Spiral spin liquid noise

An emerging concept for identification of different types of spin liquids [C. Broholm <i>et al.</i>, <i>Science</i> <b>367</b>, eaay0668 (2020)] is through the use of spontaneous spin noise [S. Chatterjee, J. F. Rodriguez-Nieva, E. Demler, <i>Phys. Rev. B</i> <b>99</b>, 104425 (2019)]. Here, we develop spin noise spectroscopy for spin liquid studies by considering Ca₁₀Cr₇O₂₈, a material hypothesized to be either a quantum or a spiral spin liquid (SSL). By enhancing techniques introduced for magnetic monopole noise studies [R. Dusad <i>et al.</i>, <i>Nature</i> <b>571</b>, 234-239 (2019)], we measure the time and temperature dependence of spontaneous flux [Formula: see text] and thus magnetization [Formula: see text] of Ca₁₀Cr₇O₂₈ samples. The resulting power spectral density of magnetization noise [Formula: see text] reveals intense spin fluctuations with [Formula: see text] and [Formula: see text]. Both the variance [Formula: see text] and the correlation function [Formula: see text] of this spin noise undergo crossovers at a temperature [Formula: see text]. While predictions for quantum spin liquids are inconsistent with this phenomenology, those from Monte-Carlo simulations of a two-dimensional (2D) SSL state in Ca₁₀Cr₇O₂₈ yield overall quantitative correspondence with the measured frequency and temperature dependences of [Formula: see text], and [Formula: see text], thus indicating that Ca₁₀Cr₇O₂₈ is an SSL.