Researchers who study and manipulate the behavior of materials at the atomic level have discovered a way to make a thin material that enhances the flow of microwave energy. The advance, which could improve telecommunications, sheds new light on structural traits, generally viewed as static and a hindrance, that, when made to be dynamic, is actually key to the material’s special ability.
The discovery, reported in the journal Nature, shows how domain walls — the naturally occurring boundaries, separating atoms with different directions of relative displacement, that create dipoles within a material — could actually be an entryway for accessing a much wider range of electromagnetic frequencies. And this access could one day expand the range of frequencies used as communications channels.
In the paper, researchers from Drexel University, Bar-Ilan University in Israel, the University of California at Berkeley, the University of California at Santa Barbara, the Carnegie Institution for Science, and the University of Pennsylvania showed how a ferroelectric material can be designed in such a way that domain walls can be used to transmit microwaves with a higher degree of frequency control than the mobile devices we currently use.
“As consumer demand for mobile communications increases the available wireless spectrum is increasingly congested and new technologies are needed to create adaptive, frequency-agile antennas,” said Robert York, Ph.D., a professor at UC Santa Barbara and a co-author of the paper. “Tunable dielectric materials could be a potential solution.”
Using the domain walls within a material to enhance its transmission quality is a particularly unexpected approach because the presence of these boundaries tends to strongly diminish a material’s ability to pass a microwave electromagnetic field. Until now, the best film materials for transmitting electromagnetic fields in radio-frequency devices were generally considered to be single-crystal materials that do not have any permanent dipole moments — let alone domain walls.
But the research team turned this perception of domain walls on its head by creating a ferroelectric material with a high density of domain walls, that can outperform single crystals when it comes to tunability and transmission quality.
The group found that the domain walls of a thin film of barium strontium titanate, a frequently studied ferroelectric material, function like vibrating guitar strings that resonate together. Rather than absorbing or scattering microwaves, the presence of a dense, but ordered, thicket of oscillating domain walls actually improves the quality of the transmission.
“Even the best-quality bulk single crystals, without permanent, re-orientable dipoles, have higher losses at higher frequencies, due to interference caused by the vibrations of atoms in the lattice,” said Jonathan Spanier, Ph.D., a materials science and engineering professor in Drexel’s College of Engineering who led the research. “Film materials with permanent dipoles form domain walls, and the loss is much worse. But films that support reversible domain wall motion and their oscillating behavior surprisingly break that trend and resonate over a wide range of frequencies.”
According to the researchers, “the proximity of and accessibility among thermodynamically predicted strain-induced, ferroelectric domain wall variants to achieve gigahertz microwave tunability and dielectric loss that surpass those for the current best film devices by 1-2 orders of magnitude, attaining values comparable to bulk single crystals, but in an intrinsically tunable material,” they write in the paper.
thumbnail courtesy of drexel.edu