Graphene’s Second Sound Enables Faster Heat Removal for Advanced Electronics

The efficient dissipation of heat remains a critical challenge in modern electronics, and understanding how heat travels at the nanoscale is paramount to overcoming this issue. Antonio Martinez Margolles and Patrick K. Schelling, both from the University of Central Florida, investigate this phenomenon by exploring the propagation of second sound, a wave-like form of heat transfer, within graphene materials. Their research establishes a simulation approach based on thermal-response functions, revealing that second-sound dissipation stems primarily from the inherent properties of the material’s atomic vibrations, rather than simple scattering as predicted by conventional theories. This discovery significantly advances our understanding of heat transport in graphene and offers insights into designing materials with enhanced thermal management capabilities, potentially leading to more efficient and reliable electronic devices.

Scientists are developing a theoretical framework to predict graphene’s thermal response to changing heat sources, aiming to understand and potentially observe second sound experimentally. They combine molecular dynamics simulations with a response function approach to model thermal behaviour, emphasizing the importance of including quantum effects for more accurate predictions. Second sound represents a high-frequency heat wave arising from the wave-like behaviour of phonons, which are quantized lattice vibrations carrying heat in solids.

The work aims to provide a theoretical basis for interpreting experimental results from time-dependent thermal measurements, such as those obtained using techniques like transient thermal reflectance or transient grating methods. Further investigation focuses on incorporating quantum statistics for accurate modelling, exploring the connection between the response function approach and the Boltzmann Transport Equation, and validating the theory through experiments. This research is significant because it provides a theoretical framework for understanding and predicting graphene’s thermal behaviour, with a particular focus on second sound. The work advances our understanding of heat transport in nanoscale materials and guides the development of new thermal management technologies. The connection to experimental techniques is crucial for verifying predictions and unlocking graphene’s full potential for thermal applications.

Molecular Dynamics Simulates Graphene Second Sound Propagation

Scientists developed a method employing molecular dynamics simulation to investigate thermal transport in graphene, focusing on second sound propagation at 300 Kelvin. This approach computes thermal response functions, allowing predictions without assuming diffusive or ballistic transport, and directly aligning with experimental techniques like transient thermal grating. The simulations model graphene using empirical potentials, enabling the study of second-sound propagation over length scales up to 68. 1 nanometres, a regime where ballistic transport is significant. The team’s method bypasses traditional approaches focused solely on thermal conductivity, instead directly addressing the physics of second sound, a phenomenon relevant to nanoscale heat dissipation.

Researchers calculated thermal response functions to identify second sound, building on previous work with one-dimensional chains and hexagonal boron nitride monolayers. The study meticulously compared simulation results with the hyperbolic heat equation, a description of second sound, to determine the lifetime and propagation velocity of temperature waves. Furthermore, calculations using the linearized Boltzmann Transport Equation and a single-mode relaxation time approximation revealed significantly longer second-sound lifetimes compared to the molecular dynamics simulations. This discrepancy highlights the importance of decoherence, arising from the details of the phonon band structure, as the primary mechanism for second-sound dissipation at 300 Kelvin, rather than anharmonic phonon scattering assumed in Boltzmann Transport Equation-based theories. The research demonstrates that the decay time for second sound depends on the wave vector of the perturbation, a finding that contrasts with the length-scale independence predicted by Boltzmann Transport Equation models.

Graphene Exhibits Ballistic Heat Transport Over Long Ranges

This work presents a detailed investigation of second sound, a ballistic heat transport phenomenon, in graphene using classical molecular dynamics simulations. Researchers accurately modelled graphene interactions using an optimized potential and computed thermal response functions at 300 Kelvin. Simulations demonstrate a strong second-sound signal over length scales up to 68. 1 nanometres, confirming the presence of ballistic heat transport in this material. The team computed the thermal conductivity using an expression to reveal the time-dependent behaviour of heat transfer within the graphene structure.

Analysis reveals that the primary mechanism governing second-sound dissipation is decoherence arising from the phonon band structure, rather than anharmonic phonon scattering as predicted by Boltzmann Transport Equation theories. Specifically, the loss of phase coherence between normal modes, due to differing propagation velocities, dictates the decay of the second-sound signal. This finding contrasts with traditional Boltzmann Transport Equation-based models, which assume a length-independent dissipation rate. The simulations demonstrate that the second-sound lifetime, or decay time, strongly depends on the wave vector characterizing the thermal excitation.

Furthermore, the study explores the response to time-dependent heat sources, providing insight into how these sources could be tuned to generate temperature oscillations. This work establishes a connection between simulation results and experimental techniques like transient thermal grating measurements, offering a framework for interpreting experimental observations of second sound in graphite crystals within the 100-200 Kelvin temperature range. The research delivers a new understanding of heat transport at the nanoscale, highlighting the importance of decoherence in determining the lifetime of second sound and challenging conventional theories based on the Boltzmann Transport Equation.

Second Sound and Nanoscale Thermal Transport

This research demonstrates the presence of second sound and significant deviations from Fourier’s law in thermal transport within single-layer graphene at length scales up to approximately 68. 1 nanometres and a temperature of 300 Kelvin. These findings align with previous observations in hexagonal boron nitride, further establishing the importance of ballistic thermal transport at the nanoscale. Specifically, the team found that the decay of second sound arises primarily from the details of the phonon band structure, leading to a loss of phase coherence, rather than solely from anharmonic phonon scattering.

This suggests that coherent ballistic transport is crucial for observing second sound, but the absence of second sound does not automatically imply diffusive thermal behaviour. The research also clarifies that spectral features sometimes referred to as “first sound” are, in fact, excitations consistent with the observed second sound phenomenon. The authors acknowledge that establishing definitive limits on the length scale and temperature at which second sound is observable remains a challenge for both theoretical and experimental investigation. Future work could focus on exploring these limits and further investigating the interplay between phonon dispersion and anharmonic scattering in determining thermal transport characteristics at the nanoscale.