Research Overview: The Power of RIXS
Courtesy of Bains Professor Steven Johnston and students Jinu Thomas and Debshikha Banerjee
Quantum materials—systems whose properties are dominated by quantum mechanical many-body effects—represent one of the most exciting frontiers of condensed matter physics. They also have the potential to revolutionize technology with applications in superconductors, magnets, and sensors.
In these systems, it is common for different degrees of freedom like spin, charge, orbital, and lattice vibrations to become intricately entangled, making it difficult to identify which is the driver of a given phenomenon. This entanglement makes quantum materials hard to model, but it also produces a wide range of exotic phenomena, including high-temperature superconductivity, various types of density waves, topological states, and more. Notably, the Bains Professor Steven Johnston’s research group has long been interested in how electrons couple to atomic vibrations and how this interaction can influence the properties of these materials.
Advanced experimental and numerical techniques are required to unravel the complex behavior present in quantum materials. Among them, resonant inelastic x-ray scattering (RIXS) has emerged as a powerful spectroscopic tool due to its ability to simultaneously probe spin, charge, orbital, and lattice excitations in a single experiment. A recent perspective piece published in Physical Review X by Johnston and Adjunct Professor Mark Dean, along with their colleagues, sheds light on applications of RIXS in quantum materials. More recently, members of Johnston’s group have published a study in Physical Review Letters, presenting state-of-the-art calculations for RIXS response for a correlated quantum material with strong interactions to phonon modes via a novel kinetic energy coupling mechanism.
First author and PhD student Debshikha Banerjee, together with Jinu Thomas, Alberto Nocera (University of British Columbia), and Johnston, used the density matrix renormalization group (DMRG) to predict the RIXS spectra for a one-dimensional Hubbard chain coupled with Su-Schrieffer-Heeger (SSH)-like electron-phonon coupling. This model has established itself for studying chain systems like Sr2CuO3, which has long served as a platform for studying quantum magnetism in low-dimensional systems.
Most studies of electron-phonon coupling have focused on simplified models (e.g., Holstein or Fröhlich), where coupling is between electron density and lattice displacements. In contrast, the SSH model captures lattice vibrations that alter atomic bond lengths, an interaction present in all materials. SSH electron-phonon interactions have gained widespread interest in recent years following predictions that SSH interactions can contribute to high-temperature superconductivity. SSH interactions have also been tied to topological edge states, a topic of interest in recent years. To test these theoretical claims, the community needs experimental protocols to identify the existence of SSH interaction in materials. Banerjee et al.’s work demonstrates how RIXS can be exploited to identify and quantify SSH interactions in quantum materials.
This study builds on a recent work led by PhD student Jinu Thomas and the same team, published in Physical Review X, which has advanced the state-of-the-art modeling of lattice excitations in RIXS.