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ItemData for the publication "Pressure control of magnetic order and excitations in the pyrochlore antiferromagnet MgCr2O4"(Georgia Institute of Technology, 2024-01) Mourigal, MartinMgCr2O4 is one of the best-known realizations of the pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic exchange interactions are perturbed by small further-neighbor exchanges such that this compound may in principle realize a spiral spin liquid (SSL) phase in the zero-temperature limit. However, a spin Jahn-Teller transition below TN≈13 K yields a complicated long-range magnetic order with multiple coexisting propagation vectors. We present neutron scattering and thermo-magnetic measurements of MgCr2O4 samples under applied hydrostatic pressure up to P=1.7 GPa demonstrating the existence of multiple close-lying nearly degenerate magnetic ground states. We show that the application of hydrostatic pressure increases the ordering temperature by around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by around 0.5 meV per GPa. We also evidence a strong tendency for the preferential occupation of a subset of magnetic domains under pressure. In particular, we show that the k=(0,0,1) magnetic phase, which is almost negligible at ambient pressure, dramatically increases in spectral weight under pressure. This modifies the spectrum of magnetic excitations, which we interpret unambiguously as spin waves from multiple magnetic domains. Moreover, we report that the application of pressure reveals a feature in the magnetic susceptibility above the magnetostructural transition. We interpret this as the onset of a short-range ordered phase associated with k=(0,0,1), previously not observed in magnetometry measurements.
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ItemRaw data and simulation code for "Quantum-to-classical crossover in generalized spin systems – the temperature-dependent spin dynamics of FeI2"(Georgia Institute of Technology, 2024-01) Mourigal, MartinSimulating quantum spin systems at finite temperatures is an open challenge in many-body physics. This work studies the temperature-dependent spin dynamics of a pivotal compound, FeI2, to determine if universal quantum effects can be accounted for by a phenomenological renormalization of the dynamical spin structure factor S(q,ω) measured by inelastic neutron scattering. Renormalization schemes based on the quantum-to-classical correspondence principle are commonly applied at low temperatures to the harmonic oscillators describing normal modes. However, it is not clear how to extend this renormalization to arbitrarily high temperatures. Here we introduce a temperature-dependent normalization of the classical moments, whose magnitude is determined by imposing the quantum sum rule, i.e. ∫dωdqS(q,ω)=NSS(S+1) for NS dipolar magnetic moments. We show that this simple renormalization scheme significantly improves the agreement between the calculated and measured S(q,ω) for FeI2 at all temperatures. Due to the coupled dynamics of dipolar and quadrupolar moments in that material, this renormalization procedure is extended to classical theories based on SU(3) coherent states, and by extension, to any SU(N) coherent state representation of local multipolar moments.
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ItemQuantum Magnetism in Quasi-Two-Dimensional Rare-Earth Oxides: Neutron Scattering and Instrumentation(Georgia Institute of Technology, 2021-07-20) Daum, Marcus J.Quantum magnetism is a rich area of hard condensed matter physics where various energy scales and exchange parameters, coupled with lattice symmetries, lead to ground states of varying degrees of complexity. Through theoretical and experimental efforts, realizations of quantum many-body phenomena are found. This thesis presents recent work on several candidate materials which have been theoretically proposed to exhibit exotic states of matter as their ground state. These materials are carefully characterized using various theoretical and experimental means such as linear spin wave theory and inelastic neutron scattering to understand their ground states. In addition, work presented here displays effort to characterize, optimize, and design new neutron scattering instruments.
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ItemVersatile Spin-Wave Approaches to the Spin Dynamics of Transition-Metal Insulators(Georgia Institute of Technology, 2020-03-16) Ge, LuweiQuantum magnetism is one of the most important branches in condensed matter physics because it serves as an excellent platform to realize model quantum many-body systems which are difficult to find elsewhere. Good understanding of the nature of magnetic excitations in such systems demands both experimental and theoretical efforts. This thesis presents comprehensive studies of the magnetic properties of several 3d transition-metal oxides for which the effective spin Hamiltonian forms quasi-1D, quasi-2D or 3D lattices. Primarily relying on advances in neutron scattering instrumentation and spin-wave theory, the work carefully examines the effectiveness of the theory of weakly interacting magnons in describing the elementary magnetic excitations of these insulators. By revealing the microscopic interactions of these systems and testing the applicability of spin-wave theory quantitatively, the work also hopes to offer useful insights or guidance to future investigations, which may extend to the entire field of quantum many-body physics.
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ItemNeutron Scattering and Quantitative Modeling of Magnetic Excitations in Frustrated Materials(Georgia Institute of Technology, 2019-11-06) Bai, XiaojianThe basic theme of my Ph.D. research is understanding exotic magnetic phases of matter and investigate their collective low-energy excitations using neutron-scattering and quantitative modeling. In this thesis, I start with an attempt to answer a list of questions that I had in the beginning of my Ph.D. study, such as why we can use a simple effective model to describe this complex world, how to synthesize and characterize samples, how to analyze the data and find a good theoretical model and many more. There is no unique answer to these questions. I speak from experience and hope to provide a road map to whoever read my thesis and is interested in starting condensed matter research using neutron-scattering. Next, I present two material projects that I assume a major role. In both projects, high resolution single-crystal inelastic neutron-scattering data enables me and my collaborators to make significantly advances in understanding complex dynamical responses of magnetic materials. In Chapter 2, I present our study on a canonical frustrated magnet MgCr2O4 in the deep cooperative paramagnetic regime. In experiment, we observe a highly structured elastic scattering pattern with continuous excitation spectrum. Using analytic and computational methods, we reveal the highly correlated spin state is proximate to a "spiral spin-liquid" phase and the collective excitations are predominantly fast harmonic precessions of spin on a slow-varying disordered background. In Chapter 3, I present our study on an enigmatic compound with prior investigations dated back to 1970s – FeI2. In experiment, we observe a bright and dispersive band with "quadrupolar" character, apparently at odds with the dipole selection rule. Using advance numerical techniques, we are able to fully account for this band via a novel hybridization mechanism involving off-diagonal symmetric exchange interactions. In Chapter 4, I introduce detailed implementations of spin dynamics simulations and application to a realistic diamond-lattice system. This technique provides a simple framework to study finite temperature and non-linear effects of complex magnetic materials and has increasingly been used to study disordered and strongly-correlated spin systems. I close this thesis in Chapter 5 with an outlook of future directions.
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ItemFrustrated Magnetism and Searching For Quantum Spin Liquid Phases in Novel Materials(Georgia Institute of Technology, 2018-05) Bender, Darian MarieIn my research, I wish to classify and identify a possible Quantum Spin-Liquid (QSL) phase on novel quantum materials. Materials of interest include the two triangular lattice materials, Li4CoTeO6 and Li4NiTeO6, in which Ni and Co ions with effective spin-1 and spin-1/2 each occupy a triangular lattice. We performed thermodynamic and magnetization measurements which indicate a possible exotic magnetic ground-state in both materials. We then performed elastic neutron scattering, providing additional evidence for exotic magnetism in these materials. Inelastic neutron scattering measurements are still necessary to probe the nature of the magnetic correlations and to confirm a QSL phase. Another material of interest is the kagomé lattice material, KFe3(OH)6(SO4)2 (known as Fe-Jarosite). This material is a popular QSL.1, 2 Small crystals of Fe-Jarosite have been created by hydrothermal synthesis in Mourigal Lab, and preliminary measurements of magnetization are in good agreement with known values.1, 3 Neutron scattering is required to study this material’s spin-dynamics, however, scattering is weak. Therefore, further synthesis attempts must be performed in order to increase the size of single-crystals of Fe-Jarosite from 2.6 mm to 1.0 cm.