| Summary: | The physics of superconducting films, and especially the role of remnant
magnetization has a defining influence on the magnetic fields used to hold and
manipulate atoms on superconducting atomchips. We magnetically trap ultracold
^{87}Rb atoms on a 200{\mu}m wide and 500nm thick cryogenically cooled niobium
Z wire structure. By measuring the distance of the atomcloud to the trapping
wire for different transport currents and bias fields, we probe the trapping
characteristics of the niobium superconducting structure. At distances closer
than the trapping wire width, we observe a different behaviour than that of
normal conducting wire traps. Furthermore, we measure a stable magnetic trap at
zero transport current. These observations point to the presence of a remnant
magnetization in our niobium film which is induced by a transport current. This
current-induced magnetization defines the trap close to the chip surface. Our
measurements agree very well with an analytic prediction based on the critical
state model (CSM). Our results provide a new tool to control atom trapping on
superconducting atomchips by designing the current distribution through its
current history.
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