Figure 1.

Local contact affects vortex configuration in a thin superconducting film. (a) Experimental configuration. Stress is applied with the tip of the SQUID chip by pushing the cantilever (inset) into the sample. (b) Vortex configuration in an NbN thin film imaged at 4.2 K with no contact. The vortices are positive (white), and the flux in each integrates to 1 Φ₀ (Supporting Information). (c) Vortex configuration imaged out of contact after dragging the tip in contact with the sample over a 30 μm × 30 μm square (dashed square). The sweep lines in the x-direction were spaced by 250 nm, progressing toward the top of the square (arrows). The strong white signal outside the dark square shows the accumulation of vortices (see Supporting Information for further discussion of the color scale). (d) Renewed vortex configuration on the same area displays no memory of previous manipulations. (e) Diamagnetic response to locally applied field by an on-chip coil, before (top) and after (bottom) the area was scanned in contact in b (dashed square). Susceptibility was measured by applying field of 0.2 G, 800 Hz. The field was not applied during the magnetometry scans in panels b–d. The area was recooled in the presence of 0.1 mG, and no vortices were present during susceptibility measurements. (f) Atomic force microscopy of the area that was scanned in contact (top) and a different area that was not scanned in contact (bottom). We detected no differences in topography or damage to the film. The roughness of the two areas, 0.26 nm root-mean-square, indicates that no damage was made by the contact. (g) Vortex configuration (top), and the same area after we made contact at one point at 8 K, where the SQUID is no longer superconducting but the vortices are still pinned (bottom). Vortices moved toward the contact location (cleared from the darker point). The SQUID was disconnected during contact. Both images were taken with no contact at 4.2 K.