# A problem with a supposedly simple Plot

I am trying to redraw the function F as a function of Jt. Where

$$F = {{(1 + 2{{\left| {{D_4}\left( t \right)} \right|}^2})} \over 3}$$

I should get the same result in the paper "Annals of Physics 355 (2015) 170–181", Figure 6 (b) solid-black line. But I get different results! Is there something wrong?

NB: D4(t) is defined by equation (12d) and F is defined at the bottom of page 7

ClearAll["Global*"]  xx = 0.1; J = 1.0; lam = J/xx; t = tau/J; k = Sqrt[lam^2 + J^2];  D4 = ((J^22)/(4*(k^2 + lam*k)))*     Exp[-1 I*(lam + k)*t] + ((J^2)/(4*(k^2 - lam*k)))*     Exp[-1 I*(lam - k)*t] - (1/2)*Cos[J*t];  F = (1 + 2*Abs[D4]^2)/3.0;  Plot[F, {tau, 0, 50}, PlotRange -> {0, 1}] `