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Topic Name: BU researchers found 'Cooper pairs' is in insulators as well superconductors
Category: Polymer Interfaces and Macromolecular Assemblies
Research persons: Thomas J. Watson, James Valles, Michael Stewart
Location: Brown University, United States
Details
Nearly a century ago, Dutch physicist Kamerlingh Onnes discovered that some
metals transform into perfect electrical conductors when cooled to temperatures
near absolute zero. Once started, their currents of electrons can flow
perpetually.
How electrons reorganize to produce this behavior remained mysterious until
1957, when theoretical physicists John Bardeen, Leon Cooper and Robert
Schrieffer unveiled their BCS (Bardeen, Cooper, Schrieffer) theory of
superconductivity. The theory shows that superconducting electrons form pairs,
now known as Cooper pairs, that correlate their motion with other electron pairs
to smoothly and infinitely flow. Cooper, currently the Thomas J. Watson, Sr.
Professor of Science at Brown
University, went on with his colleagues to win a Nobel Prize for this work.
Now, in the 50th anniversary year of BCS theory, Brown physicists are making
a surprising addi-tion to the scientific canon created by their famous
colleague. In new work appearing in Science, the team shows that Cooper pairs
not only form in superconductors, but can also form their opposite –
electrical insulators.
“Our finding is quite counterintuitive,” said James Valles, a Brown
professor of physics who led the research. “Cooper pairing is not only
responsible for conducting electricity with zero resis-tance, but it can also be
responsible for blocking the flow of electricity altogether.”
Michael Stewart is a physics graduate student at Brown and the lead author of
the Science article. Stewart started the research as a skeptic. He’d seen
scientific papers suggesting that Cooper pairs might exist in electrical
insulators under certain conditions. Stewart decided to test this unorthodox
idea. “I’d would’ve put my money down,” he said, “that the answer was
‘no’.”
To create an insulator for his experiments, Stewart chose bismuth, a rare
metal that, when thick, serves as an excellent superconductor and, when thin,
serves as an exceptional insulator. Stewart turned to Jimmy Xu, a Brown
professor of engineering and physics and a pioneering nanotechnology researcher,
to create a template for the special experimental film.
Xu supplied a template honeycombed with holes measuring only 50 nanometers in
diameter. When coated with an ultra-thin coating of bismuth just four atoms
thick, and cooled to super-low temperatures, the material could be transformed
into either a superconductor to insulator. When the material was behaving as an
insulator, and the researchers applied a magnetic field, they detected a
telltale change in electrical current, which announced the presence of Cooper
pairs.
While the team found that Cooper pairs are present in both superconductors
and insulators, they believe that they behave differently in each instance. In
superconductors, pairs link up with other pairs and move in a linear way to
create a continuous stream of electric current. Think of a conga line. But in
the insulating film, researchers believe the pairs spin solo. Think of couples
twirling on a ballroom dance floor.
The holes in their test material were the clincher, Valles and Stewart said,
allowing them to detect the electron pairs. “Cooper pairs formed, but stayed
segregated in these whirlpools,” Stewart said. “Because of that, the pairs
can’t make a continuous line of current.”
The findings could help researchers understand the limits of
superconductivity and, perhaps, push them to create insulated wires that conduct
electricity without heating up. Cooper said the work sheds important and
intriguing new light on quantum effects.
“This very interesting result reminds us that unexpected, important
discoveries await if we continue to look," Cooper said.
Aijun Yin, a senior research associate in engineering at Brown, assisted with
the research.
The National Science Foundation, the Air Force Research Laboratory, and the
Office of Naval Research funded the work. In figure 1, Conducting copper wire insulated by an outer layer of polyethylene
Note for BCS theory
BCS theory (named after its creators, Bardeen, Cooper, and Schrieffer) explains conventional superconductivity, the ability of certain metals at low temperatures to conduct electricity without electrical resistance. BCS theory views superconductivity as a macroscopic quantum mechanical effect. It proposes that electrons with opposite spin can become paired, forming Cooper pairs. Independently and at the same time, superconductivity phenomenon was explained by Nikolay Bogoliubov by means of the so-called Bogoliubov transformations.
In many superconductors, the attractive interaction between electrons (necessary for pairing) is brought about indirectly by the interaction between the electrons and the vibrating crystal lattice (the phonons). Roughly speaking the picture is the following:
An electron moving through a conductor will attract nearby positive charges in the lattice. This deformation of the lattice causes another electron, with opposite "spin", to move into the region of higher positive charge density. The two electrons are then held together with a certain binding energy. If this binding energy is higher than the energy provided by kicks from oscillating atoms in the conductor (which is true at low temperatures), then the electron pair will stick together and resist all kicks, thus not experiencing resistance.
BCS theory was developed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer, who received the Nobel Prize for Physics in 1972 as a result.
Note for Electrical insulation
Electrical insulation is a material that resists the flow of electric current. An object intended to support or separate electrical conductors without passing current through itself is called an insulator. An insulation material will have atoms with tightly bonded valence electrons. The term electrical insulation has the same meaning as the term dielectric.
Some materials such as silicon dioxide or teflon are very good electrical insulators. A much larger class of materials, for example rubber-like polymers and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).
About National Science Foundation
The National Science Foundation (NSF) is a United States government agency that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National Institutes of Health. With an annual budget of about $5.91 billion (fiscal year 2007), NSF funds approximately 20 percent of all federally supported basic research conducted by the United States' colleges and universities. In some fields, such as mathematics, computer science, economics and the social sciences, NSF is the major source of federal backing.
The NSF's director, its deputy director, and the 24 members of the National Science Board
(NSB) are appointed by the President of the United States, and confirmed by the United States Senate. The director and deputy director are responsible for administration, planning, budgeting and day-to-day operations of the foundation, while the NSB meets six times a year to establish its overall policies. The current NSF director is Dr. Arden L. Bement, Jr., and the current deputy director is Dr. Kathie L. Olsen.
About Air Force Research Laboratory
The Air Force Research Laboratory (AFRL) is a scientific research organization operated by the United States Air Force dedicated to the development of warfighting
technologies. The AFRL headquarters is at Wright-Patterson Air Force Base, Ohio. The laboratory was created in October 1997 through the consolidation of four former Air Force laboratories and the Air Force Office of Scientific Research (AFOSR).
AFRL's published mission statement is:
AFRL's mission is leading the discovery, development and integration of affordable warfighting technologies for America's aerospace forces. It is a full-spectrum laboratory, responsible for planning and executing the Air Force' science and technology program. AFRL leads a worldwide government, industry and academia partnership in the discovery, development and delivery of a wide range of revolutionary technology. The laboratory provides leading-edge warfighting capabilities keeping our air, space and cyberspace forces the world's best.
About Office of Naval Research
The U.S. Office of Naval Research (ONR), headquartered in Arlington, Virginia (Ballston), is the office within the U.S. Department of the Navy that coordinates, executes, and promotes the science and technology programs of the U.S. Navy and Marine Corps through schools, universities, government laboratories, and nonprofit and for-profit organizations.
ONR, as it is frequently referred to, reports to the U.S. Secretary of the Navy through the Assistant Secretary of the Navy for Research, Development and Acquisition. It executes its mission through:
Science & Technology Departments
ONR Corporate Programs
Naval Research Laboratory (NRL)
ONR Global Office
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