Seminars Archive

How superconductivity modifies the high-energy optical properties in Bi₂Sr₂Ca_0.92Y_0.08Cu₂O_8+I' (towards a time-resolved optical spectroscopy)

Abstract
In the BCS theory of conventional superconductors, the superconducting phase transition is accompanied by a modification of the electronic density of states over a frequency range on the order of the superconducting gap (I?SC) without significantly affecting the physical properties at higher energies. However, in strongly-correlated systems the electronic properties at the Fermi energy are intertwined with those at high energy scales. In particular, one of the pivotal challenges in the field of high-temperature superconductivity is to understand how the condensate formation modifies the electronic structure. Here we use a novel time- and spectrally-resolved optical spectroscopy to show that the photoinduced suppression of the superconducting gap in Bi₂Sr₂Ca_0.92Y_0.08Cu₂O_8+I' induces a modification of the interband electronic transitions, involving mixed Cu-O states at binding energies as high as 2 eV. This result answers the major questions whether and how the measured spectral weight variation is related to particular electronic states participating in the condensate formation. By following the spectral weight variation of these features from below to above the optimal hole concentration required to attain the maximum superconducting transition temperature (Tc), we observe a crossover from a superconductivity-induced gain to a BCS-like loss of the carrier kinetic energy.
(Rif. F. Parmigiani)