Abstract
We explain how gapped quantum spin liquids, both conventional and 'fractonic', may be unambiguously diagnosed experimentally using the technique of multidimensional coherent spectroscopy. 'Conventional' gapped quantum spin liquids (e.g. $Z_2$ spin liquid) do not have clear signatures in linear response, but do have clear fingerprints in non-linear response, accessible through the already existing experimental technique of two dimensional coherent spectroscopy. Type I fracton phases (e.g. X-cube) are (surprisingly) even easier to distinguish, with strongly suggestive features even in linear response, and unambiguous signatures in non-linear response. Type II fracton systems, like Haah's code, are most subtle, and may require consideration of high order non-linear response for unambiguous diagnosis.
Highlights
Quantum spin liquids are phases of quantum matter that feature fractionalized excitations and emergent deconfined gauge fields
We begin by noting that all the models we discuss have gapped ground states that are robust to weak perturbations, i.e., weak local perturbations do not produce phase transitions
We have identified purely spectroscopic fingerprints of both conventional gapped spin liquids and gapped fractonic phases using the technique of 2DCS spectroscopy
Summary
Quantum spin liquids are phases of quantum matter that feature fractionalized excitations and emergent deconfined gauge fields (for reviews, see [1,2]). Part of the challenge here is that it is hard to find unambiguous experimental diagnostics for these exotic phases, which are accessible using currently available experimental techniques [5] This is in contrast to gapless spin liquids (both conventional and fractonic), which do have clean signatures, e.g., in the form of “pinch points” in the dynamical structure factor, which may be probed via neutron scattering [6,7]. Gapped conventional spin liquids do not have clear diagnostics in linear response, but they do have unambiguous fingerprints.
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