Posted=A010/5/98=20
Novel Explanation Offered For Puzzling Electron 'Gas' Experiments=20
CHAMPAIGN, Ill. -- Recent experiments confirming the existence of a novel
conducting phase in a two-dimensional electron "gas" sandwiched between
semiconductors have posed a dilemma for scientists seeking to explain the=
ir
observations. Now, physicists at the University of Illinois say
superconductivity can account for what had seemed to be puzzling findings=
..=20
The experiments in question were performed on a silicon metal-oxide
semiconductor field-effect transistor. "In such a device, electrons are
confined to move at the interface between the metal oxide and the
semiconductor," explained Philip Phillips, a U. of I. professor of physic=
s
who led a team that analyzed the experiments, which were carried out
elsewhere. "Because the electrons move only at the interface, they are sa=
id
to be confined to two dimensions."=20
The experimenters probed a range of electron densities never before
examined at low temperature. In this regime, they observed that below a
certain electron density, the electrons behaved as they do in an insulato=
r.
Above a certain density, however, a conducting state was observed.=20
"The presence of this conducting state is remarkable, because standard
theory predicts that in two dimensions as you lower the temperature, the
resistivity will continue to increase and the system will become an
insulator," Phillips said. "Until now, no one had come up with an
acceptable explanation for this conducting state that was appropriate for
the description of these experiments."=20
Writing in the Sept. 17 issue of the journal Nature, Phillips, postdoctor=
al
research associate Yi Wan, and graduate students Ivar Martin, Sergey Knys=
h
and Denis Dalidovich claim that this odd conducting phase is due to a nov=
el
kind of superconductor.=20
To support their conclusion, the researchers cite several key observation=
s.
First, features of the conducting transition, such as current-voltage
characteristics and the scaling of the resistivity, resemble those of kno=
wn
insulator-superconductor phase transitions. Second, magnetoresistance
measurements offer clear evidence for a critical magnetic field above whi=
ch
the conducting phase is destroyed.=20
"There are not many states of matter that are consistent with a critical
magnetic field," Phillips said. "A critical parallel magnetic field
indicates that the electrons are paired up in spin singlet states. The on=
ly
conducting state that is compatible with this observation is a
superconducting one."=20
Because the conductivity was independent of temperature at a particular
electron density, that density marks the transition between the conductin=
g
and insulating phases, Phillips said.=20
"In this density regime, the Coulomb interactions dominate. In the
insulator, strong Coulomb interactions and disorder prevent the electrons
from moving. But as the density is increased, these strong Coulomb
interactions can lead to the formation of Cooper pairs, a prerequisite fo=
r
superconductivity."=20
While additional measurements must be performed to confirm their findings=
,
"at present, the insulator-superconductor scenario can explain the known
experimental observations," Phillips said.=20
Editor's Note: The original news release can be found at
http://www.admin.uiuc.edu/NB/98.10/supercontip.html=20