Abstract
We theoretically study the low-energy electromagnetic response of Bardeen-Cooper-Schrieffer–type superconductors focusing on propagating collective modes that are observable with terahertz near-field optics. The interesting frequency and momentum range is and , where is the gap and is the coherence length. We show that it is possible to observe the superfluid plasmons, amplitude (Higgs) modes, Bardasis-Schrieffer modes, and Carlson-Goldman modes using the terahertz near-field technique, although none of these modes couple linearly to far-field radiation. Coupling of terahertz near-field radiation to the amplitude mode requires particle-hole symmetry breaking, while coupling to the Bardasis-Schrieffer mode does not and is typically stronger. For parameters appropriate to layered superconductors of current interest, the Carlson-Goldman mode appears in the near-field reflection coefficient as a weak feature in the subterahertz frequency range. In a system of two superconducting layers with nanometer-scale separation, an acoustic phase mode appears as the antisymmetric density fluctuation mode of the system. This mode produces well-defined resonance peaks in the near-field terahertz response and has strong anticrossings with the Bardasis-Schrieffer and amplitude modes, enhancing their response. In a slab consisting of many layers of quasi-two-dimensional superconductors, realized for example in samples of high- cuprate compounds, many branches of propagating Josephson plasmon modes are found to couple to the terahertz near-field radiation.
- Received 7 February 2020
- Revised 16 April 2020
- Accepted 20 May 2020
DOI:https://doi.org/10.1103/PhysRevResearch.2.023413
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
Published by the American Physical Society
