K-theoretic classification of fermionic operator mixings in holographic   conformal field theories
  In this paper, we apply the K-theory scheme of classifying the topological insulators/superconductors to classify the topological classes of the massive multi-flavor fermions in anti-de Sitter (AdS) space. In the context of AdS/CFT correspondence, the multi-flavor fermionic mass matrix is dual to the pattern of operator mixing in the boundary conformal field theory (CFT). Thus, our results classify the possible patterns of operator mixings among fermionic operators in the holographic CFT. 
Shih-Hao Ho and Feng-Li Lin
hep-th cond-mat.str-el quant-ph