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Detecting Phonon Overtones in Correlated Materials

Carbon-based materials such as graphene, fullerene, and carbon nanotubes exhibit unique properties that could be useful in next-generation electronic devices. However, the lack of a detailed understanding about certain interactions in these materials—such as the coupling between electrons and phonons (i.e., lattice vibrations)—limits our ability to connect electronic behavior with device performance.

Resonant inelastic x-ray scattering (RIXS) has recently been shown to be a promising technique for studying electron–phonon coupling (EPC) in correlated materials. When a photoexcited electron interacts with lattice vibrations during decay, the energy loss from this inelastic process should show up in the RIXS spectra. However, until recently, such studies had been limited by insufficient resolution.

Now, at Advanced Light Source (ALS) Beamline 8.0.1, researchers have obtained RIXS spectra from samples of highly oriented pyrolytic graphite (HOPG), a model system for carbon-based materials with well-known phonon modes. The improved resolution enabled the observation of clear phonon overtones—higher-order excitations that appear as ripples in intensity. The EPC strength can be directly obtained by counting the overtones: the more there are, the stronger the coupling. Moreover, the results showed that the electrons (or excitons, because the electrons in HOPG are still bound to the holes created in the RIXS process) can couple to different phonons.

How these results relate to HOPG transport properties remains to be elucidated by further theoretical studies. Nevertheless, this RIXS study offers a new window into phonon dynamics in the presence of a strong, photon-induced exciton, an interesting topic in the photophysics of carbon materials.

Location of phonon overtones in a RIXS map.
Left: X-ray absorption spectrum of HOPG with two prominent features representing π* and σ* resonances. Center: RIXS intensity map generated by capturing and analyzing the energy of x-rays scattered from the sample, plotted against the photon excitation energy. The yellow box marks the region of interest around the σ* resonance. Right: Multiple phonon excitations (overtones) can be clearly seen in the RIXS spectra around the σ* resonance.

X. Feng, S. Sallis, Y.‑C. Shao, R. Qiao, Y.‑S. Liu, L.C. Kao, A.S. Tremsin, Z. Hussain, W. Yang, J.‑H. Guo, and Y.‑D. Chuang, “Disparate Exciton-Phonon Couplings for Zone-Center and Boundary Phonons in Solid-State Graphite,” Phys. Rev. Lett. 125, 116401 (2020), doi:10.1103/PhysRevLett.125.116401.