[超导国家重点实验室学术报告] Direct visualization of degeneracy and vortex ice in nanostructured superconductors
||Professor Joris Van de Vondel
Institute for Nanoscale Physics and Chemistry, KU Leuven,Belgium
Artificial ice systems have unique physical properties that are promising for potential applications. One of the most challenging issues in this field is to find novel ice systems that allow precise control over the geometries and many-body interactions. Superconducting vortex matter has been proposed as a very suitable candidate to study artificial ice, mainly due to the availability of tunable vortex-vortex interactions and the possibility to fabricate a variety of nanoscale pinning potential geometries. So far, a detailed imaging of the local configurations in a vortex-based artificial ice system is still lacking. In this work we directly explore the superconducting vortex matter ground state by visualizing the vortex configurations in different nanostructured superconductors via scanning Hall probe microscopy. First, the observed vortex patterns in a Kagome lattice of elongated antidots, at specific applied magnetic fields, are in good agreement with the configurations obtained using time-dependent Ginzburg-Landau simulations. Both results indicate that the long-range interaction in this nanostructured superconductor is unable to lift the degeneracy between different vortex states and the pattern formation is mainly ruled by the nearest-neighbor interaction. This simplification makes it possible to identify a set of simple rules characterizing the vortex configurations, which can explain both the observed vortex distributions and the magnetic-field-dependent degree of degeneracy. Second, vortex-ice states can be realized in double-well pinning sites arranged into a square or Kagome lattice. Indeed, a large area with the vortex-ice ground-state configuration has been detected, which confirms the recent theoretical predictions for this ice system. Besides the defects analogous to artificial spin-ice systems, other types of unique vortex defects have been visualized and identified. We also demonstrate the possibility to realize different types of defects by varying the magnetic field.
Prof.Joris Van de Vondel graduated in 2003 as ‘licenciaat in de natuurkunde’ at KU Leuven. In 2007 he obtained his PhD entitled ‘Vortex dynamics and rectification effects in superconducting films with periodic asymmetric pinning’ at KU Leuven with prof. Victor V. Moshchalkov and prof. Clécio C. de Souza Silva as promoters. From 2007 until 2013 he was a post-doctoral fellow of the FWO. He conducted a research visit at the Universidad Autonoma de Barcelona UAB in the Physics and Engineering of Nanoelectronic Devices group led by Prof. S.O. Valenzuela exploring spin transport in two dimensional materials. From October 2011until October 2016 he was appointed a part-time lecturer (10%) in the Faculty of Science, Department of Physics and Astronomy of the KU Leuven and he became associate professor embedded within the Nanoscale Superconductivity and Magnetism group. He has co-authored 58 publications on a broad range of topics including nanostructured superconductors, two-dimensional materials and spintronic devices.