Our group engages in research that covers a broad range of topics in condensed matter physics and materials science. Some of our current interests are listed below:
Excited-State Physics and Light-Matter Interactions
Exciton physics and valleytronics
Excited states describe the quantum states of a system, which have higher energies than the ground-state energy. The behaviors of these excited states not only give rise to a variety of fascinating properties and phenomena in condensed matter, but also play vital roles in modern technologies such as electronics, optoelectronics, and energy harvesting. By coupling light with materials, the study of optical excitation has enable research on excited states to make numerous fascinating discoveries.
Excitons, formed by electron-hole pairs bound by Coulomb interactions, are excited states that can be directly created by light. The excitons are responsible for sub-bandgap light absorption of semiconductors. In low-dimensional materials, the excitons are particularly important, because the Coulomb interactions are strongly enhanced and make the exciton binding energies as large as a fraction of the band gap. Due to this fact, the commonly used DFT methods fail to capture to exciton physics. In recent years, we have discovered a number interesting excitonic effects in low-dimensional materials, by using quantum theories and ab initio GW-BSE (derived from the many-body perturbation theory) calculations. These works include: tightly-bound excitons and exciton dark states, light-like exciton dispersions, valley-selective circular dichroism, and exciton-phonon replica states.
- A review summarizing these recent theory developments: K. Xie, X. Li, and T. Cao, Adv. Mater. 1904306. (2019)
Geometric effects in optical transitions
Interesting excited-state phenomena can arise from geometric effects. We discover that, in 2D semiconductors with nontrivial bands, the strength and required light polarization of an excitonic optical transition are dictated by the optical matrix element winding number, a unique and previously unrecognized topological characteristic. In gapped graphene systems, each valley hosts multiple bright excitons coupled to light of different helicity. Parts of these findings have been observed in experiments. The geometric effects still have much to offer in our understanding of excited-state phenomena.
- T. W. Fernando and T. Cao, in preparation (2019)
- Optical selection rules: T. Cao, M. Wu, and S. G. Louie, Phys. Rev. Lett. 120, 087402 (2018).
- Experimental observation: L. Ju, L. Wang, T. Cao, T. Taniguchi, K. Watanabe, S. G. Louie, F. Rana, J. Park, J. Hone, F. Wang, and P. L. McEuen, Science 358, 907-910 (2017).
Low-Dimensional Quantum Materials
Understanding and controlling magnetism
Understanding and controlling magnetism in 1D and 2D materials has been a long pursuit in condensed matter. Through ab initio calculations, we predicted tunable ferromagnetism and half-metallicity in a monolayer GaSe by hole doping.
More recently, we have been working on the ferromagnetism that arises from local magnetic moments in a 2D crystals such as Cr2Ge2Te6 and CrI3. In these materials, the ferromagnetism mainly originates from magnetic moments of Cr atoms. By using renormalized spin-wave theory, we studied the origin of the field and temperature dependent magnetic properties and interpreted experimental findings.
- Tunable magnetism in GaSe: Ting Cao, Zhenglu Li, and Steven G. Louie, Phys. Rev. Lett. 114, 236602 (2015).
- Ferromagnetism of Cr2Ge2Te6: C. Gong, L. Li, Z. Li, H. Ji, A. Stern, Y. Xia, T. Cao, W. Bao, C. Wang, Y. Wang, Z. Q. Qiu, R. J. Cava, S. G. Louie, J. Xia, and X. Zhang, Nature 546, 265–269 (2017).
- Ferromagnetism of CrI3: Z. Sun, Y. Yi, T. Song, G. Clark, B. Huang, Y. Shan, S. Wu, D. Huang, C. Gao, Z. Chen, M. McGuire, T. Cao, D. Xiao, W.-T. Liu, W. Yao, X. Xu, and S. Wu, Nature 572, 497–501 (2019).
Topological band engineering
We predict that semiconducting graphene nanoribbons (GNRs) of different width, edge, and end termination belong to different electronic topological classes. The topological phase of GNRs is protected by spatial symmetries and dictated by the terminating unit cell. These theoretical prediction of junction states and end states arising from the non-trivial topological phases has recently been experimentally observed and confirmed.
- Initial Prediction: Ting Cao, Fangzhou Zhao, and Steven G. Louie, Phys. Rev. Lett. 119, 076401 (2017).
- Experimental realization: Daniel J. Rizzo, Gregory Veber, Ting Cao, Christopher Bronner, Ting Chen, Fangzhou Zhao, Henry Rodriguez, Steven G. Louie, Michael F. Crommie, and Felix R. Fischer, Nature 560, 204 (2018).