  It is widely recognised that the effect of doping into a Mott insulator is
complicated and unpredictable, as can be seen by examining the Hall coefficient
in high $T_{\rm c}$ cuprates. The doping effect, including the electron-hole
doping asymmetry, may be more straightforward in doped organic Mott insulators
owing to their simple electronic structures. Here we investigate the doping
asymmetry of an organic Mott insulator by carrying out electric-double-layer
transistor measurements and using cluster perturbation theory. The calculations
predict that strongly anisotropic suppression of the spectral weight results in
the Fermi arc state under hole doping, while a relatively uniform spectral
weight results in the emergence of a non-interacting-like Fermi surface in the
electron-doped state. In accordance with the calculations, the experimentally
observed Hall coefficients and resistivity anisotropy correspond to the pocket
formed by the Fermi arcs under hole doping and to the non-interacting Fermi
surface under electron doping.
