Bosons in high temperature superconductors: an experimental survey

TL;DR

This paper reviews various experimental techniques revealing the bosonic spectrum in high temperature superconductors, highlighting a unified picture of bosonic excitations evolving with temperature and doping, consistent with spin fluctuations and phonons.

Contribution

It provides a comprehensive survey of experimental methods and their findings, proposing a unified understanding of bosonic spectra in high temperature superconductors.

Findings

Spectroscopic techniques reveal a broad bosonic spectrum evolving with temperature.

Evidence suggests spin fluctuations dominate, with possible phonon contributions.

Spectral features include a peak at 30-60 meV and a high-frequency background.

Abstract

We review a number of experimental techniques that are beginning to reveal fine details of the bosonic spectrum \alpha^2F(\Omega) that dominates the interaction between the quasiparticles in high temperature superconductors. Angle-resolved photo emission (ARPES) shows kinks in electronic dispersion curves at characteristic energies that agree with similar structures in the optical conductivity and tunnelling spectra. Each technique has its advantages. ARPES is momentum resolved and offers independent measurements of the real and imaginary part of the contribution of the bosons to the self energy of the quasiparticles. The optical conductivity can be used on a larger variety of materials and with the use of maximum entropy techniques reveals rich details of the spectra including their evolution with temperature and doping. Scanning tunnelling spectroscopy offers spacial resolution on the unit cell level. We find that together the various spectroscopies, including recent Raman results, are pointing to a unified picture of a broad spectrum of bosonic excitations at high temperature which evolves, as the temperature is lowered into a peak in the 30 to 60 meV region and a featureless high frequency background in most of the materials studied. This behaviour is consistent with the spectrum of spin fluctuations as measured by magnetic neutron scattering. However, there is evidence for a phonon contribution to the bosonic spectrum as well.