Quantum spin liquids on geometrically frustrated kagome latticesGomilšek, Matjaž (Avtor)
Zorko, Andrej (Mentor)
kagomequantum spin liquiddefectsthreefold symmetry breakingspinon Fermi surfacegaplessspinon metalquantum criticalitycrossoverdensity of statesKondo effectfield-induced instabilityspinon pairinggappedmu-OH(D)INSIn this Thesis we study the paradigmatic quantum kagome antiferromagnet (QKA) herbertsmithite and the recently synthesized Zn-brochantite by a variety of experimental techniques. We find that herbertsmithite possesses two types of defects at low temperatures, d_I and d_II, not just one type as previously believed. We prove that the dominant d_I defects are strongly coupled to the kagome quantum spin liquid (QSL) state of herbertsmithite, while they are usually considered quasi-free, and find that they have a substantially higher antiferromagnetic Weiss temperature (5.2 K) than previously believed. The newly discovered enigmatic d_II defects, on the other hand, are almost uncoupled from the kagome spins with a near-zero Weiss temperature. We attribute d_I defects to extra Cu2+ on the interlayer Zn2+ site, and d_II defects to spinons pinned to local distortions of the kagome planes. Unexpectedly, we find that both types of defects independently prove a subtle global structural distortion breaking global threefold symmetry in herbertsmithite thus disproving the idea of its perfect kagome lattice.
In a comprehensive study of a new QKA, Zn-brochantite, ZnCu3(OH)6SO4, we prove that it is a gapless QSL with a spinon Fermi surface, i.e. a spinon metal. We find that this QSL state is present in two temperature regions with a crossover in between where the free-spinon density of states (DOS) is changing, a unique form of QKA behaviour. We also observe a high-temperature quantum-critical region in Zn-brochantite. Importantly, we discover a quantum spin liquid version of the Kondo effect, the first observation of this effect in any 2D QSL, despite it being theoretically predicted over a decade ago. We establish an effective model of Kondo spinon pinning, where the crossover in the free-spinon DOS of the spinon metal QSL arises from an effective depletion of spinons from the QSL as they start to Kondo screen the defects below their Kondo temperature of T_K = 1.25(5) K. We also find a field-induced spinon pairing instability of the gapless spinon metal QSL resulting in a gapped QSL that might be called a "spinon superconductor".
In support of our experimental work we derive closed-form solutions for polarization curves of a muon dipolarly coupled to an arbitrary quantum spin, as well as their half-classical infinite-spin limit. We also develop a new method, in four increasingly refined variants, for extracting weak intrinsic signals from multi-temperature inelastic neutron scattering (INS) data even when a substantial background contribution is present. It enables us to apply INS to Zn-brochantite despite its signal being too weak to be observable using standard methods. We expect very wide applicability of the new method as it can dramatically shorten measurements times of weak signals, e.g. from QSL's, and enable previously infeasible inelastic scattering experiments.20182018-04-01 07:45:01Doktorsko delo/naloga100614VisID: 87210COBISS_ID: 3192420sl