Dr. Pengcheng Dai sits in an empty fifth-floor office with only a laptop to keep him company. The bare bookshelves will be filled eventually and a new computer is on the way. But on this spring afternoon the latest addition to the physics faculty isn’t concerned about the aesthetics of his working space. His enthusiasm is for his research, and the expertise he brings to the department is in a field that has great implications for East Tennessee: neutron scattering. Neutron Scattering: Understanding MaterialsNeutrons make up more than half of all visible matter and are usually bound inside the nucleus of an atom along with its positively charged counterpart, the proton. Protons and neutrons each have about the same mass and both can exist as free particles away from the nucleus. When the neutrons are freed from the nucleus via fission reactions and slowed down through the use of moderators, their energies are nearly identical to those of atoms in motion and their wavelengths are similar to interatomic lattice spacings of solids. These unique properties make neutrons a good tool to track where atoms are in a solid and how they behave when they’ve been hit with some sort of outside force like a change in temperature, pressure, or magnetism. When a powerful beam of neutrons collides with a target material, the neutrons scatter, and in the process reveal a great deal of information about the material itself. Neutrons have no charge and can penetrate bulk materials without causing any damage. Dr. Dai said this is a key advantage of this method. “Neutron scattering can go through, essentially, a wall,” he said. Neutrons behave like tiny magnets and render a “magnetic moment,” which can measure the size and direction of magnetization in solids. The applications of neutron research are many and varied, including studies of disordered crystalline materials (optical communication), complex minerals at high temperatures (earthquakes), and chemical reactions at interfaces (clean technology).
Dr. Dai, who came to East Tennessee for a post-doctoral position at Oak Ridge National Laboratory in 1993, has spent the past seven years using neutron scattering for scientific investigations in three different areas.
First, he studies the microscopic origins of superconductivity. He gains insight into the role of magnetism in superconducting materials by investigating the relationship between magnetism and cooper pairs, weakly bound electrons that result from cooling the materials below their critical temperature. Dr. Dai compares them to “lined up troops marching in an orderly fashion.” When a solid is cooled and the lattice vibrates coherently, the electrons move together in conventional superconductors such as Hg and Al. In High-Tc superconductors that have considerably higher Tc’s than the conventional superconductors, he explained, cooper pairs are formed, “but the agreement stops right there.” Using neutron scattering to study magnetic fluctuations, he tries to prove that magnetism plays a crucial role.
A second aspect of his research is the study of CMR (colossal magneto resistance). Certain materials experience an extreme change in electrical resistivity when exposed to a large magnetic field. This phenomenon is called magnetoresistance. Although all metals have some sort of magnetoresistance, the magnitude of the magnetoresistance is usually very small (less than a few percent). Dr. Dai explained that CMR materials, discovered in the 1990s, are a type of transition metal oxides that experience a “huge resistivity drop, but only at low temperatures” (e.g., below room temperature). He uses neutron scattering as a tool to study the magnetic interactions and thus address some of the most fundamental questions concerning these materials.
A final research area occupying Dr. Dai’s talents is heavy fermion physics. Discovered in the 1970s, heavy fermions are metals with electrons at the Fermi level that have a very large effective electron mass. Heavy fermions and high-temperature superconductors defy Landau’s Theory of Fermi Liquids, which applies to most materials and says that the electronic properties of a metal can be described in an effective single-particle theory.
Dr. Dai said the reason fermions deviate from this model is not yet understood, but he hopes that neutron scattering will shed some light on “why, microscopically, things behave the way they do.”
Neutron scattering is a valuable tool for a number of scientific disciplines because it reveals how atoms in materials are arranged, how they move, and how they interact. The technique complements other tools such as electron microscopy and X-ray diffraction.
“Neutron scattering is not something you do first,” Dr. Dai explained. “It’s something you do last to answer fundamental questions.”
One downside of Dr. Dai’s research is that to carry out his work, he has to go where the neutrons are.
“Beam time is expensive and hard to come by,” he said. “I travel a lot,” primarily to the ISIS facility in England or to the Institut Laue-Langevin (ILL) in France.
At present, ISIS is the world’s brightest neutron source. There are, however, two major steady neutron sources in the United States: the Oak Ridge High Flux Isotope Reactor (HFIR) and the NIST Research Reactor in Washington. With 85 megawatts of power, HFIR is “the most powerful one in the world,” Dr. Dai said. Although at present the facility is shut down for maintenance, “by November we will have neutrons,” he said.
While Europe took the lead in neutron science in the 1970s, Dr. Dai explained that the new Spallation Neutron Source being built in Oak Ridge and the upgraded HFIR will give the United States a competitive edge in this research field. The SNS, a partnership of six national laboratories with a cost of $1.4 billion, will provide the most intense pulsed neutron beams in the world and should be completed by 2006. Its addition to the ORNL facilities is a tremendous advancement for scientific research not only for the nation, but specifically for East Tennessee.
A Great Opportunity for StudentsDr. Dai, who is an associate professor at UT as well as a staff scientist in the ORNL Solid State Division, has already set to work on building a campus-based research program. He and faculty physicists Jim Thompson, Tom Callcott and David Mandrus recently learned they won a $72,500 grant from the University for a physical property measurement system. Dr. Dai said he wants to establish a UT lab, in part to study the single crystals that Dr. Mandrus will grow in his new materials laboratory, as highlighted in the last issue of Cross Sections. The plan is to grow and study samples on campus, with world-class neutron scattering facilities in nearby Oak Ridge. While this is a great addition to physics department research, it will also provide a tremendous opportunity for graduate students, who stand to gain world-class experience at Tennessee. ORNL, for example, has several neutron scattering scientists and visitors and has “between the number one and number two (group) in the world,” Dr. Dai said. He added that neutron scattering is a great field for future job prospects as there will be a tremendous shortage of scientists who work in this area, especially with the construction of the SNS, which will require more than 1,000 users per year. The neutron scattering community in the U.S. is very small (less than 600 researchers), so there will be a lot of future opportunities for neutron science. “It’s perfect timing in terms of getting a Ph.D.,” he said. “The information you obtain with neutrons is so unique” that it offers a good opportunity to publish in high-profile publications. He added that he would be happy to talk to any graduate students who think they have an interest in pursuing neutron scattering (his office is Room 502 of the Nielsen Physics Building and his e-mail address is daip@ornl.gov).And In His Spare Time . . .Although his travel schedule and twin responsibilities at UT and ORNL consume a lot of energy, Dr. Dai spends what little free time he has listening to music, collecting stamps or looking for a good badminton game. The native of China finished his Ph.D. at the University of Missouri in 1993 and then went to ORNL as a post-doc, a position he held for three years. He joined the faculty in the spring of 2001 and is a welcome addition to the physics department.Web Sites of Interest: Cross Sections, Spring/Summer 2001 Issue, Contents Page UT Physics News & Notes Page UT Physics Home Page This page was last updated on June 25, 2001. Please send comments to cal@utk.edu. |