A
University of Arkansas physicist and her colleagues have
examined dielectric susceptibilities of nanostructures - that is, the
response of their polarization to electric fields - and found novel,
seemingly contradictory properties that may change how such materials
can be used by scientists and engineers to build electronic devices.
Inna Ponomareva, Laurent Bellaiche and Raffaele Resta of the
Università de Trieste reported their findings in the journal
Physical Review Letters.
Ponomareva and her colleagues examined a property called the
dielectric susceptibility of a material, or its polarization response
to an electric field. High dielectric responses mean engineers and
scientists can build highly sensitive devices, so knowing how to
maximize this property in nanostructures will help scientists and
engineers make small, efficient electronic devices. The researchers
used physical and mathematical models to examine the effect of an
electric field on a nanostructure of lead zirconate, a ferroelectric
material -- a material that can exhibit a electrical polarization even
after the electric field has disappeared.
At the nanoscale, scientists have discovered that the
dielectric response has three different aspects, unlike in the bulk
level. These include the change of polarization with respect to the
external field, called external susceptibility, and the change in
polarization with respect to the internal field, called internal
susceptibility. Both of these are characteristic of the shape of the
material – that is, the susceptibility is dependent upon
whether the object is a nanorod, a nanodot, or a nanofilm. The third
aspect – called intrinsic susceptibility – is a
characteristic of the material.
Ponomareva and her colleagues determined that the internal
susceptibility can be negative – in other words, a positive
electric field created a negative polarization within the material.
This finding contradicted what was previously thought.
“It was believed that negative susceptibility meant
that the system was unstable,” Ponomareva said. Such negative
sign can open the door to the realization of novel technological
devices.
The researchers also wanted to see what would happen with the
material when the electric field was supplied by perfect electrodes,
that were 100 percent efficient, and also with less efficient
electrodes.
“In many practical applications, it is really hard
to find perfect electrodes,” Ponomareva said. Based on their
calculations, they found that the highest external dielectric response
occurred for electrodes that are around 90 percent efficient. This
indicates a point at which the material can be most easily manipulated
by an external electric field.
“It’s important to know what happens from
many angles,” she said. “These characteristics may
have useful applications, but right now we have more of a fundamental
interest in them.”
Ponomareva is a research assistant professor in the J. William
Fulbright College of Arts and Sciences.
Posted 8th November 2007
