arXiv subjects

Jamal Abou Haibeh

Publications and source records attributed to Jamal Abou Haibeh.

4 recordsLinked to original sources

arXiv2026

Anisotropic pressure response of thermal conductivity in crystalline polytetrafluoroethylene

Compression can change thermal conductivity differently along polymer chains and across them by altering phonon velocities and lifetimes. We examine this directional response in orthorhombic Form III polytetrafluoroethylene (PTFE) at calculated pressures of 1.503, 3.452, and 5.056 GPa, using first-principles harmonic and cubic force constants within the single-mode relaxation-time approximation (SMRTA). At 100, 200, and 300 K, the chain-axis conductivity decreases at every pressure step. Between 3.452 and 5.056 GPa, both transverse components recover: at 300 K they increase by 23.7% and 13.7%, while the longitudinal conductivity decreases by 14.5%. An exact factorization separates the conductivity into a heat-capacity--velocity factor and a weighted effective lifetime. The transverse recovery follows from larger heat-capacity-weighted squared velocities, consistent with stiffer interchain restoring forces. Along the chains, shorter effective lifetimes dominate, and both conductivity-weighted Normal and Umklapp rates increase at 300 K. Comparing the same cubic force-constant blocks at all three pressures, we find that interchain coupling is already substantial at 1.503 GPa and increases under compression.

cond-mat.mtrl-sci↗

arXiv2026

Impact of Exchange-Correlation Functionals on Predictions of Phonon Hydrodynamics: A Study of Fluorides, Chlorides, and Hydrides

We employ density functional theory calculations to examine the effect of various exchangecorrelation (XC) functionals, including the Perdew Burke Ernzerhof generalized gradient approximation (PBE), the modified Perdew Burke Ernzerhof generalized gradient approximation (PBEsol), and the local density approximation (LDA), on the electrical, mechanical, and thermal properties of sodium fluoride (NaF), lithium fluoride (LiF), potassium fluoride (KF), sodium chloride (NaCl), potassium chloride (KCl), lithium hydride (LiH), sodium hydride (NaH), and potassium hydride (KH). The lattice thermal conductivity is computed based on an iterative solution of the Boltzmann transport equation (BTE). Based on Guyer's criterion and direct solutions to the linearized BTE, we determine the ballistic, phonon hydrodynamics, and diffusive regimes as a function of length scale and temperature. In addition to confirming previous predictions of phonon hydrodynamics in NaF and LiF, we report novel predictions of phonon hydrodynamics in NaH, LiH, KH, KF, NaCl, and KCl. The impact of isotopes on the calculated lattice thermal conductivity and phonon hydrodynamics windows is also reported. The impact of Meta-GGA and hybrid functionals is also discussed. We find that the selection of a functional impacts the prediction of thermal conductivity and the window for observation of phonon hydrodynamics.

cond-mat.mes-hall↗

arXiv2025

OBELiX: A Curated Dataset of Crystal Structures and Experimentally Measured Ionic Conductivities for Lithium Solid-State Electrolytes

Solid-state electrolyte batteries are expected to replace liquid electrolyte lithium-ion batteries in the near future thanks to their higher theoretical energy density and improved safety. However, their adoption is currently hindered by their lower effective ionic conductivity, a quantity that governs charge and discharge rates. Identifying highly ion-conductive materials using conventional theoretical calculations and experimental validation is both time-consuming and resource-intensive. While machine learning holds the promise to expedite this process, relevant ionic conductivity and structural data is scarce. Here, we present OBELiX, a database of $\sim$600 synthesized solid electrolyte materials and their experimentally measured room temperature ionic conductivities gathered from literature and curated by domain experts. Each material is described by their measured composition, space group and lattice parameters. A full-crystal description in the form of a crystallographic information file (CIF) is provided for $\sim$320 structures for which atomic positions were available. We discuss various statistics and features of the dataset and provide training and testing splits carefully designed to avoid data leakage. Finally, we benchmark seven existing ML models on the task of predicting ionic conductivity and discuss their performance. The goal of this work is to facilitate the use of machine learning for solid-state electrolyte materials discovery.

cond-mat.mtrl-sci↗

arXiv2023

On the question of second sound in germanium: A theoretical viewpoint

We revisit the recent work from Beardo et al. \cite{beardo2021observation} wherein the observation of second sound in germanium is claimed. We review the requirements imposed on the collision operator (or equivalently, the full scattering matrix) of the linearized phonon Boltzmann transport equation (LBTE) for the observation of driftless second sound as established by Hardy. By performing an eigendecomposition of the full scattering matrix, we show that the requirement that the smallest nonzero eigenvalue must be associated with an odd eigenvector is not satisfied. Furthermore, direct solutions to the LBTE for a frequency modulated heat source do not reveal the presence of second sound. Finally, numerical solutions to the BTE under the relaxation time approximation (RTA) in the 1D frequency-domain thermoreflectance (1D-FDTR) experimental geometry demonstrate that phase lag alone is not a suitable experimental observable for inferring second sound. We conclude by discussing the need for a second sound `smoking gun'.

cond-mat.mes-hall↗