DATE2024.01.30 #Press Releases

When do superconducting orders react to light?

Researchers propose a theory of classifying materials with superconducting orders that respond linearly to light

January 30, 2024

International joint research including graduate student Raigo Nagashima and Associate Professor Naoto Tsuji of the University of Tokyo proposed a theory that explains what makes superconductors, materials that show no resistance and repel magnetic fields, change their order parameters linearly when excited by light. The theory contributes to understanding the microscopic properties of multiband superconductors and suggests possible methods for controlling superconducting orders by lasers. The results were published in the journal Physical Review Research.

Superconductors are not superconducting all the time. Materials that have the potential to become superconductors need to go through a phase transition at low temperatures where electrons arrange themselves so that they move between atoms in a specific fashion. One recently active line of research explores if and how superconducting orders can be induced and enhanced by applying light to materials.

In this study, the researchers found that light-induced linear change in the superconducting order parameters occurs when the Lifshitz invariant, a type of invariant related to symmetry operations, appears in a thermodynamical free energy. They also classified all the crystal symmetries and superconducting orders that make this phenomenon possible. Moreover, they learned that the linear reaction to light brings about the Leggett mode, a form of collective excitation that happens when the different phases of superconducting order parameters oscillate in alternate directions.

Superconductivity has many technological applications across a wide range of fields and could address the energy consumption issues of modern society. As such, basic research on the properties of different compounds and how they react to various kinds of excitation is vital for meeting sustainable development goals.

Figure: an illustration of a multiband superconductor whose superconducting orders are responding linearly to light. When a multiband superconductor is irradiated with light, the superconducting order parameters, which are indicated by spheres on the free energy potential for each band, move alternately in the circular direction. Such motion of the superconducting order parameters corresponds to a collective mode called the Leggett mode.

For more details, please read the article:   

Raigo Nagashima, Sida Tian, Rafael Haenel, Naoto Tsuji, and Dirk Manske. 2024. Classification of Lifshitz invariant in multiband superconductors: an application to Leggett modes in the linear response regime in Kagome lattice models. Physical Review Research. DOI: 10.1103/PhysRevResearch.6.013120