We numerically investigate critically delocalized wavefunctions in models of
2D Dirac fermions, subject to vector potential disorder. These describe the
surface states of 3D topological superconductors, and can also be realized
through long-range correlated bond randomness in artificial materials like
molecular graphene. A frozen regime can occur for strong disorder in these
systems, wherein a single wavefunction presents a few localized peaks separated
by macroscopic distances. Despite this rarefied spatial structure, we find
robust correlations between eigenstates at different energies, at both weak and
strong disorder. The associated level statistics are always approximately
Wigner-Dyson. The system shows generalized Chalker (quantum critical) scaling,
even when individual states are quasilocalized in space. We confirm analytical
predictions for the density of states and multifractal spectra. For a single
Dirac valley, we establish that finite energy states show universal
multifractal spectra consistent with the integer quantum Hall plateau
transition. A single Dirac fermion at finite energy can therefore behave as a
Quantum Hall critical metal. For the case of two valleys and non-abelian
disorder, we verify predictions of conformal field theory. Our results for the
non-abelian case imply that both delocalization and conformal invariance are
topologically-protected for multivalley topological superconductor surface
states.Comment: 17 pages, 15 figures, published versio
