Neutron-proton mass difference in isospin-asymmetric nuclear matterU. -G. Meißner, A. M. Rakhimov, A. Wirzba and U. T. YakhshievEur. Phys. J. A, 32 3 (2007) 299-309DOI: https://doi.org/10.1140/epja/i2007-10390-9