QUANTUM FIELD THEORY IN EXTERNAL SOURCE:COHERENT STATE AND EIGENSTATES OF FIELDS

The real scalar field and the spinor field with external sources present are studied. The relationship between external sources and coherent states is given. In the case of real scalar field the eigenstates of field and its canonically conjugate field are derived in terms of coherent state and functional integration techniques. For a fermion field, it is demonstrated that Schwinger's external sources technique can lead to the spinor coherent states, with which we can also derive the eigenstates of fie3d operators and their conjugate field operators.The Yang-Feldman equation with the Grassmann sources is also obtained.