Using a microscope designed to image the arrangement and interactions of electrons
in crystals, scientists have captured the first images of electrons that appear
to take on extraordinary mass under certain extreme conditions.
The technique reveals the origin of an unusual electronic phase transition
in one particular material, and opens the door to further explorations of the
properties and functions of so-called heavy fermions. Scientists from the U.S.
Department of Energy’s (DOE) Brookhaven National Laboratory, McMaster
University, and Los Alamos National Laboratory describe the results in the June
3, 2010, issue of Nature.
In this schematic diagram, individual electrons (white spheres) interact with uranium atoms (shown as yellow and blue f-electron orbitals of the uranium atoms) as they move through the URu2Si2 crystal. These interactions drastically inhibit the progress of the electrons, making them appear to take on extraordinary mass – an effect imaged for the first time in this study.
“Physicists have been interested in the ‘problem’ of heavy
fermions — why these electrons act as if they are hundreds or thousands
of times more massive under certain conditions — for thirty or forty years,”
said study leader Séamus Davis, a physicist at Brookhaven and the J.D.
White Distinguished Professor of Physical Sciences at Cornell University. Understanding
heavy fermion behavior could lead to the design of new materials for high-temperature
superconductors. Superconductivity allows materials to carry current with no
energy loss.
In the current study, the scientists were imaging electronic properties in
a material composed of uranium, ruthenium, and silicon that itself has been
the subject of a 25-year scientific mystery. In this material — synthesized
by Graeme Luke’s group at McMaster — the effects of heavy fermions
begin to appear as the material is cooled below 55 kelvin (-218 °C). Then,
an even more unusual electronic phase transition occurs below 17.5K.
Scientists had attributed this lower-temperature phase transition to some form
of “hidden order.” They could not distinguish whether it was related
to the collective behavior of electrons acting as a wave, or interactions of
individual electrons with the uranium atoms. Alexander Balatsky, a Los Alamos
theoretical physicist at the Center for Integrated Nanotechnologies, provided
guidance on how to examine this problem.
With that guidance, Davis’ group used a technique they’d designed
to visualize the behavior of electrons to “see” what the electrons
were doing as they passed through the mysterious phase transition. The technique,
spectroscopic imaging scanning tunneling microscopy (SI-STM), measures the wavelength
of electrons on the surface of the material in relation to their energy.
“Imagine flying over a body of water where standing waves are moving
up and down, but not propagating toward the shore,” said Davis. “When
you pass over high points, you can touch the water; over low points, you can’t.
This is similar to what our microscope does. It images how many electrons can
jump to the tip of our probe at every point on the surface.”
From the wavelength and energy measurements, the scientists can calculate the
effective electron mass.
“This technique reveals that we are dealing with very heavy electrons
— or electrons that act as if they are extremely heavy because they are
somehow being slowed down,” Davis said.
The detection of “heavy electron” characteristics below the second
transition temperature provides direct experimental evidence that the electrons
are interacting with the uranium atoms rather than acting as a wave.
To visualize this, imagine a team of football players running up the field
after a kickoff. If each player were free to run unimpeded, the whole team would
appear to operate as a wave of relatively independent “electrons.”
But imagine instead that the field is strewn with an array of chairs, and each
player has to sit for an instant every time he encounters a chair before continuing
up the field. In this case the chairs are analogous to the uranium atoms. Those
interactions between players and chairs (or electrons and uranium atoms) clearly
slow the progress.
In the case of the uranium material, the electron slowdown lasts only a tiny
fraction of a second at each uranium atom. But because kinetic energy and mass
are mathematically related, the slowdown makes it appear as if the electrons
are more massive than a free electron.
Besides revealing these interactions as the source of “hidden order”
in the uranium compound, Davis’ study shows that the SI-STM technique
can be used to visualize heavy electrons. That in turn opens the door to more
ways to investigate and visualize this phenomenon.
The research team is continuing to probe a variety of related compounds with
this new approach to further their understanding of heavy fermion systems.
“Heavy fermions remain mysterious in many ways, and it’s our job
as scientists to solve the problem,” Davis said.
This research was funded in the United States by the DOE Office of Science
and in Canada by the Natural Sciences and Engineering Research Council and the
Canadian Institute for Advanced Research. At Brookhaven, this research was supported
as part of the Center for Emergent Superconductivity, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of Science.