Two indicators, Brillouin's index (BI) and Hill-type index(HI), are applied to measure interdisciplinarity. Based on the data from the ESI database 2017-2018, the research reveals that the BI and HI values of hot papers in physics and chemistry are not very high, with BI<2 and HI<9. The interdisciplinarity of hot papers in chemistry is slightly higher than that in physics. Both hot papers in physics and chemistry cite papers from at least one subject other than their own subject areas. Most of the hot papers have a cross-disciplinary number of 6-10 in their references.