The spin-1/2 square-lattice J1-J2 model: the spin-gap issue Johannes Richter, Ronald Zinke and Damian J.J. Farnell Eur. Phys. J. B, 88 1 (2015) 2 Published online: 02 January 2015 DOI: 10.1140/epjb/e2014-50589-x