  Boundary effect is a widespread idea in many-body theories. However, it is
more of a conceptual notion than a rigorously defined physical quantity. One
can quantify the boundary effect by comparing two ground states of the same
physical model, which differ only slightly in system size. Here, we analyze the
quantity, which we call a boundary effect function, for an XXZ spin-1/2 model
using density matrix renormalization group calculations. We find that three
quantum phases of the model manifest as different functional forms of the
boundary effect function. As a result, the quantum phase transition of the
model is associated with a nonanalytic change of the boundary effect function.
This work thus provides and concretizes a novel perspective on the relationship
between bulk and boundary properties of ground states.
