Quasistatic magnetoelectric multipoles as order parameter for pseudogap phase in cuprate superconductors

We introduce a mechanism in which coupling between fluctuating spin magnetic dipole moments and polar optical phonons leads to a nonzero ferroic ordering of quasistatic 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 pseudogap phase and therefore propose the quasistatic magnetoelectric multipole as a possible pseudogap order parameter. Finally, we show that our mechanism embraces some key aspects of previous theoretical models, in particular the description of the pseudogap phase in terms of orbital currents.