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  Min, C.-H.; Müller, S.; Choi, W.; Dudy, L.; Zabolotnyy, V.; Heber, M.; Denlinger, J. D.; Kang, C.-J.; Kalläne, M.; Wind, N.; Scholz, M.; Lee, T.; Schlueter, C.; Gloskovskii, A.; Rienks, E.; Hinkov, V.; Bentmann, B.; Kwon, Y.; Reinert, F.; Kim, H.; Rossnagel, K.: Polaronic Quasiparticles in the Valence-Transition Compound TmSe_1-xTe_x. Physical Review Letters 135 (2025), p. 186501/1-7

10.1103/72dv-ynm2

Abstract:
Exotic quasiparticle states have been proposed in mixed-valent compounds exhibiting valence transitions. However, clear spectroscopic evidence identifying these states has remained elusive. Using synchrotron-based hard x-ray and extreme ultraviolet photoemission spectroscopy, we have probed the Tm 3⁢𝑑 and 4⁢𝑓 emissions in TmSe1−𝑥⁢Te𝑥, where a Te concentration-dependent semimetal–insulator transition occurs alongside the valence transition. Our photoemission results, which are characteristic of the bulk, track this combined transition across the critical concentration (𝑥𝑐 =0.29). Notably, our results reveal a noninteger valence for the insulating phase and a novel quasiparticle excitation in the semimetallic phase: a Holstein polaron that extends beyond the standard periodic Anderson model.