  Higher-rank electric/magnetic multipole moments are attracting attention
these days as candidate order parameters for exotic material phases. However,
quantum-mechanical formulation of those multipole moments is still an ongoing
issue. In this paper, we propose a thermodynamic definition of electric
quadrupole moments as a measure of symmetry breaking, following previous
studies of orbital magnetic dipole moments and magnetic quadrupole moments. The
obtained formulas are illustrated with a model of orbital-ordered nematic
phases of iron-based superconductors.
