US researchers studying high-temperature cuprate superconductors outside the superconducting regime have used cutting-edge X-ray scattering to detect long-predicted – but never previously observed – excitations called plasmons perpendicular to the material’s atomic planes. Researchers hope the findings may help theorists to understand these highly unusual materials better, and perhaps even guide the quest for room-temperature superconductors.
The cuprate superconductors are archetypal “strongly correlated” materials, which are difficult to describe using current approximate models. “In most theories we have today you try to catch the main interaction and treat the others as a small perturbation,” explains condensed matter physicist Wei-Sheng Lee of Stanford University in California, “But for these strongly correlated materials, all the interactions are equally important.”
The materials therefore baffle theoreticians. For example, whereas the established BCS theory of superconductivity predicts that the property should disappear above about 30 K, some cuprates remain superconducting at temperatures up to 130 K. Something seems to preserve the superconducting state at relatively high temperatures, but what it is remains unclear.
I suspect acoustic plasmons may be a necessary, but not sufficient, condition for high-temperature superconductivity
Ivan Božović, Lawrence Berkeley National Laboratory
The crystal structure of cuprate superconductors comprises well-defined planes of copper and oxygen atoms, as well as various dopants. In the non-superconducting state, they show much higher electrical conductivity along the planes than perpendicular to them. Each plane effectively contains its own two-dimensional electron gas, which means that researchers studying a material’s electrical properties often consider only a single plane. When the materials become superconductors, however, they show zero resistance in all directions. Some theorists have therefore suggested that the secret to high-temperature superconductivity may lie not within the planes but in the coupling between them.
Researchers such as Vladimir Kresin of Lawrence Berkeley National Laboratory and Ivan Božović of Brookhaven National Laboratory, both in the US, have suggested the inter-layer coupling could be mediated by quasi-particles called plasmons, which are formed from collective oscillations in the materials’ electron density. These could mediate the Coulomb interaction between the conductive planes, without electrons moving from one plane to the next. Actually detecting such plasmons, however, has previously proved impossible. Some experiments have revealed plasmons within the copper-oxide planes, but not between them.
In the new research, therefore, Wei-Sheng Lee and colleagues at Stanford University and elsewhere used a technique called RIXS (resonant inelastic X-ray scattering) to excite the plasmons. The technique advances the principles of traditional X-ray scattering, exploiting the ability of modern synchrotron radiation sources to produce a beam of X-ray photons with well-defined, tunable, energy and momentum.
The researchers bombarded samples of the electron-doped cuprate LCCO (lanthanum cerium copper oxide), held above its transition temperature, with X-ray photons from the European Synchrotron Radiation Facility in Grenoble, France. They varied the energies of incident photons and their momenta perpendicular to the copper-oxide planes, and then measured the effect on the proportion of photons absorbed by the sample, and the energies, momenta and polarizations of the scattered photons.
These experimental results were consistent with models that assume the planes interacted through acoustic plasmons. There was a clear absorption resonance, for example, at photon energies that would have excited a plasmon wavelength equal to the distance between the copper-oxide planes.
Raising the transition temperature
The researchers now plan to study hole-doped superconductors, some of which have been found to have higher transition temperatures than electron-doped ones like LCCO. “For electron-doped materials, it’s hard to investigate how the plasmons change as you go to the superconducting state because the transition temperature is much lower,” explains Lee. “But for hole-doped cuprates, the transition temperature is generally higher. Therefore it should be easier for us to investigate first whether this phenomenon is universal in the cuprate family and secondly how it changes as you enter a superconducting state. Hopefully we might get a clue about how to make a superconductor with an even higher transition temperature.”
Ivan Božović is intrigued: “I suspect acoustic plasmons may be a necessary, but not sufficient, condition for high-temperature superconductivity,” he says. He notes that a number of other materials such as nickelates, cobaltates and iridates have very similar structures to the cuprates but have shown no superconductivity at any temperature. He suggests it would be interesting to repeat the experiments with these materials to look for evidence of plasmons “Let’s see what’s common and perhaps what’s different,” he says.
“This puts on the map the peculiar, layered two-dimensional nature of these materials,” says Dirk van der Marel of the University of Geneva in Switzerland. “From that you get an entirely different interaction between the electrons. The ideas that this could be the source of the superconductivity have been worked out in some detail by the Nobel prize winner Tony Leggett, and I think this gives more fuel to those.”
The research is published in Nature.
