Abstract: No state of matter can be defined by what it is not; yet magnetic quantum systems, e.g. quantum spin liquids (QSL), are often conjectured to exist based only on nonexistence of magnetic order. An emerging concept designed to circumvent this ambiguity is to identify each type of non-ordered quantum magnetism by its spontaneous quantum spin noise spectrum.
For example, the kagome lattice of spin-1/2 Cu atoms in herbertsmithite (ZnCu3(OH)6Cl2) is conjectured to sustain a QSL state with spinon quasiparticles. Each kagome plane is separated from its homologues by a layer of spinless Zn atoms. Providentially, however, some spin-1/2 Cu atoms substitute randomly onto these inter-kagome Zn sites. We reconceptualized these ‘impurity’ atoms as ‘witness-spins’ that provide an exceptional new interrogative of the QSL state.
To explore herbertsmithite witness-spin dynamics in this context we recently introduced quantum spin noise sensing1-4 to QSL studies. It reveals the existence, slowing and intensification of spin noise, prefatory to a sharp transition at T*≈260 mK. Below T* the spin-noise power spectral density S(ω,T)∝ω--α(T) stabilizes at α≈1; the spin noise variance σM2(T) diminishes precipitously; the ultra-low-field magnetic susceptibility χ(T) undergoes a sharp transition into a phase exhibiting an Edwards-Anderson order-parameter and ultra-slow spin-state ageing.
To understand these phenomena, we use a model of QSL spinon-mediated witness-spin interactions and, when compared to empirical phenomenology of witness-spin noise, it reveals the spinon spectrum in herbertsmithite. Time permitting, I will discuss other types of quantum spin noise spectroscopy.