Strong Coulomb interaction between electrons solids gives rise to remarkable emergent behavior that is absent in free electrons. Prominent examples are the self-organization into charge density waves, fractionalization of elementary spin and charge, quantum entanglement across macroscopic length scales, and most famously, the formation of Cooper pairs and superconductivity.
This breathtaking variety of collective quantum phenomena is exemplified in the copper oxide high-temperature (high-Tc) superconductors. My postdoctoral research is focused on the ladder material, Sr14Cu24O41. Ladders are the simplest structural units that exhibit the phenomenology of the high-Tc cuprates, and offer an ideal platform for rigorous comparisons between experiments and many-body theory. My goal is twofold: first, to use this material to better understand the microscopic physics of high-Tc superconductivity, and second, to create new, light-driven nonequilibrium phases of matter. I achieve this using ultrafast THz and resonant X-ray spectroscopy.
Beyond-Hubbard pairing in a cuprate ladder
A central challenge in understanding high-temperature superconductivity in cuprates is identifying the mechanism that drives electron pairing. While the widely studied Hubbard model captures many aspects of cuprate behavior, advanced numerical studies suggest that it fails to produce a robust superconducting state, suggesting that crucial interactions are missing. Since magnetic fluctuations are thought to play a key role in pairing, examining how spins behave when the material is doped provides an important window into the underlying physics. In this work, we use resonant inelastic x-ray scattering (RIXS) and theoretical modeling to uncover evidence of strong hole pairing beyond predictions of the simple Hubbard model.
Our findings suggest that strong attraction beyond the Hubbard model is necessary to describe superconducting pairing in cuprates. This discovery paves the way for improved theoretical frameworks to understand and design new superconducting materials.
H. Padma, J. Thomas, S. TenHuisen, W. He, Z. Guan, J. Li, B. Lee, Y. Wang, S. H. Lee, Z. Mao, H. Jang, V. Bisogni, J. Pelliciari, M. P. M. Dean, S. Johnston, M. Mitrano. Physical Review X 15, 021049 (2025).
Resonant inelastic x-ray scattering on the cuprate ladder Sr14Cu24O41 reveals enhanced hole pairing beyond the Hubbard model.
A powerful tool to study driven quantum materials: time-resolved resonant inelastic x-rays scattering
A dominant paradigm in the study of quantum materials is to measure the elementary excitations of a system (e. g. phonons, magnons), and use this to reconstruct the underlying interactions. Time-resolved resonant inelastic X-ray scattering (trRIXS), a recently developed technique, expands this powerful approach to the realm of light-driven nonequilibrium systems. Collaborating with scientists at X-ray free electron laser facilities, I use trRIXS to quantify transient electronic interactions in light-driven high-Tc cuprates.
More coming soon!
The time-resolved RIXS spectrometer at the Furka endstation in SwissFEL at the Paul Scherrer Institut, where we conducted a pilot experiment measuring the transient magnetic RIXS spectrum of Sr14Cu24O41.