We explore novel topological responses and axion-like phenomena in
three-dimensional insulating systems with spacetime-dependent mass terms
encoding domain walls. Via a dimensional-reduction approach, we derive a new
axion-electromagnetic coupling term involving three axion fields. This term
yields a topological current in the bulk and, under specific conditions of the
axions, real-space topological defects such as magnetic-like monopoles and
hopfions. Moreover, once one the axions acquires a constant value, a nontrivial
boundary theory realizes a (2+1)-dimensional analog of the Witten effect, which
shows that point-like vortices on the gapped boundary of the system acquire
half-integer electric charge. Our findings reveal rich topological structures
emerging from multi-axion theories, suggesting new avenues in the study of
topological phases and defects.

Cet article explore les excursions dans le temps et leurs implications.

Télécharger PDF: