The high integration density of modern energy and information devices often results in high power density and intense heat flux. Depending on the operating and optimal temperature range of the device, heat must be either effectively dissipated or retained. Precise regulation of heat flow is essential for the advancement of next-generation energy and information technologies. Dynamic heat flow control and nonlinear thermal transport open new avenues for developing smart battery thermal management systems, solid-state refrigeration devices, and thermal logic elements analogous to electronic circuits. Due to their unique capability to actively modulate heat transfer and exhibit thermal rectification behavior, thermal switches and thermal diodes have shown great potential in managing heat and/or maintaining thermal stability beyond the limits of conventional passive thermal materials and devices. Here, we review recent progress in the design principles, fundamental mechanisms, and applications of thermal switches and thermal diodes for energy and information technologies, and evaluate their potential for practical deployment. Furthermore, we discuss the emerging demands in these sectors and provide future perspectives to inspire applied research toward solving real engineering challenges.

We investigate how metallic rattling modes in Sr₂HgSn simultaneously suppress particle-like (κ_p) and enhance wave-like (κ_c) thermal conductivity via a combined first-principles, force-constant modulation, and Wigner transport analysis. Weak Hg-Sn bonds generate flat phonon bands that relax momentum conservation, intensifying both three- and four-phonon scattering and shortening phonon lifetimes. This dual scattering-coherence mechanism reveals a frequency-selective κ_p - κ_c crossover, leading to a weak temperature-dependent κ_L. Our work establishes rattling as a tunable design strategy for controlling phonon transport in thermoelectrics.

Phonon hydrodynamics is a theoretical framework for predicting nondiffusive heat transport processes in solids at the nanoscale or under high-frequency excitations. This article presents the microscopic and thermodynamic foundations of the theory and reviews its applications. First, we discuss historical and modern derivations of hydrodynamic heat transport equations from the phonon Boltzmann transport equation (BTE), and highlight advanced methods to predict hydrodynamic effects from direct solutions of the BTE beyond the Relaxation Time Approximation. Then, we review the main experiments that uncovered nondiffusive heat transport effects and their interpretation from the hydrodynamic perspective. Overall, the developments summarized in this work establish phonon hydrodynamics as a vital tool for understanding and engineering thermal transport at the nanoscale in data-processing and energy-conversion devices.

The two-temperature model (TTM) has been widely employed in describing ultrafast relaxation dynamics, providing a simple yet powerful framework to study energy relaxation in photoexcited systems. Recently, the time-dependent Boltzmann equation (TDBE) has revealed the limitations of TTM. However, current implementations of the TDBE assume instantaneous electronic thermalization. In this work, we employ first-principles Boltzmann transport simulations to explicitly examine the impact of non-thermal electronic distributions on relaxation processes. By comparing gold, silver, and aluminum as representative cases, we show that while phonons can indeed remain far from equilibrium, the neglect of non-thermal electrons is far more consequential. For gold, the absence of strongly coupled scattering channels makes the influence of non-thermal electrons negligible, rendering the TTM valid. For silver, deviations from the TTM stem mainly from non-thermal electronic effects, and for aluminum, both non-thermal electrons and phonons lead to substantial discrepancies. These findings demonstrate that non-thermalized electrons play a decisive role in out-of-equilibrium ultrafast energy transfer between electrons and phonons, offering a new perspective on the limitations of the TTM.