Physicist at CNRS-Thales Lab, France
Superconducting vortices (magnetic flux quanta) are interesting objects. Technologically, because their motion causes dissipation and determines the electrical transport in superconductors. Fundamentally, because they constitute a system of interacting particles and, as such, provide a model to investigate general problems that are common to particle manifolds (colloids, charge density waves, bacteria, skyrmions, etc). The latter is my main motivation to contribute to this field. A few examples are given below.
Ratchets and guided motion – A ratchet is an asymmetric potential on which the particles’ motion is easier in one sense than in the opposite. Thus, the particles can acquire a net velocity under the effect of an alternative unbiased force. This mechanism is crucial in protein motion (biological motors), among other natural systems. Some years ago we realized artificial ratchets using vortices. We created an asymmetric vortex energy landscape by intorducing periodic arrays of magnetic nano-triangles in a superconducting thin film. Interestingly, this artificial system shows a reversible ratchet effect, in which the net direction of motion can be tuned via the applied magnetic field.
Vortex ratchets, in combination with guided vortex motion, allow vortex manipulation, and have opened the door “fluxtronics” –proposals to use vortices for information storage and computation.
Quasicrystals and fractals – What happens when a periodic elastic lattice is put on top of a quasiperiodic or a fractal substrate? Does the lattice deform to adapt to the quasiperiodic energy landscape? How? What happens with the lattice long-range correlations? To answer these questions, we studied vortices on quasiperiodic and fractal energy landscapes. We found unexpected commensurability effects: for instance the magneto-resistance showed a series of minima at fields that followed the Fibonacci series, the fractal structure appeared as a hierarchical series of oscillations following the Golden ratio… Strikingly, these imply that the vortex lattice adopts a fractal structure with long-range correlations.
Spaghetti vortices – While the most of the experiments with artificial ordered pinning use low-temperature superconductors (see above), vortices in high-temperature ones are even more interesting. The presence of thermal fluctuations and anisotropy makes vortices soft, which results in a much richer vortex phase diagram than in low-temperature superconductors. However, creating ordered pinning potentials in high-temperature materials is challenging, especially if one wants to control the geometry at the nanoscale.
We have implemented a masked irradiation technique that allows us to engineer -with truly nanometric resolution- the strength and the geometry of the vortex energy landscape in high-temperature superconductors. This has opened new possibilites, such as the design of energy landscapes whose geometry can be switched using temperature as a knob.