LAUSR

We show that NV centers could provide distinct signatures of emergent gauge fields in certain phase transitions in spin liquids. We consider the relaxation rate $1/T_1$ of a diamond NV center placed a distance $z_0$ above a crystal consisting of two dimensional layers which hosts a $U(1)$ quantum spin liquid with spinon Fermi surface. We show that during a spinon pairing transition from a $U(1)$ to a $\mathbb{Z}_2$ spin liquid, the relaxation rate exhibits a rapid fall just below the pairing transition. The drop is much faster than what can be accounted for by the opening of the energy gap. Instead, the rapid fall with a unique $z_0$ dependence of its height and width, is the signature of the Meissner effect of the gauge field. We identify the organic ET compound which has a phase transition at 6K as a candidate for the experimental observation of this phenomenon.