Christoph W. Zollitsch, Kai Mueller, David P. Franke, Sebastian T. B. Goennenwein, Martin S. Brandt, Rudolf Gross, Hans Huebl: High cooperativity coupling between a phosphorus donor spin ensemble and a superconducting microwave resonator. 2015Appl. Phys. Lett.: 107. http://scitation.aip.org/content/aip/journal/apl/107/14/10.1063/1.4932658