Weak ferromagnetism accompanying loop current order in underdoped cuprate superconductors

We discuss the necessary symmetry conditions and the different ways in which they can be physically realized for the occurrence of ferromagnetism accompanying the loop current orbital magnetic order observed by polarized neutron-diffraction experiments (or indeed any other conceivable principal order) in the underdoped phase of cuprates. We contrast the Kerr effect experiments in single crystals observing ferromagnetism with the direct magnetization measurements in large powder samples, which do not observe it. We also suggest experiments to resolve the differences among the experiments, all of which we believe to be correct.

Figure 1

(A) The crystal structure of YBCO and (B) the current pattern in the observed time reversal violating states.

Figure 2

Schematic spin order in YBCO for the domain shown in the presence of exchange interaction. The spins need not be collinear and the angle between the net oxygen spins (summed over all oxygens in a unit cell) and the copper atom is dictated by the relative strength of the exchange and ordering fields.