  We study the textures of generalized "charge densities" (scalar objects
invariant under time reversal), in the vicinity of non-magnetic impurities in
square-lattice quantum anti-ferromagnets, by order parameter field theories.
Our central finding is the structure of the "vortex" in the generalized density
wave order parameter centered at the non-magnetic impurity. Using exact
numerical data from quantum Monte Carlo simulations on an antiferromagnetic
spin model, we are able to verify the results of our field theoretic study. We
extend our phenomenological approach to the period-4 bond-centered density wave
found in the underdoped cuprates.
