Spin quantum Hall transition in disordered superconductors

We study a delocalization transition for non-interacting quasiparticles mov ing in two dimensions, which belongs to a new symmetry class. This symmetry class can be realized in a dirty, gapless superconductor in which time-rev ersal symmetry for orbital motion is broken, but spin rotation symmetry is intact. We find a direct transition between two insulating phases with quan tized Hall conductances of zero and two for the conserved quasiparticles. T he energy of quasiparticles acts as a relevant, symmetry-breaking field at the critical point, which splits the direct transition into two conventiona l plateau transitions. (C) 2001 Elsevier Science B.V. All rights reserved.