Physicists Capture First Direct Evidence of a Light-Induced Floquet Topological State in Tin Telluride

Overview

Physicists have captured the first direct experimental evidence of a Floquet topological state, a light-induced phase of matter that until now existed only in theory and simulation, according to Phys.org. The team, led by Professor Claude Monney of the University of Fribourg in Switzerland, used ultrafast laser pulses to briefly convert the semiconductor tin telluride (SnTe) into a topological conductor, publishing the result in Nature Physics.

What We Know

Topological insulators are materials that conduct electricity along their surface while remaining insulating throughout their bulk, according to Phys.org. Monney told the outlet, “The key finding is that we showed that a semiconductor can be changed into a metal just through the interaction with light,” adding that “the chemical composition and atomic structure of the materials do not change.”

To produce and observe the effect, the researchers cooled SnTe crystals to 30 K and fired a short pump laser pulse tuned close to the material’s bandgap energy at the sample, according to Phys.org. They tracked the result with time-resolved angle-resolved photoemission spectroscopy, or TR-ARPES, described by the outlet as “a technique that images a material’s electronic structure with extreme time resolution.” Monney explained the method: “In the so-called pump-probe scheme, we can follow how the band structure evolves as a function of the delay time of the light pump pulses,” and “we could see a change in the dispersion of the conduction band when the pump light pulses were shining on the solid.”

That change took the form of a Dirac cone — the signature of a topological surface state — appearing within SnTe’s bandgap while the pump laser was active, then vanishing about 100 femtoseconds after the pulse passed, according to Phys.org. “This is tightly related to the topological state of SnTe,” Monney said. “We managed to get the electronic structure of SnTe to mimic this phase under the influence of light.” He added: “I use the word ‘mimic’ because we could also prove that the ion positions in this transient phase do not change… However, Floquet engineering could achieve the same impact on the electrons.”

The underlying paper, titled “Floquet topological state induced by light-driven band inversion in SnTe,” reports “a concomitant renormalization of the band dispersions that reveals the generation of Floquet states connecting to the topological state,” according to the study in Nature Physics and its arXiv preprint. The effect was strongest specifically when the pump laser’s photon energy matched the bandgap at the Γ point of the crystal, ruling out a simpler laser-assisted photoemission artifact, according to the Nature Physics paper. Theoretical calculations carried out by Professor Jan Minár’s group at the University of West Bohemia in Pilsen, with contributions from researcher Akki Pulkkinen, matched the experimental observations, supporting a genuine light-induced band inversion rather than a measurement artifact, according to Phys.org.

Because the topological surface state is tied to band symmetry rather than to surface chemistry or defects, it is difficult to disrupt — a property Phys.org notes is why topological insulators have long drawn interest for spintronics and quantum-computing applications.

What We Don’t Know

The Floquet topological state currently persists for only about 100 femtoseconds, and Phys.org reports that sustaining the effect for longer without heating the material remains unresolved. Monney said he hopes to identify other materials that could be converted into Floquet topological insulators by the same light-driven mechanism, telling the outlet: “Furthermore, it would be nice to find other similar materials that could also be changed into a Floquet topological insulator by the same mechanism. This would confirm that we understood the right mechanism and that it can be generalized to a whole class of materials.” Whether the technique can be extended beyond cryogenic conditions, or scaled toward practical spintronic or quantum-computing devices, remains an open question the paper does not resolve.