Rohit R. Kakodkar | Searcharxiv

arXiv2017

Probing the validity of the diffuse mismatch model for phonons using atomistic simulations

Due to it's simplicity the diffuse mismatch model (DMM) remains a popular description of phonon transmission across solid-solid boundaries. However, it remains unclear in which situations the DMM should be expected to be a valid model of the underlying physics. Here, its validity is investigated mode-by-mode using a 3-dimensional extension of the frequency domain, perfectly matched layer (FD-PML) method, to study the interface between face-centered cubic solids with interdiffused atoms. While submonolayer levels of interdiffusion are found to increase the number of available modes for transmission, consistent qualitatively with the DMM, we do not find quantitative or qualitative convergence toward the DMM at higher levels of interdiffusion. In particular, contrary to the fundamental assumption of the DMM, modes are not found to lose memory of their initial polarization and wavevector. The transmission coefficients of randomly interdiffused and smoothly-graded interfaces are also compared. While smoothly graded interfaces show strong anti-reflection properties, selection rules still prohibit transmission of many modes, whereas interdiffused interfaces are not subject to such rules and achieve similar thermal interface conductance by transmitting with lower probability but using a wider range of modes.

cond-mat.mes-hall↗

arXiv2015

A framework for solving atomistic phonon-structure scattering problems in the frequency domain using Perfectly Matched Layer boundaries

We present a numerical approach to the solution of elastic phonon scattering problems based on a frequency domain decomposition of the atomistic equations of motion and the use of perfectly matched layer or PML boundaries. Unlike MD wavepacket analysis, the current approach has the ability to simulate scattering from individual phonon modes, including wavevectors in highly dispersive regimes. Like the Atomistic Green's Function method, the technique reduces scattering problems to a system of linear algebraic equations via a sparse, banded matrix. However, the use of PML boundaries enables rapid absorption of scattered wave energies at the boundaries, and provides a simple and inexpensive interpretation of the scattered phonon energy flux calculated from the energy dissipation rate in the PML. The accuracy of the method is demonstrated on connected monoatomic chains, for which an analytic solution is known. The parameters defining the PML are found to affect the performance and guidelines for selecting optimal parameters are given. The method is used to study the energy transmission coefficient for connected diatomic chains over all available wavevectors for both optical and longitudinal phonons; it is found that when there is discontinuity between sublattices, even connected chains of equivalent acoustic impedence have near-zero transmission coefficient for short wavelengths. The phonon scattering cross section of an embedded nanocylinder is calculated for a wide range of frequencies to demonstrate the extension of the method to high dimensions. The calculations match continuum theory for long wavelength phonons and large cylinder radii, but otherwise show complex physics including Mie oscillations which terminate when incident phonon frequencies exceeds the maximum available frequency in the embedded nanocylinder, and scattering efficiencies larger than two near the Brillouin zone edge.

cond-mat.mes-hall↗