Enhancement of penetration eld due to surface Andreev bound states in vortex nanocrystals of Bi2Sr2CaCu2O8􀀀

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

Natal

Workshop; Charla invitada en el 16th International Workshop on Vortex Matter in Superconductors; 2017

International Institute of Physics, Natal

The nucleation of nanocrystalline vortex matter with a large surface-to-volume vor-tex ratio in micron-sized superconducting samples opens the possibility of studying theinuence of surface eects on its magnetic properties. For instance, theoretical studiespredict that the penetration and depinning elds for vortices in d-wave superconductorsis altered by fermionic Andreev-bound states appearing at the sample surface. [1] Thesestates, that can exist at zero excitation energy depending on the relative orientation ofthe the sample-surface to the d-wave gap function, generate an anomalous Meissner cur-rent running opposite to the supercurrents. The Bean-Livingston surface barrier limitingvortex entry at intermediate temperatures is altered by these surface states and as a con-sequence the penetration eld Hp depends on the orientation of the surface of the sampleto the d-wave superconducting order parameter. [1] An experimental study indeed de-tected 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 rationucleated in cuboid Bi2Sr2CaCu2O8􀀀 samples of 50 to 20 m width and roughly 2 mthickness (see Fig. 1). The engineering method we use [3, 4] allowed us to generate twosets of micro-cuboids, one with the edges aligned along the nodal and another along theanti-nodal d-wave order parameter directions. By means of Hall probe magnetometerswith active areas of 16 16 m2 we measure local dc and ac-magnetization. We detectthat the penetration eld is enhanced when the edges of the micro-cuboids are parallelto the nodal direction of the d-wave order parameter. This result, in agreement with thetheoretical predictions, has only been detected thanks to the combination of low-noiselocal magnetic techniques and the nucleation of vortex nanocrystals with a signicantsurface-to-volume vortex ratio.Figure 1: Nanocrystalline vortex matter nucleated in eld-cooling processes inBi2Sr2CaCu2O8􀀀 cuboids with 30 m sides and 2 m thickness. Magnetic decorationswere performed at 4.2 K and at applied elds of 20 (left panel) and 40 Oe (right panel).[1] C. Iniotakis, T. Dahm, and N. Schopohl, Phys. Rev. Lett. 100, 037002 (2008).[2] A.E. Bohmer, 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 Low Temp.Phys. 179, 35 (2015)