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Abstract: 

Quantum spin liquids are often identified simply by the absence of long-range magnetic order when the energy scale of magnetic interactions J greatly exceeds that of temperature kT. However, because there are many types of quantum spin fluids lacking long-range order (e.g. magnetic monopole fluids, spiral spin liquids, quantum spin liquids, random singlet phases, quantum Griffiths phases etc.) such an assignment is highly ambiguous. Ideally, a new approach is required  for the definite identification of distinct types of quantum spin systems.

One of the most intriguing approaches would be to determine the full spectrum of spin fluctuations, which is distinct for each system. For that purpose, we developed high-sensitivity, SQUID based spin noise spectrometers. Their first use was to measure the frequency and temperature dependence of the power spectral density of spin noise,   in Dy₂Ti₂O₇ spin-ice samples. This revealed all the predicted features of the spin noise spectrum predicted for a fluid of magnetic monopoles (Nature 571, 234(2019).  I will then discuss our recent generalization of this approach to the spiral spin liquid system Ca₁₀Cr₇O₂₈ and  future prospects for spin noise spectroscopy in gapless quantum spin liquids.