The HERO (Hidden, Entangled and Resonating Orders) project aims to identify novel kinds of hidden orders in materials, both to expose new fundamental physics and to engineer new properties with potential technological applications. In condensed matter systems, the atoms, electrons or spins sometimes arrange themselves in ways that result in unexpected properties that cannot be detected by conventional experimental probes. A historical example is the case of apparently non-magnetic manganese oxide (MnO) in which an unexpected experimental observation was made almost one hundred years ago: A cusp in the specific heat as a function of temperature, indicative of a phase transition, was found to coincide with a cusp in the magnetic susceptibility, suggesting that the phase transition had a magnetic origin. An explanation was proposed by Neel, who showed theoretically that the observed behaviour was consistent with hidden antiferromagnetic order, but verification required the development of a new characterization technique (neutron diffraction), which was able to directly measure the antiparallel alignment of neighbouring Mn magnetic moments.
In his Nobel Prize lecture, Neel made the statement that “while a large number of antiferromagnetic materials are now known, they are extremely interesting from the theoretical viewpoint, but do not seem to have any applications." Today, antiferromagnetic materials underpin multi-billion dollar industries as the exchange bias component in magnetic sensors and are promising for possible future spintronic devices.
In the HERO project, we are searching systematically for new forms of hidden order in three previously unexplored directions.
First, composite orders that are derived from correlations between conventional classical order parameters. Here, an example is motivated by multiferroic materials, which are simultaneously ferromagnetic and ferroelectric. That is, the order parameters describing the magnetic moment and
the electric dipole are non-zero. Instead of this revealed composite order, we are identifying scenarios in which both magnetic and electric dipole orders are zero but their product is not. Such materials have no net magnetization or electric polarization, but exhibit complex magnetic and dielectric susceptibilities associated with the hidden coupled magneto-electric order.
Second, entangled orders between quantum variables, such as at quantum multicritical points where different forms of order simultaneously appear near zero temperature.
Third, orders arising from dynamical effects such as quantum fluctuations or from external ac drive fields.
