Magneto-optical probes shed light on the unconventional resistivity of EuCd2P2

Understanding the mechanisms and consequences of the coupling between disparate degrees of freedom is at the forefront of quantum materials research. A prominent example is coupling between magnetic and electronic degrees of freedom, offering the exciting opportunity to control the near Fermi level electronic structure and topology by manipulating magnetic order. However, the same coupling may promote a complex and inhomogeneous magnetic state, which is challenging to experimentally identify.

Here we introduce a new experimental technique designed to sensitively detect optical birefringence proportional to an applied magnetic field, which we refer to as linear magneto-birefringence (LMB). We discuss the role of LMB as a powerful probe of magnetic order, as it is allowed by Onsager’s relations only in systems which simultaneously break time-reversal symmetry and three-fold rotational symmetry. We measure LMB in EuCd2P2, a material which stands out due to the unusual temperature dependence of its resistivity: metallic behavior at high temperatures is followed by a 100-fold increase of resistivity with decreasing temperature. Remarkably, resistivity peaks at 18K, well above the Neel temperature of 11K. Using a powerful combination of our new technique and resonant x-ray scattering we reveal a far richer picture of EuCd2P2 magnetism than has been previously reported, helping to resolve this mystery.