Dominant Role of Quantum Anharmonicity in the Stability and Optical Properties of Infinite Linear Acetylenic Carbon Chains

Abstract

Carbyne, an infinite-length straight chain of carbon atoms, is supposed to undergo a second order phase transition from the metallic bond-symmetric cumulene (CC) ∞ toward the distorted insulating polyyne chain (−CC−) ∞ displaying bond-length alternation. However, recent synthesis of ultra long carbon chains (∼6000 atoms, [Nat. Mater., 2016, 15, 634]) did not show any phase transition and detected only the polyyne phase, in agreement with previous experiments on capped finite carbon chains. Here, by performing first-principles calculations, we show that quantum-anharmonicity reduces the energy gain of the polyyne phase with respect to the cumulene one by 71%. The magnitude of the bond-length alternation increases by increasing temperature, in stark contrast with a second order phase transition, confining the cumulene-to-polyyne transition to extremely high and unphysical temperatures. Finally, we predict that a high temperature insulator-to-metal transition occurs in the polyyne phase confined in insulating nanotubes with sufficiently large dielectric constant due to a giant quantum-anharmonic bandgap renormalization.

Renormalized phonon spectrum in the Su–Schrieffer–Heeger model

“…A finite-T generalization is in principle straightforward and would allow us to investigate the effects of the quadratic coupling on the transition temperature to an undimerized state [37]. We note that unusual behavior with T, attributed to quantum anharmonic effects, has been recently observed in carbyne chains by Romanin et al [38], wherein, counter-intuitively, dimerization actually increases with temperature: given that β deepens the potential energy well, it is possible that some of the behavior observed in [38] (obtained in ab initio), while unattainable with linear SSH coupling, could be manifested with quadratic coupling. We leave such calculations for future work.…”

Raman spectroscopy of isolated carbyne chains confined in carbon nanotubes: Progress and prospects

“…4(b). [58] Such a transitions would lift the correspondence of C-mode and bandgap as shown in Fig. 1(d), and is therefore readily detectable by 127801-9…”

“…127801-8 [58]. A phase transition from polyynic semiconducting carbyne to polyynic metallic carbyne is expected to occur at a temperature ∼ 1200 K.…”

“…A recent study by Romanin et al explains this discrepancy as a consequence of strong quantum anharmonicity, as the bond length alteration in carbyne is too big to be considered in the small displacement limit (7%-10% of the total bond length). [58] Thus, the temperature of the transition from polyynic to cumulenic carbyne is predicted to be extremely high and unphysical. However, for confined carbyne surrounded by a high dielectric environment, as it occurs in LCC@CNTs, they predict a transition from semiconducting polyynic carbyne to metallic, but surprisingly also polyynic carbyne at T = 1200 K due to the effects of optical vibrations on the gap, with a saturation of the longitudinal optical phonon (C-mode) frequency, as depicted in Fig.…”

Highly tunable optics across a topological transition in organic polymers

“…S3). Additionally, based on the findings of previous work on carbynes 33 and finite-sized acene polymers 13 , the intriguing change from acetylenic to cumulenic bridging at the transition suggests that phonons and anharmonicity could also play an important, temperature-dependent role in bringing these systems to the critical point where, if stabilized, an intrinsic organic metal is expected to emerge. 1 Supplemental Materials: Highly tunable optics across a topological transition in organic polymers…”