Graphene: Has the strongest pseudomagnetic field been generated?

Graphene has once again caused surprises: by merely stretching and compressing nanobubbles of this carbon form, researchers have generated extremely strong pseudo-magnetic fields.

Magnetic Fields Also Change the Behavior of Electrons

In these, electrons behaved as if under the influence of 300 Tesla – more than the strongest magnetic fields ever generated in the laboratory. This method, which has now been published in “Science”, opens up completely new possibilities for specifically influencing the behavior of electrons.

Magnetic fields not only influence metals, they also change the behavior of electrons. Until now, however, it has not been possible to generate stronger magnetic fields than around 85 tesla in the laboratory – and only for fractions of a second. By comparison, magnetic resonance tomographs use a magnetic field of around ten tesla, while the Earth’s magnetic field measures just 31 microtesla at the level of the Earth’s surface.

But now physicists at the University of California at Berkeley and the Lawrence Berkeley National Laboratory (LBNL) have succeeded in making electrons behave in the laboratory in a way that normally only occurs in extremely strong magnetic fields.

Stretching creates Nanobubbles

The effect was discovered by accident during an experiment in which a graduate student “grew” graphene on the surface of a platinum crystal. Graphene is a form of carbon in which the atoms are arranged in a hexagonal formation in a monatomic layer, similar to a chain-link fence.

When grown on platinum, the atoms of graphene do not lie flat on the crystal structure of the metal at all points, thus creating stresses – similar to a rubber skin stretched tightly over an uneven object.

Circling Electrons with Quantized Energies

These tensile stresses create tiny, triangular graphene bubbles four to ten nanometers in size, in which, it turns out, electrons behave in extremely unusual ways. Their energies do not occupy broad regions, as is normally the case with graphene, but are divided into separate, quantized energy blocks a behavior that is otherwise only common in extremely strong magnetic fields.

In magnetic fields, electrons normally move in a spiral around magnetic field lines. In the stretched nanobubbles of graphene, they also showed characteristic circular motions, as if a magnetic field of about 300 tesla were acting perpendicular to the atomic layer of the graphene. But there was no magnetic field – apparently it is a pseudomagnetic field, an effect that, unlike real magnetic fields, affects only the motion of the electrons, not other properties of the particles, such as spin.

New Possibilities for Electron Control

“This gives us new ways to control the motion of electrons in graphene by stretching them, and thus the electronic properties of graphene,” explains Michael Crommie, professor of physics at the University of California at Berkeley and a researcher at LBNL. “By influencing where the electrons gather and at what energy, we can make them move more easily or less easily through the graphene, in principle controlling its conductivity, optical and microwave properties. Controlling electron movement is an essential part of any electronic device.”

“Observing these massive pseudo-magnetic fields opens a door to room-temperature ‘spraintronics’ – the idea of using mechanical deformations in graphene to tailor its behavior for various electronic applications,” Crommie continued. In addition to the practical applications of this discovery, the researchers now want to test how this unusual property of graphene can be used to observe the behavior of electrons under field strengths that have never before been generated under laboratory conditions. “When you amplify a magnetic field, you start to see very interesting behavior because the electrons then spin in tiny circles,” Crommie explains. “This effect gives us a whole new way to trigger this behavior, even in the absence of a real magnetic field.”

Effect already Predicted Theoretically

Interestingly, researchers from Spain, the Netherlands and the United Kingdom had already theoretically predicted this very effect – which they christened the “pseudo-quantum Hall effect.” “Theorists often come up with ideas and explore them theoretically before experiments are done, and sometimes they come up with predictions that seem a little crazy at first,” Crommie said. “The exciting thing is that we now have data showing that these ideas are not so crazy.”