Spiral Spin Liquid Noise

No state of matter can be defined categorically by what it is not; yet spin liquids are often conjectured to exist based on the nonexistence of magnetic order as T → 0. An emerging concept designed to circumvent this ambiguity is to categorically identify each spin liquid type by using its spectrum of spontaneous spin noise. Here we introduce such a spectroscopy to spin liquid studies by considering Ca_10Cr_7O_28. This is a spin liquid, but whether classical or quantum and in which specific state, are unknown. By enhancing the flux-noise spectrometry techniques introduced for magnetic monopole noise studies, here we measure the time and temperature dependence of spontaneous flux (t,T) and thus magnetization M(t,T) of Ca_10Cr_7O_28 samples. The resulting power spectral density of magnetization noise S_M(ω,T) along with its correlation function C_M(t,T), reveal intense spin fluctuations spanning frequencies 0.1Hz≤ω/2π≤ 50 kHz, and that S_M(ω,T)∝ω^-α(T) with 0.84 < α(T) < 1.04. Predictions for quantum spin liquids yield a frequency-independent spin-noise spectrum, clearly inconsistent with this phenomenology However, when compared to Monte Carlo simulations for a 2D spiral spin liquid state that are accurately parameterized to describe Ca_10Cr_7O_28, comprehensive quantitative correspondence with the data including S_M(ω,T), C_M(t,T) and magnetization variance σ_M^2(T) fingerprint the state of Ca_10Cr_7O_28 as a spiral spin liquid.