Purdue physicist Erica Carlson stands in front of an illustration of the fractal clusters present in copper-oxygen-based superconducting material. Photo: Purdue University/Mark Simons

Many researchers studying superconductivity strive to create
a clean, pure, perfect sample, but a team of physicists found that some flaws
might hold the key to a material’s unique abilities.

Erica Carlson, a Purdue University associate professor of
physics, led a team that mapped seemingly random, four-atom-wide dark lines of electrons
seen on the surface of copper-oxygen-based superconducting crystals. The team
uncovered a pattern in these flawed lines, which are separate from the expected
structure of the material, and discovered that they exist throughout the
crystal. The findings suggest the lines could play a role in the material’s
superconductivity at much higher temperatures than others.

“This material is ceramic, like your dinner plates, and
it has no business conducting electricity, but under the right conditions it
conducts electricity perfectly with zero energy loss,” Carlson says.
“A better understanding of how and why this superconductor works could
help us design better ones. If we can create a superconductor that works at
high enough temperatures, it could transform how we use and generate
energy.”

For instance, room-temperature superconductive wires could
lead to power lines that do not leak electricity in transit, saving tremendous
amounts of energy and money. Superconductors also have special magnetic
properties that could allow for levitated, frictionless trains and stronger,
more durable permanent magnets like those used in wind turbines, she says.

The electrons confined to these mysterious lines behave
radically different than those that freely move throughout the crystal, and it
had been suggested that they could play a role in superconductivity. However,
whether the lines were merely surface effects or extended into the material was
unknown because the scanning tunneling microscopy that reveals them can only be
used on the material’s surface, she says.

Carlson collaborated with Karin Dahmen from the University
of Illinois and Purdue graduate student Benjamin Phillabaum on a study to
examine the tiny, ladder-like lines that point vertically or horizontally
across the surface of the crystal. By treating sets of neighboring parallel
lines as part of a single “cluster,” the team discovered that the
patterns formed by the clusters are fractal in nature, meaning they fall
between two dimensions of space, Carlson says.

“When thinking of a fractal, imagine a crumpled piece
of paper,” she says. “It is more than the 2D flat piece of paper and
it enters three dimensions, but it does not quite fill that space. It is not a
true and solid ball. Fractals can occupy fractions of a dimension. They also
have a pattern that is maintained even as you view smaller and smaller pieces
of it.”

Fractal patterns are mathematically understood, and the
patterns present on the surface of the copper-oxygen-based high-temperature superconductors
could be compared to known models of how that pattern can arise. Using this
information, the team was able to determine that these patterns originate from
deep inside the material. A paper detailing the NSF and Research Corporation
for Science Advancement-funded research was published in Nature
Communications.

The fractal patterns also offer information about the space
they occupy, and through this work the team developed a new analysis method to
help understand the materials and the interactions going on inside of them,
Carlson says.

“If you see pattern formation, then you can try to
identify the fundamental physics causing its formation,” she says.
“It can apply to a lot of systems and materials beyond
superconductors.”

Carlson says the method could be used to identify the
dominant class of disorder present in a material, which helps in understanding
its unique characteristics.

“We want to move beyond trying to get rid of disorder,
striving for unattainable purity in the materials we examine, and instead take
the disorder into account and use it to our advantage,” Carlson says.
“These little patches of imperfection where things aren’t lined up in a
perfect crystal lattice are important, and old methods that overlooked them
fail to capture important physics. The flaws in the lines are like a
fingerprint. They reveal the identity deeper inside.”

The team next plans to look at pattern formation in other
superconductive materials.

Source: Purdue University