Feng Xu UC Riverside Papers $N=2$ superconformal nets Sebastiano Carpi, Robin Hillier, Yasuyuki Kawahigashi, Roberto Longo, Feng Xu July 10, 2012 open access link Communications in Mathematical Physics 336 (2015), no. 3, 1285-1328 Topological Sectors and a Dichotomy in Conformal Field Theory Roberto Longo, Feng Xu September 23, 2003 open access link Commun.Math.Phys.251:321-364,2004