Regularization and Interpolation of Positive Matrices
  We consider certain matricial analogues of optimal mass transport of positive definite matrices of equal trace. The framework is motivated by the need to devise a suitable geometry for interpolating positive definite matrices in ways that allow controlling the apparent tradeoff between "aligning up their eigenstructure" and "scaling the corresponding eigenvalues". Indeed, motivation for this work is provided by power spectral analysis of multivariate time series where, linear interpolation between matrix-valued power spectra generates push-pop artifacts. Push-pop of power distribuion is objectionable as it corresponds to unrealistic response of scatterers. 
Kaoru Yamamoto, Yongxin Chen, Lipeng Ning, Tryphon T. Georgiou, and   Allen Tannenbaum
math.OC