Chiral charge as hidden order parameter in URu$_2$Si$_2$

Abstract

We propose a chiral charge (electric toroidal monopole) as the hidden order parameter in URu$_{2}$Si$_{2}$, which satisfies all the symmetry conditions accumulated by many experimental and theoretical efforts since its discovery. By using the minimal effective $d$-$f$ hybridized model in the itinerant picture, we demonstrate expected cross-correlated phenomena under the chiral charge ordering, and spatial distribution of the order parameter with surface or domain boundary. Based on the results, we discuss possible experiments by using NQR and diffraction measurements under an electric field, or optical measurements near the surface or domain boundary, although the proposed fully rotational symmetric order parameter is hard to detect as it is.