Light Pulses Transform Mott Insulators

Scientists from the RIKEN Cluster for Pioneering Research have used light pulses to transform Mott insulators into superconducting materials. Through numerical simulations, the researchers showed that pulse irradiation could induce unconventional superconductivity in a Mott insulator of the Hubbard model and found that this superconductivity occurred as a result of a phenomenon known as eta pairing.

Schematic of eta pairing. Courtesy of RIKEN.

The researchers further found that the superconductivity from eta pairing, which is believed to take place under nonequilibrium conditions in strongly correlated materials, was different from the superconductivity observed in the same correlated materials under equilibrium conditions, and was also different from traditional superconductivity. Also, in superconductivity from eta pairing, the nonlinear optical response was found to be essential to increase the number of η pairs and thus enhance the superconducting correlation in the photoexcited state.

The concept of eta pairing was proposed 30 years ago by the mathematical physicist Chen-Ning Yang. But because it was a purely mathematical concept, it was understood as a virtual phenomenon that did not take place in the “real world.” But when the RIKEN team used nonequilibrium dynamics to analyze the effect of pulses of light on a Mott insulator, they demonstrated that eta pairing was more than a nonphysical phenomenon. “What is interesting is that our calculations showed that this takes place based on the beautiful mathematical structure that Yang and his followers formulated so many years ago,” said researcher Tatsuya Kaneko.

The team's findings could provide an alternative mechanism for enhancing superconductivity. According to Seiji Yunoki, who led the research team, “This work provides new insights not only into the phenomenon of nonequilibrium dynamics, but also could lead to the development of new high-temperature superconductors, which could be useful in applications. What remains is to perform actual experiments with Mott insulators to verify that this process actually takes place.”

The research was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.122.077002).