Instanton analysis for the spin quantum Hall symmetry class: Nonperturbative corrections to physical observables and generalized multifractal spectrum

Recently, there has been renewed interest in studies of criticality in the spin quantum Hall effect, realized in the Altland-Zirnbauer symmetry class C of disordered, noninteracting fermions in two spatial dimensions. In our study, we develop a nonperturbative analysis of the replica two-dimensional nonlinear sigma model in class C. We explicitly construct the instanton solution with a unit topological charge. By treating fluctuations around the instanton at the Gaussian level, we calculate the instanton correction to the disorder-averaged logarithm of the partition function. We compute nonperturbative corrections to the anomalous dimensions of pure power-law scaling local operators, which determine the spectrum of generalized multifractality. We also calculate instanton corrections to the renormalized longitudinal and Hall spin conductivities and determine the topology of the phase diagram for class C. Our results demonstrate that the spin quantum Hall effect is indeed a close cousin of the integer quantum Hall effect.