Giant and tuneable thermal diffusivity observed in the Dirac-fluid regime in graphene devices

by Virginia Greco

The study of heat transport in graphene at room temperature, in both the diffusive and the hydrodynamic regime, is described in a paper recently published in "Nature Nanotechnology". The very high and controllable thermal diffusivity revealed using ultrafast spatiotemporal thermoelectric microscopy can find application in heat management of nanoscale electronic devices.

Charge transport in conducting solids has been studied extensively in the most common diffusive regime, where charge carriers –electrons and holes– move randomly, before a collision occurs that changes their momentum. If observed at ultrasmall length scales or ultrafast timescales, which means before the momentum loss due to collisions takes place, charges move in a straight line and this is named the ballistic regime. Under special conditions, though, a hydrodynamic regime emerges, characterised by viscous charge flow, with electrons obeying laws similar to those of classical fluid transport. This so-called Fermi-liquid regime has been observed, in recent years, in graphene and other 2D materials.

A second hydrodynamic regime appears in stricter conditions, under which the viscous fluid behaviour does not follow classical fluid laws, instead an ultra-relativistic description needs to be used. Together with charge, electrons and holes carry electronic heat. In this Dirac-fluid regime, where the system becomes a quantum-critical fluid, electronic heat can flow more efficiently than charge, which is usually not the case in more conventional transport regimes. Attempts to observe this phenomenon at room temperature had not been successful so far.

A study just published in Nature Nanotechnology describes the observation of the electronic heat flow in graphene, both in the diffusive and in these hydrodynamic regimes at room temperature, including a controllable transition between the Fermi-liquid and the Dirac-fluid regime. This work was coordinated by Dr Klaas-Jan Tielrooij, leader of the ICN2 Ultrafast Dynamics in Nanoscale Systems Group, and the first author of the paper is Dr. Alexander Block from this group. Three more research groups that belong to the Barcelona Institute of Science and Technology (BIST) made important contributions to the project: the Theoretical and Computational Nanoscience Group at ICN2 via ICREA Prof. Stephan Roche and Aron Cummings, as well as the groups of ICREA Profs. Niek van Hulst and Frank Koppens from the Institute of Photonic Sciences of Barcelona (ICFO). The work also involved researchers from the University of Manchester (UK), and the National Institute for Materials Science of Tsukuba (Japan).

This investigation of the heat transport in graphene —specifically, in a hexagonal boron nitride- encapsulated graphene device— was made possible by the introduction of ultrafast spatiotemporal thermoelectric microscopy, a technique in which ultrafast laser pulses are used to generate localised electronic heat spots and their evolution in the device is tracked. Measurements in the stricter hydrodynamic regime showed a highly efficient initial heat spreading, which corresponds to a giant thermal diffusivity of graphene electrons (up to 70,000 cm²/s), in agreement with theoretical predictions for the Dirac fluid.

The authors of the study were also able to observe transitions from the Fermi-liquid to the quantum-critical Dirac-fluid regime and back, at room temperature, and to control this phenomenon by controlling the incident laser power and the voltage applied to the device. They found values of the electron thermal conductivity that are about three orders of magnitude higher than the one obtained in the Dirac-fluid regime at cryogenic temperatures and larger than the record thermal conductivity of phonons in graphene (which, in turn, is higher than that of diamond, for example).

Even though the ultrafast heat spreading observed in this system is short-lived (its lifetime is of a few hundreds of femtoseconds), it is potentially very useful, as it can extract heat from small hot spots extremely efficiently. Therefore, this property of graphene and other materials could find application in thermal management of nanoscale devices, such as transistors and chips, in mobile phones, computers, etc. In addition, the optoelectronic technique employed to carry out the research presented in the paper proved to be a very valuable tool to be used for analysing and understanding the thermal behaviour of a broad range of quantum materials.

Image: Artistic illustration of conventional heat spreading (left) vs. ultra-efficient heat spreading in the Dirac fluid regime (right).

Reference article:

Alexander Block, Alessandro Principi, Niels C. H. Hesp, Aron W. Cummings, Matz Liebel, Kenji Watanabe, Takashi Taniguchi, Stephan Roche, Frank H. L. Koppens, Niek F. van Hulst, Klaas-Jan Tielrooij, Observation of giant and tunable thermal diffusivity of a Dirac fluid at room temperature. Nature Nanotechnology, August 2021. DOI: 10.1038/s41565-021-00957-6