Yves Noat

Local tunneling spectroscopy of the electron-doped cuprate superconductor Sm1.85Ce0.15CuO4

We present local tunneling spectroscopy in the optimally electron-doped cuprate Sm2-xCexCuO4 x=0.15. A clear signature of the superconducting gap is observed with an amplitude ranging from place to place and from sample to sample (Delta~3.5-6meV). Another spectroscopic feature is simultaneously observed at high energy above \pm 50meV. Its energy scale and temperature evolution is found to be compatible with previous photoemission and optical experiments. If interpreted as the signature of antiferromagnetic order in the samples, these results could suggest the coexistence on the local scale of antiferromagnetism and superconductivity on the electron-doped side of cuprate superconductors.

Pair–pair interactions as a mechanism forhigh-Tc superconductivity

The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above Tc, pre-existing but fluctuating Cooper pairs give rise to the unconventional pseudogap (PG) state, well-characterized by experiment. At the critical temperature, the pair–pair interaction induces a Bose-like condensation of these preformed pairs leading to the superconducting (SC) state. Below Tc, both the condensation energy and the pair– pair interaction β are proportional to the condensate density Noc(T), while the usual Fermi-level spectral gap Dp is independent of temperature. The new order parameter b (T), can be followed as a function of temperature, carrier concentration and disorder—i.e. the phase diagrams. The complexity of the cuprates, revealed by the large number of parameters, is a consequence of the coupling of quasiparticles to Cooper-pair excitations. The latter interpretation is strongly supported by the observed quasiparticle spectral function

From Cooper-pair glass to unconventional superconductivity: a unified approach to cuprates and pnictides

Abstract We report a microscopic model wherein the unconventional superconductivity emerges from an incoherent ‘Cooper-pair glass’ state. Driven by the pair-pair interaction, a new type of quasi-Bose phase transition is at work. The interaction leads to the unconventional coupling of the quasiparticles to excited pair states, or ‘super-quasiparticles’, with a non-retarded energy-dependent gap. The model describes quantitatively the quasiparticle excitation spectra of both cuprates and pnictides, including the universal ‘peak-dip-hump’ signatures, and for the pseudogap phase above T c . The results show that instantaneous pair-pair interactions account for the SC condensation without a collective mode.

Cooper pairs without “glue” in high-Tc superconductors: A universal phase diagram

The phase diagram of high- T c cuprates can be understood, without resorting to a boson mode, in terms of a single energy scale , the antiferromagnetic (AF) exchange energy at the metal-insulator transition. As a result, holes form a new quantum object, the “pairon”, i.e. , a pair of holes localized within their local antiferromagnetic environment on the scale of the finite AF correlation length, . In the incoherent pseudogap phase, above T c or within the vortex core, the pairon binding energies are distributed statistically, forming a “Cooper-pair glass”. Contrary to conventional superconductors it is the mutual pair-pair interaction that is responsable for their condensation. We give a natural explanation for the ergodic rigidity of the excitation gap, the latter being constant with respect to a perturbation such as temperature or magnetic field, and determined only by the carrier concentration p and J .