In strongly correlated materials, electrons cannot maneuver freely and their motions are strongly influenced by others, like cars moving in heavy traffic. When these materials are driven to non-equilibrium, intriguing phenomena such as metal-insulator transition and superconductivity can further emerge. Understanding the behaviors of correlated electrons out of equilibrium remains a challenging task. Overcoming the challenge could open up revolutionary opportunities for novel device applications. In this project, the PI have utilized advanced quantum simulations with high-performance supercomputing to tackle the challenging problem of modeling non-equilibrium strongly correlated systems.

Intellectual Merit:

In two high-profile publications [Phys. Rev. Lett. 120, 246402 (2018) and Phys. Rev. X 11, 041028 (2021)], the PI and collaborators have studied the non-equilibrium dynamics of correlated electron systems coupled with phonons (quanta of lattice vibrations). It is found that the d-wave superconductivity pairing correlation can be enhanced by a pulse laser, when the original equilibrium system resides near a quantum phase boundary. However, the pairing correlation length is heavily suppressed, indicating that the induced electron Cooper pairs are essentially local, and that light-enhanced superconductivity may be of fluctuating nature.

To study the above problems, a novel quantum many-body approach was developed to simulate non-equilibrium electron-phonon systems. This approach separates the electronic wave function, the phonon wave function, and their entanglement. This allows solving the dynamics of each component using respectively exact diagonalization and variational methods while evolving the entire wave function self-consistently. The study is an important step towards understanding ultrafast light-induced phenomena in strongly correlated materials with prominent lattice effects.

In another publication [Phys. Rev. B 98, 245106 (2018)], the PI and collaborators have developed the theory for non-resonant time-resolved Raman scattering, and simulated the pump-probe dynamics of a correlated Mott-Hubbard model. It is found that for high-frequency and off-resonance pumps, the Floquet theory can capture the softening of magnon. Moreover, at small pump fluences, effective heating can dominate, while many-body effects take over at larger pump amplitudes and frequencies on resonance to the Mott gap. The results demonstrated that time-resolved Raman scattering provides the platform to explore various ultrafast processes and engineer fundamental interactions out of equilibrium. 

Broader of Impacts:

During the project period, the PI has mentored 4 PhD, 8 undergrad, and 1 high-school students. The students are of diversified backgrounds, including female, African American, and veteran students. They have received training on materials modeling, quantum many-body theory, high-performance supercomputing, and data-driven machine learning. In addition to scientific writing and publication, the students were supervised to present in local/regional/national conferences to improve their presentation/communication skills and to disseminate research findings. The PI has developed new courses "Computational Solid-State Physics" and "Machine Learning Applications in Physics and Materials Science" for his Department's computational physics track. The courses covered interdisciplinary topics in computational physics, data sciences, and materials modeling, where part of the educational materials come directly from activities supported by this grant. The PI has served as lecturers at the Birmingham Summer Institute held by the non-profit organization College Admissions Made Possible for 100 African American elementary-school students, and in the Tech Track Camp organized by American Association of University Women for 65 eighth-grade girls from 17 Alabama counties. The PI's extensive student mentoring, education, and outreach efforts will help build up advanced STEM workforce for Alabama and the U.S., by engaging them at various stages and by capturing America's strength in diversity.

The project topics directly address two of the Five Grand Challenges in Basic Energy Sciences, including understanding emergent phenomena due to correlation effects and characterizing material properties away from equilibrium. Using innovative computation techniques and data-driven discoveries to study non-equilibrium complex quantum systems also concerns several of the 10 Big Ideas for Future NSF Investments. Therefore, the project outcomes have potential impacts on an energy-sustainable future and help push forward the frontier of U.S. science leadership.

Last Modified: 05/24/2022
Modified by: Cheng-Chien Chen