def xi_func_theory(xi0, T, Tc, nu):
    """
    Parameter
    ---------
    xi0 : float
        correlation length 0 (in m)
    T   : float
        lattice constant of the cubic unit cell (in K)
    Tc  : float
        critical temperature of the system (in K)
    nu  : float
        critical exponent of the correlation length (nu = gamme_eff/2)

    Return
    ------
    xi  : float
        temperature dependent correlation length (in m)
    """

    return xi0 * (T/Tc - 1.0)**(-nu)