UCLA

Quantum Spin Liquids (QSLs) are novel, exotic states of matter that have been proposed to be the key to topological quantum computing and high temperature superconductivity. The QSL is defined, in theory, to be a nonmagnetic insulator with quantum disordered, highly entangled spins. Unfortunately, the connection between theory and experiment in this field is still incomplete. While lack of magnetic order is easy to detect experimentally, quantum disorder and long range entanglement are more difficult to prove. Consequently, there are many candidate QSL materials, but there are no undisputed QSLs to date. This dissertation investigates the first quasi-2D QSL candidate ever to be discovered, κ-(BEDT-TTF)2Cu2(CN)3, and its cousins, κ-(BEDT-TTF)2Hg(SCN)2Cl, and κ-(BEDT-TTF)2Hg(SCN)2Br, through the lens of Nuclear Magnetic Resonance (NMR) in an attempt to answer some of the outstanding questions still present almost 20 years after the discovery of κ-(BEDT-TTF)2Cu2(CN)3. First, the effects of charge degrees of freedom on the ground state of κ-(BEDT-TTF)2X materials are examined via the study of κ-(BEDT-TTF)2Hg(SCN)2Cl and κ-(BEDT-TTF)2Hg(SCN)2Br, both of which are more likely to be charge ordered than κ-(BEDT-TTF)2Cu2(CN)3. Then, the question of whether or not the NMR signature of κ-(BEDT-TTF)2Cu2(CN)3 is compatible with a spinon fermi surface and the nature of the “6 K anomaly” in κ-(BEDT-TTF)2Cu2(CN)3 is revisited via low temperature, high field NMR. In the case of κ-(BEDT-TTF)2Hg(SCN)2Cl, it is found that impurity spins dominate the low temperature NMR relaxation signature masking the intrinsic behavior. The 1/T1 relaxation of κ-(BEDT-TTF)2Hg(SCN)2Cl is very similar to that of κ-(BEDT-TTF)2Cu2(CN)3, and it is reasonable to believe the same “masking” may apply in the latter case. κ-(BEDT-TTF)2Hg(SCN)2Br, on the other hand, behaves quite differently from the other two. While none of the materials studied exhibit any signs of long range magnetic order, signatures for short range magnetic order were detected in κ-(BEDT-TTF)2Hg(SCN)2Br. κ-(BEDT-TTF)2Cu2(CN)3 is revisited at high fields in order to test the theory that impurity spins mask intrinsic relaxation behavior, as NMR signatures from impurities should be frozen out at sufficiently high fields. The relaxation signature of κ-(BEDT-TTF)2Cu2(CN)3 is found to be gapped, inconsistent with a spinon fermi surface. The nature of this gap remains unclear. Some exploration into the Inverse Laplace Transform (ILT) method as a tool for analyzing the stretched exponential in the context of κ-(BEDT-TTF)2Cu2(CN)3 is discussed as well.