Parametric number variance of disordered systems in the multifractal regime

We study the effect of an external field X on the energy levels of a disordered system by evaluating the parametric number variance (PNV). The weak disorder regime is studied within the Gaussian random matrix theory, while the multifractal regime is studied by considering the q-deformed random matrices. The PNV at both small and large values of X has distinct features in the weak disorder and multifractal regimes that should be observable in numerical studies.