Physics graduate students Karl-Anton Lorenzer (left) and Andrey Sidorenko adjust equipment at Vienna University of Technology. Photo: F. Aigner/TU Wien |
New evidence this week supports a theory developed five years ago at Rice University
to explain the electrical properties of several classes of materials—including
unconventional superconductors—that have long vexed physicists.
The findings in Nature Materials
uphold a theory first offered in 2006 by physicist Qimiao Si, Rice’s Harry C.
and Olga K. Wiess Professor of Physics and Astronomy. They represent an
important step toward the ultimate goal of creating a unified theoretical
description of the quantum behavior of high-temperature superconductors and
related materials.
“We now have a materials-based global phase diagram for heavy-fermion
systems—a kind of road map that helps relate the predicted behavior of several
different classes of materials,” Si said. “This is an important step
on the road to a unified theory.”
High-temperature superconductivity is one of the greatest unsolved mysteries
of modern physics. In the mid-1980s, experimental physicists discovered several
compounds that could conduct electricity with zero resistance. The effect
happens only when the materials are very cold, but still far above the
temperatures required for the conventional superconductors that were discovered
and explained earlier in the 20th century.
In searching for a way to explain high-temperature superconductivity,
physicists discovered that the phenomenon was one of a larger family of
behaviors called “correlated electron effects.”
In correlated electron processes, the electrons in a superconductor behave
in lockstep, as if they were a single entity rather than a large collection of
individuals. These processes bring about tipping points called “quantum
critical points” at which materials change phases. These phase changes are
similar to thermodynamic phase changes that occur when ice melts or water
boils, except they are governed by quantum mechanics.
Materials at the border of magnetism and superconductivity—including
heavy-fermion metals and high-temperature superconductors—are the prototype
systems for quantum critical points.
In 2001, Si and colleagues proposed what has now become the dominant theory
to explain correlated electron effects in heavy-fermion systems. Their
“local quantum critical” theory concluded that both magnetism and
charged electron excitations play a role in bringing about quantum critical
points.
Experiments over the past decade have provided overwhelming evidence for the
role of both effects. In addition, experiments have shown that quantum critical
points fall into different classes for different types of materials, including
several nonsuperconductors.
“In light of the experimental evidence, an important question arose as
to whether a unifying principle might exist that could explain the behavior of
all the classes of quantum critical points that had been observed in
heavy-fermion materials,” Si said.
Vienna University of Technology graduate students Hannes Winkler (left) and Andrey Sidorenko are coauthors of a new paper that sheds light on “correlated electron effects” in heavy fermion materials. Photo: F. Aigner/TU Wien |
In 2006, Si put forward a new theory aimed at doing just that. Experiments
two years ago confirmed that the theoretical global phase diagram could explain
the quantum critical behavior of YRS—composites of ytterbium, rhodium, and
silicon that are among the most-studied quantum critical materials.
In the new Nature Materials paper,
a group led by experimental physicist Silke Paschen of Vienna University of Technology
in Vienna
examined a new material made of cerium, palladium, and silicon (CPS). Both YRS
and CPS are heavy-fermion compounds; however, YRS is a composite of stacked 2D
layers, and CPS has a 3D crystalline structure.
“In YRS, the collapse of charged electronic excitations occurs at the
onset of magnetic order,” Paschen said. “In CPS, we established a
similar collapse of the electronic excitations but inside an ordered
phase.”
To explain the difference between the observations in CPS and YRS, Si and
co-author Rong Yu, a Rice postdoctoral researcher, invoked the effect of
dimensionality.
“In systems like YRS, reduced dimensionality enhances the quantum
fluctuations between the electrons, and that enhancement influences their
collective behavior,” Yu said. “In the 3D material, we found that the
quantum fluctuations were reduced, and this affected the quantum critical point
and the correlated behavior in a way that was predicted by theory.”
Si said the linkage between the quantum critical points of CPS and YRS is
important for the ultimate question of how to classify and unify quantum
criticality.
“Our study not only highlights a rich variety of quantum critical
points but also indicates an underlying universality,” he said.
Si said it is important to test the theory’s ability to correctly predict
the behavior of even more materials, and his group is working with Paschen and
other experimentalists via the International
Collaborative Center
on Quantum Matter to carry out those tests.
