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New insight into high-temperature superconductors


Researchers have found evidence for an acoustic plasmon or "sound wave", which has been predicted for layered systems and suggested to play a role in mediating high temperature superconductivity.

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When electrical current propagates through a conducting material, energy dissipates due to the conductor’s electrical resistance. In a superconductor, however, the resistance can vanish completely if the material is cooled to extremely low temperatures. Such dissipationless supercurrent would be highly desirable for a plethora of electronic and technological applications, and has spawned decades of intense research dedicated to finding materials with superconducting properties at elevated temperatures.

While all superconducting materials reported until the 1980’s had to be cooled below 30 K, the game changed in 1986, when the first superconductors based on copper oxide materials were discovered. These so-called high-temperature superconductors are composed of stacked layers of copper-oxygen planes and some show zero electrical resistance well above 100 K. By understanding the mechanisms mediating superconductivity in the copper oxides, the scientific community hopes to become able to devise novel materials that show zero resistance even at room temperature. However, a comprehensive understanding of these mechanisms has yet remained elusive. Nonetheless, superconductors are used already today in some technological applications, such as magnetic resonance imaging devices in the field of medicine. Future applications of room temperature superconductors could revolutionize the fields of electrical power storage and transmission, and enable rapid public transport by magnetically levitated trains.  

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Overview of the beamline ID32 at the ESRF. Credits: P. Jayet

A team of researchers led by Dr. Matthias Hepting and Dr. Wei-Sheng Lee, from Stanford University (USA), came to the ESRF to investigate so-called electron-doped copper oxides, which are part of the family of high-temperature copper oxide superconductors. The researchers were in the quest to find evidence for the acoustic plasmon, a collective electronic excitation, hypothesized by some theories to play a substantial role in mediating high-temperature superconductivity.

With this goal, the team, which includes scientists from the Politecnico di Milano (Italy), Binghamton University (USA), the SLAC National Accelerator Laboratory (USA), the University of Maryland (USA) and the ESRF, performed a set of Resonant Inelastic X-ray scattering (RIXS) experiments at ID32. “It was probably the only beamline where we had a chance to find the plasmon. The special capabilities of high energy resolution, full polarization analysis, and continuous rotation of the RIXS spectrometer allowed us to probe the relevant energy-momentum region, which was inaccessible elsewhere. ”, explains Hepting, who has moved to the Max-Planck-Institute for Solid State Research in Stuttgart (Germany) in August 2018.

The results of the experiments ticked all the boxes indicating the presence of the acoustic plasmon in layered copper oxide superconductors. First of all, the scientists used the polarisation analyser of ID32 to separate magnetic and charge contributions in the RIXS spectra and revealed a pure charge character of the relevant spectral features, as expected for plasmons. Secondly, the ID32 instrument not only allowed RIXS measurements probing excitations within the copper-oxygen planes, like most previous RIXS studies, but also along the out-of-plane direction. Interestingly, the characteristics of the plasmon excitation along the out-of-plane direction suggested that interplane Coulomb interaction gives rise to coherent out-of-plane charge dynamics and its three-dimensional propagation is consistent with the long-sought acoustic plasmon.

“As a key result we have demonstrated that three-dimensional plasmon excitations and coherent out-of-plane charge dynamics exist in the normal state electron-doped cuprates, which was not evident in previous optical studies with a limited range of accessible momenta.” said Hepting.

Furthermore, the assignment of the excitations observed in the RIXS spectra to the acoustic plasmon was supported by theory calculations conducted for the study. 

“With momentum resolved RIXS, the acoustic plasmon bands were explored for the first time. This could shed new light on long-standing theories that connect acoustic plasmons to enhanced superconducting transition temperatures, or perhaps more importantly, to suggestions that the Coulomb energy stored between copper-oxygen planes plays a part in the energy savings associated with the superconducting transition.” Hepting noted. “This finding tells us that the layered structure itself affects how the electrons behave in a very profound way that should not be overlooked.” added Wei-Sheng Lee.


Hepting, M., et al, Nature, 31 October 2018.


Top image: An illustration depicts the repulsive energy (yellow flashes) generated by electrons in one layer of a cuprate material repelling electrons in the next layer. Theorists think this energy could play a critical role in creating the superconducting state, leading electrons to form a distinctive form of “sound wave” that could boost superconducting temperatures. Scientists have now observed and measured those sound waves for the first time. Credits: Greg Stewart/SLAC National Accelerator Laboratory.