Theta electromagnetism in quantum spin ice: Microscopic analysis of improper symmetries

Abstract

$U(1)$ gauge theories, including conventional Maxwell electromagnetism, allow $\hat{I}¸$-terms when parity and time-reversal symmetry are broken. In condensed matter systems, the physics of $\hat{I}¸$ as a magnetoelectric response has been explored extensively within the context of topological insulators and multiferroics. We show how $\hat{I}¸$-terms can arise in the internal dynamics of the emergent electromagnetism in a $U(1)$ quantum spin liquid. In its Coulomb phase, the minimal model of pyrochlore quantum spin ice is governed by a six-spin ring exchange Hamiltonian. We identify the next-order contribution to the microscopic Hamiltonian when parity, time-reversal, and all improper spatial symmetries are broken -- a seven-spin term which leads to a two-parameter lattice gauge theory with a $\hat{I}¸$-electromagnetic phase. We derive how the seven-spin term is generated perturbatively within each of the three symmetry classes of short-range pyrochlore spin ice. Within a complete microscopic symmetry analysis, we find that the most general nearest-neighbor Hamiltonians fail to generate the seven-spin term, and one must include next-nearest-neighbor interactions to obtain an emergent $\hat{I}¸$. Using gauge mean-field theory we compute additional contributions to the $\hat{I}¸$-term from the spinon sector. Finally, we determine the conditions required for an internal $\hat{I}¸$-term to generate a significant external magnetoelectic response.

26 pages, 5 figures