Isotopic effects on phonon anharmonicity in layered van der Waals crystals: Isotopically pure hexagonal boron nitride

• Published in: Physical review. B 2018-04, Vol.97 (15), Article 155435
• Main Authors: Cuscó, Ramon, Artús, Luis, Edgar, James H., Liu, Song, Cassabois, Guillaume, Gil, Bernard
• Format: Article
• Language: English
• Subjects: Absorption cross sections; Anharmonicity; Anisotropy; Boron; Boron 10; Boron nitride; Channels; Condensed Matter; Crystals; Decay; Enrichment; Graphene; Neutron counters; Nuclear capture; Other; Perturbation methods; Perturbation theory; Phonons; Physics; Polaritons; Raman spectra; Single crystals; Solid state; Temperature dependence; Thermal neutrons
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.97.155435

Hexagonal boron nitride (h-BN) is a layered crystal that is attracting a great deal of attention as a promising material for nanophotonic applications. The strong optical anisotropy of this crystal is key to exploit polaritonic modes for manipulating light-matter interactions in 2D materials. h-BN has also great potential for solid-state neutron detection and neutron imaging devices, given the exceptionally high thermal neutron capture cross section of the boron-10 isotope. A good knowledge of phonons in layered crystals is essential for harnessing long-lived phonon-polariton modes for nanophotonic applications and may prove valuable for developing solid-state BN10 neutron detectors with improved device architectures and higher detection efficiencies. Although phonons in graphene and isoelectronic materials with a similar hexagonal layer structure have been studied, the effect of isotopic substitution on the phonons of such lamellar compounds has not been addressed yet. Here we present a Raman scattering study of the in-plane high-energy Raman active mode on isotopically enriched single-crystal h-BN. Phonon frequency and lifetime are measured in the 80–600-K temperature range for B10-enriched, B11-enriched, and natural composition high quality crystals. Their temperature dependence is explained in the light of perturbation theory calculations of the phonon self-energy. The effects of crystal anisotropy, isotopic disorder, and anharmonic phonon-decay channels are investigated in detail. The isotopic-induced changes in the phonon density of states are shown to enhance three-phonon anharmonic decay channels in B10-enriched crystals, opening the possibility of isotope tuning of the anharmonic phonon decay processes.