Abstract: The ability to control Josephson vortices is instrumental for developing superconducting cryoelectronics. Abrikosov and Josephson vortices (AVs and JVs) represent topological objects in superconductors and Josephson junctions (JJs), respectively. Both carry a flux quantum, 0, and have a 2π phase rotation. However, despite similarities they have significant qualitative and quantitative differences.

Qualitatively unlike AVs, JVs are coreless, elongated along the junction quasi-one-dimensional objects. In an applied magnetic field, multiple JVs form a one-dimensional vortex chain due to mutual repulsion. Quantitatively, the size of the AV is determined by the short London penetration depth L ~ 100 nm, while the size of the JV along the junction is given by the much longer Josephson penetration depth J,  typically in the range of several microns. The small L leads to large magnetic signatures of AVs: the maximal field ~ 100 Oe and the field gradient ~107 Oe/cm. This leads to the relative ease of observing AVs, which are very well studied, visualized, and even monitored in real time by using a variety of techniques, including scanning probe (STM and MFM) microscopy. Concurrently, the large J makes the magnetic signatures of JVs much weaker and more difficult to observe. Furthermore, the small field gradient together with the coreless structure of JVs makes them almost pinning-less and mobile. Those factors make it difficult to immobilize JVs during the time needed for their detection and visualization. Recently, we have demonstrated that the dynamic response of a low-temperature magnetic force microscope (MFM) carries information about JVs in planar JJs.

In this talk, I will show that the MFM technique can be used for direct visualization of individual JVs, their internal structure and spatial configurations within the junction, as well as for probing of mutual interaction between JVs. I will discuss the mechanism of JV detection due to a nontrivial junction-tip interaction and the possibility of developing novel scanning probe sensors with spatial resolution not limited by sensor size.

Bio: Vladimir Krasnov is a head of the Experimental Condensed Matter Physics group in the Department of Physics at Stockholm University. His research interests include superconductivity, low-temperature physics, quantum electronics, spintronics, and nanotechnology.