Physicists advance theory for new class of quantum phase transition

CONTACT: Lia Unrau
PHONE:
(713) 348-6778
EMAIL: unrau@rice.edu

PHYSICISTS ADVANCE THEORY FOR NEW CLASS OF QUANTUM PHASE
TRANSITION
Framework shows how metals transform when quantum mechanics is
‘fine-tuned’

The
complete workings of quantum mechanics and how it affects the universe is still
a mystery, but Rice University-led physicists have made a key advancement in
understanding how complex quantum fluctuations play a role in the transformation
of metals from one electronic state to another.

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.

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 that
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 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 widely known heavy fermions are the
plutonium metals.

In strongly
correlated electron systems, the interaction between neighboring electrons is so
strong that the electrons cannot 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.

Editors:
For more information about Qimiao Si visit: <http://dacnet.rice.edu/depts/ricephys/faculty/index.cfm?FDSID=568>.