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Dirk van der Marel - Département de Physique de la matière condensée, Université de Genève

Optical determination of the relation between the electron-boson coupling function and the critical temperature in high Tc cuprates


Pairing of electrons can occur through the virtual exchange of bosons. We deduced from our optical spectra the bosonic glue-spectrum as a function of energy using the Kubo¬Holstein-Allen formalism.
The main novelty is that we prove that aforementioned formalism enables us to make detailed predictions : Firstly, if for a given sample this relation is determined from the optical data taken at room temperature, we can predict exactly what the optical spectra will look like at all other temperatures in the normal state. Secondly, knowing the glue-spectrum we are able to predict Tc and compare it to the actual value. Our predictions for 10 crystals from strongly underdoped to highly overdoped are ’only’ two times higher than the actual Tc’s, which is very good considering that, among other things, all of the measured ‘glue’ was taken to mediate d-wave pairing, which is certainly too optimistic.
A related hotly debated issue is, whether aforementioned bosons are phonons or spin-fluctuations. While we obtain indications in our data for both types, we also observe in the overdoped samples that the electron-phonon coupling is far too weak to explain Tc, while Tc is still very high. We therefore can eliminate phonon mediated pairing as "the mechanism of high Tc" in favor of pairing mediated by spin-fluctuations.

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