The hedgehog pathway and ocular developmental anomalies.

Mutations in effectors of the hedgehog signaling pathway are responsible for a wide variety of ocular developmental anomalies. These range from massive malformations of the brain and ocular primordia, not always compatible with postnatal life, to subtle but damaging functional effects on specific eye components. This review will concentrate on the effects and effectors of the major vertebrate hedgehog ligand for eye and brain formation, Sonic hedgehog (SHH), in tissues that constitute the eye directly and also in those tissues that exert indirect influence on eye formation. After a brief overview of human eye development, the many roles of the SHH signaling pathway during both early and later morphogenetic processes in the brain and then eye and periocular primordia will be evoked. Some of the unique molecular biology of this pathway in vertebrates, particularly ciliary signal transduction, will also be broached within this developmental cellular context

Homogeneous superconducting state at 8.1 K under ambient pressure in the organic conductor β-(BEDT-TTF)2I3

We report the observation of the first narrow and complete superconducting transition yet obtained in β-(BEDT-TTF)2I3 at 8.1 K and ambient pressure after pressurization up to 1.5 kbar and a release of the helium gas pressure at low temperature. We show experimental evidences indicating that the metastable state giving rise to homogeneous superconductivity at ambient pressure is not stable above 250 K

Determining ethylene group disorder levels in κ\kappa-(BEDT-TTF)2_2Cu[N(CN)2_2]Br

We present a detailed structural investigation of the organic superconductor $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br at temperatures $T$ from 9 to 300 K. Anomalies in the $T$ dependence of the lattice parameters are associated with a glass-like transition previously reported at $T_g$ = 77 K. From structure refinements at 9, 100 and 300 K, the orthorhombic crystalline symmetry, space group {\it Pnma}, is established at all temperatures. Further, we extract the $T$ dependence of the occupation factor of the eclipsed conformation of the terminal ethylene groups of the BEDT-TTF molecule. At 300 K, we find 67(2) %, with an increase to 97(3) % at 9 K. We conclude that the glass-like transition is not primarily caused by configurational freezing-out of the ethylene groups

Mott transition in one dimension

I review some of the results on the Mott transition in one dimensional systems. In particular I discuss the phase diagram and critical properties of both Mott transitions at fixed filling and upon doping, as well as the dc and ac conductivity. Application of these results to organic conductors is discussed.Comment: Proceedings of the SCES96 conference (August 96), 6 pages, 6 figures, uses epsfi

Phase diagram of quarter-filled band organic salts, [EDT-TTF-CONMe2]2X, X = AsF6 and Br

An investigation of the P/T phase diagram of the quarter-filled organic conductors, [EDT-TTF-CONMe2]2X, is reported on the basis of transport and NMR studies of two members, X=AsF6 and Br of the family. The strongly insulating character of these materials in the low pressure regime has been attributed to a remarkably stable charge ordered state confirmed by 13C NMR and the only existence of 1/4 Umklapp e-e scattering favoring a charge ordering instead of the 1D Mott localization seen in (TM)2X which are quarter-filled compounds with dimerization. A non magnetic insulating phase instead of the spin density wave state is stabilized in the deconfined regime of the phase diagram. This sequence of phases observed under pressure may be considered as a generic behavior for 1/4-filled conductors with correlations

Towards a consistent picture for quasi-1D organic superconductors

The electrical resistivity of the quasi-1D organic superconductor (TMTSF)2PF6 was recently measured at low temperature from the critical pressure needed to suppress the spin-density-wave state up to a pressure where superconductivity has almost disappeared. This data revealed a direct correlation between the onset of superconductivity at Tc and the strength of a non-Fermi-liquid linear term in the normal-state resistivity, going as r(T) = r0 + AT + BT2 at low temperature, so that A goes to 0 as Tc goes to 0. Here we show that the contribution of low-frequency antiferromagnetic fluctuations to the spin-lattice relaxation rate is also correlated with this non-Fermi-liquid term AT in the resistivity. These correlations suggest that anomalous scattering and pairing have a common origin, both rooted in the low-frequency antiferromagnetic fluctuations measured by NMR. A similar situation may also prevail in the recently-discovered iron-pnictide superconductors.Comment: ISCOM'09 proceedings to be published in Physica

Insulator-Metal Transition in One Dimension Induced by Long-Range Electronic Interactions

The effects of a long range electronic potential on a one dimensional commensurate Charge Density Wave (CDW) state are investigated. Using numerical techniques it is shown that a transition to a metallic ground state is reached as the range of the electron-electron repulsion increases. In this metallic state, the optical conductivity exhibits a large Drude weight. Possible interpretations of our results are discussed.Comment: 5 pages, Revtex, minor misprints corrected and a reference to earlier work by V. Emery and C. Noguera adde

The high-Tc superconducting state of β-(BEDT-TTF)2I3 at atmospheric pressure: bulk superconductivity and metastability

The AC susceptibility study of β-(BEDT-TTF)2I3 reveals that the high-Tc metastable superconducting state which can be stabilized at atmospheric pressure after a particular pressure-temperature cycling procedure, exhibits bulk superconductivity resembling very closely that of the high-Tc state, which is stabilized above 1 kbar. Annealing experiments show that the high-Tc state remains stable at low temperature as long as the annealing temperature does not exceed 125 K