Microscopic description of thermal-phonon coherence: From coherent transport to diffuse interface scattering in superlattices (original) (raw)
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Coherent Phonon Heat Conduction in Superlattices
Science, 2012
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2009
Phonon transport in superlattices is investigated using anharmonic and quasi-harmonic lattice dynamics calculations. Within the lattice dynamics framework, we develop a method for predicting the properties of both coherent and incoherent phonons. The method is implemented for test systems consisting of Stillinger-Weber silicon-germanium superlattices. In these systems the mode dependent frequencies, heat capacities, group velocities, transmission coefficients, and relaxation times of the phonons are computed and used to predict the thermal conductivity. We relate changes in the superlattice structure (e.g., period length and interface roughness) to the predicted phonon properties and, for each structure, identify the phonon modes that dominate thermal transport.
Thermal transport in nanostructures
AIP Advances, 2012
This review summarizes recent studies of thermal transport in nanoscaled semiconductors. Different from bulk materials, new physics and novel thermal properties arise in low dimensional nanostructures, such as the abnormal heat conduction, the size dependence of thermal conductivity, phonon boundary/edge scatterings. It is also demonstrated that phonons transport super-diffusively in low dimensional structures, in other words, Fourier's law is not applicable. Based on manipulating phonons, we also discuss envisioned applications of nanostructures in a broad area, ranging from thermoelectrics, heat dissipation to phononic devices.
Nature Communications, 2015
Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures. However, the role of coherent boundary scattering in such structures has become a matter of some debate. Here we report on the first experimental observation of coherent phonon boundary scattering at room temperature in 2D phononic crystals formed by the introduction of air holes in a silicon matrix with minimum feature sizes 4100 nm. To delaminate incoherent from coherent boundary scattering, phononic crystals with a fixed minimum feature size, differing only in unit cell geometry, were fabricated. A suspended island technique was used to measure the thermal conductivity. We introduce a hybrid thermal conductivity model that accounts for partially coherent and partially incoherent phonon boundary scattering. We observe excellent agreement between this model and experimental data, and the results suggest that significant room temperature coherent phonon boundary scattering occurs.
Decomposition of coherent and incoherent phonon conduction in superlattices and random multilayers
Physical Review B, 2014
Nonequilibrium molecular dynamics (NEMD) simulations on conceptual binary Lennard-Jones systems show that the thermal conductivity (κ) of a superlattice (SL) can be significantly reduced by randomizing the thicknesses of its layers, by which a SL becomes a random multilayer (RML). Such reduction in κ is a clear signature of coherent phonon that can be localized in RMLs. We build a two-phonon model that divides the overall heat conduction into coherent and incoherent phonon contributions. In SL both coherent and incoherent phonons contribute to heat conduction, while in RML coherent phonons are localized so only incoherent phonons contribute. This model can fit the length dependence of the thermal conductances predicted in our NEMD simulations very well. The ballistic-limit thermal conductance and the intrinsic mean free path (MFP) of both coherent and incoherent phonons, and the localization length of coherent phonons, are obtained by fitting our model to the NEMD simulation results. The significant increase in κ of SL with total length is due to the long MFP of coherent phonons, and the lower κ of RML than SL is caused by the localization of coherent phonons.
Semi-coherent Heat Conduction in Bulk and Single-Nanowire Twinning Superlattices
arXiv: Materials Science, 2015
Coherent twin boundaries, which form periodic lamellar twinning in a wide variety of semiconductor nanowires, are often viewed as near-perfect interfaces with reduced phonon and electron scattering behaviors. Such unique characteristics are of practical interest for high-performance thermoelectrics and optoelectronics; however, insufficient evidence for the existence of coherent heat conduction in nanotwinned materials poses significant limitations for potential applications. Here, using atomistic simulations and ab-initio calculations, we report direct computational observations showing intrinsic nanotwin effects on thermal conductivity of twinning superlattices in prototypical bulk and nanowire Si examples. Incoherent phonon transport is identified for twin periods geq\geqgeq 15.1 nm and coherent transport for twin periods leq\leqleq 3.8 nm. Remarkably, a regime of semi-coherent phonon transport is unveiled for twin periods spreading between these two limits, described by quasi-ballistic ...
Anomalous Thermal Transport in Nanostructures
Fluctuation Relations and Beyond, 2013
Thermal transport in nanoscale structures has attracted an increasing attention in last two decades. Here we give a brief review of the recent developments in experimental and theoretical studies of heat transport in nano materials such as nanotube and nanowire. In particular, we will demonstrate that the phonons in nanotube and nanowires transport super-diffusively, which leads to a length dependent thermal conductivity. In other words, heat conduction in low dimensional nanostructures does not obey the Fourier's law.
Crossover from incoherent to coherent phonon scattering in epitaxial oxide superlattices
Nature Materials, 2013
Elementary particles such as electrons 1,2 or photons 3,4 are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxialgrowth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.