Chirality-Resolved Spectroscopy Probes Superconducting Vortex Cores
Researchers used terahertz light to directly separate and measure two distinct quasiparticle groups within superconducting vortices, a breakthrough for engineering quantum materials.
Scientists have used invisible terahertz light to distinguish two groups of particles trapped inside superconducting vortices, revealing how material composition affects their behavior. The new chirality-resolved spectroscopy technique directly separates contributions from electron-like and hole-like quasiparticles, a capability earlier methods lacked. This advance enables the examination of internal properties in superconductors with narrow vortex cores, a critical step for developing materials for strong magnetic field applications like MRI scanners and particle accelerators.
Experimental Breakthrough in Vortex Core Analysis
The study employed atomically precise 40-nanometer-thick superconducting films of FeTe₁₋ₓSeₓ, cooled to 3.5 Kelvin, to enable direct measurement of quasiparticle properties within narrow vortex cores. The films were deposited via molecular beam epitaxy on aluminum oxide substrates, with both samples becoming superconducting at approximately 13 Kelvin (minus 261 degrees Celsius). Researchers used a mercury-arc lamp, a Martin-Puplett interferometer, and a superconducting magnet capable of reaching 15 Tesla. Unlike scanning tunneling spectroscopy, which measures static local density of states, this technique measures dynamics, providing direct, chirality-resolved data on quasiparticle lifetimes, resonance frequencies, and coherence lengths.
Chirality-Dependent Response of Particle Groups
Two particle groups within vortices responded to opposite handedness of circularly polarized terahertz light, allowing their individual properties to be distinguished based on angular momentum transfer. Left-handed, or counterclockwise, circularly polarized terahertz light resonates specifically with electron-like quasiparticle transitions. The method exploits the angular momentum carried by terahertz photons to discriminate between quasiparticle types, a process governed by charge-conjugation and time-reversal symmetry.
Girsh Blumberg, a distinguished professor in the Department of Physics and Astronomy at Rutgers, said, "To improve superconductors, scientists need to understand what vortices contain and how they behave."
Tunable Material Properties Through Composition
Varying selenium content in the iron-based superconductor altered the ratio of particle groups, energy requirements for transitions, and response times, demonstrating precise control over quasiparticle behavior. Changing the composition from x = 0.15 to x = 0.38 shifted the ratio of electron-like to hole-like carriers from nearly four-to-one to roughly equal numbers. This compositional tuning also changed the energy needed for particle level transitions and the speed at which the light response decayed.
The technique can be applied to other superconductors with sufficiently narrow vortex cores and provides the baseline characterization needed to identify genuine Majorana zero modes, which are sought for topological quantum computing. Blumberg added, "This study provides a new way to look inside them." The work, supported by the National Science Foundation and the European Research Council, demonstrates a new method to examine microscopic physics in superconductors with narrow vortex cores, offering potential applications for improving materials used in MRI scanners and particle accelerators.
