Study Shows How Quantum Magnets Mimic Emergent Light

A theory about the quantum properties of magnets forms the basis for a hypothesis about emergent light.

Several years ago, researchers from the Okinawa Institute of Science and Technology (OIST) predicted a characteristic signature that could signal the presence of emergent light inside a quantum spin ice. The researchers report that they have now observed this signature in a material called praseodymium hafnate (Pr₂Hf₂O₇).

Graphical representation of theoretical neutron scattering on a quantum spin ice. Note the characteristic pinch point (circled), a bow-tie-shaped pattern of neutron reflection. Courtesy of OIST.

To test the theory, researchers from the Paul Scherrer Institut (PSI) worked at temperatures as low as 50 mK (millikelvin) — less than a tenth of a degree above absolute zero — with crystals free of any dirt and imperfections, to generate a perfect crystal of a quantum spin ice material, which was used to test the hypothesis.

The crystal was transported to the Oak Ridge National Laboratory (ORNL), where an array of 960 supermirrors, coated with alloys that could selectively reflect different types of neutrons, was used with ORNL’s HYSPEC instrument to obtain a 3D analysis of the neutrons’ reflection patterns.

Models of the atomic lattice structure of the quantum spin ice, praseodymium hafnate (Pr₂Hf₂O₇). Courtesy of OIST.

The scattered neutrons were mapped using an instrument at the European Institut Laue-Langevin (ILL). Researchers measured the polarization of the scattered particles and mapped the energy signatures that those particles produced.

The graphical representation of neutron reflection displayed so-called pinch points, which are characteristic features of a quantum spin ice. When the spin ice was scanned at low temperatures, the pinch points disappeared in a way that strongly suggested emergent light.

This is a crystal of the quantum spin ice candidate Pr2Hf2O7 used in the study. Courtesy of Monica Ciomaga Hatnean.

Researcher Han Yan analyzed the experimental data to determine the speed of the emergent light — 3.6 m per second, about as fast someone running a marathon in four hours. Conventional photons could cover the same distance in less than a thousandth of a second.

“At present, we don't know any way of explaining these results without invoking quantum mechanics," said OIST professor Nic Shannon, "so it really does look like we have seen emergent light.”

Nic Shannon (left) and Han Yan of the OIST Theory of Quantum Matter Unit. Courtesy of OIST.

In the late 20th century, physicists began exploring the phenomenon called emergence, which describes how particles in large groups can behave in unexpected ways. The investigation into emergent light began by focusing on the family of magnetic systems known as spin ice, because researchers found that when an emergent electric field was introduced in spin ice, the emergent electric and magnetic fields combined to produce magnetic excitations that behaved like photons.

The research was published in Nature Physics (doi:10.1038/s41567-018-0116-x).