| Summary: | Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2009. === This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. === Cataloged from student submitted PDF version of thesis. === Includes bibliographical references (p. 201-214). === The magnetic properties of the geometrically frustrated quantum magnets clinoatacamite, Cu2(OH)3Cl, and herbertsmithite, ZnCu3(OH)6Cl2, are studied by means of neutron scattering measurements as well as specific heat, susceptibility, and mag-netization measurements. These materials are studied to investigate the nature of the ground state of the spin-1 2 kagomé lattice antiferromagnet, as such a system is considered ideal for the emergence of spin liquid physics. Clinoatacamite, a distorted kagomé lattice antiferromagnet with weak inter-plane coupling, undergoes a Néel or- dering transition at TN ~/~ 6.2 K and shows evidence of a static local moment in the disordered phase below 18 K. Our experiments suggest two-dimensional Ising fluctuations at the Néel transition. A proposed spin ordering model is developed that suggests an order structure below TN and two-dimensional short range order of the kagomé plane spins up to 18 K. The inelastic spectrum is analyzed in terms of spin waves in an ordered kagomé lattice antiferromagnet with a Dzyaloshinskii-Moriya interaction. Herbertsmithite is the first structurally perfect spin- 1 2 kagomé lattice antiferromagnet. Susceptibility, specific heat, and neutron scattering measurements show no sign of any spin freezing or transition to a long range ordered state down to 50 mK. The data shows magnetic excitations extending adjacent to the ground state, suggesting the lack of any measurable spin gap. Several hypotheses are explored as possible explanations for the apparent lack of a spin gap. === (cont.) Dynamic susceptibility data display an unusual scaling relation, suggesting proximity to a quantum critical point. In sum, a wide range of data suggest that herbertsmithite displays a disordered gapless spin liquid ground state. === by Joel Strader Helton. === Ph.D.
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