Kavli Affiliate: J. C. Seamus Davis
| First 5 Authors: Hiroto Takahashi, Hiroto Takahashi, , ,
| Summary:
The kagome lattice of spin-1/2 Cu atoms in herbertsmithite (ZnCu3(OH)6Cl2)
may sustain a quantum spin liquid (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 reconceptualize these ‘impurity’ atoms as quantum
‘witness-spins’, an exceptional new interrogative of the conjectured
Z2-gauge-symmetric QSL state. Thus we introduce spin-noise spectroscopy to
explore herbertsmithite witness-spin dynamics for QSL studies. It reveals the
existence, slowing and intensification of spin noise, prefatory to a sharp
transition at T* approx 260 mK. Below T* the spin-noise power spectral
density S_M(omega,T) propto omega^-alpha(T) stabilizes at
alpha approx 1; the spin noise variance sigma_M^2(T) diminishes
precipitously; the ultra-low-field magnetic susceptibility chi(T) undergoes
a sharp transition into a phase exhibiting an Edwards-Anderson order-parameter
and ultra-slow spin-state relaxation. A Z2 QSL theory of spinon-mediated
witness-spin interactions corresponds best to all these experimental
observations, predicting slowing and intensification of witness-spin
fluctuations and noise spectrum S_M(omega,T) with cooling, with a transition
into a unique spinon-mediated phase signified by rapidly diminishing spin
noise, with S_M(omega,T) propto omega^-1, a sharp cusp in the DC
magnetic susceptibility chi(T), and the appearance of an Edwards-Anderson
order-parameter. We rule out numerous other mechanisms for these effects, so
that only spinon-mediation by either a Z2 or U(1) QSL is consistent with all
present herbertsmithite empirics, with the former model providing a closer
match to data.
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