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Theories of high-temperature superconductivity violate Pauli Principle
James E.
Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
3/24/2005
CHAMPAIGN, Ill. —
Scientists seeking to explain high-temperature superconductivity have
been violating the Pauli exclusion principle, a team of researchers
from the University of Illinois at Urbana-Champaign and Rutgers University
report. Any theory that does not embrace the Pauli principle has a lot
of explaining to do, they say.
The basic organizing precept behind the periodic table is the Pauli
principle, which says that electrons with the same spin cannot occupy
the same energy state. The Pauli principle leads to the shell structure
of atoms, and is inviolate for electronic systems. Many researchers,
however, have been breaking this important rule when proposing theories
to explain the mechanism behind high-temperature superconductivity.
“Within a class of materials known as doped Mott insulators, such
as the high-temperature copper-oxide superconductors, the Pauli principle
emerges as a sum-rule connecting high- and low-energy scales,”
said Philip Phillips, a professor of physics at the University of Illinois at Urbana-Champaign, who will present
the team’s findings at the spring meeting of the American Physical
Society, to be held in Los Angeles, March 21-25. This work also appeared
in the Dec. 31, 2004, issue of the journal Physical Review Letters.
“It is standard practice in physics to separate high- and low-energy
scales through a procedure known as renormalization,” Phillips
said. “We have shown that this procedure changes the statics of
the excitations within doped Mott insulators, resulting in a violation
of the Pauli principle. Since such a violation is not possible, we conclude
that high- and low-energy scales are inextricably linked in doped Mott
insulators.”
Unlike low-temperature superconductors, which are metals, high-temperature
superconductors are insulators in their normal state. Even more puzzling,
half of the electron states are empty.
“Since there are plenty of available positions for electrons,
you would think these materials should be metallic,” Phillips
said. “Even though there are many unoccupied states, strong electron
interactions cause them to be insulators.”
Strong electron interaction is the key to understanding Mott insulators,
Phillips said. “The interactions cause a mixing of the high- and
low-energy scales. Because the electrons at all energy levels are interconnected,
performing renormalization will be done at a price – in this case
at the expense of the Pauli exclusion principle.”
Phillips and his colleagues – Illinois graduate student Dimitrios
Galanakis and former graduate student Tudor D. Stanescu (now a postdoctoral
research associate at Rutgers University) – also suggest that
the mixing of high- and low-energy scales might explain the absence
of well-resolved electron-like features in the normal state of the copper-oxide
superconductors.
Experiments have demonstrated that removing an electron from a metal
results in a very narrow peak in the photoemission spectrum. Removing
an electron from the normal state of a high-temperature superconductor
results in a very broad feature.
“If you remove the high-energy scale through the process of renormalization,
the spectral features are very sharp,” Phillips said. “But
if you retain it, the features are very broad. If the physics changes
when you remove the high-energy scale, then renormalization is out the
window. The Pauli principle can not be violated.”
Funding for the work was provided by the National Science Foundation
and the American Chemical Society.
