Purely Electronic Interactions Possibly Cause High-Temperature Superconductivity

Determining the relationship between cause and effect proves to be challenging when multiple processes occur at the same time. This condition is also applicable for cuprates—a category of high-temperature superconductors.

Cuprates are copper-oxygen compounds discovered almost 35 years ago. They have the ability to conduct electricity without any resistance under specific conditions.

It is essential to modify them chemically (“doped”) with extra atoms that contribute electrons or holes (electron vacancies) into the copper-oxide layers and cool them to temperatures less than 100 K (−280 °F)—considerably warmer than those required for traditional superconductors.

However, one of the most important unresolved questions in condensed matter physics is precisely how electrons surpass their mutual repulsion and combine to flow freely in these materials. High-temperature superconductivity (HTS) is one of several phenomena that arise from strong interactions between electrons, making it a challenge to identify where it originates.

Physicists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, who are investigating a familiar cuprate that consists of layers made of bismuth oxide, strontium oxide, calcium, and copper oxide (BSCCO), decided to pay attention to the less complex “overdoped” side by doping the material to an extent that superconductivity disappears over time.

According to their study, published recently in Nature Communications, this technique has helped them discover that HTS is possibly caused by purely electronic interactions.

“Superconductivity in cuprates usually coexists with periodic arrangements of electric charge or spin and many other phenomena that can either compete with or aid superconductivity, complicating the picture,” explained Tonica Valla, the first author of the study, who is a physicist in the Electron Spectroscopy Group of Brookhaven Lab’s Condensed Matter Physics and Materials Science Division.

But these phenomena weaken or completely vanish with overdoping, leaving nothing but superconductivity. Thus, this is the perfect region to study the origin of superconductivity. Our experiments have uncovered an interaction between electrons in BSCCO that correlates one to one with superconductivity. Superconductivity emerges exactly when this interaction first appears and becomes stronger as the interaction strengthens.

Tonica Valla, Physicist, Electron Spectroscopy Group, Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory

Overdoping of cuprate samples above the point where superconductivity disappears has been achieved only very recently. Earlier, the concentration of oxygen (the dopant material) in a bulk crystal of the material would be increased by annealing (heating) the material in high-pressure oxygen gas.

The new technique—first demonstrated by Valla and other Brookhaven researchers nearly a year ago at OASIS, a new on-site instrument for preparing and characterizing samples—involves annealing cleaved samples by using ozone in the place of oxygen. Cleaving is the process in which the crystal is broken in a vacuum to form clean, perfectly flat surfaces.

The oxidation power of ozone, or its ability to accept electrons, is much stronger than that of molecular oxygen. This means we can bring more oxygen into the crystal to create more holes in the copper-oxide planes, where superconductivity occurs. At OASIS, we can overdope surface layers of the material all the way to the nonsuperconducting region and study the resulting electronic excitations.

Ilya Drozdov, Study Coauthor and Physicist, Oxide Molecular Beam Epitaxy Group, Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory

The electronic structure of these films is examined by OASIS by using an OMBE system for developing oxide thin films in combination with angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging–scanning tunneling microscopy (SI-STM) instruments.

Using this instrument, it is possible to grow and examine materials using the same linked ultrahigh vacuum system to prevent contamination and oxidation by water, carbon dioxide, and other molecules in the atmosphere. Because SI-STM and ARPES are highly surface-sensitive methods, very clean surfaces are crucial to achieving precise measurements.

Bulk BSCCO crystals for this study were grown by Genda Gu, a study co-author and a physicist in the division’s Neutron Scattering Group. Drozdov increased the doping up to the point where superconductivity was totally lost by annealing the cleaved crystals in ozone within the OMBE chamber at OASIS.

Subsequently, the same sample was annealed in vacuum to slowly reduce the doping and raise the transition temperature at which superconductivity appears. The electronic structure of BSCCO was analyzed over this doping-temperature phase diagram by Valla using ARPES.

“ARPES gives you the most direct picture of the electronic structure of any material,” stated Valla. “Light excites electrons from a sample, and by measuring their energy and the angle at which they escape, you can recreate the energy and momentum of the electrons while they were still in the crystal.”

While quantifying the energy-versus-momentum relationship, Valla identified an anomaly in the electronic structure that relies on the superconducting transition temperature. The anomaly is more prominent and transfers to higher energies with an increase in this temperature and with stronger superconductivity. However, it vanishes outside the superconducting state.

Based on this information, Valla was aware that electron-phonon coupling could not be the interaction that generates the electron pairs needed for superconductivity as proposed for traditional superconductors. According to this theory, phonons (that is, vibrations of atoms in the crystal lattice) act as an attractive force for otherwise repulsive electrons by exchanging energy and momentum.

Our result allowed us to rule out electron-phonon coupling because atoms in the lattice can vibrate and electrons can interact with those vibrations, regardless of whether the material is superconducting or not. If phonons were involved, we would expect to see the kink in both the superconducting and normal state, and the kink would not be changing with doping.

Tonica Valla, Physicist, Electron Spectroscopy Group, Condensed Matter Physics and Materials Science Division, Brookhaven National Laboratory

The team believes that something similar to electron-phonon coupling is going on in this case, but instead of phonons, another excitation gets exchanged between electrons. It appears that electrons are interacting through spin fluctuations, which are related to electrons themselves. Spin fluctuations are changes in electron spin or the way that electrons point either up or down as tiny magnets.

Furthermore, the researchers discovered that the anomaly’s energy is less when compared to the characteristic energy at which a sharp peak (resonance) appears in the spin fluctuation spectrum. From this discovery, it is clear that the onset of spin fluctuations (rather than the resonance peak) causes the observed anomaly and could be the “glue” that combines the electrons into the pairs needed for HTS.

As a next step, the researchers intend to gather more evidence to show that spin fluctuations are associated with superconductivity by achieving SI-STM measurements. They will also perform similar experiments on another well-known cuprate, lanthanum strontium copper oxide (LSCO).

“For the first time, we are seeing something that strongly correlates with superconductivity,” stated Valla. “After all these years, we now have a better grasp of what may be causing superconductivity in not only BSCCO but also other cuprates.”

Source: https://www.bnl.gov/world/