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A molecular model of the material studied by
Rutgers physicists. In this representation of the crystal
structure of CeIrIn5, the red, gold and gray spheres
correspond to cerium, iridium and indium.
Image � by
Rutgers University
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In a paper posted to Science Express, a Web
site of research reports slated for upcoming print editions of
Science, the researchers describe how electrons interact with other
particles in these compounds to morph into what physicists call a
fluid of �heavy quasiparticles� or a �heavy fermion fluid.� While this
effect has been previously observed in some materials, the Rutgers
work employs new materials to provide a level of detail that has
eluded scientists so far.
�In this paper, we essentially track the fate of electrons as we lower
the temperature,� said Gabi Kotliar, Board of Governors Professor of
Physics in the School of Arts and Sciences. �Experimental physicists
may have seen different aspects of this behavior, or they may have
seen behaviors they did not understand. Our calculations reconcile
what they�ve seen.�
The Rutgers researchers based their models on experiments using a new
metallic crystalline compound made of the elements cerium, indium and
iridium. This and similar compounds that substitute cobalt and rhodium
for iridium are excellent test beds for observing heavy electron
behavior.
Earlier investigations used high-temperature superconducting materials
called cuprates, which failed to give physicists a clear view of
electron behavior because of disorders in the crystalline structure
caused by doping. The new cerium-based compounds are simpler to study
because they are free of dopants.
�The new compounds are for us what fruit flies are for genetics
researchers,� said Kristjan Haule, assistant professor of physics and
astronomy. �Fruit flies are easy to breed and have a simple gene
makeup that�s easy to change. Likewise, these compounds are easy to
make, structurally straightforward and adjustable, giving us a clearer
view into the many properties of matter that arise at low temperatures.
For example, we can use a magnetic field to kill superconductivity and
examine the state of matter from which superconductivity arose.�
These compounds are examples of strongly correlated materials, or
materials with strongly interacting electrons, that can�t be described
by theories that treat electrons as largely independent entities. The
terms �heavy quasiparticles� refers to how electrons interact with
each other and, as a result of those interactions, form a new type of
particle called a �quasiparticle.�
In explaining how this effect appears at low temperatures and vanishes
at higher ones, Haule noted that electrons in f-orbitals are tightly
bound to cerium atoms at room temperature. But as the temperature
drops, the electrons exhibit coherent behavior, or delocalization from
their atoms. At 50 degrees above absolute zero, or 50 degrees Kelvin,
the researchers clearly observe quasiparticles as electrons interact
with each other and other electrons in the metal known as conduction
electrons.
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