Emergent moments and random singlet physics in a Majorana spin liquid

Abstract

We exhibit an exactly solvable example of a SU(2) symmetric Majorana spin liquid phase, in which quenched disorder leads to random-singlet phenomenology. More precisely, we argue that a strong-disorder fixed point controls the low temperature susceptibility $\ddot{I}‡(T)$ of an exactly solvable $S=1/2$ model on the decorated honeycomb lattice with quenched bond disorder and/or vacancies, leading to $\ddot{I}‡(T)=\mathcal{C}/T+\mathcal{D}{T}^{\hat{I}\pm (T)-1}$ where $\hat{I}\pm (T)\to 0$ as $T\to 0$. The first term is a Curie tail that represents the emergent response of vacancy-induced spin textures spread over many unit cells: it is an intrinsic feature of the site-diluted system, rather than an extraneous effect arising from isolated free spins. The second term, common to both vacancy and bond disorder (with different $\hat{I}\pm (T)$ in the two cases) is the response of a random singlet phase, familiar from random antiferromagnetic spin chains and the analogous regime in phosphorus-doped silicon (Si:P).

two-column format; 4+pages; 3 figures