Vibrational Modes in LiBC: Theory Compared with Experiment

TL;DR

This paper presents a theoretical calculation of the phonon spectrum in LiBC, comparing it with experimental Raman and IR data, to understand its vibrational properties relevant for superconductivity.

Contribution

It provides the first detailed phonon spectrum calculation for LiBC and predicts IR-active mode splittings, aiding future experimental and theoretical studies.

Findings

Calculated Raman-active modes match recent observations

Predicted small LO-TO splitting for B-C bond stretching mode

Large LO-TO splitting for modes at 540 cm^{-1} and 620 cm^{-1}

Abstract

The search for other superconductors in the MgB2 class currently is focussed on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with the potential to be a better superconductor than MgB2. Here we present the calculated phonon spectrum of the parent semiconductor LiBC. The calculated Raman-active modes are in excellent agreement with a recent observation, and comparison of calculated IR-active modes with a recent report provides a prediction of the LO--TO splitting for these four modes, which is small for the B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes at 540 cm^{-1} and 620 cm^{-1}.