E. Wilson and J.T Haraldsen
Dr. Jason Haraldsen | College of Arts and Sciences | Department of Physics
The new field of Dirac quantum matter has produced a lot of interesting theories and materials, especially in the dynamics of magnetic materials. One such material is RuCl3, which is a S = 1/2 zigzag honeycomb lattice. Through inelastic neutron scattering, this material has demonstrated spin waves with an energy scale of 1.5-8.0 meV. According to literature, RuCl3 may be the realization of a new theoretical phase of matter called a spin liquid. This materials seems to fit the profile and has been investigated using a Kitaev model. In this study, we re-examine the data for RuCl3 using a standard Heisenberg spin-spin exchange model with easy axis anisotropy. By imposing a Holstein-Primakoff expansion and utilizing competing exchange interactions within the zigzag magnetic configurations of RuCl3, we provide insight into the evolution of the spin dynamics. By analyzing the system by adding frustration, we are able to demonstrate that a standard Heisenberg model can produce an accurate model of the observed spin waves is produced. Therefore, with a simpler model describing the spin dynamics of RuCl3, we ultimately shed some doubt on the current considerations of RuCl3 as a quantum spin liquid.
Hi, I’m Evan Wilson and I’m going to give a brief overview of the research that I am working on within the field of Dirac quantum matter. Dirac quantum matter is a sub-field within Solid State physics that has grown an immense amount of interest as of late. This has produced a lot of interesting materials accompanied by rather complicated theories. One such material, which we are investigating, is RuCl3, which is described as the realization of a new phase of matter called a quantum spin liquid. This sounds rather complicated, and that’s because it is! As it is described by a complex model called the Kitaev interaction seen in the top right. This description comes from data produced from inelastic neutron scattering, which demonstrates a spin wave or a disturbance within the magnetic configurations that compose RuCl3. In this study we ask one simple question and that is, do we need such a complicated model to describe the behavior observed in RuCl3? Is there a way that the observed spin-wave can be modeled simply? In science, when we make an observation we always start with the simplest base model. If it doesn’t work, we build on the model, until it accurately describes what we are observing. So we start at the base model called a Heisenberg spin exchange model, which is used to describe the spin-wave dynamics and excitations for the zig-zag magnetic configuration of the honeycomb lattice in RuCl3. We observe two models of the interactions between the neighboring atoms within RuCl3, one where the atoms have opposite polarity and one with the same polarity. This configuration can be seen underneath the Kitaev figures. The no frustation model is the starting point that most would take; however, as seen in the No Frustration model, the behavior observed at different energy exchanges does not match the data. Naturally, this leads the observer to say this model doesn’t work, which would lead most to the Kitaev interaction. Nonetheless, we observe that there is no structural reason for RuCl3 to have opposite polarities, so we introduce a frustration between the interactions by making the polarities the same. This frustration causes the resultant spin wave to flip, giving us behavior similar to what we see in the data as seen in the bottom right. By changing the J and J’ interactions described in the model similarly to what we did in the no frustration model, we can see how the behavior of the spin wave changes. Utilizing the evolution of this behavior, we model effectively and accurately the observed spin-waves in RuCl3 as can be seen in the middle. By successfully doing this we cite occam’s razor which states that when you have two competing theories that make exactly the same predictions, the simpler one is the better. This ultimately calls into question the candidacy of RuCl3 as a quantum spin liquid as well as raises more questions into the behavior observed in RuCl3. Future calculation of the spin wave intensities will hopefully clarify this model as we continue to map the evolution of RuCl3’s spin dynamics. Thank you for your interest in my research, and thank you for listening.