MAE Professor Yongjie Hu led a research team that has reported for the first time thermal measurements in twisted graphene and uncovered polarized phonon spectral transport with quantum coupling in Moiré pattern.

Twisted van der Waals materials featuring Moiré patterns present new design possibilities and demonstrate unconventional behaviors in electrical, optical, spintronic, and superconducting properties. However, experimental exploration of thermal transport across Moiré patterns has not been as extensive, despite its critical role in nanoelectronics, thermal management, and energy technologies.

Here the UCLA researchers conducted the first experimental study on thermal transport across twisted graphene, demonstrating a phonon polarizer concept from the rotational misalignment between stacked layers. The direct thermal and acoustic measurements, structural characterizations, and atomistic modeling, reveal a thermal modulation with various Moiré angles, while maintaining a high acoustic transmission. By comparing experiments with density functional theory and molecular dynamics simulations, mode-dependent phonon transmissions are quantified based on the angle alignment of graphene band structures and attributed to the coupling among flexural phonon modes. The agreement confirms the dominant tuning mechanisms in adjusting phonon transmission from high-frequency thermal modes while having negligible effects on low-frequency acoustic modes near Brillouin zone center.

This study provides crucial insights into the fundamental thermal transport in Moiré structures, opening avenues for the invention of quantum thermal devices and new design methodologies based on manipulations of vibrational band structures and phonon spectra.

The paper’s first author, Zihao Qin, a PhD candidate from the MAE program in Hu’s group, stated, “I am very excited about our findings on the fundamental thermal transport of moiré structures through atomic-level materials control and am really looking forward to our new structural designs for novel quantum materials and devices.”

The co-first author, Lingyun Dai, also a PhD candidate in Hu’s lab, added, “The understanding of phonon transport across twisted graphene has hitherto lacked a clear physical picture. I’m very pleased that we were able to bridge this crucial gap to quantum mechanics with extensive measurements and rigorous atomistic simulations.”

The researchers detailed their discovery in a recent paper published in Advanced Materials. The authors include PhD students and postdocs Zihao Qin, Lingyun Dai, Man Li, Suixuan Li, and Huan Wu from Professor Yongjie Hu’s group, and Katherine E. White and Gilad Gani from Professor Paul S. Weiss’ group.

Reference: Z. Qin, L. Dai, M. Li, S. Li, H. Wu, K. E. White, G. Gani, P. S. Weiss, Y. Hu, Moiré Pattern Controlled Phonon Polarizer Based on Twisted Graphene. Adv. Mater. 2024, 2312176.