Thermal Conductivity of BAs under Pressure

Boron arsenide (BAs) is an ultrahigh-thermal-conductivity material with special phonon-phonon scattering behaviors. At ambient pressure, the bunching of acoustic phonon branches in BAs is believed to result in a small phase space for three-phonon scattering. Density functional theory predicts that this acoustic phonon bunching effect is sensitive to pressure and leads to an unusual pressure dependence of thermal conductivity. To explore this physics, we measure the thermal conductivity of BAs from 0 to 25 GPa using time-domain thermoreflectance in a diamond anvil cell. We characterized two BAs samples with ambient thermal conductivities of 350 and 480 W m-1 K-1. Our experiments show that the thermal conductivity of both samples depends weakly on pressure from 0 to 25 GPa. We attribute the weak pressure dependence of the thermal conductivity of BAs to the weak pressure dependence of total phonon-phonon scattering rates. Our experimental results are consistent with DFT predictions that three-phonon scattering rates increase from 0 to 25 GPa, while four-phonon scattering rates decrease.

Electron-LO-phonon Intrasubband Scattering Rates in a Hollow Cylinder Under the Influence of a Uniform Axial Applied Magnetic Field

Scattering rates arising from the interactions of electrons with bulk longitudinal optical (LO) phonon modes in a hollow cylinder are calculated as functions of the inner radius and the uniform axial applied magnetic field. Now, the specific nature of electron-phonon interactions mainly depends on the character of the energy spectrum of electrons. As is well known, in cylindrical quantum wires, the application of a parallel magnetic field lifts the double degeneracy of the non-zero azimuthal quantum number states; m≠0; irrespective of all electron's radial quantum number l states. In fact, this Zeeman splitting is such that the m < 0 electron's energy subbands initially decrease with the increase of the parallel applied magnetic field. In a solid cylinder, the lowest-order; {l = 1; m = 0} subband is always the ground state. In a hollow cylinder, however, as the axial applied magnetic field is increased, the electron's energy subbands take turns at becoming the ground state; following the sequence {m=0,-1,-2...-N} of azimuthal quantum numbers. Furthermore, in a hollow cylinder, in general, the electron's energy separations between any two subbands are less than the LO phonon energy except for exceptionally high magnetic fields, and some highest-order quantum number states. In view of this, the discussion of the energy relaxation here is focused mainly on intrasubband scattering of electrons and only within the lowest-order {l = 1; m = 0} electron's energy subband. The intrasubband scattering rates are found to be characterized by shallow minima in their variations with the inner radius, again, for a fixed outer radius. This feature is a consequence of a balance between two seemingly conflicting effects of the electron's confinement by the inner and outer walls of the hollow cylinder. First; increased confinement of the charge carriers generally leads to the enhancement of the rates. Second; the presence of a hole in a hollow cylinder leads to a significant suppression of the scattering rates. The intrasubband scattering rates also show a somewhat parabolic increase in their variations with the applied magnetic field; an increase which is more pronounced in a relatively thick hollow cylinder.