We have studied the nonlinear current-voltage characteristic of a two dimensional lattice Coulomb gas by Monte Carlo simulation. We present three different determinations of the power-law exponent $a(T)$ of the nonlinear current-voltage characteristic, $V

Hereλisthevortexscreeninglength,and (k)isthepartofthedielectricfunction, (k),

describingthepolarizationoftheboundpairs.Thetwo arerelatedby

1

(k)k2

2

2πVl(k)eik·r.

WecanobtainanapproximateexpressionforVl(r)bymakinguseofthefactthat (k)only

dependsweakly(inmostofkspace)onk.Foragivendistancer,theFourierintegralpicks

upitsmaincontributionfromthekvaluesaround2π/r.Hence

Vl(r) Vl(r=1)≈ 1

(2π/r)ln(r/λ)(9)

whereweuse insteadof ,asthetemperatureisbelowTc.

Accordingtothisdiscussionthecoe cientE1isgivenby[20]

E1=1