Images a–c represent conductance maps at varying energy levels. The high-resolution images d to l were produced by Fourier transform scanning tunneling spectroscopy; the bright square reveals the presence of an electron with a particular wavelength. In the superconducting state, these lights would disappear when electrons pair up to form Cooper pairs. Images: Vidya Madhavan, University of Illinois at Urbana-Champaign, Department of Physics and Frederick Seitz Materials Research Laboratory.
Images a–c represent conductance maps at varying energy levels. The high-resolution images d to l were produced by Fourier transform scanning tunneling spectroscopy; the bright square reveals the presence of an electron with a particular wavelength. In the superconducting state, these lights would disappear when electrons pair up to form Cooper pairs. Images: Vidya Madhavan, University of Illinois at Urbana-Champaign, Department of Physics and Frederick Seitz Materials Research Laboratory.

Since the discovery two decades ago of the single-layered ruthenate Sr2RuO4 as an unconventional topological superconductor, scientists have extensively investigated its properties at temperatures below its 1K critical temperature (Tc). This is the temperature at which the ruthenate undergoes a phase transition from a metal state to a superconducting state.

Now, experiments performed at the University of Illinois at Urbana-Champaign, in collaboration with researchers at six institutions in the US, Canada, the UK and Japan, have shed new light on the electronic properties of this material at temperatures 4K above its Tc. The team's findings, which are reported in a paper in Nature Physics, may elucidate yet-unresolved questions about Sr2RuO4's emergent properties in the superconducting state.

Vidya Madhavan, a physics professor and member of the Frederick Seitz Materials Research Lab at the University of Illinois, led the experiment. "We began from the widely held assumption that, in Sr2RO4's normal metallic state above its Tc, the interactions of electrons would be sufficiently weak, so that the spectrum of excitations or electronic states would be well defined," she explains

"However, and this is a big surprise, our team observed large interaction effects in the normal metallic state. Electrons in metals have well defined momentum and energy. In simple metals, at low temperatures the electrons occupy all momentum states in a region bounded by a 'Fermi surface'. Here, we found that the velocity of electrons in some directions across the Fermi surface were reduced by about 50%, which is not expected. We saw similar interaction effects in the tunneling density of the states. This is a significant reduction, and it was a great surprise. We thought we would just find the shape of the Fermi surface, but instead we get these anomalies."

"The basic electronic properties of this material have been known for some time," continues Eduardo Fradkin, a physics professor and director of the Institute for Condensed Matter Theory at the University of Illinois. "Scientists study this material because it's supposed to be a simple system for testing scientific effects. But the material has also been a source of ongoing debate in the field: this is a p-wave superconductor, with spin-triplet pairing. This has suggested that the superconducting state may be topological in nature. Understanding how this system becomes superconducting is an open and intriguing question."

The breakthrough to understanding the puzzling properties of the material's superconducting state may lie in its anomalous normal (non-superconducting) state. In a conventional normal metallic state at low temperatures, the electronic states behave as well defined quasi-particles, as described by what is known as the Landau-Fermi liquid theory. But the researchers found anomalies in the particle interactions at 5K that actually characterize Sr2RuO4 as a ‘strongly correlated metal’.

In the experiment, Madhavan's team passed electrons into the material using an electronic metallic tip, then measured the resultant current using two highly advanced and complementary techniques: Fourier transform scanning tunneling spectroscopy and momentum resolved electron energy loss spectroscopy. In four experimental runs, the scientists found a significant change in the probability of electron tunneling near zero energy, as compared with Fermi-liquids.

"We were surprised to see so much rich information," says Madhavan. "We started talking to Eduardo about the theory and to Peter Abbamonte about his experiments. Abbamonte's group, applying the technique of momentum resolved electron energy loss spectroscopy, also finds interactions with collective modes at the same energies."

"The open question now: we found something interesting at 4K above the superconducting phase transition; what significance does this have to what's happening below the superconducting temperature?" Madhavan adds. The team plans to delve into this question next.

"When Vidya goes to the superconducting state, we will know more," Fradkin says. "These findings will enable her to take a unique approach to revealing the superconducting order parameter of this material in upcoming experiments."

This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.