Quantum Stirling heat engine and refrigerator with single and coupled spin systems Xiao-Li Huang, Xin-Ya Niu, Xiao-Ming Xiu and Xue-Xi Yi Eur. Phys. J. D, 68 2 (2014) 32 Published online: 20 February 2014 DOI: 10.1140/epjd/e2013-40536-0