Control of the phonon band gap with isotopes in hexagonal boron nitride

TL;DR

This study demonstrates that isotopic variation in hexagonal boron nitride can significantly control the phonon band gap, enabling potential manipulation of terahertz wave propagation without chemical interfaces.

Contribution

The paper provides experimental evidence that isotopic mass changes can tune the phonon band gap in hexagonal BN, a novel approach for phononic device engineering.

Findings

Phonon band gap size varies with boron isotope mass.

In $^{11}$BN the gap is about 7 meV.

In $^{10}$BN the gap nearly closes below 0.5 meV.

Abstract

The isotopic mass of constituent elements of materials has a well-known effect on the energy of vibrational modes. By means of monochromated scanning transmission electron microscopy we have experimentally studied the phonon bandstructure of hexagonal BN, where a phonon band gap appears between in-plane optical phonon modes and the lower energy part of the phonon spectrum. The size of the phonon band gap can be manipulated by the isotopic mass of the boron. While in $^{11}$BN the phonon band gap is about 7 meV wide, in $^{10}$BN the gap nearly closes, being an order of magnitude smaller (below 0.5 meV). This opens exciting options for manipulating terahertz wave propagation through isotopically structured devices having otherwise no interfaces between chemically distinct components.