Direct observation of thermal fluctuations near the superconducting transition
Superconducting transitions are driven by thermal fluctuations close to the transition temperature, Tc. We used the scanning SQUID to image thermal superconducting fluctuations in Nb, and observe them in both space and time. An important outcome of these measurements is the observation that the magnetic susceptibility of a type II superconductor decreases to zero in quantized steps irrespectively to sample geometry.
Modulations in the superconductivity of thin films generated by the substrate
Our Scanning SQUID measurements revealed large-scale modulations of the superfluid density and the critical temperature in superconducting Nb, NbN, and underdoped YBCO films deposited on STO. We showed that these modulations are a result of the STO domains and domain walls, forming below the structural phase transition of STO. We found that the flow of normal current is also modulated over the same domain structure, suggesting a modified carrier density. Modulated superconductivity over mobile channels offers the opportunity to locally control superconducting properties and better understand the relations between superconductivity and the local structure.
Mechanical Control of Individual Superconducting Vortices
We Manipulate individual vortices in thin superconducting films via local mechanical contact without magnetic field or current. We used a scanning SQUID to image vortices and move them by applying local vertical stress with the tip of our sensor.
Vortices were attracted to the contact point, relocated, and were stable at their new location. Mechanical manipulation of vortices provides a local view of the interaction between strain and nanomagnetic objects as well as controllable, effective, and reproducible manipulation technique.
Modulated superfluid density in twinned high Tc superconductors
Several physical properties like the local strain, the bond angle, and the magnetic order may change on twin boundaries in orthorhombic crystals. These properties are known to affect superconductivity in bulk measurements. We use scanning SQUID to image the local magnetometry and susceptibility on the surface of twinned superconductors. We observe increased diamagnetic susceptibility in underdoped, but not overdoped, single crystals of the pnictide superconductor Ba(Fe1-xCox)2As2 , consistent with enhanced superfluid density on twin boundaries. Interesting information is also acquired by following the vortex behavior. Individual vortices avoid pinning on or crossing the twin boundaries, and prefer to travel parallel to them. These results help us connect the magnetic properties with the local changes in the crystal.
PRB 81 184513 (2010); viewpoint; PRB 81 184514 (2010); PRB 83 064511 (2011)
With: Kam Moler, John Kirtley, Ian Fisher beena@stanford
Dynamics of single vortices on grain boundaries in YBCO thin films
Above the critical current vortices travel in a type-II superconductor allowing for dissipation. Using scanning Hall probe microscopy, we have detected the hopping of individual vortices between pinning sites along grain boundaries in YBCO thin ?lms. The hopping frequency increased with the applied current, which drives the vortices faster. Detecting the motion of individual vortices allowed us to probe the current-voltage (I-V) characteristics of the grain boundary with voltage sensitivity below a femtovolt. We found a very sharp onset of dissipation that shows essentially no dependence on temperature or grain boundary angle.
APL 94 202504 (2009)
With: Kam Moler, Eli Zeldov, John Kirtley, Hans Hilgenkamp beena@stanford