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
Resonant inelastic x-ray scattering on the cuprate ladder Sr14Cu24O41 reveals enhanced hole pairing beyond the Hubbard model.
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).
Electronic metastability protected by symmetry
Optical excitation results in a nonequilibrium electronic distribution that is trapped by symmetry.
Optically excited quantum materials exhibit non-equilibrium states with remarkable emergent properties, but these phenomena are usually transient, decaying on picosecond timescales and limiting practical applications. Advancing the design and control of non-equilibrium phases requires the development of targeted strategies to achieve long-lived, metastable phases. In this work, we report the discovery of symmetry-protected electronic metastability in the model cuprate ladder Sr14Cu24O41. Using femtosecond THz and resonant X-ray spectroscopy, we show that this metastability is driven by a transfer of holes from chain-like charge reservoirs into the ladders. This ultrafast charge redistribution arises from the optical dressing and activation of a hopping pathway that is forbidden by symmetry at equilibrium. Relaxation back to the ground state is, hence, suppressed after the pump coherence dissipates.
Our findings highlight how dressing materials with electromagnetic fields can dynamically activate terms in the electronic Hamiltonian, and provide a rational design strategy for non-equilibrium phases of matter.
H. Padma, F. Glerean, S. TenHuisen, Z. Shen, H. Wang, L. Xu, J. D. Elliott, C. C. Homes, E. Skoropota, H. Ueda, B. Liu, E. Paris, A. Romaguera, B. Lee, W. He, Y. Wang, S-H. Lee, H. Choi, S. Park, Z. Mao, M. Calandra, H. Jang, E. Razzoli, M. P. M. Dean, Y. Wang, M. Mitrano. Nature Materials (2025).