Since the discovery two decades ago of the unconventional topological superconductor Sr2RuO4, scientists have extensively investigated its properties at temperatures below its 1°K critical temperature (Tc), at which a phase transition from a metal to superconducting state occurs. Now experiments done at the University of Illinois at Urbana-Champaign in the Madhavan and Abbamonte laboratories, in collaboration with researchers at six institutions in the U.S., Canada, United Kingdom, and Japan, have shed new light on the electronic properties of this material at temperatures 4°K above Tc. The team's findings 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 U. of I., led the experiment. She explains, "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."
This research relies on current- measurement techniques that are highly sensitive, yielding very precise results. Images a-c represent conductance maps at varying energy levels. The high-resolution images d through l were taken with 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 into Cooper pairs. Image courtesy of Vidya Madhavan, University of Illinois at Urbana-Champaign, Department of Physics and Frederick Seitz Materials Research Laboratory
Credit: University of Illinois Department of Physics
Madhavan continues, "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 percent, 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."
Eduardo Fradkin, a physics professor and the director of the Institute for Condensed Matter Theory at the U. of I., comments, "The basic electronic properties of this material have been known for some time. 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 this anomalous normal (non-superconducting) state. In a conventional normal metallic state at low temperature, the electronic states behave as well defined quasi-particles, as described by the Landau-Fermi liquid theory. But the researchers found anomalies in the particle interactions at 5°K 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 data runs, the scientists found a significant change in the probability of the electron tunneling near zero energy, as compared with Fermi-liquids.
"We were surprised to see so much rich information," shares 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 4°K above the superconducting phase transition. What significance does this have to what's happening below the superconducting temperature?" Madhavan continues. The team plans to delve into that question next: "When Vidya goes to the superconducting state, we will know more," Fradkin affirms. "These findings will enable her to take a unique approach to revealing the superconducting order parameter of this material in upcoming experiments."
Vidya Madhavan | EurekAlert!
Structured light and nanomaterials open new ways to tailor light at the nanoscale
23.04.2018 | Academy of Finland
On the shape of the 'petal' for the dissipation curve
23.04.2018 | Lobachevsky University
At the Hannover Messe 2018, the Bundesanstalt für Materialforschung und-prüfung (BAM) will show how, in the future, astronauts could produce their own tools or spare parts in zero gravity using 3D printing. This will reduce, weight and transport costs for space missions. Visitors can experience the innovative additive manufacturing process live at the fair.
Powder-based additive manufacturing in zero gravity is the name of the project in which a component is produced by applying metallic powder layers and then...
Physicists at the Laboratory for Attosecond Physics, which is jointly run by Ludwig-Maximilians-Universität and the Max Planck Institute of Quantum Optics, have developed a high-power laser system that generates ultrashort pulses of light covering a large share of the mid-infrared spectrum. The researchers envisage a wide range of applications for the technology – in the early diagnosis of cancer, for instance.
Molecules are the building blocks of life. Like all other organisms, we are made of them. They control our biorhythm, and they can also reflect our state of...
University of Connecticut researchers have created a biodegradable composite made of silk fibers that can be used to repair broken load-bearing bones without the complications sometimes presented by other materials.
Repairing major load-bearing bones such as those in the leg can be a long and uncomfortable process.
Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.
Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...
Novel highly efficient and brilliant gamma-ray source: Based on model calculations, physicists of the Max PIanck Institute for Nuclear Physics in Heidelberg propose a novel method for an efficient high-brilliance gamma-ray source. A giant collimated gamma-ray pulse is generated from the interaction of a dense ultra-relativistic electron beam with a thin solid conductor. Energetic gamma-rays are copiously produced as the electron beam splits into filaments while propagating across the conductor. The resulting gamma-ray energy and flux enable novel experiments in nuclear and fundamental physics.
The typical wavelength of light interacting with an object of the microcosm scales with the size of this object. For atoms, this ranges from visible light to...
13.04.2018 | Event News
12.04.2018 | Event News
09.04.2018 | Event News
24.04.2018 | Life Sciences
24.04.2018 | Materials Sciences
24.04.2018 | Trade Fair News