Based on the Barton–Nakajima–Namikawa
relationship linking dc conductivity, σdc, to the hopping time,
τ e, σdc = 1/τ e, and employing Dyre’s hopping model [65]
for ac conduction in disordered solids, to account for the
conductivity in the bulk and in the EP layer, they came to
some interesting conclusions as to the onset of EP in these
samples.