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arXiv · 2210.14207

Observation of the massive Lee-Fukuyama phason in a charge density wave insulator

Soyeun Kim·Yinchuan Lv·Xiao-Qi Sun·Chengxi Zhao·Nina Bielinski·Azel Murzabekova·Kejian Qu·Ryan A. Duncan·Quynh L. D. Nguyen·Mariano Trigo·Daniel P. Shoemaker·Barry Bradlyn·Fahad Mahmood
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Abstract

The lowest-lying fundamental excitation of an incommensurate charge density wave (CDW) material is widely believed to be a massless phason -- a collective modulation of the phase of the CDW order parameter. However, as first pointed out by Lee and Fukuyama, long-range Coulomb interactions should push the phason energy up to the plasma energy of the CDW condensate, resulting in a massive phason and a fully gapped spectrum. Whether such behavior occurs in a CDW system has been unresolved for more than four decades. Using time-domain THz emission spectroscopy, we investigate this issue in the material (TaSe$_4$)$_2$I, a classical example of a quasi-one-dimensional CDW insulator. Upon transient photoexcitation at low temperatures, we find the material strikingly emits coherent, narrow-band THz radiation. The frequency, polarization and temperature-dependence of the emitted radiation imply the existence of a phason that acquires mass by coupling to long-range Coulomb interaction. Our observations constitute the first direct evidence of the massive "Lee-Fukuyama" phason and highlight the potential applicability of fundamental collective modes of correlated materials as compact and robust sources of THz radiation.

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Soyeun Kim, Yinchuan Lv, Xiao-Qi Sun, Chengxi Zhao, Nina Bielinski, Azel Murzabekova, Kejian Qu, Ryan A. Duncan, Quynh L. D. Nguyen, Mariano Trigo, Daniel P. Shoemaker, Barry Bradlyn, Fahad Mahmood. 2022-10-25. Observation of the massive Lee-Fukuyama phason in a charge density wave insulator. https://doi.org/10.1038/s41563-023-01504-5

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