Effects of a string cloud on the criticality and efficiency of AdS black holes as heat engines J. P. Morais Graça, Iarley P. Lobo, Valdir B. Bezerra and H. Moradpour Eur. Phys. J. C, 78 10 (2018) 823 Published online: 12 October 2018 DOI: 10.1140/epjc/s10052-018-6277-z