As metallic nanostructures shrink towards the size of the electronic mean free path, thermal conductivity decreases due to increased electronic scattering rates. Matthiessen’s rule is commonly applied to assess changes in electron scattering rates, although this rule has not been validated experimentally at typical operating temperatures for most of the electronic systems (e.g., near room temperature). In this study, we experimentally evaluate the validity of Matthiessen’s rule in determining the thermal conductivity of thin metal films by measuring the in-plane thermal conductivity and electronic scattering rates of copper (Cu) films with varying thicknesses (27 nm — 5 µm), microstructures, and grain boundary segregation. Comparing total electron scattering rates measured with infrared ellipsometry to infrared ultrafast pump-probe measurements, we find that the electron-phonon coupling factor is independent of film thickness, whereas the total electronic scattering rate increases with decreasing film thickness. Our findings provide experimental validation of Matthiessen’s rule for electron transport in thin metal films at room temperature and also introduce an approach to discern critical heat transfer processes in thin metal interconnects, which holds significance for the advancement of future CMOS technology.

Thermal transport across solid–solid interfaces is vital for advanced electronic and photonic applications, yet conventional conduction pathways often restrict performance. In polar crystals, hybridized vibrational modes called phonon polaritons offer a promising avenue to overcome the limitations of intrinsic phonon heat conduction. Here our work demonstrates that volume-confined hyperbolic phonon polariton (HPhP) modes can transfer energy across solid–solid interfaces at rates far exceeding phonon–phonon conduction. Using pump–probe thermoreflectance with a mid-infrared, tunable probe pulse with subpicosecond resolution, we remotely and selectively observe HPhP modes in hexagonal boron nitride (hBN) via broadband radiative heating from a gold source. Our measurements ascertain that hot electrons impinging at the interface radiate directly into the HPhPs of hBN in the near field, bypassing the phonon–phonon transport pathway. Such polaritonic coupling enables thermal transport speeds in solids orders of magnitude faster than possible through diffusive phonon processes. We thereby showcase a pronounced thermal transport enhancement across the gold–hBN interface via phonon–polariton coupling, advancing the limits of interfacial heat transfer.

Multicomponent oxides, such as many minerals and high entropy oxides, show promise as materials for protection in extreme environments. Similar to other phononically dominated materials, the spectrum of vibrational carriers and phonon scattering heavily influences thermal transport in multi-cation oxides. In this work, we experimentally and computationally investigate the nature of phonon scattering and thermal transport in a series of single and multi-cation rare earth sesquioxides and zirconates. A reduction in thermal conductivity was observed from the single to multi-cation oxides, which is directly correlated to measured optical mode lifetimes. Via spectroscopic ellipsometry, we observe red shifting of the optical modes from local bonding distortion. Density functional theory calculation was used to evaluate how bonding distortions influence the phononic scattering rate observed through modal broadening and reduced thermal conductivity. Compared to single-cation oxides, the multi-cation oxides, especially those with larger cation size variance, exhibited lower effective coordination number and greater bond distortion.

Graphene's near-field radiative heat transfer is determined from its electrical conductivity, which is commonly modeled using the local (wave-vector independent) Kubo and Drude formulas. In this paper, we analyze the nonlocality of the graphene electrical conductivity using the Lindhard model combined with the Mermin relaxation time approximation. We also study how the variation of the electrical conductivity with the wave vector affects near-field radiative conductance between two graphene sheets separated by a vacuum gap. It is shown that the variation of the electrical conductivity with the wave vector, $k_{\rho }$, is appreciable for $k_{\rho }\mathrm{s}$ greater than $100k_0$, where $k_0$ is the magnitude of the wave vector in the free space. The Kubo model is obtained by assuming $k_{\rho }\to 0$, and thus is not valid for $k_{\rho }>100k_0$. The Kubo electrical conductivity provides an accurate estimation of the spectral radiative conductance between two graphene sheets except for around the surface-plasmon-polariton frequency of graphene and at separation gaps smaller than 20 nm where there is a non-negligible contribution from electromagnetic modes with $k_{\rho }>100k_0$ to the radiative conductance. The Drude formula proves to be inaccurate for modeling the electrical conductivity and radiative conductance of graphene except for at temperatures much below the Fermi temperature and frequencies much smaller than $\frac{2{\mu }_c}{\hslash }$, where ${\mu }_c$ and $\hslash$ are the chemical potential and reduced Planck's constant, respectively. It is also shown that the electronic scattering processes should be considered in the Lindhard model properly, such that the local electron number is conserved. A simple substitution of $\omega$ by $\omega +i\gamma$ ($\omega , i$, and $\gamma$ being the angular frequency, imaginary unit, and scattering rate, respectively) in the collisionless Lindhard model does not satisfy the conservation of the local electron number and results in significant errors in computing the electrical conductivity and radiative conductance of graphene.

Silicon carbide (SiC) supports surface phonons in the infrared region of the electromagnetic spectrum where these modes can be thermally emitted. Additionally, the magnitude, spectrum, and direction of thermal radiation from SiC can be controlled by engineering this material at the sub-wavelength scale. For these reasons, SiC nanopillars are of high interest for thermal-radiation tuning. So far, theoretical and experimental studies of thermal emission from SiC nanopillars have been limited to long-pitch arrays with a microscale interpillar spacing. It is not clear how far-field thermal emission from SiC nanopillars is affected when the interparticle spacing reduces to the nanometer scale, where the near-field interaction between adjacent nanopillars arises and the array becomes zero order. In this Letter, we study physical mechanisms of far-field thermal radiation from zero-order arrays of silicon-carbide nanopillars with a nanoscale interpillar spacing. We show that the increased volume of thermal emitters and thermal radiation of the hybrid waveguide-surface-phonon-polariton mode from zero-order arrays increase the spectral emissivity of silicon carbide to values as large as 1 for a wide range of angles. The enhanced, dispersion-less thermal emission from a zero-order SiC array of nano-frustums with an optimized interspacing of 300 nm is experimentally demonstrated. Our study provides insight into thermal radiation from dense nanostructures and has significant implications for thermal management of electronic devices and energy harvesting applications.