Transport properties of tetrathiotetracene iodide (TTT2I3−δ) at low temperatures

We have performed a series of transport measurements on tetrathiotetracene iodide ($\mathrm{TT}{\mathrm{T}}_2{\mathrm{I}}_{3-\delta }$) samples with varying degrees of disorder. These measurements include electrical conductivity from 300 to 0.2 K, magnetoresistance from 30 to 1.5 K, and magnetothermopower from 30 to 4.2 K. Microwave conductivity from 300 to 10 K and electric-field-dependent conductivity were measured to investigate frequency and voltage dependences. Our results indicate that disorder smears the metal-insulator transition so that the most highly disordered samples are best characterized at low temperatures as semiconductive with a high density of localized states in their gap. A sizable increase in the low-temperature resistance and thermopower with applied magnetic field indicates that the magnetic field reduces the density of localized states at the Fermi energy.