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Anisotropic semi-Dirac electrons in atomically-precise trilayers

In Arthur C. Clarke’s classic SF novel Against the Fall of Night, there is a description of the “moving ways”, the powered sidewalks on which people rode around the city, as being made of a material that would have baffled an engineer of our own times because it was solid in one direction and liquid in another.

We don’t have anything like that yet, but in this Eurekalert report is the discovery that “Depending on direction of travel, constrained electrons in thin layer of vanadium dioxide behave either like particles with mass or without mass”:

“Our model is demonstrating a new kind of band structure [dynamics of electrons], which no one has been aware of before,” said Warren Pickett, professor and chair of the physics department at UC Davis. “We think that some of the transport properties we’re seeing in the material — electrical conduction and conduction in a magnetic field — will be different than anything seen before.”

The discovery comes five years after a group at the University of Manchester in England first isolated graphene, a single-layer lattice of carbon atoms. That material, too, had unique electronic properties, and it sparked a huge surge of interest among physicists and materials scientists, who have published hundreds of papers on it. The team termed the behavior of electrons in graphene “Dirac-like” because of its similarity to the behavior of massless particles as described in an equation formulated by the illustrious theoretical physicist Paul Dirac.

Now Pickett and co-author Victor Pardo, a professor at the University of Santiago de Compostela in Spain who was a visiting professor at UC Davis when he did the work, have coined the term “semi-Dirac” to characterize the behavior of electrons in their multilayered vanadium dioxide lattice.

In this nanomaterial, Pickett explained, the sandwiching layers of the insulating titanium dioxide confine the vanadium, enforcing two-dimensional motion on its electrons. When the electrons move in one direction, they behave in the usual fashion, as particles with mass, but movement in the other direction produces behavior characteristic of particles without mass.

“It’s important that we use precisely three layers of vanadium dioxide,” Pickett said. “Using one or two layers only produces a magnetic insulator, while anything more than three layers produces a fairly normal magnetic metal that exhibits conducting behavior. The semi-Dirac system is neither conducting nor insulating.”

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