The Mysterious Pseudogap in High Temperature Superconductivity, an Infrared View

TL;DR

Infrared spectroscopy reveals the pseudogap's characteristics and its relation to other phenomena in high-temperature superconductors, showing a universal scattering mode linked to superconducting transition temperature.

Contribution

This review highlights the role of infrared spectroscopy in uncovering the pseudogap and scattering mechanisms, establishing their universality across high-Tc cuprates.

Findings

Pseudogap appears as a depression in c-axis conductivity and persists near room temperature.

A scattering mode at 300 cm-1 correlates with the superconducting transition temperature.

Infrared evidence shows the scattering mode's presence across various high-Tc cuprates.

Abstract

We review the contribution of infrared spectroscopy to the study of the pseudogap in high temperature superconductors. The pseudogap appears as a depression of the frequency dependent conductivity in the c-axis direction and seems to be related to a real gap in the density of states. It can also be seen in the Knight shift, photoemission and tunneling experiments. In underdoped samples it appears near room temperature and does not close with temperature. Another related phenomenon that has been studied by infrared is the depression in the ab-plane scattering rate. Two separate effects can be discerned. At high temperatures there is broad depression of scattering below 1000 cm-1 which may be related to the gap in the density of states. At a lower temperature a sharper structure is seen, which appears to be associated with scattering from a mode at 300 cm-1, and which governs the carrier life time at low temperatures. This mode shows up in a number of other experiments, as a kink in ARPES dispersion, and a resonance at 41 meV in magnetic neutron scattering. Since the infrared technique can be used on a wide range of samples it has provided evidence that the scattering mode is present in all high temperature cuprates and that its frequency in optimally doped materials scales with the superconducting transition temperature. The lanthanum and neodymium based cuprates do not follow this scaling and appear to have depressed transition temperatures.