Researchers from Oak Ridge National Laboratory, Helmholtz-Zentrum Berlin (HZB) and University of Amsterdam have used inelastic neutron scattering and methods of integrability to experimentally observe and theoretically describe a local, coherent, long-lived, quasiperiodically oscillating magnetic state emerging out of the distillation of propagating excitations following a local quantum quench in a Heisenberg antiferromagnetic chain.
This “quantum wake” displays similarities to Floquet states, discrete time crystals and nonlinear Luttinger liquids. The team also showed how this technique reveals the non-commutativity of spin operators, and is thus a model-agnostic measure of a magnetic system’s “quantumness.”
Neutron scattering is considered the method of choice to study magnetic structures and excitations in quantum materials. Now, for the first time, the evaluation of measurement data from the 2000s using new methods has provided significantly deeper insights into a model system - the 1D Heisenberg spin chains. This provides a new toolbox for research into future quantum materials.
Potassium copper fluoride KCuF3 is considered to be the simplest model material for a so-called Heisenberg quantum spin chain: The spins interact with their neighbors antiferromagnetically along a single direction (one-dimensional) and are subject to the laws of quantum physics.
"We carried out the measurements on this simple model system at the ISIS spallation neutron source some time ago, when I was still a postdoc," says Prof. Bella Lake, who heads the HZB Institute for Quantum Phenomena in New Materials. "We compared our results, which we published in 2005, 2013 and again in 2021, with new theories," she says. With new and advanced methods, a team led by Prof. Alan Tennant and Dr. Allen Scheie has now succeeded in gaining significantly deeper insights into the interactions between the spins and their spatial and temporal development.
"In neutron scattering, you nudge a spin in such a way that it flips over. This creates a dynamic, like a wake when a ship goes through the water, that can affect its neighbors and their neighbors," explains Tennant.
"Neutron scattering data is measured as a function of energy and wave vector," says Scheie. "Our breakthrough was to map the spatial and temporal evolution of the spins using mathematical methods such as the back Fourier transform." In combination with other theoretical methods, the physicists received information about the interactions between the spin states and their duration and range as well as insights into what is known as quantum coherence.
The work offers a new toolbox for the analysis of neutron scattering data in order to deepen the understanding of technologically relevant quantum materials.