We introduce a mechanism in which coupling between fluctuating spin magnetic dipole moments and polar optical phonons leads to a non-zero ferroic ordering of quasi-static magnetoelectric multipoles. Using first-principles calculations within the LSDA+U method of density functional theory, we calculate the magnitude of the effect for the prototypical cuprate superconductor, HgBa2CuO4. We show that our proposed mechanism is consistent, to our knowledge, with all experimental data for the onset of the pseudo-gap phase and therefore propose the quasi-static magnetoelectric multipole as a possible pseudo-gap order parameter. Finally, we show that our mechanism embraces some key aspects of previous theoretical models, in particular the description of the pseudo-gap phase in terms of orbital currents.
@article{arxiv.1510.04844,
title = {Quasi-static magnetoelectric quadrupoles as the order parameter for the pseudo-gap phase in cuprate superconductors},
author = {M. Fechner and M. J. A. Fierz and F. Thöle and U. Staub and N. A. Spaldin},
journal= {arXiv preprint arXiv:1510.04844},
year = {2016}
}