Low-temperature dipolar echoes in amorphous dielectrics: Significance of relaxation and decoherence free two-level systemsA. L. Burin, J. M. Leveritt, G. Ruyters, C. Schötz, M. Bazrafshan, P. Fassl, M. von Schickfus, A. Fleischmann and C. EnssEPL, 104 5 (2013) 57006DOI: https://doi.org/10.1209/0295-5075/104/57006