We report the influence of the field effect on the dc resistance and Hall coefficient in the strain-induced Mott insulating state of an organic superconductor $\kappa \mathrm{\text{−}}(\mathrm{BEDT}\mathrm{\text{−}}\mathrm{TTF}{)}_2\mathrm{Cu}[\mathrm{N}(\mathrm{CN}{)}_2]\mathrm{Br}$. Conductivity obeys the formula for an activated transport ${\sigma }_{\square }={\sigma }_0\exp (-W/k_{B}T)$, where ${\sigma }_0$ is a constant and $W$ depends on the gate voltage. The gate-voltage dependence of the Hall coefficient shows that, unlike in conventional field-effect transistors, the effective mobility of dense hole carriers ($\sim 1.6\times {10}^{14}{\mathrm{cm}}^{-2}$) is enhanced by a positive gate voltage. This implies that carrier doping involves delocalization of intrinsic carriers that were initially localized due to electron correlation.

• Received 22 May 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.116801