Non-magnetic impurities as probes of insulating and doped Mott   insulators in two dimensions
  We characterize paramagnetic Mott insulators by their response to static, non-magnetic impurities. States with spinon deconfinement (or spin-charge separation) are distinguished from those with spinon confinement by distinct impurity susceptibilities and finite-size spectra. We discuss the evolution of physical properties upon doping to a d-wave superconductor, and argue that a number of recent experiments favor spinon confinement in the reference Mott insulating state. 
Subir Sachdev (Yale) and Matthias Vojta (Augsburg)