Spiral spin liquid noise

An emerging concept for identification of different types of spin liquids [C. Broholm et al., Science 367, eaay0668 (2020)] is through the use of spontaneous spin noise [S. Chatterjee, J. F. Rodriguez-Nieva, E. Demler, Phys. Rev. B 99, 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 et al., Nature 571, 234–239 (2019)], we measure the time and temperature dependence of spontaneous flux ϕ(t, T)and thus magnetization M(t, T) of Ca₁₀Cr₇O₂₈ samples. The resulting power spectral density of magnetization noise S_M(ω, T)reveals intense spin fluctuations with S_M(ω, T) ∝ ω^−α(T) and 0.84 < α(T) < 1.04. Both the variance σ²_M(T) and the correlation function _M(t, T) of this spin noise undergo crossovers at a temperature ^∗ ≈ 450 mK. 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 S_M(ω, T), C_M(t, T), and σ²_M (T), thus indicating that Ca₁₀Cr₇O₂₈ is an SSL.