An exotic new superconductor based on the element ytterbium displays unusual properties that could change how scientists understand and create materials for superconductors and electronics. In a paper published Jan. 21 in the journal Science, Univ. of Tokyo and Rutgers Univ. researchers report that this material, beta-YbAlB4, can reach a point where seemingly contradictory electrical and magnetic properties coexist, without being subject to massive changes in pressure, magnetic fields, or chemical impurities. This point, which physicists call “quantum critical,” often defines whether and how a material can become superconducting—a valued property where all resistance to electrical flow vanishes. Credit: Courtesy Science/AAAS |
In 2008, an
international team of scientists studying an exotic new superconductor based on
the element ytterbium reported that it displays unusual properties that could
change how scientists understand and create materials for superconductors and
the electronics used in computing and data storage.
But a key
characteristic that explains the material’s unusual properties remained
tantalizingly out of reach. So members of that team from the Univ. of Tokyo
reached out to theoretical physicists at Rutgers Univ.
to help uncover the material’s secrets.
In a paper published in
the Science, the Tokyo and Rutgers researchers now report that the material can
reach a point where seemingly contradictory electrical and magnetic properties
coexist, without being subject to massive changes in pressure, magnetic fields,
or chemical impurities.
This point, which
physicists call “quantum critical,” often defines whether and how a
material can become superconducting—a valued property where all resistance to
electrical flow vanishes. Superconductivity, discovered 100 years ago, has
since been put to work in a variety of applications, from physics research to
medical MRI scanners.
Scientists have long
been able to “tune” materials toward quantum criticality by altering
the materials’ properties. This is done by exposing them to high magnetic
fields and pressures, or by adding certain atomic impurities to the materials.
The material studied by the Tokyo and Rutgers researchers, however, appears to be the first to
exhibit quantum criticality in its natural state, without tuning.
“This is a
completely unexpected result,” said Piers Coleman, professor of physics
and astronomy, School of Arts and Sciences, at Rutgers.
“It could be the first example of what physicists describe as a ‘strange’
metallic phase of matter, manifesting itself intrinsically, without any tuning
of the material’s properties.”
The material
synthesized and studied by the Japanese experimental physicists is an exotic
crystal made up of the elements ytterbium, boron, and aluminum. It has the
chemical formula YbAlB4 but the physicists gave it the nickname
“YBAL” (pronounced “why-ball”). Superconductivity had earlier
been observed in YBAL, in a particular crystalline form called the
“beta” structure. The Tokyo
physicists suspected they could find a quantum critical point in the material;
however, its superconducting behavior that kicks in slightly above absolute
zero masked their ability to pinpoint it.
Coleman and
postdoctoral researcher Andriy Nevidomskyy examined the data from the Tokyo experiments at a
wide range of temperatures and magnetic field strengths. All the data, they
found, collapsed onto a curve that pointed to the unobservable quantum critical
point (QCP) hidden by the superconducting phase. The QCP was within hair’s
breadth of zero magnetic field, with no externally applied tuning of pressure
or other parameters.
“It’s kind of a
dream system,” said Coleman, also a member of the Rutgers Center
for Materials Theory. “We’ve found a material that is intrinsically
quantum critical with very simple behavior. It’s puzzling, because there’s
nothing simple about the material’s structure. We’re not sure why this
happens.”
Nevidomskyy, now an
assistant professor of physics and astronomy at Rice Univ.,
likened the discovery of the QCP to finding a black hole in outer space.
“You can’t see a
black hole because light can’t escape from its grip; however, you can observe
the gravitational pull that a black hole has on nearby stars,” he said.
“Similarly, we couldn’t see the quantum critical point directly, but we
could see evidence of it in the material’s magnetic properties and thereby deduce
its position underneath the veil of superconductivity.”
The discovery that most
intrigues the physicists is that beta-YBAL could be revealing an exotic new
phase of matter known as the “critical strange metal” phase. At the
quantum critical point, the material can shift between conventional electrical
behavior, which physicists call a Fermi liquid, to superconducting behavior,
and to a condition that resembles neither, called “strange metal”
behavior. This behavior has been observed in superconducting materials, but
it’s not known whether it occurs only in the vicinity of a QCP or whether it
can exist over an extended range of physical conditions, which would
essentially make it a phase of matter.
Proposed by Nobel
laureate Philip Anderson, the idea of strange metal phases has been long
debated by physicists. “It is extremely controversial,” said Coleman.
“The experiments our Tokyo
colleagues are doing right now might provide more evidence. It could change our
basic understanding of materials going forward.”
“We are very
excited,” said Satoru Nakatsuji, professor and leader of the Tokyo research team.
“If true, this would be an amazing discovery, opening new horizons in our
understanding of quantum criticality.”
Coleman praised the
working relationship that he and Nevidomskyy have with Nakatsuji’s team,
including the paper’s primary author, Yosuke Matsumoto, and five other
researchers: K. Kuga, Y. Karaki, N. Horie, Y. Shimura, and T. Sakakibara. The
physicists are affiliated with the Univ.
of Tokyo’s Institute for Solid State
Physics in Kashiwa, Japan.
“In modern
science, this interplay between theory and experiment is extremely
important,” Coleman said. “If you can get a powerful current of ideas
going, you can take physics much further. A lot of our work has been done by
video conference. Unfortunately with the time difference, it means one of our
groups had to get up early while the other had to stay late at night.”
