High-temperature superconductivity: Exploring quadratic electron-phonon coupling

A new study published in Physical Review Letters (PRL) explores the potential of quadratic electron-phonon coupling to enhance superconductivity through the formation of quantum bipolarons.

Electron-phonon coupling is the interaction between electrons and vibrations in a lattice called phonons. This interaction is crucial for superconductivity (resistance-free electrical conductance) in certain materials as it facilitates the formation of Cooper pairs.

Cooper pairs are pairs of electrons bound together via attractive interactions. When these Cooper pairs condense into a coherent state, we get superconducting properties.

Electron-phonon coupling can be categorized based on its dependence on phonon displacement, which means how much the lattice vibrates. The most commonly considered case is when electron density linearly couples to lattice displacements, causing a lattice distortion to surround every electron.

The researchers wanted to study if superconductivity can be enhanced for materials exhibiting quadratic coupling, which is when the interaction energy is proportional to the square of the phonon displacement.

Phys.org spoke to the co-authors of the study, Zhaoyu Han, a Ph.D. candidate at Stanford University and Dr. Pavel Volkov, Assistant Professor at the Department of Physics, University of Connecticut.

Speaking of his motivation behind pursuing this research, Han said, "It has been one of my dreams to identify and propose new mechanisms that may help achieve high-temperature superconductivity."

Dr. Volkov said, "The superconductivity of doped strontium titanate was discovered more than 50 years ago, however, its mechanism remains an open question, with conventional mechanisms being unlikely. This is why I started looking into alternative electron-phonon coupling mechanisms."

Linear coupling and its challenges for superconductivity

As mentioned earlier, coupling can be categorized as linear or quadratic coupling.

Linear coupling refers to the scenario when the coupling is proportional to the displacement of the phonons. On the other hand, quadratic coupling depends on the square of phonon displacement.

They can be identified by studying the symmetry of the material, experimental observations, and theoretical frameworks. Their implications for superconductivity, however, appear quite different.

Linear coupling, seen in most superconducting materials, is extensively studied because of its prevalence in many materials and has a theoretical framework.

However, conventional superconductors with linear electron-phonon coupling face limitations. These materials have a low critical temperature, which is the temperature below which the material can exhibit superconductivity.

Han explained, "The critical temperatures for these superconductors are usually below 30 Kelvin or -243.15 degrees Celsius. This is partly because the Cooper pair binding energy and the kinetic energy are exponentially suppressed in the weak and strong coupling regimes, respectively."

In weak coupling, the electron-phonon interactions are weak due to the low binding energy. In strong coupling, the interactions are stronger, leading to a higher effective mass of the Cooper pairs, suppressing superconductivity.

However, the suppression hinders any efforts to improve the critical temperatures in such materials by just increasing the coupling strength, encouraging the researchers to explore materials with quadratic electron-phonon coupling, which are not as well understood.