Pressure-dependent optical investigations of α\alpha-(BEDT-TTF)2_2I3_3: tuning charge order and narrow gap towards a Dirac semimetal

Infrared optical investigations of $\alpha$-(BEDT-TTF)$_2$I$_3$ have been performed in the spectral range from 80 to 8000~cm$^{-1}$ down to temperatures as low as 10~K by applying hydrostatic pressure. In the metallic state, $T > 135$~K, we observe a 50\% increase in the Drude contribution as well as the mid-infrared band due to the growing intermolecular orbital overlap with pressure up to 11~kbar. In the ordered state, $T<T_{\rm CO}$, we extract how the electronic charge per molecule varies with temperature and pressure: Transport and optical studies demonstrate that charge order and metal-insulator transition coincide and consistently yield a linear decrease of the transition temperature $T_{\rm CO}$ by $8-9$~K/kbar. The charge disproportionation $\Delta\rho$ diminishes by $0.017~e$/kbar and the optical gap $\Delta$ between the bands decreases with pressure by -47~cm$^{-1}$/kbar. In our high-pressure and low-temperature experiments, we do observe contributions from the massive charge carriers as well as from massless Dirac electrons to the low-frequency optical conductivity, however, without being able to disentangle them unambiguously.Comment: 13 pages, 17 figures, submitted to Phys. Rev.

Structure and in vivo requirement of the yeast Spt6 SH2 domain.

During transcription elongation through chromatin, the Ser2-phosphorylated C-terminal repeat domain of RNA polymerase II binds the C-terminal Src homology 2 (SH2) domain of the nucleosome re-assembly factor Spt6. This SH2 domain is unusual in its specificity to bind phosphoserine, rather than phosphotyrosine and because it is the only SH2 domain in the yeast genome. Here, we report the high-resolution crystal structure of the SH2 domain from Candida glabrata Spt6. The structure combines features from both structural subfamilies of SH2 domains, suggesting it resembles a common ancestor of all SH2 domains. Two conserved surface pockets deviate from those of canonical SH2 domains, and may explain the unusual phosphoserine specificity. Differential gene expression analysis reveals that the SH2 domain is required for normal expression of a subset of yeast genes, and is consistent with multiple functions of Spt6 in chromatin transcription