The findings provide insight into the electronic structure of strongly correlated materials — materials that are potentially significant for far-reaching technological applications in nanotechnology and high-temperature superconductors.
Rice University theoretical physicist Qimiao Si and his team of researchers report their discovery of an entirely new class of critical point — the point at which a complex system undergoes a change between two distinctive phases — marking a substantial advance in the study of phase transitions. Familiar examples of thermal phase transitions, those driven by changes in temperature, are when water changes to steam or to ice.
Quantum phase transitions, on the other hand, are those driven by quantum fluctuations and dictated by Heisenberg’s famous uncertainty principle.
"The findings clarify, for the first time, what experimentalists have observed but were not able to explain because the results apparently contradict the traditional theory for quantum-critical metals — a theory that has held sway since the mid-1970’s," said Qimiao Si, associate professor of physics and astronomy at Rice University.
Their research is published in the Oct. 25 issue of the journal Nature. Authors on the report are Si of Rice; Silvio Rabello, postdoctoral fellow at Rice; Kevin Ingersent, associate professor of physics at the University of Florida, Gainesville; and Llewellun Smith, graduate student at Rice.
The researchers’ theory provides a basis for the quantum mechanism that gives seemingly conventional metals unconventional properties. In effect, they discovered a new quantum critical metallic state of matter. It is "quantum critical" because the transformation is dependent upon quantum fluctuations.
"The theoretical findings show that, under suitable conditions, quantum critical metals contain ‘critical local excitations’ — collective electronic objects which have very low energy, yet occur at one point in space," Si said.
The notion of local criticality could be applicable to a range of strongly correlated metals, including high-temperature superconductors. There is a growing realization that the apparent breakdown of the standard theory of metals — Landau’s Fermi-liquid theory — in high-temperature superconductors and related systems may result from proximity to quantum criticality.
Si, associate professor of physics, and his colleagues looked at a class of strongly correlated electron systems: heavy fermion metals, which contain the so-called rare earth elements and actinides, or radioactive metals. Among the most famous heavy fermions are the plutonium metals.
In strongly correlated electron systems, the interaction between neighboring electrons is so strong that the electrons can not be considered separately, as is done in describing simple metals and insulators.
Theoretically, it is very difficult to study complex behavior so Si and his team looked for benchmarks where they could understand the behavior.
Physicists have learned how to manipulate, or fine-tune, the degree to which the uncertainty principle comes into play in strongly correlated electron systems, allowing them to observe a quantum critical point. Experimentalists have previously done just that in heavy fermion metals.
When electrons are strongly interacting, even a small change in some external variable can have a dramatic impact, resulting in a change from one type of electronic or magnetic state to another.By changing the parameters of the system, Si and his colleagues were able to tune the system to be exactly at or on the cusp of the transition, where electrons behave most collectively and paradoxically, where accurate theoretical treatment is easier to carry through. Taking into account both quantum fluctuations and strong electron-electron interactions, they discovered the surprising "locally critical point."
In correlated electron physics, a frontier of condensed matter physics, scientists are trying to get an understanding of all of the electronic processes governing natural materials and man-made ones.
"Our field is still very much in its infancy," Si said. "We are looking for some very basic principles that govern how new electronic states of matter emerge as a result of quantum fluctuations and electron-electron interactions."
This research was supported by the National Science Foundation, the Texas Center for Superconductivity at the University of Houston and the Alfred P. Sloan Foundation.
Journal
Nature