Browsing by Author "Nevidomskyy, Andriy H"
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Item Frustrated Magnetism in Strongly Correlated Electron Systems(2016-04-21) Wang, Zhentao; Nevidomskyy, Andriy HA deep understanding of magnetism is essential for its application in magnetic semiconductors, spintronic devices and unconventional superconductors. In this work, we study magnetic structures and their corresponding excitations in several strongly correlated electron systems, where exotic orderings can be induced as a result of magnetic frustration and quantum fluctuations. We show that emergent spin textures can arise close to a magnetic field-induced quantum critical point, when the single magnon excitations have several degenerate non-coplanar minima. In this case, quantum fluctuations can lift such degeneracy and lead to the crystallization of the magnetic vortex strings. Magnetic frustration also plays an important role in Fe-based superconductors. We analyze the spin excitations in the ordered as well as paramagnetic phase of these materials, and find that higher order spin exchanges are essential for understanding the inelastic neutron scattering experiments (INS). The presence of such higher order spin interactions has far-reaching consequences, potentially resulting in more exotic phases, such as the multipolar orders. In particular, we find propensity to ferro-quadrupolar order, which we propose as a candidate for the ground state of the iron selenide FeSe. We find that the calculated spin excitations in this quadrupolar state closely resemble the results of recent INS measurements. In addition to electron spins, orbital physics also plays a prominent role in Fe-based superconductors. We study the interplay between spin and orbital degrees of freedom and show that the so-called nematic order can be naturally understood as the decoupling of the two transitions, when orbital ordering preempts long-range magnetic spin order. Our results reveal that magnetic frustration plays an important role in several strongly correlated electron systems, and elucidating its consequences is crucial for the understanding and potential application of these materials.Item Ground States and Phase Transitions in Frustrated Spin Models: Investigations Using Classical and Quantum Monte Carlo(2022-04-19) Butcher, Matthew; Nevidomskyy, Andriy HMagnetic frustrations are the fundamental cause of many interesting phenomena in materials that have been intensely studied since the middle of the 20th century. Magnetic systems with competing interactions, ranging from insulating antiferromagnets to arrays of superconducting qubits, exhibit nontrivial quantum e ects that arise from the need to satisfy a jagged free energy landscape. The search for and characterization of quantum spin liquids, nontrivial topological phases, and quantum phase transitions is conducted through investigations of such frustrated models. Within the large umbrella of frustrated magnetism I have studied some speci c models with the purpose of answering the following questions: (1) What are the interesting phases and less-interesting phases, and how do they fall in the phase diagram relative to one another? (2) What properties do the phases have, and how can we characterize what happens when one phase transitions to another? Armed with the answers to these questions, researchers can further explore these interesting phenomena and apply them towards new materials and devices. In this thesis, I rst develop a computational methodology for studying both classical and quantum spin models using various forms of Monte Carlo simulation. I use classical Monte Carlo to study highly-frustrated models of Ising spins both at nite temperatures and in the T = 0 limit, to map the phase diagram and determine the universality class of phase transitions. Using these methods, I show that in a dissipative chain of qubits, bath-induced qubit-qubit interactions lead to collective decoherence above a critical dissipation strength. This causes the qubits to lose all stored information and become fully correlated with one another, an e ect that one would need to mitigate in order to perform quantum computation. In another study, I show that in antiferromagnetic insulators with anisotropic interactions, frustration can cause an intermediate paramagnetic phase with oblong \droplets" of correlated spins. This phenomenon is quite general and the presence of these droplets can help explain experimental signatures that occur at unexpected temperatures in such materials. In addition to the classical simulations, I develop a variational Monte Carlo (VMC) technique to search for ground states in Heisenberg-type models, speci cally for spins S = 1. The case of S = 1 is more complicated than the case for S = 1=2 due to an additional local degree of freedom, which has interesting implications for the possible states that can be realized. Using semiclassical energy minimization as well as VMC, I show that spin S = 1 diamond-lattice antiferromagnets experience higher-order spinspin interactions that destroy previously theorized liquid-like spiral spin orderings. However, the presence of these higher-order interactions can also potentially allow other phases such as quantum spin liquids and topological paramagnets. Such results can provide an avenue for explaining anomalous phenomena like low-temperature paramagnetism in future experimental materials.