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

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.

6 pages, 4 figures. SI available on request

References in corpus (2)

• A Charge-Density-Wave Topological Semimetal
• Does (TaSe4)2I really harbor an axionic charge density wave?

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• Centrosymmetric-noncentrosymmetric Structural Phase Transition in Quasi one-dimensional compound, (TaSe${}_4$)${}_3$I
• Large moiré superstructure of stacked incommensurate charge density waves
• Memristive switching in the surface of a charge-density-wave topological semimetal
• Collective modes in the charge-density wave state of K${}_{0.3}$MoO${}_3$: The role of long-range Coulomb interactions revisited
• Anomalous amplitude mode dynamics below the expected charge-density-wave transition in 1$T$-VSe${}_2$
• RKKY interaction in the presence of a charge-density-wave order
• Emergence of a hidden-order phase well below the charge density wave transition in a topological Weyl semimetal (TaSe${}_4$)${}_2$I