The velocity–velocity correlation function σ xx ( ω ) versus the frequency ω for different values of the in-plane magnetic field: (a) B y = 0 T, (b) B y = 2 T, (c) B y = 4 T, and (d) B y = 6 T. The real part is plotted as the black solid curve, and the imaginary part is plotted as the red dashed curve. The de-polarization effect is taken into account.