Bosons in high-temperature superconductors: an experimental survey
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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.
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