  We study the effects of symmetry-breaking defects at continuous quantum
transitions (CQTs), which may arise from localized external fields coupled to
the order-parameter operator. The problem is addressed within
renormalization-group (RG) and finite-size scaling frameworks. We consider the
paradigmatic one-dimensional quantum Ising models at their CQT, in the presence
of defects which break the global ${\mathbb Z}_2$ symmetry. We show that such
defects can give rise to notable critical crossover regimes where the
ground-state properties experience substantial and rapid changes, from
symmetric conditions to symmetry-breaking boundaries. An effective
characterization of these crossover phenomena driven by defects is achieved by
analyzing the ground-state fidelity associated with small changes of the defect
strength. Within the critical crossover regime, the fidelity susceptibility
shows a power-law divergence when increasing the system size, related to the RG
dimension of the defect strength; in contrast, outside the critical defect
regime, it remains finite. We support the RG scaling arguments with numerical
results.
