The coexistence of the negative and positive electrocaloric effect in ferroelectric thin films for solid-state refrigerationB. Li, J. B. Wang, X. L. Zhong, F. Wang, Y. K. Zeng and Y. C. ZhouEPL, 102 4 (2013) 47004DOI: https://doi.org/10.1209/0295-5075/102/47004