWSKI; C. J. VAN DER BEEK

Natal

Workshop; VORTEX2017; 2017

The nucleation of nanocrystalline vortex matter with a large surface-to-volume vortex ratio in micron-sized superconducting samples opens the possibility of studying the influence of surface effects on its magnetic properties. For instance, theoretical studies predict that the penetration and depinning fields for vortices in d-wave superconductors is altered by fermionic Andreev-bound states appearing at the sample surface [1]. These states, that can exist at zero excitation energy depending on the relative orientation of the the sample surface to the d-wave gap function, generate an anomalous Meissner current running opposite to the supercurrents. The Bean-Livingston surface barrier limiting vortex entry at intermediate temperatures is altered by these surface states and as a consequence the penetration fieldHp depends on the orientation of the surface of the sample to the d-wave superconducting order parameter [2]. An experimental study indeed detected a tiny crystal-orientation dependence of Hp in relatively macroscopic samples [2]. In this work we study vortex nanocrystals with 10-1% surface-to-volume vortex ratio nucleated in cuboidBi2Sr2CaCu2O8−d samples of 50 to 20 μm width and roughly 2μm thickness (see Fig. 1). The engineering method we use [3,4] allowed us to generate two sets of micro-cuboids, one with the edges aligned along the nodal and another along the anti-nodal d-wave order parameter directions. By means of Hall probe magnetometers with active areas of 16 × 16 μm2 we measure local dc and ac-magnetization. We detect that the penetration field is enhanced when the edges of the micro-cuboids are parallel to the nodal direction of the d-wave order parameter. This result, in agreement with the theoretical predictions, has only been detected thanks to the combination of low-noise local magnetic techniques and the nucleation of vortex nanocrystals with a significant surface-to-volume vortex ratio. Nanocrystalline vortex matter nucleated in field-cooling processes in Bi2Sr2CaCu2O8−d cuboids with 30 μm sides and 2 μm thickness. Magnetic decorations were performed at 4.2 K and at applied fields of 20 (left panel) and 40 Oe (right panel).[1] C. Iniotakis, T. Dahm, andN. Schopohl, Phys. Rev. Lett. 100,037002( 2008).[2] A.E. Böhmer, M. Konczykowski and C.J. van der Beek, arXiv 1004.5309( 2010).[3] M. I. Dolz et al., Phys. Rev. Lett. 115, 137003( 2015).[4]N. R. Cejas Bolecek et al., Journal of LowTemp.Phys. 179,35(2015).