Graphene's Quantum Leap: Electrons Flow as Frictionless Dirac Fluid, Defying Textbook Physics

In a groundbreaking discovery, researchers from the Indian Institute of Science in collaboration with the National Institute of Material Sciences in Japan have observed electrons in graphene behaving like a frictionless, fluid-like state, defying conventional physics principles. In their Nature Physics study “Universality in quantum critical flow of charge and heat in ultraclean graphene”, Majumdar, et al. unveils a quantum phenomenon where electrons exhibit properties akin to a perfect fluid, governed by a universal quantum number, offering new insights into quantum mechanics and potential applications in advanced technologies.

Graphene, a single layer of carbon atoms arranged in honeycomb lattice, has been a focal point of scientific research since its discovery in 2004 due to its remarkable electrical and mechanical properties. The IISc team engineered ultra-clean graphene samples, free from atomic defects and impurities, to investigate electron behavior under controlled conditions. By simultaneously measuring electrical and thermal conductivity, they uncovered an inverse relationship between these properties: as electrical conductivity increased, thermal conductivity decreased and vice versa. This observation starkly contradicts the Wiedemann-Franz law, a cornerstone of metal physics that predicts a direct proportionality between electrical and thermal conductivity. In their experiments, the team observed a deviation from this law by a factor exceeding 200 at low temperatures, indicating a significant decoupling of charge and heat transport mechanisms.

The decoupling it tied to a unique state known as the Dirac point, where graphene’s electronic structure is neither metallic nor insulating. At this critical point, electrons no longer behave as individual particles but more collectively resembling a fluid with minimal viscosity, or a Dirac fluid. The researchers liken this state to the quark-gluon plasma, a highly energetic state of matter observed in particle accelerators like CERN. The viscosity of this Dirac fluid approaches the theoretical minimum, making it one of the closest approximations to a perfect fluid observation in a lab setting. This fluid-like behavior is governed by a universal constant, the quantum of conductance, which dictates both charge and heat transport independently of the material’s properties.

The implications of this discovery extend beyond condensed matter physics. Graphene’s Dirac fluid provides a tabletop platform for exploring high-energy physics concepts, such as blackhole thermodynamics and entanglement entropy, which are typically studied in extreme astrophysics or particle physics contexts. For example, Muller et al. state in their 2009 Physics Review Letter study “Relativistic hydrodynamics in two-dimensional Dirac systems” that the fluid-like electron flow in graphene mirrors theoretical models of relativistic hydrodynamics, offering a controlled environment to test predictions about quantum critical systems. In addition, the quantized conductivity observed in graphene aligns with findings in other 2D materials, where electron interactions lead to exotic quantum slates, as observed by Cao, et al. in their 2018 study “Corelated insulator behavior at half-filling in magic-angle graphene superlattices”.

The study builds on decades of research into electron transport in low-dimensional systems. Previous work on graphene has shown its potential to host exotic quantum states, but the observation of a Dirac fluid with such clarity is unprecedented. The ultra-clean graphene samples used in this study were critical to minimizing scattering from impurities, which typically obscures such quantum effects. This achievement highlights the importance of material purity in uncovering fundamental physics phenomena. Similar behaviors have been hinted at in other systems such as topological insulators, but graphene’s simplicity and accessibility make it an ideal candidate for further exploration, states Hasan, & Kane in their 2010 study “Colloquium: Topological insulators” in the Reviews of Modern Physics.

Looking forward, the discovery opens new avenues for both theoretical and applied research. The ability to replicate high-energy physics phenomena in a low-cost, laboratory-friendly material like graphene could democratize access to cutting-edge quantum research. Moreover, the technological potential of Dirac fluids in quantum sensors and electronics underscores the need for continued investment in graphene-based technologies. As Professor Arindam Ghosh notes, even after two decades of research, graphene continues to surprise, revealing new facets of quantum behavior that challenge our understanding of the subatomic world.