Liang, Will - Johns Hopkins University

Investigating Amplitude Mode in Lanthanum Cuprates Using THz 2D Coherent Spectroscopy

Signatures of the amplitude mode has been reported in various conventional and unconventional superconductors. Many of these recent studies utilize optical techniques to probe the non-linear response of these systems, but most of them find it challenging to distinguish the response of the amplitude mode from that of quasi-particle excitations and other processes. Recently, THz 2D coherent spectroscopy has been shown to be uniquely sensitive to the amplitude mode in an s-wave superconductor NbN. Our study adapts this method to probe the presence of amplitude mode in electron-doped and hole-doped lanthanum cuprates. Utilizing controlled high-intensity broad-band and narrow-band THz pulses, we find the coherent 2D NL response from lanthanum class cuprates to be significantly different from that of conventional superconductors and attribute this mainly to the over-damping of the amplitude mode and the anisotropic d-wave superconducting gap.