arXiv subjects

Nikhil Malviya

Publications and source records attributed to Nikhil Malviya.

4 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

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↗

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↗