Recent experiments of polarized inelastic neutron scattering have identified the amplitude mode (ie, Higgs) in C9H18N2Cubr4, a nearly quantum quantum 2-D spin ladder compound that exhibits a weak easy-axis exchange anisotropy. Inspired by these findings, researchers from RWTH Aachen University, the Harbin Institute of Technology and the University of Erlangen-Nürnberg conducted a study examining the dynamic spin factor structure of planar coupled spin-ladder systems using the quantum Monte Carlo (QMC) on a large scale. simulations.
"Observation and understanding of the Higgs amplitude mode in quantum magnets is exciting because it connects research in high energy physics (Nobel 2013 for Higgs particle observation) to similar concepts in condensed matter physics," Kai Phillip Schmidt and Stefan Wessel, two of the researchers who conducted the study, said Phys.org via email. "However, this mode is quite fragile in many planar magnets, so its potential experimental detection on a planar quantum magnet of spin ladders coupled by inelastic neutron scattering was a surprise."
For the purpose of the study, Schmidt developed an approximate theory, which was yet to be rigidly confirmed via quantitative modeling. To achieve this, Wessel, with whom Schmidt was well acquainted, and Tao Ying, a postdoctoral student under the supervision of Wessel, decided to try to apply Monte Carlo simulations to this problem.
Essentially, they set out to examine the dynamic spin-ladder factor of planar coupled spin-ladder systems previously identified using QMC simulations. Their combined study, published in Physical Review Letters (PRL), allowed them to gain a quantitative understanding of the mode of Higgs amplitude described in previous research.
"The dynamic structure factor is important because it contains complete information about magnetic excitations (such as the Higgs amplitude mode) and is the essential quantity that is measured by the dispersion of inelastic neutrons," said Schmidt and Wessel. "Quantum Monte Carlo (QMC) simulations are a very powerful numerical tool for studying certain classes of quantum magnets and for extracting the dynamic structure factor, which is typically very difficult to obtain by other means."
Using the latest generation QMC simulation techniques, Schmidt, Wessel and Ying were able to compare the numerical values of certain excitation energies with those measured in spreading inelastic neurons. This allowed them to identify the magnetic interactions present in a particular quantum magnet.
"The quantitative modeling of the specific experimental quantum magnet and the possibility of interpreting the nature of the magnetic excitations observed in theory allow the rigorous identification of the Higgs amplitude mode in a two-dimensional system of coupled spin ladders," said Schmidt and Wessel. "In addition, we were able to trace the properties of the Higgs amplitude mode over a large parameter space in our model, which allowed us to follow this particle to the so-called Ising limit, which is one of the most paradigmatic models of physics."
In their study, Schmidt, Wessel and Ying were able to explicitly understand the mode of Higgs amplitude observed in previous experiments as a connected state of two conventional magnetic excitations, which is in analogy to a molecule being made from atoms. His work demonstrates the feasibility of formulating a quantitative theory to understand the spin dynamics of near-quantum 2-D magnets using state-of-the-art QMC simulation techniques. While they specifically applied their theory to compound C9H18N2Cubr4They believe it can also be used to understand the quantum dynamics of other similar magnetic compounds.
"There are several interesting routes to follow in the future," said Schmidt and Wessel. "In particular, it will be important to understand the fate of the Higgs amplitude mode when it is tuned closer to the quantum critical points, for example, by applying a magnetic field or external pressure, as this excitation behaves and becomes unstable"
Neutrons detect the indefinable mode of Higgs amplitude in quantum material
T. Ying et al. Higgs Mode of Planar Coupled Spin Ladders and their Observation in C9H18N2CuBr4, Physical Review Letters (2019) DOI: 10.1103 / PhysRevLett.122.127201
Tao Hong et al. Higgs amplitude mode in a two-dimensional quantum antiferromagne near the quantum critical point, Physics of Nature (2017). DOI: 10.1038 / nphys4182
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