Systematic study of the hidden order in URu$_{2}$Si$_{2}$ as a multipolar order; the role of triakontadipoles

Abstract

A systematic search for possible order parameters of the so-called hidden order of URu$_{2}$Si$_{2}$ is conducted. Among the possible candidates that do fulfill the experimental symmetry restrictions on the hidden order parameter, we find one candidate that stand out -- one of the components of a triakontadipole multipole tensor that belongs to the $A_{1u}$ irreducible representation of the point group $D_{4h}$. This solution is characterized by a $\vec{Q}=(0,0,1)$ ordering of the triakontadipoles and has a symmetry that forbids magnetic moments as well as most other multipole ordering on the uranium sites. This hidden order phase is closely related to the the antiferromagnetic phase, which is manifested in the similarities of the geometries of the calculated Fermi surfaces. Finally is is found that this non-magnetic solution allows for another secondary superimposed order parameter that belong to $B_{2u}$, which gives rise to an anisotropic in-plane susceptibility without breaking the tetragonal crystal symmetry.