We present a detailed analysis of the temperature dependence of the interlayer shear mode that reveals a variation of the four-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 four-phonon scattering contribution in quartic anharmonicity. The change in the quartic anharmonicity coefficient between the isotopically pure samples is larger than would be purely expected from the mass variation. © 2019 American Physical Society.

This work has been supported by the Spanish MINECO/FEDER under Contract No. MAT2015-71035-R. B.G. acknowledges the Russian Megagrant program (Ioffe Institute of RAS, Contract No. 14.W03.31.0011). Support for the h -BN crystal growth from the Materials Engineering and Processing program of the National Science Foundation, Award No. CMMI 1538127, is greatly appreciated.

Peer reviewed