  Ever since the high thermal conductivity in cubic boron arsenide (c-BAs) was
predicted theoretically by Lindsay et. al in 2013, countless studies have
zeroed in on this particular material. Most recently, c-BAs has been confirmed
experimentally to have a thermal conductivity of around 1,100 W/m-K. In this
study, we investigate the seldom studied two dimensional hexagonal form of
boron arsenide (h-BAs) using a first-principles approach and by solving the
Boltzmann Transport Equation for phonons. Traditionally, a good indicator of a
high thermal conductivity material is its high Debye temperature and high
phonon group velocity. However, we determine h-BAs to have a much lower Debye
temperature and average phonon group velocity compared to the other monolayer
boron-V compounds of boron nitride (h-BN) and boron phosphide (h-BP), yet
curiously it possesses a higher thermal conductivity. Further investigation
reveals that this is due to the phonon frequency gap caused by large mass
imbalances, which results in a restricted Umklapp phonon-phonon scattering
channel and consequently a higher thermal conductivity. We determine the
intrinsic lattice thermal conductivity of monolayer h-BAs to be 362.62 W/m-K at
room temperature, which is considerably higher compared to the other monolayer
boron-V compounds of h-BN (231.96 W/m-K), h-BP (187.11 W/m-K), and h-BSb (87.15
W/m-K). This study opens the door for investigation into a new class of
monolayer structures and the properties they possess.
