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Posted: May 4, 2009
Nano-sandwich triggers novel electron behavior
(Nanowerk News) A material just six atoms thick in which electrons appear to be
guided by conflicting laws of physics depending on their direction of
travel has been discovered by a team of physicists at the University
of California, Davis. Working with computational models, the team has
found that the electrons in a thin layer of vanadium dioxide
sandwiched between insulating sheets of titanium dioxide exhibit one
set of properties when moving in forward-backward directions, and
another set when moving left to right.
With its unique properties, the material could open up a new world of
possibilities in the emerging field of spintronics technology, which
takes advantage of the magnetic as well as the electric properties of
electrons in the design of novel electronic devices.
"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
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."
A big advantage that the vanadium lattice has over the one-layer
thick graphene is greater rigidity, which will make it easier to etch
into experimental or functional shapes, Pickett said.
For the time being, the material exists only as a computational
model. Yet many of the basic, underlying processes and principles of
physics are first established theoretically, with or without
computational analysis, Pickett said.
Pickett and Pardo have teamed with UC Davis physics professor Rajiv
Singh and graduate student Swapnonil Banerjee to investigate the
material's properties. The team has constructed a classical
mathematical model called a "tight-binding" model that they expect
will promote a theoretical understanding of the material at the most
basic level. "We're pretty confident that this nanostructure can be
made, and made clean enough to demonstrate the properties the model
has demonstrated," Pickett said.
The group has already achieved a basic understanding of the low
energy behavior of semi-Dirac systems and has submitted a second
paper for publication describing the peculiar behavior.