Superconducting energy gap versus pseudogap in hole-doped cuprates as revealed by infrared spectroscopy

TL;DR

This study uses infrared spectroscopy to distinguish between the superconducting energy gap and pseudogap in hole-doped cuprates, showing they are separate phenomena with different energy scales and temperature appearances.

Contribution

It provides direct infrared evidence that the pseudogap is not a precursor to superconductivity and challenges previous assumptions about the clean limit in cuprate superconductors.

Findings

Superconducting gap is distinct from pseudogap.

Pseudogap appears at higher temperatures than the superconducting gap.

Infrared spectroscopy can detect the superconducting gap in cuprates.

Abstract

We present in-plane infrared reflectance measurement on two superconducting cuprates with relatively low T$_c$: a nearly optimally-doped Bi$_2$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta}$ with T$_c$=33 K and an underdoped La$_{1.89}$Sr$_{0.11}$CuO$_4$ with T$_c$=30 K. The measurement clearly reveals that the superconducting energy gap is distinct from the pseudogap. They have different energy scales and appear at different temperatures. The results suggest that the pseudogap is not a precursor to the superconducting state. The data also challenge the longstanding viewpoint that the superconductivity within the ab-plane is in the clean limit and the superconducting pairing energy gap could not be detected by in-plane infrared spectroscopy.