Observables compatible to the toroidal moment operator

Abstract

The quantum operator $\hat{T}_3$, corresponding to the projection of the toroidal moment on the $z$ axis, admits several self-adjoint extensions, when defined on the whole $\mathbb{R}^3$ space. $\hat{T}_3$ commutes with $\hat{L}_3$ (the projection of the angular momentum operator on the $z$ axis) and they have a \textit{natural set of coordinates} $(k,u,\phi)$ where $\phi$ is the azimuthal angle. The second set of \textit{natural coordinates} is $(k_1,k_2,u)$, where $k_1 = k\cos\phi$, $k_2 = k\sin\phi$. In both sets, $\hat{T}_3 = -i\hbar\partial/\partial u$, so any operator that is a function of $k$ and the partial derivatives with respect to the \textit{natural variables} $(k, u, \phi)$ commute with $\hat{T}_3$ and $\hat{L}_3$. Similarly, operators that are functions of $k_1$, $k_2$, and the partial derivatives with respect to $k_1$, $k_2$, and $u$ commute with $\hat{T}_3$. Therefore, we introduce here the operators $\hat{p}_{k} \equiv -i \hbar \partial/\partial k$, $\hat{p}^{(k1)} \equiv -i \hbar \partial/\partial k_1$, and $\hat{p}^{(k2)} \equiv -i \hbar \partial/\partial k_2$ and express them in the $(x,y,z)$ coordinates. One may also invert the relations and write the typical operators, like the momentum $\hat{\bf p} \equiv -i\hbar {\bf \nabla}$ or the kinetic energy $\hat{H}_0 \equiv -\hbar^2\Delta/(2m)$ in terms of the "toroidal" operators $\hat{T}_3$, $\hat{p}^{(k)}$, $\hat{p}^{(k1)}$, $\hat{p}^{(k2)}$, and, eventually, $\hat{L}_3$. The formalism may be applied to specific physical systems, like nuclei, condensed matter systems, or metamaterials. We exemplify it by calculating the momentum operator and the free particle Hamiltonian in terms of \textit{natural coordinates} in a thin torus, where the general relations get considerably simplified.