Sophisticated neutron scattering techniques have been used by a team headed by the Department of Energy’s Oak Ridge National Laboratory in order to detect a mysterious quantum state called the Higgs amplitude mode in a two-dimensional material.
During the neutron scattering experiment, the sample containing copper ions exhibited exotic quantum properties as certain quasiparticles spin in a wave-like configuration, eventually revealing the Higgs amplitude mode. Credit: Oak Ridge National Laboratory
The Higgs amplitude mode is referred to as a condensed matter cousin of the Higgs boson, the storied quantum particle, which was theorized in the 1960s and experimentally proved in 2012. It is one amongst a number of collective, quirky modes of matter discovered in materials at the quantum level. Condensed matter Researchers have studied these modes and thus recently discovered new quantum states called quasiparticles, including the Higgs mode.
These studies offer unique opportunities for exploring quantum physics and applying its striking effects in enhanced technologies such as quantum computing, and spin-based electronics, or spintronics.
To excite a material’s quantum quasiparticles in a way that allows us to observe the Higgs amplitude mode is quite challenging.
Tao Hong, an Instrument Scientist, ORNL’s Quantum Condensed Matter Division
Even though the Higgs amplitude mode has been observed in different systems,
“the Higgs mode would often become unstable and decay, shortening the opportunity to characterize it before losing sight of it,” Hong said.
An alternative method was offered by the ORNL-headed team. A crystal made up of copper bromide was selected by the Researchers, since the copper ion is perfect for studying exotic quantum effects, Hong stated. The Researchers started the fragile task of “freezing” the material’s agitating quantum-level particles by bringing down its temperature to 1.4 Kelvin, which is almost -457.15 °F.
The team fine-tuned the experiment until the particles arrived at the place located close to the desired quantum critical point, which is the sweet spot where collective quantum effects spread over wide distances in the material, which develops the best conditions in which a Higgs amplitude mode can be observed without decay.
By performing neutron scattering at ORNL’s High Flux Isotope Reactor, the Researchers succeeded in observing the Higgs mode with an infinite lifetime: no decay.
There’s an ongoing debate in physics about the stability of these very delicate Higgs modes. This experiment is really hard to do, especially in a two-dimensional system. And, yet, here’s a clear observation, and it’s stabilized.
Alan Tennant, Chief Scientist of ORNL’s Neutron Sciences Directorate
The team’s observation offers new insights into the basic theories underlying exotic materials including antiferromagnets, ultracold bosonic systems, charge-density wave systems and superconductors.
“These breakthroughs are having widespread impact on our understanding of materials’ behavior at the atomic scale,” Hong added.
The study, titled, “Direct observation of the Higgs amplitude mode in a two-dimensional quantum antiferromagnet near the quantum critical point,” was published in Nature Physics. ORNL’s Tao Hong, Sachith E. Dissanayake, Harish Agrawal and David A. (Alan) Tennant, and Scientists from Shizuoka University, the National Institute of Standards and Technology, University of Maryland, University of Jordan, Clark University, Helmholtz-Zentrum Berlin for Materials and Energy and Lehrstuhl für Theoretische Physik I Co-Authored the study.
Cold neutron triple-axis spectrometer beams were used by the Researchers to study exotic magnetic effects and they also examined low-energy excitations in the copper bromide compound. The unpolarized neutron scattering measurements were carried out at ORNL’s HFIR and at Helmholtz-Zentrum Berlin for Materials and Energy. A high-intensity multi-axis crystal spectrometer was also used at NIST’s Center for Neutron Research by the team in order to compare data from polarized neutron-scattering measurements.
The DOE Office of Science funded this work, which was carried out ORNL’s HFIR, a DOE Office of Science User Facility.
ORNL is managed by UT-Battelle DOE’s Office of Science. The Office of Science is considered to be the single largest supporter of fundamental research in the Physical Sciences in the United States, and is working on addressing a few of the most pressing challenges that are currently available.