Influence of isotopic substitution on the anharmonicity of the interlayer shear mode of h -BN

Abstract

We present a detailed analysis of the temperature dependence of the interlayer shear mode that reveals a variation of the 4-phonon scattering coupling with isotopic mass. Phonon anharmonic decay and isotopic disorder effects are very weak for the low-energy interlayer shear mode, and the overall temperature dependence is mainly governed by the thermal lattice expansion. This allows us to observe systematic differences in the temperature dependence of the low energy mode with the isotopic mass that are related to the 4-phonon scattering contribution in quartic anharmonicity. The change in the quartic anharmonicity coefficient between the isotopically pure samples is larger than it would be purely expected from the mass variation.

Long lifetime of the E1u in-plane infrared-active modes of h -BN

“…The effective anharmonic potentials used to obtain this curve are listed in Table I. The values of the anharmonic potentials are very similar to those found for the E high 2g phonon [18]. Since the E 1u (LO) phonon lies at a significantly higher frequency, the dominant decay pathways are different from those of the E 1u (TO) phonon.…”

“…The increase of the damping parameter with temperature is due to anharmonic phonon decay processes, as thoroughly discussed in Refs. [16,18,19]. By inspection of the phonon dispersion shown in Fig.…”

“…3, the dominant phonon decay paths that conserve energy and crystal momentum can be identified. The effects of phonon anharmonicity can then be modeled using a perturbation theory approach described in detail elsewhere [16,18]. Since the E 1u (TO) phonon is nearly degenerate with the E high 2g Raman-active mode, essentially the same channels contribute to the anharmonic phonon decay of both modes.…”

“…Being isotopic disorder the principal mechanism that limits the phonon lifetime in h-BN [15], a threefold increase in the polariton propagation length was demonstrated in nearly isotopically pure h-BN [9], opening prospects for the control of optical losses, which is essential for the development of phonon-polariton nanophotonic devices. In previous works, we extensively studied in detail the different mechanisms that limit the optical phonon lifetime by means of temperature-dependent Raman scattering measurements [15][16][17][18][19]. However, those studies addressed only the Raman-active E 2g mode.…”

Pressure dependence of the interlayer and intralayer E2g Raman-active modes of hexagonal BN up to the wurtzite phase transition

“…As expected, the GGA gives a superior account of the valence bonds in the hexagonal layers compared to the LDA, which clearly overestimates the E high 2g frequency. The experimental E low 2g frequency is, however, somewhat lower than the GGA + vdW prediction, even if we take into account the temperature effect on the frequency measured at room temperature, which would amount to a downshift of ∼2 cm −1 relative to the T = 0 K value provided by the ab initio calculations [46]. This suggests that the DFT-D3(BJ) density functional dispersion correction may be overestimating the interlayer van der Waals forces.…”