Researcher Heike Pfau attaches an experimental setup at the base of a cooling unit at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany. The experiment allowed Pfau and colleagues to measure thermal conductivity in a sample of ytterbium dirhodium disilicide—one of the most-studied varieties of the exotic “heavy fermion metals.” Image: Ulrike Stockert/Max Planck Institute, Dresden |
A new study finds that “quantum critical points” in exotic
electronic materials can act much like polarizing “hot button issues” in an
election. Reporting in Nature,
researchers from Rice University, two Max Planck Institutes in Dresden, Germany,
and the University of California, Los
Angeles find that on either side of a quantum critical
point, electrons fall into line and behave as traditionally expected, but at
the critical point itself, traditional physical laws break down.
“The beauty of the quantum critical point is that even
though it’s only one point along the zero temperature axis, what happens at
that point dictates how electrons will interact in the material under a broad
set of physical conditions,” said study co-author Qimiao Si, a theoretical
physicist at Rice
University. The new study
involved “heavy-fermion metals,” magnetic materials with many similarities to
high-temperature superconductors.
Flowing electrons power all the lights, computers, and
gadgets that are plugged into the world’s energy grids, and physicists have
spent more than a century describing how these electrons behave. But
long-standing theories that describe how electrons interact in traditional
metals and semiconductors have yet to explain the strange electronic properties
of heavy-fermion metals, man-made composites that contain precise atomic
arrangements of transition metals and rare earth elements.
In the new study, Si collaborated with a group of
experimental physicists led by Frank Steglich at the Max Planck Institute for
Chemical Physics of Solids. The researchers examined several physical
properties at extremely cold temperatures—some as much as 10 times colder than
any such previous measurements—to show exactly how the standard theory of
electron correlations in metals breaks down at the quantum critical point
(QCP). That theory, Landau’s Fermi liquid theory, was first introduced in 1956.
“By measuring the ratio of the thermal to electrical
transport near the QCP in one of the most-studied heavy-fermion
metals—ytterbium dirhodium disilicide—we found a breakdown in the fundamental
concepts of Landau-Fermi liquid theory,” said Steglich, the founding director
of the Max Planck Institute for Chemical Physics of Solids.
Quantum particles come in two main varieties—bosons and
fermions. Bosons are the quantum equivalent of extroverts; they enjoy one
another’s company and can occupy the same quantum space. Fermions are the
opposite; no two can occupy the same quantum space, and this defines much of
their behavior.
Electrons are fermions, and their tendency to seek quantum
elbow room affects the way they organize. It’s important for scientists to
understand how they behave in concert because even a small electric current in
a tiny wire involves billions upon billions of individual electrons.
Landau-Fermi liquid theory is a mathematical system that
allows physicists to describe the actions of many billions of electrons with
just a handful of variables. Landau’s vehicle for collapsing the actions of so
many particles is something he dubbed a “quasiparticle,” a placeholder that acts
like an individual but describes the collective fate of many physical
particles.
“One of the tenets of the Landau theory is that this
quasiparticle carries the same amount of quantum units of charge and spin as an
electron in isolation,” said Si, Rice’s Harry C. and Olga K. Wiess Professor of
Physics and Astronomy. “It is not an actual electron, but it behaves like an
electron and has the physical status of an electron.”
This microscope image shows thermometers (top and bottom) and a heater (right) connected via 50-um-wide gold wires to a black rectangle of the ytterbium dirhodium disilicide (center) that is only three-quarters of a millimeter wide. Using this setup, researchers at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany, induced a thermal current by setting up a small difference in temperature at the two ends of the sample. The proportionality coefficient between this temperature difference and the thermal power provided by the heater defined the thermal conductivity of the sample, which was found to violate traditional laws of physics when the material was cooled to a “quantum critical point.” Image: Heike Pfau/Max Planck Institute, Dresden |
To show how Landau’s theory breaks down, the new study
demonstrated that quasiparticles near a QCP behaved in a way that electrons
could not. Electrons have the ability to convey energy as either heat or
electricity. Setting up either a temperature or voltage difference in the
material provides the means to measure the thermal or electrical conductivity,
and the experimental team measured the ratio of the two conductivities at the
QCP and found that the quasiparticles there were carrying about 10% less
thermal conduction than expected.
From the data, Si and fellow theorists Elihu Abrahams and
Stefan Kirchner were able to show that the violation in the accepted ratio
between heat and electrical conduction occurred only at the QCP; electrons on
either side behaved normally.
“This is important because it shows that the breakdown of
traditional electron organization occurs at the QCP,” said Kirchner, a theorist
from the Max Planck Institute for the Physics of Complex Systems and former
postdoctoral fellow at Rice.
The QCP is the point at which the material passes from one
phase to another, like ice melting into water, except that the QCP marks a
difference between quantum phases.
Abrahams, professor of physics at the University
of California, Los Angeles, said, “The finding is
unambiguous; new physics is occurring, and the QCP is the culprit.”
The finding adds to the growing body of experimental
evidence in support of a theory Si and colleagues offered in 2001 to explain
the correlated electron behavior at the QCP.
“At the QCP, magnetism drives quantum fluctuations,” Si
said. “Our theory accounts for these in a way that traditional theories like
Landau-Fermi liquid theory cannot.”
Si said these quantum fluctuations at the QCP drive the
strange electronic behavior that has often been measured in heavy fermion
metals, and they may also play a key role in other exotic materials like
high-temperature superconductors.