The Signal
Terahertz Light Separates Electron and Hole States in Vortex Cores
A Rutgers and Estonian team used terahertz Faraday spectroscopy to distinguish electron-like and hole-like quantum states in iron telluride–selenide superconductors. The measurements reveal how those vortex-core states change with selenium
Quantum.world Team · Written with AI · fact-checked ·
“By exploiting polarization-selective optical transitions, we directly resolve the helicity and band origin of vortex-core quasiparticles.”
T. Rõõm, A. Glezer Moshe, R. Nagarajan, U. Nagel, Hee Taek Yi, Seongshik Oh and G. Blumberg, Physical Review Letters
Terahertz light can distinguish two kinds of quantum states trapped inside a superconductor’s magnetic vortices, according to a study published in Physical Review Letters on October 23, 2026. The researchers measured iron telluride–selenide films, separating responses associated with electron-like and hole-like particles by the circular polarization each absorbs. Rutgers University described the work on October 28.
Light reads the vortex states’ handedness
Vortices form when magnetic fields penetrate many superconductors. Within each narrow core, where superconductivity is weakened, quasiparticles occupy discrete energy levels known as Caroli–de Gennes–Matricon states. The levels can be difficult to resolve in materials where they sit too close together; the team chose iron-based films with short coherence lengths, which help keep the states distinct.
The team used terahertz Faraday magneto-optical spectroscopy, sending light through a film and measuring how its polarization changed. The paper describes opposite circular-polarization responses for the two bands: electron-like carriers respond to left-handed light and hole-like carriers to right-handed light. “By looking for these transitions with light of different handedness, you can separate them,” Girsh Blumberg, a distinguished professor in Rutgers’ Department of Physics and Astronomy and a study co-author, said in the university’s account.
Two films, nearly matching transition temperatures
The measurements used two 40-nanometer-thick epitaxial films, FeTe1−xSex, with selenium compositions x = 0.15 and x = 0.38. Their fitted superconducting transition temperatures were 12.8 kelvin and 12.9 kelvin, respectively. Despite that close match, the band contributions inside their vortex cores differed substantially.
For the x = 0.15 film, the fitted electron-like carrier density in the vortex core was 0.26 × 1027 per cubic meter, compared with 0.07 × 1027 for hole-like carriers—a ratio of about 3.7 to 1. In the x = 0.38 film, the corresponding values were 0.17 × 1027 and 0.19 × 1027, close to an even split. Those are parameters inferred by fitting the optical response, not direct counts of individual particles.
Resonances expose different lifetimes
The fitted resonance frequencies and lifetimes also varied by band and composition. In the x = 0.15 film, the electron-like and hole-like resonances were 0.36 and 0.99 terahertz, with fitted lifetimes of 0.41 and 0.71 picoseconds. For x = 0.38, the resonances were 0.89 and 0.72 terahertz, while the lifetimes were 0.80 and 0.61 picoseconds.
At 15 tesla, the x = 0.38 film produced a Faraday rotation near 75 milliradians around 0.3 terahertz, the paper reports. The researchers used the polarization-resolved response to estimate band-specific vortex masses, coherence lengths and upper critical fields as well as quasiparticle lifetimes. The measurements make the vortex’s internal dynamics accessible through light rather than only its static local electronic structure.
The analysis leaves band coupling open
The composition comparison shows that nearly equal superconducting transition temperatures do not guarantee similar vortex-core properties. Changing the tellurium-to-selenium ratio shifts the fitted balance between electron-like and hole-like carriers and changes their characteristic transition frequencies and lifetimes. That gives researchers a way to examine how composition reshapes the separate band contributions inside a superconducting vortex.
The authors also set a boundary on their interpretation: their analysis treats the two condensates as noninteracting. They note that coupling between condensates, which the model leaves out, could hybridize Caroli–de Gennes–Matricon states from different bands. The reported band-resolved values therefore come from a model that does not include that interband mixing.