Superconductivity close to the Mott state: From condensed-matter systems to superfluidity in optical lattices

Since the discovery of high-temperature superconductivity in 1986 by Bednorz and Mueller, great efforts have been devoted to finding out how and why it works. From the d-wave symmetry of the order parameter, the importance of antiferromagnetic fluctuations, and the presence of a mysterious pseudogap phase close to the Mott state, one can conclude that high-Tc superconductors are clearly distinguishable from the well-understood BCS superconductors. The d-wave superconducting state can be understood through a Gutzwiller-type projected BCS wave-function. In this review article, we revisit the Hubbard model at half-filling and focus on the emergence of exotic superconductivity with d-wave symmetry in the vicinity of the Mott state, starting from ladder systems and then studying the dimensional crossovers to higher dimensions. This allows to confirm that short-range antiferromagnetic fluctuations can mediate superconductivity with d-wave symmetry. Ladders are also nice prototype systems allowing to demonstrate the truncation of the Fermi surface and the emergence of a Resonating Valence Bond (RVB) state with preformed pairs in the vicinity of the Mott state. In two dimensions, a similar scenario emerges from renormalization group arguments. We also discuss theoretical predictions for the d-wave superconducting phase as well as the pseudogap phase, and address the crossover to the overdoped regime. Finally, cold atomic systems with tunable parameters also provide a complementary insight into this outstanding problem.Comment: 98 pages and 18 figures; Final version (references added and misprints corrected

Two-photon excitation and photoconversion of EosFP in dual-color 4Pi confocal microscopy

Recent years have witnessed enormous advances in fluorescence microscopy instrumentation and fluorescent marker development. 4Pi confocal microscopy with two-photon excitation features excellent optical sectioning in the axial direction, with a resolution in the 100 nm range. Here we apply this technique to cellular imaging with EosFP, a photoactivatable autofluorescent protein whose fluorescence emission wavelength can be switched from green (516 nm) to red (581 nm) by irradiation with 400-nm light. We have measured the two-photon excitation spectra and cross sections of the green and the red species as well as the spectral dependence of two-photon conversion. The data reveal that two-photon excitation and photoactivation of the green form of EosFP can be selectively performed by choosing the proper wavelengths. Optical highlighting of small subcellular compartments was shown on HeLa cells expressing EosFP fused to a mitochondrial targeting signal. After three-dimensionally confined two-photon conversion of EosFP within the mitochondrial networks of the cells, the converted regions could be resolved in a 3D reconstruction from a dual-color 4Pi image stack