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Measurements in an electron-doped copper oxide directly link superconductivity to antiferromagnetic spin fluctuations. The result resolves an important part of the puzzle, but not the mechanism behind the highest transition temperatures.
Superconductivity allows electrical current to flow without resistance, but most superconductors require extremely low temperatures. Copper oxides, or cuprates, can remain superconducting at much higher temperatures than conventional materials, and the mechanism behind this behavior has been a central unsolved problem in physics for decades.
In conventional superconductors, electrons pair through vibrations of the crystal lattice. In cuprates, researchers have long suspected that magnetic fluctuations in electron spins could play an analogous role, but a direct connection between the strength of that interaction and the temperature at which superconductivity appears has been missing.
An international team led from the University of Bristol now reports such a connection in the electron-doped cuprate lanthanum cerium copper oxide, or LCCO.
The researchers made thin LCCO films in which the amount of cerium changed gradually along the sample. That allowed them to examine many doping levels within the same material without differences between separate samples dominating the comparison.
They measured electrical resistance and magnetoresistance in strong magnetic fields. The analysis allowed the team to extract how electrons were being scattered as they moved through the material.
The results point to low-energy antiferromagnetic spin fluctuations as the dominant relevant scattering mechanism. More importantly, the strength of the anisotropic, temperature-linear scattering tracked the superconducting critical temperature, Tc.
Across the studied doping range, the strength of this scattering fell by roughly an order of magnitude. Over the same range, Tc dropped from near 25 kelvin to zero.
That correlation is the team's central evidence that the same spin fluctuations scattering electrons in the normal state also provide the interaction that binds electrons together in the superconducting state.
The Nature Physics study therefore identifies low-energy antiferromagnetic spin fluctuations as the primary pairing glue for superconductivity in electron-doped cuprates such as LCCO.
The result is not a general solution to high-temperature superconductivity.
Cuprates can be made superconducting in two broad ways: by adding electrons or by creating so-called holes, meaning missing electrons. This study examines the electron-doped side. The highest critical temperatures, however, occur in hole-doped cuprates.
The researchers point out that magnetotransport in the hole-doped materials behaves fundamentally differently. Their higher Tc therefore cannot be explained simply by strengthening the coupling to the same spin fluctuations.
The work substantially narrows the problem. In electron-doped cuprates, the researchers now have experimental support for a specific mechanism and have shown how its strength follows superconductivity as doping changes.
At the same time, the finding makes the divide within the cuprate family sharper. If hole-doped materials require an additional ingredient, researchers still need to identify it before they can explain the full phenomenon and understand why some cuprates remain superconducting at much higher temperatures.
This is therefore not a new superconductor or a material that suddenly carries current without loss at room temperature. The advance is a much more direct experimental link between one important part of the superconducting mechanism and measurable transport behavior.
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