A research team from the Universities of Manchester, Nottingham and Loughborough has discovered a quantum phenomenon that helps to understand the basic limitations of graphene electronics.
The work published in Nature Communications describes how electrons in a single atomic thin graphene layer dissipate the vibrating carbon atoms that make up the hexagonal crystal lattice.
Applying a magnetic field perpendicular to the graphene plane forces the current-carrying electrons to move in closed, circular "cyclotron" orbits. In pure graphene, an electron can only escape this orbit by rebounding a "phonon" in a scattering event. These phonons are particulate energy and momentum bundles and are the "quanta" of the sound waves associated with the vibrating carbon atom. The phonons are generated in increasing numbers when the graphene crystal is heated from very low temperatures.
By directing a small electrical current through the graphene layer, the team was able to accurately measure how much energy and momentum were transferred between the two, an electron and a phonon during a scattering event.
Their experiment revealed that two types of phonons scatter the electrons: transversal acoustic (TA) phonons in which the carbon atoms oscillate perpendicular to the direction of phonon propagation and wave motion (somewhat analogously) surface waves on water) and longitudinal acoustic (LA) phonons in which the carbon atoms oscillate back and forth along the direction of the phonon and the wave motion; (This movement roughly corresponds to the movement of sound waves through the air.)
The measurements provide a very accurate measure of the velocity of both phonon types, a measurement that is otherwise difficult to perform for a single atomic layer. An important result of the experiments is the discovery that TA phonon scattering dominates LA phonon scattering.
The observed phenomenon, commonly referred to as magnetophonon vibration, was measured in many semiconductors years before the discovery of graphene. It is one of the oldest quantum transport phenomena known before more than 50 years before the quantum Hall effect. While graphene has a number of novel, exotic electronic properties, this rather fundamental phenomenon has remained hidden.
Laurence Eaves & Roshan Krishna Kumar, co-authors of the work, said: "We were pleasantly surprised to discover such prominent magnetophonon oscillations in Graphene, and we wondered why people had not seen them before, given the extensive literature to quantum transport in graphene. "
Their appearance requires two main components. First, the team at the National Graphene Institute had to produce high-quality graphene transistors with large areas. If the device dimensions are smaller than a few microns, the phenomena can not be observed.
Piranavan Kumaravadivel of the University of Manchester, lead author of the paper, said: "At the beginning of the quantum transport experiments, humans examined macroscopic, millimeter-sized crystals, and most of the quantum transport work on graphene is typically only a few microns in size It seems that the fabrication of larger graphene devices is important not only for applications but now also for fundamental investigations. "
] The second component is the temperature. Most graphene quantum transport experiments are performed at ultracold temperatures to slow the vibrating carbon atoms and "freeze" the phonons that normally break quantum coherence. Therefore, the graph is warmed up because the phonons must be active to trigger the effect.
Mark Greenaway of the University of Loughborough, who worked on the quantum theory of this effect, said, "This result is extremely exciting ̵
Fast Magnetism: Electron-phonon interactions studied at BESSY II
P. Kumaravadivel et al. Strong Magnetophonon Oscillations in Extra-Large Graphene, Nature Communications (2019). DOI: 10.1038 / s41467-019-11379-3
New Quantum Phenomenon Helps Understand Basic Limits of Graphene Electronics (2019, July 26)
retrieved on July 26, 2019
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