  Two-dimensional ferromagnetic electron gases subject to random scalar
potentials and Rashba spin-orbit interactions exhibit a striking quantum
criticality. As disorder strength $W$ increases, the systems undergo a
transition from a normal diffusive metal consisting of extended states to a
marginal metal consisting of critical states at a critical disorder $W_{c,1}$.
Further increase of $W$, another transition from the marginal metal to an
insulator occurs at $W_{c,2}$. Through highly accurate numerical procedures
based on the recursive Green's function method and the exact diagonalization,
we elucidate the nature of the quantum criticality and the properties of the
pertinent states. The intrinsic conductances follow an unorthodox
single-parameter scaling law: They collapse onto two branches of curves
corresponding to diffusive metal phase and insulating phase with correlation
lengths diverging exponentially as $\xi\propto\exp[\alpha/\sqrt{|W-W_c|}]$ near
transition points. Finite-size analysis of inverse participation ratios reveals
that the states within the critical regime $[W_{c,1},W_{c,2}]$ are fractals of
a universal fractal dimension $D=1.90\pm0.02$ while those in metallic
(insulating) regime spread over the whole system (localize) with $D=2$ ($D=0$).
A phase diagram in the parameter space illuminates the occurrence and evolution
of diffusive metals, marginal metals, and the Anderson insulators.

Mesoscale and Nanoscale Physics