Research

"It doesn't matter how beautiful your theory is, it doesn’t matter how

smart you are. If it doesn't agree with experiment, it's wrong."

—— Richard Feynman

Correlation-enhanced photoexcited phenomena beyond the linear response regime

Recent extraordinary advances in light sources have enabled us to probe quantum materials and engineer new nonequilibrium quantum phases through light-matter interaction, in an unprecedented way. Understanding the phenomena driven by light is of tremendous interest and importance. However, it is usually difficult to have an accurate description because photo processes often involve many-body correlations and high-intensity pulses of light that go beyond the regime of linear response

Our group is interested in investigating photo-excited physics, including nonlinear optics, exciton dynamics, pump-probe spectroscopy, light-dressed states, etc. To have a predictive description, we develop and utilize advanced ab initio methods based on many-body perturbation theory, such as GW, Bethe-Slapter equation, and time-dependent GW. We also want to explore the interplay between light-matter interactions, band topology, and quantum geometry, and to elucidate the roles of many-electron correlations in these novel phenomena.

Related works

J. Ruan, Y.-H. Chan, S. G. Louie, Excitonic effects in nonlinear optical responses: Exciton-state formalism and first-principles calculations. arXiv:2310.09674, (Accepted by Nano Letter) (2024)
J. Ruan, W. Tang, J. Jiang, C. Hu, Y.-H. Chan, S. G. Louie, Exciton-enhanced tunable spin photocurrent in graphene nanoribbons. Submitted (2024)

Understanding and controlling quantum states in 2D materials

Two-dimensional materials offer an ideal platform for exploring and manipulating emergent quantum states. The reduced dimensionality diminishes electron screening, thereby amplifying the Coulomb interaction and resulting in pronounced many-body effects. These materials present numerous opportunities for designing novel structures through stacking and twisting.

We are interested in investigating 2D materials and their moiré heterostructures, with a focus on exotic excited states (such as excitons, trions, and biexcitons ) and emergent correlated states in moiré superlattices.

Related works

J. Ruan, Z. Li, C. S. Ong, S. G. Louie, Optically controlled single-valley exciton doublet states with tunable internal spin structures and spin magnetization generation. Proc. Natl. Acad. Sci. 120, e2307611120 (2023)
S. Craig*, J. Ruan*, C. Wang*, Z. Li*, Y. Deng, S. G. Louie, C. Shi, Layer-resolved flat-band excitations in magic-angle twisted bilayer phononic graphene. Under review by PRL (2024) (* denotes equal contribution)
C. Hu, M. Naik, Y.-H. Chan, J. Ruan, Steven G. Louie, Light-induced shift current vortex crystals in moiré heterobilayers. Proc. Natl. Acad. Sci. 120, e2314775120 (2023)

Topological phases and topological materials

Topological phases have opened a new chapter in condensed matter physics and materials science. These new phases of matter exhibit topologically protected phenomena that are immune to local perturbations, leading to unique and robust transport behaviors.

One of our research focuses is discovering new topological quantum phases and the real material candidates. We are also interested in non-Hermitian topology, which extends the traditional understanding of topological states of matter to non-Hermitian system.

Related works

J. Ruan, S.-K. Jian, H. Yao, H. Zhang, S.-C. Zhang, D. Xing, Symmetry-protected ideal Weyl semimetal in HgTe-class materials. Nat. Commun. 7, 111368 (2016)
J. Ruan, S.-K. Jian, D. Zhang, H. Yao, H. Zhang, S.-C. Zhang, D. Xing, Ideal Weyl Semimetals in the Chalcopyrites CuTlSe2, AgTlTe2, AuTlTe2, and ZnPbAs2. Phys. Rev. Lett. 116, 226801 (2016)
H. Wang, J. Ruan, H. Zhang, Non-Hermitian nodal-line semimetals with an anomalous bulk-boundary correspondence. Phys. Rev. B. 99, 75130 (2019)
J. Jiang, J. Ruan, S. G. Louie, Non-Hermitian topology induced unidirectional electron transport in graphene nanoribbons. Under review by PRL (2024)