Junior Strings is a seminar series where DPhil students present topics of common interest that do not necessarily overlap with their own research area. This is primarily aimed at PhD students and post-docs but everyone is welcome.

In this talk, I will discuss Figalli and Gigli’s formulation of optimal transport between non-negative Radon measures in the setting of metric pairs. This framework allows for the comparison of measures with different total masses by introducing an auxiliary set that compensates for mass discrepancies. Within this setting, classical characterisations of optimal transport plans extend naturally, and the resulting spaces of measures are shown to be complete, separable, geodesic, and non-branching, provided the underlying space possesses these properties. Moreover, we prove that the spaces of measures 
equipped with the $L^2$-optimal partial transport metric inherit non-negative curvature in the sense of Alexandrov. Finally, generalised spaces of persistence diagrams embed naturally into these spaces of measures, leading to a unified perspective from which several known geometric properties of generalised persistence diagram spaces follow. These results build on recent work by Divol and Lacombe and generalise classical results in optimal transport.

We are going to study the Ginzburg-Landau energy for two specific geometries, related to the very experiments on fermionic condensates: annuli and strips 

The specific geometry of a strip provides connections between solitons and vortices, called solitonic vortices, which are vortices with a solitonic behaviour in the infinite direction of the strip. Therefore, they are very different from classical vortices which have an algebraic decay at infinity. We show that there exist stationary solutions to the Gross-Pitaevskii equation with k vortices on a transverse line, which bifurcate from the soliton solution as the width of the strip is increased. This is motivated by recent experiments on the instability of solitons by imposing a phase shift in an elongated condensate for bosonic or fermionic atoms.

For annuli, we prescribe a very large degree on the outer boundary and find that either there is a transition from a giant vortex to vortices also in the bulk but tending to the outer boundary.

This is joint work with Ph. Gravejat and E.Sandier for solitonice vortices and Remy Rodiac for annuli.
 

In this talk we consider the four-waves spatially homogeneous kinetic equation arising in weak wave turbulence theory from the microscopic Fermi-Pasta-Ulam-Tsingou (FPUT) oscillator chains.  This equation is sometimes referred to as the Phonon Boltzmann Equation. I will discuss the global existence and stability of solutions of the kinetic equation near the Rayleigh-Jeans (RJ) thermodynamic equilibrium solutions. This is a joint work with Pierre Germain (Imperial College London) and Joonhyun La (KIAS).