Superfluid density in cuprates: hints on gauge compositeness of the   holes
  We show that several features (the three-dimensional XY universality for moderate underdoping, the almost-BCS behaviour for moderate overdoping and the critical exponent) of the superfluid density in hole-doped cuprates hint at a composite structure of the holes. This idea can be implemented in a spin-charge gauge approach to the $t - t' - J$ model and provides indeed good agreement with available experimental data. 
P. A. Marchetti and G. Bighin
cond-mat.supr-con hep-th