arXiv subjects

Navaneetha K. Ravichandran

Publications and source records attributed to Navaneetha K. Ravichandran.

12 recordsLinked to original sources

arXiv2026

Examining the microscopic origin of a computationally inexpensive thermal-conductivity-based indicator for phonon hydrodynamics

Hydrodynamic heat flow, where out-of-equilibrium phonons collectively drift in response to a temperature gradient, has attracted renewed interest following its experimental observation in graphite from cryogenic to room temperatures. To rapidly screen for other materials exhibiting this unconventional non-Fourier regime, the computationally inexpensive thermal conductivity ratio $κ_{LPBE}/κ_{RTA}$ obtained from a complete solution of the linearized Peierls-Boltzmann equation (LPBE) for phonon transport and the relaxation time approximation (RTA) for phonon decay, has often been used as an indicator for phonon hydrodynamics. Yet, a clear connection between $κ_{LPBE}/κ_{RTA}$ and the signatures of drifting hydrodynamic phonon populations has not been established in the literature. Here we show that $κ_{LPBE}/κ_{RTA}$ directly correlates with the microscopic hydrodynamic signatures arising from the spectral properties of the phonon collision operator, which is often computationally expensive to compute, thus establishing the former as a reliable low-cost indicator for phonon hydrodynamics. On the other hand, other indicators in the literature that are derived only from the phonon scattering rates do not correlate with $κ_{LPBE}/κ_{RTA}$, and so, are inadequate to predict phonon hydrodynamics. Our study also reveals that $κ_{LPBE}/κ_{RTA}$, and therefore the strength of hydrodynamic signatures, decrease with increasing Brillouin zone (BZ) sampling density for several ultrahigh-$κ$ materials at low temperatures, thus underscoring the need for careful BZ sampling for robust predictions of phonon hydrodynamics. Our work justifies the use of $κ_{LPBE}/κ_{RTA}$ as a computationally inexpensive indicator of phonon hydrodynamics, thus enabling accelerated search for new materials that exhibit such unconventional heat flow regimes.

cond-mat.mtrl-sci↗

arXiv2026

Semiconductor nanofilms as thermal phonon polarizers: competing effects of scattering selection rules and boundary mode conversion

Phonon scattering selection rules are known to control heat flow through bulk solids. Here we show that these selection rules also modulate heat flow through nanoscale semiconductor films, although through a previously-unexplored mechanism. Using first-principles calculations, we expose a competition between these selection rules and phonon mode conversion at boundaries of nanoscale films, that drives mode-polarized heat currents at cryogenic temperatures ($\le$ 100 K). This polarizing effect is stronger in materials like indium phosphide, where selection rules based on large velocity differences between phonon branches amplifies the longitudinal acoustic (LA) phonon contribution to thermal conductivity by restricting their intrinsic scattering events, while boundary mode conversion in nanoscale films suppresses it by depopulating the LA phonons. The resulting transverse-polarized non-equilibrium phonons will enable symmetry-selective engineering of phonon coupling to electrons, strains and defects in nanoscale films, that is difficult to achieve in bulk solids.

cond-mat.mtrl-sci↗

arXiv2026

Bridging the continuum and the kinetic-Boltzmann theories of heat flow through generalized Knudsen numbers

Heat conduction in semiconductor crystals is fundamentally governed by the linearized Peierls-Boltzmann equation (LPBE) for phonon transport, that arises out of a kinetic theory for phonon quasiparticles. Yet, continuum theories such as the Fourier's heat diffusion, weakly quasiballistic and hydrodynamic heat equations are often used to explain the experimental observations of heat flow in these materials. Here, we show that a systematic reduction of the LPBE into such equivalent continuum descriptions are possible only for the limiting values of a set of generalized Knudsen numbers. We further show that all of these continuum heat flow regimes, along with the ballistic heat flow, can be described by a single continuum equation for the temperature field that originates from the eigenmode analysis of the LPBE, thus offering a unified picture of all possible heat flow regimes in semiconducting crystals. Using quantitative examples on twenty three technologically important semiconductors, we show that several previously-unidentified features of the non-Fourier heat flow regimes emerge from this generalized Knudsen number framework such as (1) the mutual exclusivity of the weakly quasiballistic and the hydrodynamic heat flow regimes, (2) length-dependent velocity of the hydrodynamic second sound temperature wave and a characteristic heating length for the strongest hydrodynamic second sound, (3) characteristic frequency-domain temperature response distinguishing the hydrodynamic second sound from the ballistic heat flow regime and, (4) a new non-oscillatory signature of transient hydrodynamic heat flow. Our work formally bridges the continuum and the particulate descriptions of heat flow, and provides insights into the important signatures of temperature dynamics in each of these heat flow regimes, that will aid in their unambiguous experimental observations in the future.

cond-mat.mtrl-sci↗

arXiv2026

Elasticity reshapes heat flow in graphene

Classical thermal transport theories that preserve rotational symmetry, predict strong anharmonic scattering of out-of-plane lattice vibrational modes called flexural phonons in flat suspended graphene sheets. Such strong scattering processes cause a breakdown of the phonon quasiparticle picture, which remains valid only when several cycles of lattice vibrations occur before the mode decays. Here we show that the renormalization of elastic bending rigidity ($D$), caused by the coupling between the in-plane and the out-of-plane thermal lattice fluctuations, restores phonon quasiparticles in suspended graphene. Importantly, this $D$-renormalization weakens the momentum-dissipating Umklapp phonon scattering processes, resulting in improved thermal conductivity and amplified phonon hydrodynamics in suspended graphene. Our results unveil a previously-unrecognized connection between the macroscopic elasticity and the microscopic flexural phonon scattering in two-dimensional (2D) materials that does not occur in three-dimensional bulk crystals, thereby motivating a re-examination of the classical theories and opening up new avenues to engineer the thermal as well as the phonon-limited electronic transport and relaxation in two- and lower-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2025

Efficient calculation of phonon dynamics through a low-rank solution of the Boltzmann equation

Exotic nondiffusive heat transfer regimes such as the second sound, where heat propagates as a damped wave at speeds comparable to those of mechanical disturbances, often occur at cryogenic temperatures (T) and nanosecond timescales in semiconductors. First-principles prediction of such rapid, low-T phonon dynamics requires finely-resolved temporal tracking of large, dense, and coupled linear phonon dynamical systems arising from the governing linearized Peierls-Boltzmann equation (LPBE). Here, we uncover a rigorous low-rank representation of these linear dynamical systems, derived from the spectral properties of the phonon collision matrix, that accelerates the first-principles prediction of phonon dynamics by a factor of over a million without compromising on the computational accuracy. By employing this low-rank representation of the LPBE, we predict strong amplification of the wave-like second sound regime upon isotopic enrichment in diamond - a finding that would have otherwise been computationally intractable using the conventional brute-force approaches. Our framework enables a rapid and accurate discovery of the conditions under which wave-like heat flow can be realized in common semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

Dramatic Failure of the Callaway Description of Heat Flow in Boron Arsenide and Boron Antimonide Driven by Phonon Scattering Selection Rules

Callaway's simplified heat flow model is often used to confirm experimental realizations of unconventional, hydrodynamic and Poiseuille phonon transport in ultrahigh thermal conductivity ($κ$) materials, due to its simplicity and low computational cost. Here, we show that the Callaway model works exceptionally well for most ultrahigh-$κ$ materials like diamond and boron nitride, but fails dramatically for boron arsenide (BAs) and boron antimonide (BSb). This failure is driven by the inability of the Callaway model to effectively describe the severely restricted phonon scattering in BAs and BSb, where many scattering selection rules are activated simultaneously. Our work highlights the powerful predictive capability of the Callaway model, and gives insights into the nature of phonon scattering in ultrahigh-$κ$ materials and the suitability of the Callaway's description of heat flow through them.

cond-mat.mtrl-sci↗

arXiv2020

Exposing the hidden influence of selection rules on phonon-phonon scattering by pressure and temperature tuning

Using ab initio calculations, we show that the hidden influence of selection rules on three-phonon scattering can be exposed through anomalous signatures in the pressure ($P$) and temperature ($T$) dependence of the thermal conductivities, $κ$, of certain compounds. Boron phosphide reveals such underlying behavior through an exceptionally sharp initial rise in $κ$ with increasing $P$, which may be the steepest of any material, and also a peak and decrease in $κ$ at high $P$. These features are in stark contrast to the measured behavior for many solids, and occur at experimentally accessible conditions. Similar anomalous behavior is predicted for silicon carbide and other related materials.

cond-mat.mtrl-sci↗

arXiv2020

Phonon-Phonon Interactions in Strongly Bonded Solids: Selection Rules and Higher-Order Processes

We show that the commonly used lowest-order theory of phonon-phonon interactions frequently fails to accurately describe the anharmonic phonon decay rates and thermal conductivity ($κ$), even among strongly bonded crystals. Applying a first principles theory that includes both the lowest-order three-phonon and the higher-order four-phonon processes to seventeen zinc blende semiconductors, we find that the lowest-order theory drastically overestimates the measured $κ$ for many of these materials, while inclusion of four-phonon scattering gives significantly improved agreement with measurements. We have identified new selection rules on three-phonon processes that help explain many of these failures in terms of anomalously weak anharmonic phonon decay rates predicted by the lowest-order theory competing with four-phonon processes. We also show that zinc blende compounds containing boron (B), carbon (C) or nitrogen (N) atoms have exceptionally weak four-phonon scattering, much weaker than in compounds that do not contain B, C or N atoms. This new understanding helps explain the ultrahigh $κ$ in several technologically important materials like cubic boron arsenide, boron phosphide and silicon carbide. At the same time, it not only makes the possibility of achieving high $κ$ in materials without B, C or N atoms unlikely, but it also suggests that it may be necessary to include four-phonon processes in many future studies. Our work gives new insights into the nature of anharmonic processes in solids and demonstrates the broad importance of higher-order phonon-phonon interactions in assessing the thermal properties of materials.

cond-mat.mtrl-sci↗

arXiv2016

Experimental metrology to obtain thermal phonon transmission coefficients at solid interfaces

Interfaces play an essential role in phonon-mediated heat conduction in solids, impacting applications ranging from thermoelectric waste heat recovery to heat dissipation in electronics. From the microscopic perspective, interfacial phonon transport is described by transmission coefficients that link vibrational modes in the materials composing the interface. However, direct experimental determination of these coefficients is challenging because most experiments provide a mode-averaged interface conductance that obscures the microscopic detail. Here, we report a metrology to extract thermal phonon transmission coefficients at solid interfaces using ab-initio phonon transport modeling and a thermal characterization technique, time-domain thermoreflectance. In combination with transmission electron microscopy characterization of the interface, our approach allows us to link the atomic structure of an interface to the spectral content of the heat crossing it. Our work provides a useful perspective on the microscopic processes governing interfacial heat conduction.

cond-mat.mes-hall↗

arXiv2015

The Role of Thermalizing and Non-thermalizing Walls in Phonon Heat Conduction along Thin Films

Phonon boundary scattering is typically treated using the Fuchs-Sondheimer theory, which assumes that phonons are thermalized to the local temperature at the boundary. However, whether such a thermalization process actually occurs and its effect on thermal transport remains unclear. Here we examine thermal transport along thin films with both thermalizing and non-thermalizing walls by solving the spectral Boltzmann transport equation (BTE) for steady state and transient transport. We find that in steady state, the thermal transport is governed by the Fuchs-Sondheimer theory and is insensitive to whether the boundaries are thermalizing or not. In contrast, under transient conditions, the thermal decay rates are significantly different for thermalizing and non-thermalizing walls. We also show that, for transient transport, the thermalizing boundary condition is unphysical due to violation of heat flux conservation at the boundaries. Our results provide insights into the boundary scattering process of thermal phonons over a range of heating length scales that are useful for interpreting thermal measurements on nanostructures.

cond-mat.mtrl-sci↗

arXiv2015

Fresnel transmission coefficients for thermal phonons at solid interfaces

Interfaces play an essential role in phonon-mediated heat conduction in solids, impacting applications ranging from thermoelectric waste heat recovery to heat dissipation in electronics. From a microscopic perspective, interfacial phonon transport is described by transmission and reflection coefficients, analogous to the well-known Fresnel coefficients for light. However, these coefficients have never been directly measured, and thermal transport processes at interfaces remain poorly understood despite considerable effort. Here, we report the first measurements of the Fresnel transmission coefficients for thermal phonons at a metal-semiconductor interface using ab-initio phonon transport modeling and a thermal characterization technique, time-domain thermoreflectance. Our measurements show that interfaces act as thermal phonon filters that transmit primarily low frequency phonons, leading to these phonons being the dominant energy carriers across the interface despite the larger density of states of high frequency phonons. Our work realizes the long-standing goal of directly measuring thermal phonon transmission coefficients and demonstrates a general route to study microscopic processes governing interfacial heat conduction.

cond-mat.mes-hall↗

arXiv2014

Coherent and Incoherent Thermal Transport in Nanomeshes

Coherent thermal transport in nanopatterned structures is a topic of considerable interest, but whether it occurs in certain structures remains unclear due to poor understanding of which phonons conduct heat. Here, we perform the first fully three-dimensional, frequency-dependent simulations of thermal transport in nanomeshes by solving the Boltzmann transport equation with a novel, efficient Monte Carlo method. From the spectral information in our simulations, we show that thermal transport in nanostructures that can be created with available lithographic techniques is dominated by incoherent boundary scattering at room temperature. Our result provides important insights into the conditions required for coherent thermal transport to occur in artificial structures.

cond-mat.mtrl-sci↗