Weak Ferromagnetism Accompanying Loop current order in Underdoped Cuprates

TL;DR

This paper explores the symmetry conditions for ferromagnetism co-occurring with loop current order in underdoped cuprates, analyzing experimental discrepancies and proposing future experiments.

Contribution

It clarifies the symmetry requirements for ferromagnetism in cuprates and discusses how different experiments may observe or miss this phenomenon.

Findings

Symmetry analysis of ferromagnetism and loop current order

Contrast between Kerr effect and magnetization measurements

Proposed experiments to resolve experimental discrepancies

Abstract

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 under-doped 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.