plotting – 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})