Evidence of a subenergy gap in the overdoped regime of Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-\delta} thin films from THz Spectroscopy

TL;DR

This study uses THz spectroscopy to reveal a sub-energy gap in overdoped Ca-doped YBCO thin films, indicating changes in pairing symmetry and node removal as doping increases.

Contribution

It provides experimental evidence of a sub-gap in overdoped YBCO films, highlighting a potential new feature in the superconducting state related to pairing symmetry.

Findings

Evidence of a sub-gap at 1 meV in overdoped samples

Sub-gap becomes more prominent with increased doping

Supports d-wave pairing with imaginary component suggesting node removal

Abstract

We measured the terahertz (THz) complex conductivity of Ca doped YBa_{2}Cu_{3}O_{7-\delta} thin films in the frequency range of 0.1 to 3 THz (3 to 100 cm^{-1}) and at a temperature range of 20 to 300 K. The films were measured using both time domain and frequency domain THz methods. We showed evidence for the existence of a sub-gap in overdoped Y_{1-x}Ca_{x}Ba_{2}Cu_{3}O_{7-\delta} samples doped with 5% and 10% Ca. Evidence for the opening of this sub-gap appears as a sharp decrease in the spectrum of the real part of conductivity at frequencies equivalent to a gap energy of 1 meV and is more prominent with increased doping. This decrease in conductivity can be explained by using d-wave pairing symmetry with an imaginary part of is or id_{xy} which suggests node removal.