Erratum to: npj Quantum Materials (2017); doi:10.1038/s41535-017-0032-9; Published 13 June 2017

In the original version of this paper, the formula for ω 0 used in equation 10 incorrectly contained “k”, which should be “κ”. Equation 16c incorrectly contained the term 3(C2/3), which should be C2. 3(C2/3) has also been replaced by C2 on page 6 of the PDF in relation to LO mode frequency.

These corrections have been made to the HTML and PDF versions of this article.