One of the main challenges in experimentally identifying a quantum spin liquid (QSL) state is in understanding the influence of disorder. Chemical and structural imperfections exist in many promising QSL candidate materials, and can lead to a spatially inhomogeneous behaviour that obfuscates the interpretation of sample-averaged measurements. This issue highlights the importance of nuclear magnetic resonance (NMR) which can locally probe the intrinsic spin susceptibility χspin (separate from defect contributions) and low-energy spin fluctuations via the Knight shift K and nuclear spin-lattice relaxation rate 1/T1, respectively. The value of 1/T1 is typically ascertained by fitting the net nuclear magnetization M(tD) with an appropriate decay function. However, the M(tD) measured at a given frequency has contributions from many nuclei, which in a disordered material, can exhibit a broad distribution of 1/T1. Analogous to how variations in local χspin are reflected in the distribution of Knight shifts which make up the inhomogeneously broadened NMR lineshape, the distribution of 1/T1 that make up a single M(tD) curve can represent multiple environments whose local magnetic ground states are qualitatively distinct. We developed a program which computes the inverse Laplace transform (ILT) of our measured M(tD) data, in order to deduce a probability density function P(1/T1) representing the 1/T1 distribution. Our ILT algorithm primarily employs Tikhonov regularization, which iv limits the instability of numerically inverting data with finite noise. This 1/T1 analysis method offers significant advantages over the traditional method of fitting M(tD) against a phenonmenological stretched exponential function, which provides only a crude approximation of the spatial average of the 1/T1 distribution. In contrast, our approach of calculating P(1/T1) using ILT can delineate the behavior of multiple distinct 1/T1 components, and hence preserve vital information on the position-by-position distribution of local spin dynamics. In this thesis, we report on our 7Li NMR measurements of the Kitaev honeycomb iridate Ag3LiIr2O6, our 63Cu nuclear quadrupole resonance (NQR) measurements on the kagome Heisenberg antiferromagnets ZnCu3(OD)6Cl2 (herbertsmithite) and ZnCu3(OD)6FBr (Zn-barlowite), and further measurements of ZnCu3(OD)6FBr with 19F NMR. Using ILT, we provide crucial insight into both the intrinsic and disorder-induced low-energy spin excitations of these materials. Firstly, we elucidate the effect of stacking faults and unwanted Ag inclusion by comparing the 7Li NMR lineshape and P(1/T1Li) of Ag3LiIr2O6 samples with varying levels of disorder. Next, we observe in P(1/T1Cu) a fraction of spin singlets with spatially inhomogeneous energy gaps emerging below ∼30 K within the kagome planes of ZnCu3(OD)6Cl2 and ZnCu3(OD)6FBr. Finally, we develop a novel method using ILT to obtain the two-dimensional correlation map between 19K and 1/T1F at the 19F sites of ZnCu3(OD)6FBr, and evidence the existence of spin-polarized domains emerging near interlayer Cu2+ defects. / Thesis / Doctor of Science (PhD) / A quantum spin liquid (QSL) is an exotic state of matter whose magnetism fundamentally differs from those of ordinary materials. At temperatures near absolute zero, the electron spins which make up an ordinary magnet generally freeze in place, whereas in a QSL, they form a highly-entangled quantum superposition. A theoretically attainable QSL state was famously proposed in 1973 by Philip W. Anderson. Since then, several candidate materials have been discovered, and research on QSLs became a major focus in the field of condensed matter physics. The realization of a QSL is predicted to have applications in quantum computing (by hosting more robust quantum bits), and can help us understand the physics of other quantum materials, such as high-temperature superconductors. In this thesis, we report our experimental findings on the QSL candidates Herbertsmithite, Zn-barlowite, and Ag3LiIr2O6, where we use nuclear magnetic resonance (NMR) spectroscopy to probe the behaviour of their spins. Hindering past attempts to study these materials is the ever-present influence of disorder, such as chemical and structural imperfections. To combat this, we developed a novel technique for acquiring and analyzing NMR data, known as inverse Laplace transform (ILT) 1/T1 analysis, and used it to make unprecedented discoveries about the heterogeneous physics of these disordered materials.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/30078 |
| Date | January 2024 |
| Creators | Wang, Jiaming |
| Contributors | Imai, Takashi, Physics and Astronomy |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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