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 their observations. Now, physicists at the University of Illinois say superconductivity can account for what had seemed to be puzzling findings.
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 physics who led a team that analyzed the experiments, which were carried out elsewhere. "Because the electrons move only at the interface, they are said to be confined to two dimensions."
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 insulator. Above a certain density, however, a conducting state was observed.
"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."
Writing in the Sept. 17 issue of the journal Nature, Phillips, postdoctoral research associate Yi Wan, and graduate students Ivar Martin, Sergey Knysh and Denis Dalidovich claim that this odd conducting phase is due to a novel kind of superconductor.
To support their conclusion, the researchers cite several key observations. First, features of the conducting transition, such as current-voltage characteristics and the scaling of the resistivity, resemble those of known insulator-superconductor phase transitions. Second, magnetoresistance measurements offer clear evidence for a critical magnetic field above which the conducting phase is destroyed.
"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 only conducting state that is compatible with this observation is a superconducting one."
Because the conductivity was independent of temperature at a particular electron density, that density marks the transition between the conducting and insulating phases, Phillips said.
"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 for superconductivity."
While additional measurements must be performed to confirm their findings, "at present, the insulator-superconductor scenario can explain the known experimental observations," Phillips said.