Microscopic Compensation of Circular Optical Response in Altermagnets

Overview & Synopsis

A weak or vanishing circular optical signal need not imply weak helicity-sensitive transitions: it can be the residual of much larger opposing microscopic contributions. We resolve the finite-frequency interband Kubo response before summing over transitions and momentum, separating cancellation among transitions at fixed momentum from cancellation across the Brillouin zone. An exactly compensated $d$-wave model establishes the symmetry benchmark. In MnTe, first-principles Wannier calculations reveal a symmetry-dark Néel orientation with an extended sign-changing circular kernel whose bulk integral vanishes. Reorienting the Néel vector removes this extinction, yet the allowed response remains a small imbalance between large positive- and negative-momentum sectors; the imbalance reverses with photon energy and flips the macroscopic sign. Thus magnetic symmetry decides whether circular response is allowed, whereas microscopic compensation determines how much survives and with what sign. This distinction provides a general framework for interpreting weak, vanishing, and sign-changing magneto-optical signals in compensated magnets.