Oral: Witnessing Disorder in Quantum Magnets

There are no clean samples in nature. Thus, when discussing entanglement in quantum materials, disorder effects must be considered. This is particularly relevant for quantum spin liquids, which lie outside the Landau paradigm for classifying phases.

In this talk, we explore what experimentally-accessible measures, in the form of concurrence, residual tangle (RT), and quantum Fisher information (QFI), can teach us about the entanglement in the presence of disorder. As a representative example, we consider the Tomonaga-Luttinger liquids (TLL) and disorder-driven random singlet (RS) states in an antiferromagnetic quantum spin chain. Using QFI and RT, we demonstrate that both TLL and RS states exhibit multipartite entanglement. In the case of the RS state, we attribute this to entanglement localized below a crossover length scale.

We further show that the order of disorder average matters in calculating measures like concurrence, and that this can lead to false inferences when interpreting experiments. Nonetheless, correctly interpreted, these witnesses provide useful information about the effects of disorder. We show how information about the central charge of the TLL can be extracted from the low temperature behavior of concurrence, and conjecture that this analysis can be extended to the effective central charge of the RS state. Finally, we show how RS and TLL states can be distinguished through the growth of multipartite entanglement, as witnessed by the equal-time structure factor.