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Title: Structure and magnetic properties of model S=1/2 and S=1 kagome magnets
Author: Connolly, Eamonn
ISNI:       0000 0004 7965 1550
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2019
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Quantum spin-liquids (QSLs) are exotic electronic states that do not break crystallographic symmetry operations and possess spin dynamics as T !0. Predicted to underpin high-TC superconductivity by Anderson, the search for QSLs has been a cornerstone of condensed matter physics. The kagome antiferromagnets (KAFM) are ideal candidates to produce QSLs as the strong geometric frustration of the lattice produces large ground state degeneracies that can couple with the zero-point energy fluctuations of S = 1 2 and S = 1 spin systems to destabilise magnetic order and allow QSLs to form. The crystal structure and preliminary magnetic data for a series of S = 12 KAFM copper-vanadates with the general formula ACu2.5V2O7(OH)2 H2O (A = K+,NH4 +, Rb+) are reported here. Despite geometric distortions and some site depletion, bulk magnetic data evidences frustration through the suppression of magnetic order below jqWj and the build up of superparamagnetic-like regimes. Structural analysis reveals competing magnetic exchange terms and dynamic orbital-fluctuations that make them candidate materials to map the phase space of spin-orbital frustrated magnets. Two S = 1 KAFMs, NH4Ni2:5V2O7(OH)2.H2O and NH4Ni2Mo2O10H3, are also presented in this work, where both have the 3-fold symmetry of the kagome lattice but significant site depletion, with 77 % and 66 % site occupancy, respectively. Both materials evidence frustration through the suppression of order below jqWj and competition between ferromagnetic and antiferromagnetic interactions, where ferromagnetic response observed in the ordered states are in contrast to the antiferromagnetic character of the mean-field and hint at unconventional ground states.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available