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Explaining the mechanism of superconductivity in the high-$T_c$ cuprates requires an understanding of what causes electrons to form Cooper pairs. Pairing can be mediated by phonons, the screened Coulomb force, spin or charge fluctuations, excitons, or by a combination of these. An excitonic pairing mechanism has been postulated, but experimental evidence for coupling between conduction electrons and excitons in the cuprates is sporadic. Here we use resonant inelastic x-ray scattering (RIXS) to monitor the temperature dependence of the $underline{d}d$ exciton spectrum of Bi$_2$Sr$_2$CaCu$_2$O$_{8-x}$ (Bi-2212) crystals with different charge carrier concentrations. We observe a significant change of the $underline{d}d$ exciton spectra when the materials pass from the normal state into the superconductor state. From theoretical modeling, we determine the strength of the coupling between the electrons and the excitons. Our observations show that the coupling to excitons can be strong enough to play an important role in stabilizing the superconducting state.
We show that the resistivity in each phase of the High-Tc cuprates is a special case of a general expression derived from the Kubo formula. We obtain, in particular, the T-linear behavior in the strange metal (SM) and upper pseudogap (PG) phases, the
Angle-resolved photoemission spectroscopy with low-energy tunable photons along the nodal direction of oxygen isotope substituted Bi2Sr2CaCu2O8+delta reveals a distinct oxygen isotope shift near the electron-boson coupling kink in the electronic disp
We have studied the doping dependence of the in-plane and out-of-plane superfluid density, rho^s(0), of two monolayer high-Tc superconductors, HgBa_2CuO_{4+delta} and La_{2-x}Sr_xCuO_4, using the low frequency ac-susceptibility and the muon spin rela
Large pulsed magnetic fields up to 60 Tesla are used to suppress the contribution of superconducting fluctuations (SCF) to the ab-plane conductivity above Tc in a series of YBa2Cu3O6+x single crystals. The fluctuation conductivity is found to vanish
The mechanism of high temperature superconductivity is not resolved for so long because the normal state of cuprates is not yet understood. Here we show that the normal state pseudo-gap exhibits an unexpected non-monotonic temperature dependence, whi