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A New Source Of Magnetism Discovered By Accident
August 2, 2010 | 6 Comments
A new magnetic effect was discovered by accident when a UC Berkeley postdoctoral researcher and several students grew graphene on the surface of a platinum crystal. Graphene is a one atom-thick sheet of carbon atoms arranged in a hexagonal pattern, that looks like chicken wire. Examination showed when grown on platinum, the carbon atoms do not perfectly line up with the metal surface’s triangular crystal structure, which creates a strain pattern in the graphene as if it were being pulled from three different directions.
Michael Crommie, professor of physics at UC Berkeley and a faculty researcher at Lawrence Berkeley National Laboratory runs the lab where the discovery was made. Charles Kane and Eugene Mele of the University of Pennsylvania first predicted the appearance of a “pseudomagnetic” field in response to strain in graphene for carbon nanotubes in 1997. Nanotubes are a rolled up form of graphene.
Crommie explains the strain produces small, raised triangular graphene bubbles 4 to 10 nanometers across in which the electrons occupy discrete energy levels rather than the broad, continuous range of energies allowed by the band structure of unstrained graphene. This new electronic behavior was detected spectroscopically by scanning tunneling microscopy. These so-called Landau levels are reminiscent of the quantized energy levels of electrons in the simple Bohr model of the atom.
Graphene Strain Induced Nanobubbles. Click image for more info.
Crommie said, “This gives us a new handle on how to control how electrons move in graphene, and thus to control graphene’s electronic properties, through strain. By controlling where the electrons bunch up and at what energy, you could cause them to move more easily or less easily through graphene, in effect, controlling their conductivity, optical or microwave properties. Control of electron movement is the most essential part of any electronic device.”
Inventive engineers take note – this opens a new field.
What happens is the electrons within each nanobubble segregate into quantized energy levels instead of occupying energy bands, as in unstrained graphene. The energy levels are identical to those that an electron would occupy if it were moving in circles in a very strong magnetic field, as high as 300 tesla, which is stronger than any laboratory can produce except in brief explosions, said Crommie. For comparison, a magnetic resonance imager uses magnets running at less than 10 tesla, while the Earth’s magnetic field at ground level is only 31 microtesla. The scale, while atom sized on one dimension – is incredible.
Meanwhile over the last year Francisco Guinea of the Instituto de Ciencia de Materiales de Madrid in Spain, Mikhael Katsnelson of Radboud University of Nijmegen, the Netherlands, and A. K. Geim of the University of Manchester, England predicted what they termed a pseudo quantum Hall effect in strained graphene. This is the very quantization that Crommie’s research group has experimentally observed. Boston University physicist Antonio Castro Neto, who was visiting Crommie’s laboratory at the time of the discovery, immediately recognized the implications of the data, and subsequent experiments confirmed that it reflected the pseudo quantum Hall effect predicted earlier.
This is pretty cheerful stuff. Crommie observes, “Theorists often latch onto an idea and explore it theoretically even before the experiments are done, and sometimes they come up with predictions that seem a little crazy at first. What is so exciting now is that we have data that shows these ideas are not so crazy. The observation of these giant pseudomagnetic fields opens the door to room-temperature ‘straintronics,’ the idea of using mechanical deformations in graphene to engineer its behavior for different electronic device applications.”
The catch in all the excitement is the nanobubble experiments performed in Crommie’s laboratory were performed at very low temperature. Crommie notes that the pseudomagnetic fields inside the nanobubbles are so high that the energy levels are separated by hundreds of millivolts, much higher than room temperature. Thus, thermal noise would not interfere with this effect in graphene even at room temperature.
Normally, electrons moving in a magnetic field circle around the field lines. Within the strained nanobubbles, the electrons move in circles in the plane of the graphene sheet, as if a strong magnetic field has been applied perpendicular to the sheet even when there is no actual magnetic field. Apparently, Crommie said, the pseudomagnetic field only affects moving electrons and not other properties of the electron, such as spin, that are affected by real magnetic fields.
There’s a lot of pseudo so far in the press release and the paper’s abstract at Science. But the research effort is measuring the Tesla force. That point focuses attention is a major way. Getting to 10 Tesla requires lots of power and a source without such a power input thirty times as strong is prey worthy of the best minds in science. Should the effect make it beyond microelectronics in scale to say motors, the impact would be huge.
The long term potential isn’t known in precise terms. There is a great deal of further exploration and experimentation to come. Yet the early theory ideas have borne fruit – by accident.
The serendipitous post doc remains un named, but add to paper’s author list Castro Neto and Francisco Guinea, Sarah Burke, now a professor at the University of British Columbia; Niv Levy, now a postdoctoral researcher at the National Institute of Technology and Standards; and graduate student Kacey L. Meaker, undergraduate Melissa Panlasigui and physics professor Alex Zettl of UC Berkeley. It’s a paper that might be worth the reading fee for the inventive engineer.
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