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• 1. [Article] Theoretical Study of Spin-wave Effects in Quantum Ferromagnets

  In this dissertation, we examine quantum ferromagnets and determine various effects of the magnetic Goldstone modes or "magnons'' in these systems. Firstly, we calculate the magnon contribution to the ...

  Citation × Citation Citation Abstract Copy Title: Theoretical Study of Spin-wave Effects in Quantum Ferromagnets Author: Bharadwaj, Sripoorna Paniyadi Krishna Year: 2017 In this dissertation, we examine quantum ferromagnets and determine various effects of the magnetic Goldstone modes or "magnons'' in these systems. Firstly, we calculate the magnon contribution to the transport relaxation rate of conduction electrons in metallic ferromagnets and find that at asymptotically low temperatures, the contribution behaves as T^2 exp(-T_0/T) and not as T^2 predicted previously. To perform these calculations, we derive and use a very general effective theory for metallic ferromagnets. This activation barrier-like behavior is due to the fact that spin waves only couple electrons from different Stoner subbands that arise from the splitting of the conduction band in presence of a nonzero magnetization. The T^2 behavior is found to be valid only in a pre-asymptotic temperature window. The temperature scale T_0 is the energy of the least energetic ferromagnon that couples electrons of different spins. Second, we discuss magnon-induced long-range correlation functions in quantum magnets. In the ordered phases of both classical ferromagnets and antiferromagnets, the long-range correlations induced by the magnons lead to a singular wavenumber dependence of the longitudinal order-parameter susceptibility in spatial dimensions 2<d<4. We investigate the quantum analog of this singularity using a nonlinear sigma model. In a quantum antiferromagnet at $T=0$, a weaker nonanalytic behavior is obtained, which is consistent with power counting. The analogous result for a quantum ferromagnet is absent if the magnon damping is neglected. This is due to the lack of magnon number fluctuations in the quantum ferromagnetic ground state. Magnon damping due to quenched disorder restores the expected nonanalyticity. Finally, we use an effective field theory for clean, strongly interacting electron systems to calculate the magnon contribution to the density of states, the longitudinal magnetic susceptibility and the conductivity in an itinerant ferromagnet. Utilizing a loop expansion that does not assume the electron-electron interaction to be a small parameter, we obtain the leading nonanalytic corrections to the Stoner saddle-point results for these observables, as functions of the frequency and wavenumber in the hydrodynamic limit. The dissertation includes previously published and unpublished co-authored material. Title: Theoretical Study of Spin-wave Effects in Quantum Ferromagnets Author: Bharadwaj, Sripoorna Paniyadi Krishna Year: 2017 In this dissertation, we examine quantum ferromagnets and determine various effects of the magnetic Goldstone modes or "magnons'' in these systems. Firstly, we calculate the magnon contribution to the transport relaxation rate of conduction electrons in metallic ferromagnets and find that at asymptotically low temperatures, the contribution behaves as T^2 exp(-T_0/T) and not as T^2 predicted previously. To perform these calculations, we derive and use a very general effective theory for metallic ferromagnets. This activation barrier-like behavior is due to the fact that spin waves only couple electrons from different Stoner subbands that arise from the splitting of the conduction band in presence of a nonzero magnetization. The T^2 behavior is found to be valid only in a pre-asymptotic temperature window. The temperature scale T_0 is the energy of the least energetic ferromagnon that couples electrons of different spins. Second, we discuss magnon-induced long-range correlation functions in quantum magnets. In the ordered phases of both classical ferromagnets and antiferromagnets, the long-range correlations induced by the magnons lead to a singular wavenumber dependence of the longitudinal order-parameter susceptibility in spatial dimensions 2<d<4. We investigate the quantum analog of this singularity using a nonlinear sigma model. In a quantum antiferromagnet at $T=0$, a weaker nonanalytic behavior is obtained, which is consistent with power counting. The analogous result for a quantum ferromagnet is absent if the magnon damping is neglected. This is due to the lack of magnon number fluctuations in the quantum ferromagnetic ground state. Magnon damping due to quenched disorder restores the expected nonanalyticity. Finally, we use an effective field theory for clean, strongly interacting electron systems to calculate the magnon contribution to the density of states, the longitudinal magnetic susceptibility and the conductivity in an itinerant ferromagnet. Utilizing a loop expansion that does not assume the electron-electron interaction to be a small parameter, we obtain the leading nonanalytic corrections to the Stoner saddle-point results for these observables, as functions of the frequency and wavenumber in the hydrodynamic limit. The dissertation includes previously published and unpublished co-authored material. Close