摘要
Abstract We examine the contribution of electromagnetic field to the atomic spin, by adopting two different, both gauge invariant definitions of the electromagnetic angular momentum：→JI≡fd3x∈0→r×（→E×→B）and→JII≡fd3x（∈0→E×→A⊥＋∈0Ei→τ× A⊥i）.Notably, at the classical level, →JI gives an exactly null result while →JI gives a finite value. This suggests that Jn leads to a simpler and more reasonable picture of the atomic spin, therefore qualifies as a more appropriate definition of the electromagnetic angular momentum. Our observation gives important hint on the delicate issue of gluon contribution to the nucleon spin.
