MSCI minimum variance portfolios
Robeco's Blitz and van Vliet on Minimum variance portfolios
Unigestion's minimum variance portfolio
Haugen and Baker on the inefficiency of the market portfolio
Clarke, De Silva and Thorley on Minimum variance portfolios
Tal Schwartz on minimum variance portfolios
Christopher Jones Heteroskedascitity Consistent Factor Extraction
My data on Betas, minimum variance portfolios (free, but registration required)