June 5, 2015

A team of physicists from the University of Pittsburgh, the University of Wisconsin-Madison, and the U.S. Naval Research Laboratory (NRL) has discovered electron pairing in strontium titanate far above the superconducting transition temperature. The phase is a long-postulated state of matter in which electrons form pairs that do not condense into a superconducting phase.

At low temperatures, many materials enter a superconducting phase, where they exhibit precisely zero electrical resistance. Superconductors are used for many applications including in magnetic resonance imaging devices and for magnetic energy storage. The basis for all superconductors is the formation of electron pairs.

In the normal non-superconducting phase, the electrons in most metals move independently—the scattering of electrons causes electrical resistance. In a superconductor, the paired electrons move in a highly coordinated fashion that has zero electrical resistance. The new research identified an intermediate phase, in which electrons form pairs, but the pairs move independently. The independent pairs are able to scatter, and the phase exhibits electrical resistance.

The researchers used quantum dots in strontium titanate to observe the electron pairs. Quantum dots are small regions of a material in which the number of electrons can be precisely controlled, in this case using an electrostatic gate. The quantum dots were large enough to support a superconducting phase at low temperatures, but the researchers observed that the dots always preferred an even number of electrons in the new phase at higher temperatures. When the researchers applied a magnetic field, they observed breaking of the electron pairs one at a time.

A theory of electron pairing without formation of a superconducting state was first published by David M. Eagles in 1969. C. Stephen Hellberg, a physicist in NRL's Material Science and Technology Division and the team's theorist, observed "the results are well described by a simple model with attractive interactions between electrons. We still don't know the origin of the attractive interaction: possibilities include 'Negative-U' defect centers and bipolarons."

The team created and measured 58 quantum dots with varying dimensions and barriers between the quantum dots and the leads. The new pairing phase was observed in all of the dots. The discovery provides clues about the mechanisms causing superconductivity in strontium titanate, which may eventually help researchers to the discovery of a material that superconducts at room temperature.

These images show differential conductance through the quantum dot as a function of the gate voltage that controls the number of electrons in the dot (x-axis) and the applied magnetic field (y-axis). Blue regions have low differential conductance and a constant number of electrons; green, yellow, and brown show higher differential conductance, indicating a change in the number of electrons in the dot. The top panel shows the measured differential conductance; the bottom panel shows the theoretical calculation (which has no disorder). Both experiment and theory show splitting of the electron pairs with increasing field and reentrant pairing at higher fields (the merging of pairs of boundaries into vertical boundaries).