I have referred to Emily Bount's solution to this question. The sampling time is alot for the code. Hence decrease the sampling time. Instead of 0.07 make it like 0.0005.
The changed code is
F0 =13;
T0 =1/F0;
Ts = 0.0005;
t = 0:Ts:13*T0;
x = real(exp(j*(2*pi*F0*t-pi/2)));
plot(t,x).
The changed diagram is
