ORNL used lasers and neutrons to observe a long-lived nonequilibrium state of magnons in a magnet

Researchers at the US Department of Energy's Oak Ridge National Laboratory (ORNL) and their collaborators used lasers and neutrons to observe a long-lived quantum state in a magnetic material, according to a release published on newswise.com. newswise.comnewswise.comnewswise.comOpen source ↗ The work, published in Nature Communications, describes a way to study material systems driven far from equilibrium. The experiment combined rapid laser pulses with ORNL's Spallation Neutron Source, a DOE Office of Science user facility.

The team studied magnons, quantum quasiparticles whose wave-like excitations are critical to magnetic behavior in materials. Magnons can carry information, and quantum magnets can support long lifetimes and memory for data transfer.

Detailed balance violation

Laser pulses drove the material out of its normal resting state, while neutron scattering revealed the microscopic details of the magnetic behavior inside it. According to the release, this combined approach is not typically used for nonequilibrium studies.

The measurements showed that the material violated the rule of detailed balance. In a system at rest, this rule links energy-gaining and energy-losing events in a predictable way. Under the laser pulses the balance broke, and the neutron data showed extra signals only on one side. newswise.comnewswise.comnewswise.comOpen source ↗ ORNL's Chengyun Hua said that no one had directly probed a detailed balance violation in a condensed matter system before.

A persistent steady state

The nonequilibrium state did not quickly disappear. The lopsided excess of magnons persisted, which, according to the researchers, indicates that this was not a brief heating effect but a long-lived nonequilibrium steady state sustained by periodic laser pulsing.

The authors say the system can serve as a platform for studying other nonequilibrium quantum physics in real time. They also describe the study as a new benchmark for what neutron scattering can reveal in materials observed while they are being driven rather than at rest. The approach was first envisioned by Hua, Barry Winn and Alan Tennant, and reflects a multi-year effort at ORNL to build the specialized infrastructure for such measurements.

For ORNL researchers, the result helps build an experimental foundation for understanding new quantum materials and for exploring how such materials may eventually be studied with quantum computers and used in future information technologies. newswise.comnewswise.comnewswise.comOpen source ↗