UC Davis researchers made a surprising discovery while exploring the unusual electronic properties of a multi-layered nanoscopic structure. Depending on which way its electrons move, the material either behaves like a typical semiconductor or exhibits peculiar features unique to a single-layer carbon material that is now touted for its promising potential in electronic applications.
At a subatomic level, the researchers found that electrons in a vanadium dioxide network act like particles with mass while traveling in one direction, and particles without mass while moving in the perpendicular direction. The vanadium is layered between insulating sheets of titanium dioxide so that movement of its electrons is restricted to two dimensions.
“It’s almost as if the laws of physics governing their dynamics are different along the forward direction and the sideways direction,” said Warren Pickett, professor and chair of the physics department at UC Davis.
Researchers once thought that mass-less particle behavior was limited to situations where particles travel near the speed of light such as in accelerators and outer space. This view changed five years ago when physicists at the University of Manchester isolated graphene, a one-atom-thick sheet of carbon arrayed in a honeycomb shaped lattice. The equivalent behavior of electrons in graphene was dubbed “Dirac-like” in honor of the physicist who coined an equation for mass-less particle behavior.
Unlike graphene, the three-tiered vanadium oxide lattice does not purely exhibit Dirac-like properties, so Pickett and his colleagues termed the phrase “semi-Dirac” to describe the distinctive dual behavior of its electrons moving in two dimensions.
The new nanostructure currently exists as a computational model, but Pickett is certain that its electrical transport properties – particularly conduction in a magnetic field – will be unlike anything seen before in real world materials.
In the past few years, researchers have discovered interesting yet poorly understood “conducting interfaces” located between the two insulating materials in similar oxide nanostructures, Pickett said.
“We think semi-Dirac and Dirac behavior may be much more widespread than initially thought, especially in nanoscale layered materials, in which case it should find its way into applications in electronic devices,” said Rajiv Singh, UC Davis physics professor, who was not involved in the study. “But we do not know yet how or where exactly it will happen.“
According to Pickett, new methods that harness the tiny scale and unusual properties of such materials have been proposed to surmount physical limitations in the development of smaller, more complex microchips.
Instead of growing nanostructures into active devices such as transistors in a computer chip, the nanomaterial itself could serve as the base on which circuits are designed and etched into using a very fine tip to “provide conducting strips in an otherwise insulating material,” Pickett said.
The multi-layer vanadium oxide lattice is more rigid than one-layer graphene, which makes it particularly well-suited for this purpose, he added.
Pickett and Rajiv recently devised a mathematical model to further examine and describe the material’s macroscopic properties such as how temperature and magnetic fields affect its ability to conduct an electric current.
Pablo Jarillo-Herrero, a physics professor at Massachusetts Institute of Technology, said he agrees that more theoretical and experimental work is needed to determine the material’s basic characteristics and to test predictions about electron dynamics before proposing real world applications.
“The theoretical results seem quite interesting … but the implications for electronic devices are not clear since transport in these systems occur often in a diffusive manner,” said Jarillo-Herrero, who was not involved in the study.
Details about the material and its properties were published in the Apr. 22 issue of Physical Review Letters.
ELAINE HSIA can be reached at firstname.lastname@example.org.