Direct Measurement of Ballistic and Diffusive Electron Transport in Gold.

We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump-probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron-phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron-phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.

Electron-electron scattering limits thermal conductivity of metals under extremely high electron temperatures.

We report on the thermal transport properties of noble metals (gold, silver and copper) under conditions of extremely high electron temperatures (that are on the order of the Fermi energy). We perform parameter-free density functional theory calculations of the electron temperature-dependent electron-phonon coupling, electronic heat capacities, and thermal conductivities to elucidate the strong role played by the excitation of the low lyingd-bands on the transport properties of the noble metals. Our calculations show that, although the three metals have similar electronic band structures, the changes in their electron-phonon coupling at elevated electron temperatures are drastically different; while electron-phonon coupling decreases in gold, it increases in copper and, it remains relatively unperturbed for silver with increasing electron temperatures of up to ∼60 000 K (or 5 eV). We attribute this to the varying contributions from acoustic and longitudinal phonon modes to the electron-phonon coupling in the three metals. Although their electron-phonon coupling changes with electron temperature, the thermal conductivity trends with electron temperature are similar for all three metals. For instance, the thermal conductivities for all three metals reach their maximum values (on par with the room-temperature values of some of the most thermally conductive semiconductors) at electron temperatures of ∼6000 K, and thereafter monotonically decrease due to the enhanced effect of electron-electron scattering for electronic states that are further away from the Fermi energy. As such, only accounting for electron-phonon coupling and neglecting electron-electron scattering can lead to large over-predictions of the thermal conductivities at extremely high electron temperatures. Our results shed light on the microscopic understanding of the electronic scattering mechanisms and thermal transport in noble metals under conditions of extremely high electron temperatures and, as such, are significant for a plethora of applications such as in plasmonic devices that routinely leverage hot electron transport.

Exceptionally Enhanced Thermal Conductivity of Aluminum Driven by Extreme Pressures: A First-Principles Study.

Extreme pressure conditions reveal fundamental insights into the physical properties of elemental metals that are otherwise not evident under ambient conditions. Herein, we use the density functional perturbation theory to demonstrate that the change in thermal conductivity as a result of large hydrostatic pressures at room temperature for aluminum is the largest of any known material. More specifically, in comparison to ambient conditions, we find that the change in thermal conductivity for aluminum is greater than the relative changes in thermal conductivities of diamond and cubic boron nitride combined, which are two of the most thermally conductive bulk materials known to date. We attribute this to the relatively larger increase in mean free paths and lifetimes of electrons in aluminum as a result of weaker electron-phonon coupling at higher pressures. Our work reveals direct insights into the exceptional electronic transport properties of pressurized aluminum and advances a broad paradigm for understanding thermal transport in metals under extreme pressure.

Tailoring the thermal conductivity of two-dimensional metal halide perovskites.

Proper thermal management of solar cells based on metal halide perovskites (MHPs) is key to increasing their efficiency as well as their durability. Although two-dimensional (2D) MHPs possess enhanced thermal stability as compared to their three-dimensional (3D) counterparts, the lack of comprehensive knowledge of the heat transfer mechanisms dictating their ultralow thermal conductivities is a bottleneck for further improvements in their thermal performance. Here, we experimentally and computationally study the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) phases of MHPs (n = 1) to demonstrate that the length of the organic spacers has a negligible influence on their thermal transport properties; we experimentally measure thermal conductivities of 0.19 ± 0.03 W m-1 K-1 and 0.18 ± 0.03 W m-1 K-1 for the RP and DJ phases with 13.6 Å and 6.3 Å interlayer inorganic separations, respectively. In contrast, we show that thermal conductivity is mainly dependent on the separation between the adjacent organic cations. Decreasing the intermolecular distance (by up to 40%) leads to drastically enhanced overall heat conduction (with monotonically increasing thermal conductivity by more than threefold) which is mainly driven by the vibrational hardening of the organic spacers. Although these 2D layered materials constitute a high density of hybrid organic-inorganic interfaces, our results also show that a substantial portion of heat is conducted through coherent phonon transport and that the thermal conductivity of these materials is not solely limited by incoherent interfacial scattering.