Condensed Matter Seminar: Alex von Hoegen, OSU Dept. of Physics
Two-dimensional superfluid plasmons as a probe of finite-momentum phase coherence
Event Details:
- Date: Monday, April 20, 2026
- Time: 10:00 - 11:00 AM
- Location: 1080 Physics Research Building
Abstract
The collective electrodynamics of a superconductor is shaped not only by pairing, but also by dimensionality, Coulomb screening, and electromagnetic boundary conditions. In layered cuprates, these ingredients can qualitatively reorganize the low-energy, terahertz (THz) spectrum, giving rise to collective modes that are absent in the bulk. Accessing such modes experimentally, however, requires probing the superconducting response at finite momentum, beyond the reach of conventional far-field THz spectroscopy.
In this talk, I will present a THz near-field approach that provides direct access to finite-momentum superconducting electrodynamics in few-layer Bi2Sr2CaCu2O8+x. By placing the flake in the near field of a spintronic THz emitter, we launch and detect a deeply sub-diffractive in-plane collective mode. Our experiment reveals that this mode, is a below-gap two-dimensional superfluid plasmon that emerges only in the superconducting state and is absent in bulk BSCCO. Spatial imaging directly resolves its confined wave character, while polarization- and geometry-dependent measurements establish its plasmonic dispersion.
A central result is that this plasmon offers a direct view of the superconducting transition in two dimensions, which is akin to a momentum-dependent dynamical BKT-like transition: finite-momentum phase distortions of the condensate melt through vortex–antivortex unbinding before the long-wavelength limit reaches Tc.