Taking advantage of graphene’s special properties, physicists have experimentally observed a stream of record-breaking electrons that move with a conductance exceeding the limit theorists established decades ago. The researchers reported their findings recently in Nature Physics.
For more than half a century, physicists who study the flow of electrons have wondered how high conductance can go in a material. Conductance is the opposite of resistance; it describes how easily current passes through. Physicists study conductance to investigate fundamental properties of electrons. From a more practical standpoint, devices with high conductance make appealing components for future electronic devices.
The standard assumption, based on pioneering work from the 1960s, predicts a natural maximum for free electrons. Electrons lose momentum as they interact with the impurities, walls, and vibrations of the material, all sources of resistance. But even if they travel ballistically—unimpeded and without scattering—a quantum limit should apply. But new experiments have revealed streams of electrons flagrantly exceeding that upper bound as they flowed through tiny channels in a graphene device—not unlike water or gas streaming through a pinhole.
The work, led by physicist Andre Geim at the University of Manchester and with theoretical support from Marco Polini of Istituto Italiano di Tecnologia, in Genoa, reports the first experimental evidence of higher-than-expected conductance, with the electrons flowing like a fluid more viscous than honey. They describe the behavior as “superballistic” because it exceeds the limit for ballistic travel.
Finding such high conductance was “one of the big surprises in these experimental findings,” says physicist Kin Chung Fong at Harvard University, who was not involved in the study.
Theorists have long predicted that such electron fluids can form when interactions between the charged particles dominate over the interactions of the electrons with the material itself. That’s because when electrons encounter impurities or barriers, they lose momentum in the collisions. When they hit other electrons, momentum is conserved. Coaxing that liquid into existence experimentally has been a challenge, says Geim. Ordinary metals have too many impediments.
“In order to create a collective behavior, and viscosity is a collective behavior, you need to allow time and space for the molecules to interact,” says Geim. At low temperatures, in which electrons interact with each other each other over longer distances, normal metals have too many imperfections to allow the electron-electron behavior to dominate. If the temperature is raised, the electron interaction scale decreases—but vibrations can occur in the crystal structure of the material. These can act effectively like obstacles, leading to momentum losses in the electrons.
Graphene, though, changes the game. Made of a single layer of carbon atoms, graphene is stiff and strong, and demonstrates a spectrum of surprising electronic properties. Geim, together with Konstantin Novoselov, first isolated graphene in 2004, and they were awarded the 2010 Nobel Prize in Physics for their pioneering work.
Geim says graphene makes a good playground for super-ballistic electrons because it’s free of impurities. But he says “the real breakthrough” came with the realization that electron-phonon interactions are very weak in graphene. Phonons are quasiparticles associated with thermal vibrations in a material, but because “graphene is the stiffest material we know,” Geim notes, vibrations pose less of a problem.
In February 2016, Science published a trio of papers—including one led by Fong, and one led by Geim—reporting hydrodynamic behavior of electrons. They described structures associated with the flow of ordinary gases and liquids, such as turbulence, whirlpools and vortices, but in the electron fluids.
In March of this year, in PNAS, theorists led by Leonid S. Levitov at MIT predicted that super-ballistic behaviors should arise in materials like graphene. The new paper, says Fong, not only verifies those predictions but also shows that physicists have to consider a new limit associated with the viscous flow.
“Graphene provides a basic model to understand these physics because we have a well-controlled environment to study strongly-interacting electron gas now,” says Fong. “These are things that we couldn’t study a year ago.”