Searching for hidden order with muons, neutrons and photons

In condensed matter systems often the electronic degrees of freedom are entangled to form complex composites that give rise to unusual properties of matter, while escaping detection in the conventional experiments. Such entangled orders are known as hidden orders, a classical example of which was the anti-ferromagnetic ordering, that lies at the root of present-day technology based on magnetic sensors. A rather recent example of such hidden orders is the magneto-electric multipole, that breaks both space-inversion and time-reversal symmetries. While the magneto-electric multipoles [1] have already been recognized to be foundational to understand the magneto-electric effect, i.e., the generation of magnetization (electric polarization) in response to an applied electric field (magnetic field), the detection of such multipoles is still quite challenging and requires purposely designed experiments.