The role of magnetism in forming the c-axis spectral peak at 400 cm-1 in high-temperature superconductors

TL;DR

This paper investigates the 400 cm-1 spectral peak in underdoped high-temperature superconductors, linking it to magnetic processes and transverse plasmons, and explores its temperature dependence and relation to spin dynamics.

Contribution

It demonstrates that the c-axis spectral peak is governed by magnetic processes rather than superconducting fluctuations, supported by a model fitting experimental data.

Findings

Peak amplitude correlates with the spin gap temperature Ts

Mode tracks the 41 meV neutron resonance amplitude

Temperature dependence linked to in-plane scattering processes

Abstract

We discuss the peak at 400 cm-1, which is seen in c-axis conductivity spectra of underdoped high temperature superconductors. The model of van der Marel and Munzar, where the peak is the result of a transverse plasmon arising from a low frequency conductivity mode between the closely spaced planes, fits our data well. Within the model we find that the temperature dependence of the peak amplitude is controlled by in-plane scattering processes. The temperature range where the mode can be seen coincides with Ts, the spin gap temperature, which is lower than T*, the pseudogap temperature. As a function of temperature, the amplitude of the mode tracks the amplitude of the 41 meV neutron resonance and the spin lattice relaxation time, suggesting to us that the mode is controlled by magnetic processes and not by superconducting fluctuations which have temperature scale much closer to Tc, the superconducting transition temperature.