Imaging a terahertz superfluid plasmon in a two-dimensional superconductor.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• The superconducting gap is crucial for dissipationless transport in superconductors.
• Terahertz (THz) spectroscopy reveals collective superfluid response perpendicular to cuprate layers.
• In-plane superfluid response is obscured by dissipation at higher energies.

Purpose of the Study:

• To provide spectroscopic evidence of a below-gap, two-dimensional (2D) superfluid plasmon in cuprate superconductors.
• To spatially resolve the THz electrodynamics of this 2D superfluid plasmon.
• To investigate the momentum- and frequency-dependent superconducting transition in 2D.

Main Methods:

• Utilized terahertz (THz) spectroscopy with a spintronic THz emitter.
• Placed the superconductor (Bi2Sr2CaCu2O8+x) in the near field of the emitter.
• Mapped the geometric anisotropy and dispersion of the observed resonance.

Main Results:

• Spectroscopic evidence of a below-gap, 2D superfluid plasmon was found in few-layer Bi2Sr2CaCu2O8+x.
• This plasmon resonance was absent in bulk samples and only observed in the superconducting phase.
• The plasmonic nature was confirmed by mapping its geometric anisotropy and dispersion.

Conclusions:

• The study reveals a distinct below-gap, 2D superfluid plasmon in cuprate superconductors.
• This discovery allows direct observation of the 2D superconducting transition.
• The findings provide crucial insights into collective charge oscillations within the CuO2 planes.