Science Alliance Annual Report
2008–2009
UT-ORNL Distinguished Scientists
UTK materials science and engineering and physics and astronomy; ORNL materials science and technology
Takeshi Egami
High-temperature superconductivity
Superconductors allow current to flow without resistance in low temperatures.
How low? Well, the first observed superconductivity in 1911 by Dutch physicist Heike Kamerlingh Onnes took place at a chilly 4 Kelvins above absolute zero, theoretically the coldest temperature possible. For comparison, ice melts at 273.15 Kelvin; water boils at 373.15.
Seventy-five years later, 1987 Nobel Laureates K. Alexander Müller and J. Georg Bednorz created a brittle copper oxide (cuprate) compound that superconducted at a breakthrough 30 Kelvins.
A widely accepted theory for the superconductivity in elements and simple alloys at near absolute zero temperatures came in 1957, but proved inadequate to explain the phenomenon in more complex compounds at higher temperatures. In spite of all the work since then, and the variety of compounds found to exhibit this phenomenon, the microscopic mechanisms propelling high-temperature superconductivity remain a puzzle.
Takeshi Egami uses neutron scattering and other experimental techniques and theoretical modeling to study the local structure and dynamics of the complex compounds that exhibit high-temperature superconducting behavior.
This year his work with a new family of superconductors, iron (Fe) pnictides (compounds in the nitrogen group on the periodic table), led to a new idea explaining why superconductivity in these particular compounds is unusually strong. Egami says this exciting class of superconducting compounds, first reported by a Japanese group led by Hideo Hosono in 2008, appears both electronically and structurally simpler than their cuprate counterparts, and thus might be easier to understand.
Atomic-scale dynamics of liquids and glasses
The viscosity of liquids can change by as much as 15 orders of magnitude (1015) over a temperature range of a few hundred degrees, while the structure remains nearly the same. The atomic origin of this dramatic behavior is unknown.
Phonons, or lattice waves, that give us a basic understanding of how atoms vibrate in crystals, cannot help scientists resolve questions about the structure and dynamics of liquid and glass. Liquid structures lack the distinct regularity found in crystals. Consequently the phonons are scattered haphazardly throughout the fluid and only have a lifetime of 10-12 seconds.
For some time, Egami has pursued the idea that the dynamics of local atomic-level stresses—a stress each atom experiences due to interaction with neighboring atoms—might play a similar role to that of crystalline phonons. Indeed his group’s computer simulations verified that they play an equivalent role as phonons in crystals.
They also demonstrated that the relationships among atomic-level stresses develop correlations in both space and time, as the liquid cools, explaining the rapid rise in viscosity.
Egami says a fully developed theory explaining the correlations among atomic-level stresses should bring the solution of this long-standing problem in liquid physics within reach.