High Pressure Investigation Of An Organic Three-Dimensional Dirac Semimetal Candidate Having A Diamond Lattice

(ET)Ag-4(CN)(5) [ET = BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] is a rare example of organic conductors that crystallizes in a diamond structure due to the tetrahedral arrangement of neighboring ET molecules. Because each ET has a +1 site charge, the band is half filled, which in combination with strong Coulomb interactions, turn (ET)Ag-4(CN)(5) into a Mott insulator. Recently, it was realized that this material may realize a nodal Dirac semimetal with a diamondlike lattice, whose state can be induced by suppressing the strong electron correlations. In this report, we present high pressure transport and optical studies on (ET)Ag-4(CN)(5). Our transport study indicates that the activation energy is highly suppressed by the application of hydrostatic pressure. At high pressures, the system varies from activated behavior into Coulomb gap variable range hopping.