We have noticed that the two equations appear in the manuscript1 were typed with errors. Here are the correct forms of Eqs. (1) and (2):

σ=0,
(1)
D=0.
(2)

The typos occurred during the production of the manuscript and do not change the results or conclusion of the paper.

1.
Z. Y.
Gao
,
J.
Zhou
,
Y. D.
Gu
,
P.
Fei
,
Y.
Hao
,
G.
Bao
, and
Z. L.
Wang
,
J. Appl. Phys.
105
,
113707
(
2009
).