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Motion of vortices for the extrinsic Ginzburg-Landau flow for vector fields on surfaces

We consider the gradient flow of a Ginzburg-Landau functional of the type \[ F_\varepsilon^{\mathrm{extr}}(u):=\frac{1}{2}\int_M \left|D u\right|_g^2 + \left|\mathscr{S} u\right|^2_g +\frac{1}{2\varepsilon^2}\left(\left|u\right|^2_g-1\right)^2\mathrm{vol}_g \] which is defined for tangent vector fields (here $D$ stands for the covariant derivative) on a closed surface $M\subseteq\mathbb{R}^3$ and includes extrinsic effects via the shape operator $\mathscr{S}$ induced by the Euclidean embedding of~$M$. The functional depends on the small parameter $\varepsilon>0$. When $\varepsilon$ is small it is clear from the structure of the Ginzburg-Landau functional that $\left|u\right|_g$ ''prefers'' to be close to $1$. However, due to the incompatibility for vector fields on $M$ between the Sobolev regularity and the unit norm constraint, when $\varepsilon$ is close to $0$, it is expected that a finite number of singular points (called vortices) having non-zero index emerges (when the Euler characteristic is non-zero). This intuitive picture has been made precise in the recent work by R. Ignat \& R. Jerrard [7]. In this paper we are interested the dynamics of vortices generated by $F_\varepsilon^{\mathrm{extr}}$. To this end we study the behavior when $\varepsilon\to 0$ of the solutions of the (properly rescaled) gradient flow of $F_\varepsilon^{\mathrm{extr}}$. In the limit $\varepsilon\to 0$ we obtain the effective dynamics of the vortices. The dynamics, as expected, is influenced by both the intrinsic and extrinsic properties of the surface $M\subseteq\mathbb{R}^3$.