March 18, 2002
Brookhaven Spotlights: News From the March 2002 American Physical Society Meeting
NOTE TO EDITORS: “Brookhaven Spotlights” is issued periodically to bring you up to date on some of the latest newsworthy developments at the U.S. Department of Energy’s Brookhaven National Laboratory. The selected briefings below describe research that Brookhaven scientists presented at the American Physical Society meeting March 18-22, 2002 at the Indiana Convention Center in Indianapolis, Indiana.
Giant Nanomolecule Discovered by Accident
"Giant nanomolecules" may sound like an oxymoron, but these relatively large inorganic structures may provide big benefits for nanoscience, the study of chemical, physical, electrical, and magnetic interactions on the nanometer scale. Measuring 5.1 billionths of a meter in diameter, the molecules have a spherical, cagelike structure, similar to now-famous 60-carbon "buckyballs."
Covered with large pores that allow ions to move in and out, the hollow spheres may be useful as "containers" for studying chemical reactions at the nanoscale. In addition, these molecules are magnetically active because each has a large number of magnetic molybdenum ions in its structure. Even stronger magnetic materials can be created by loading other compounds inside the huge cages. Such materials might find applications as contrast agents in magnetic resonance imaging (MRI).
to Brookhaven physicist Tianbo Liu, this polyoxomolybdate giant molecule
was discovered by accident while using x-ray diffraction at the
National Synchrotron Light Source
at Brookhaven, as well as transmission electron microscopy, to study
nanoclusters of ions in complex polymer systems. "We found something very
special: The particles arranged themselves in an orderly fashion over long
distances," Liu says. Such order is impossible for ordinary nanoparticles,
Liu says, because they vary in size. But because polyoxomolybdate is a
molecule, with a definite molecular structure, "it's made the exact same
way every time, with a uniform size." Having one-size particles is an
advantage when studying and finding applications for nanoscale properties,
which are generally dependent on particle size.
New Spin on High-Temperature Superconductors
Understanding what holds electron pairs together in high-temperature (high-Tc) superconductors is "one of the biggest problems in condensed matter physics," says Peter Johnson, a Brookhaven physicist who is searching for the explanation. Johnson will present his group's latest findings, which indicate that electron "spin" plays an important role.
Like traditional superconductors, high-Tc superconductors can carry electrical current with no resistance, or loss. But to do so, traditional superconductors must be kept at temperatures just above absolute zero (0 kelvin, or -273 °C) by surrounding them with expensive liquid helium. High-Tc superconductors, however, operate at temperatures around 90 kelvins (-183 °C), where less-expensive liquid nitrogen can do the cooling. "This difference would decrease the cost of using superconducting materials and open up a wide range of potential applications," says Johnson, including, possibly, power lines that lose no power during transmission. Understanding the mechanism behind high-Tc superconductivity is the first step, he says.
In both types of materials, conventional and high-Tc
superconductors, superconductivity is achieved by pairs of electrons
carrying the current. But studies at the
National Synchrotron Light Source
at Brookhaven reveal that rather than pairing by exchanging vibrations
with the crystal lattice (the mechanism for traditional superconductors),
electron pairs in high-Tc materials interact by affecting the
spin of atoms in the lattice. "The role of spin has the potential to
revolutionize our thinking about the transfer of electrical current,"
Electron Excitations in High-Temperature Superconducting Materials
Unlike Johnson, who is studying individual electrons in high-temperature (high-Tc) superconductors, Young-June Kim, another Brookhaven physicist, is studying the collective behavior of electrons in materials closely related to high-Tc superconductors. He uses resonant inelastic x-ray scattering (RIXS), a technique developed at Brookhaven's National Synchrotron Light Source but used at Argonne National Laboratory's brighter Advanced Photon Source, to understand how electrons are moving around in the system.
By comparing the energy of x-rays beamed into a sample and those coming out, this sensitive technique measures how much energy is transferred to the electrons in the material. The absorbed energy can result in a variety of excitations, which can be distinguished by RIXS. Kim is using the technique to study lanthanium copper oxide, an insulating material, and looking at how the excitations change as the material is transformed to a high-Tc superconductor by gradually substituting strontium atoms for lanthanium atoms.
"The technique is very new, and we are working to improve its
sensitivity, possibly to the level where we'll be able to study
excitations and interactions of electrons participating in
superconductivity," says Kim. This greater sensitivity may help reveal the
mechanisms behind high-Tc superconductivity.
Material with Unusually High Dielectric Constant Holds Up to Scrutiny
Further optical studies of a material with an unusually high dielectric constant -- discovered at Dupont and described last July by Brookhaven scientists and their collaborators -- will be presented by Brookhaven physicist Christopher Homes. The material -- a perovskite-related oxide containing calcium (Ca), copper (Cu), titanium (Ti), and oxygen (O) in the formula CaCu3Ti4O12 -- may have applications in high-performance capacitors and miniaturized electronics.
A material's dielectric constant determines its ability to separate positive and negative electrical charges (i.e., to become polarized). This material's dielectric constant remains large over a wide range of temperatures, but drops 1,000-fold at temperatures below 100 kelvins (-173 °C). Optical, structural, and electron transport studies of this material at the National Synchrotron Light Source at Brookhaven indicate that this change is related to an anomalous increase in the strengths of some of the vibrational modes of the crystalline lattice at low temperatures (see previous press release).
The latest studies show that the high dielectric constant and the drop
with temperature are apparent even in thin films of the material, which
are the form necessary for applications in microelectronics. The
scientists are now studying how the material changes when cadmium is
swapped for calcium. "This will allow us to separate intrinsic from
extrinsic effects and to zero in on the mechanism responsible for the
large dielectric constant," Homes says.
Seeing Hidden Structure in Liquid Crystals
Liquid crystals, the materials used in laptop computer screen displays, optical networking devices, and other applications, are composed of rod-shaped molecules with the ability to change their orientation (the direction in which they "point") in response to an electric field. In many cases, these oriented molecules also form layers.
"There are a range of ways the molecules may orient themselves into phases with different symmetries that exhibit different electro-optical responses," says Brookhaven physicist Ron Pindak. Understanding how the underlying structure of these phases affects their optical properties will allow scientists to improve the design of liquid crystal materials, possibly leading to higher-definition video displays or faster optical conditioning devices.
Until now, some of the structural details of the liquid crystal phases have been "hidden." Conventional x-ray scattering can detect the layers, but not the orientation of individual molecules within the layers. Moreover, often the orientation changes over very short distances, making it invisible to an optical microscope. But "resonant polarized x-ray diffraction" experiments at the National Synchrotron Light Source at Brookhaven are starting to reveal these hidden structural details.
The key, says Pindak, is to tune the energy of the x-rays to match the threshold energy needed to excite electrons in certain atoms in the molecule to higher energy levels. "At this particular energy, the x-ray scattering pattern will be sensitive to the orientation of the molecules in the layers," Pindak says.
Related Link: Complete information about the March 2002 APS meeting.