Browsing by Subject "Scattering"
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Item 1D SAXS indexing macro for Igor Pro(2021-09-08) Lindsay, Aaron P; Mueller, Andreas J; Mahanthappa, Mahesh K; Lodge, Timothy P; Bates, Frank S; bates001@umn.edu; Bates, Frank S; UMN Polymer GroupThis code was developed for the facile analysis of 1D SAXS data collected from ordered materials in Igor Pro. A robust file loading algorithm is included, allowing for rapid generation of publication quality stack plots. Also included is a straightforward indexing macro, enabling indexing of 1D SAXS data to a variety of phases. New phases can be added with minimal effort and multiple indexing options are included (e.g., ticks, lines, color, etc.), minimizing time spent analyzing data and producing plots for presentations or publications.Item Elastic transmission of identical particles through a strongly correlated Bose-Einstein condensate(2008-12) Lutsyshyn, YaroslavAtomic transmission experiments on superfluid helium-four may provide information about its structure. It was proposed in the past that a transmission channel is possible in which the impinging atoms couple directly to the condensate fraction in helium-II. Such a mechanism would provide an important direct probe of the off-diagonal long-range order in helium-II. We have successfully developed a method based on the diffusion Monte Carlo technique to simulate elastic transmission of atoms through a slab of helium-four at zero temperature. The scattering process is presented as a sum of appropriate standing wave scattering states. The phase factors for each scattering state are determined by matching the diffusion Monte Carlo results with correct energy of the scattering state. The scattering states effectively set the boundary conditions for the problem and in this way determine a phase factor and momentum of the incoming particle. Diffusion Monte Carlo is then performed in its fixed-node flavor. Our results suggest a possibility of complete transparency of small unbound helium films for a broad range of incoming particle's energy. Wavepacket analysis of the computed transmission coefficient's phase dependence on the incoming particle's wavevector was used to obtain times of the transmission process. Time delay analysis suggests the presence of anomalously fast transmission. Such results are strongly supportive of the original condensate mediated transmission hypothesis.Item Electrical detection of the spin Hall effect in ferromagnet-semiconductor heterostructures.(2010-06) Garlid, Eric ScottThis thesis describes the effects of spin-orbit coupling on electron transport in Fe/InGaAs heterostructures. Spin-orbit coupling is a relativistic phenomenon that couples the spin of an electron to its momentum by means of a momentum-dependent effective magnetic field. The spin-orbit coupling in bulk InGaAs is determined by measuring the spin Hall effect. In the spin Hall effect, an applied charge current induces a spin current due to spin-orbit coupling. The spin current flows in a direction that is perpendicular to the charge current, with a spin orientation that is perpendicular to the flow direction of both the charge current and the spin current. The spin Hall effect leads to an out-of-plane spin accumulation that is opposite in sign at opposite edges of the channel. Lateral Fe/InGaAs devices are fabricated using standard semiconductor processing techniques. The interface between the Fe and InGaAs is a highly doped Schottky tunnel barrier for efficient electrical spin injection and detection. Measurements of the spin valve and Hanle effect are performed in the non-local geometry to confirm that the Fe electrodes are sensitive to spin polarization in the InGaAs channel and its dephasing by precession in an applied magnetic field. The spin accumulation due to the spin Hall effect is identified through the observation of a Hanle effect in the Hall voltage measured by pairs of ferromagnetic contacts at the channel edges. The data are fit using a model which includes spin diffusion, precession, and relaxation. We use the parameters determined from the fit to calculate the spin Hall conductivity. We find that the magnitude of the spin Hall conductivity is in agreement with models of the extrinsic SHE due to ionized impurity scattering. By analyzing the dependence of the spin Hall signal on channel conductivity we determine the contributions of both skew and side jump scattering to the total spin Hall conductivity. We calculate that the spin-orbit coupling parameter is larger than predicted by standard k · p perturbation theory.Item Electron spin-flip scattering in graphene due to substrate impurities(2013-01) Goswami, AditiGraphene has long been known for its peculiar Dirac-like band structure which lends it many of its remarkable properties. It is a promising material for electronic and spintronic applications due to its high carrier mobility, low intrinsic spin-orbit interaction and small hyperfine coupling. However, extrinsic effects may easily dominate intrinsic mechanisms. The scattering mechanisms investigated here are those associated with non-magnetic, charged impurities in the substrate (e.g. SiO2) beneath a planar n-type graphene layer. Such impurities cause an electric field that extends through the graphene and has a non-vanishing perpendicular component. Consequently, the impurity, in addition to the conventional spin-conserving scattering can give rise to spin-flip processes. The latter are a consequence of a spatially varying Rashba spin-orbit interaction caused by the electric field of the impurity in the substrate. This work focuses on the calculation of the elastic scattering cross-sections for these mechanisms. Additionally, relaxation times are estimated for assumed impurity concentrations.Item Supporting Data for Salt-Dependent Structure in Methylcellulose Fibrillar Gels(2022-06-09) Liberman, Lucy; Schmidt, Peter W; Coughlin, McKenzie L; Matatyaho Ya'akobi, Asia; Davidovich, Irina; Edmund, Jerrick; Ertem, Sedef P; Morozova, Svetlana; Talmon, Yeshayahu; Bates, Frank S; Lodge, Timothy P; lodge@umn.edu; Lodge, Timothy P; Materials Research Science & Engineering Center (MRSEC)Data files used to generate all figures in the manuscript "Salt-Dependent Structure in Methylcellulose Fibrillar Gels" and its supplementary information.Item Time- and phase-resolved spectroscopy of three-magnon scattering(2023-06) Hamill, AlexIn ferromagnets, scattering processes between magnon modes have been an active platform for the investigation of nonlinear wave interactions and chaotic dynamics for over six decades. Despite this rich history, questions remain regarding the nature of these interactions. In this regard, three-magnon scattering of the ferromagnetic resonance (FMR) mode is of particular appeal: the threshold FMR magnon population at which scattering occurs is distinctly low, enabling its investigation over a wide range of excitation powers. This particular scattering process requires the availability of magnon modes at half of the frequency of the FMR mode. This requirement is readily fulfilled in magnetic films of micrometer thickness, as the associated dipolar interactions lead to a dip in their magnon dispersion. Existing studies of three-magnon scattering have largely focused on its influence on the magnon populations and on the steady-state behavior. A comprehensive understanding of its transient behavior (i.e. how it evolves in time as it approaches steady state) is missing. Similarly, little is known about the influence of three-magnon scattering on the magnons' phases. This is largely the case for other magnon scattering processes as well. There is also a lack of a formal understanding of the relationship between the forward and backward three-magnon scattering processes, i.e. between splitting and confluence. These gaps are, in part, owing to the fact that there is a lack of a comprehensive time- and phase-resolved experimental investigation of the three-magnon scattering process. Magnon scattering processes are most commonly investigated through diode-based techniques, which are relatively insensitive and lack phase resolution. They are also most commonly investigated through Brillouin light scattering spectroscopy; this technique is typically employed for large excitation powers, and its phase-sensitive implementations have not been applied to three-magnon scattering. Motivated by the above, I have assembled a time- and phase-resolved homodyning spectrometer that is operable over six orders of magnitude in microwave power. This spectrometer demonstrates a time resolution of 2 ns, and its sensitivity enables measurement of the transient behavior down to an excitation power of 10 microwatts. Upon measuring the resonance peak of the FMR mode, I observed satellite peaks near that of the FMR resonance. Such satellite peaks are observed in the literature as well. I found that they originate from the excitation of magnon modes with finite in-plane wavevectors, due to the inhomogeneity of the microwave field throughout the sample. To address this inhomogeneity, I created a microstrip waveguide with a signal line width of approximately 3.4 mm, such that it is appreciably wider than the 2 mm-wide sample. This ensures a highly uniform microwave field and, therefore, the highly isolated excitation of the FMR mode. Isolating the excitation of the FMR mode in this manner enables a clear interpretation of the measured transient behavior, and contributes toward the strong agreement observed between my experiment and my semianalytical model. With the above developments, I have investigated the transient behavior of the FMR mode during this scattering process over five orders of magnitude in power. In addition to my observing the expected transient behavior, in which the scattering monotonically suppresses the FMR magnon population to its threshold value, I find a second nonlinear in which the FMR magnon population exhibits transient oscillations about its threshold value. I find that both these oscillations and the timescale of the initial transient peak are highly dependent on the excitation power. At high powers, I find a third nonlinear regime in which the scattering generates 180-degree phase shifts of the FMR magnons. Moreover, I find that both these phase shifts and the transient oscillations reappear upon removing the microwave excitation (i.e. after turn-off). To supplement the experiment, and to understand my findings, I have derived a simplified semianalytical model of this scattering process based on the Landau-Lifshitz-Gilbert equation. Upon linearizing my model, I found that the oscillatory regime corresponds to a transition of the nonlinear regime's fixed point from a stable node to a stable spiral. I also extracted the predicted scaling of the oscillation frequency with the microwave field amplitude. Upon extracting the associated scaling of the experimental data, I found it to be in quantitative agreement with the predicted scaling over several orders of magnitude in power. To investigate the 180-degree phase shifts observed in the experiment, I generalized my model to allow for phase dynamics by omitting the standard assumption of harmonic time dependence. Numerically solving this generalized model, I found that it predicts these 180-degree phase shifts. In order to derive the equations of motion of the magnon populations, one begins with the generalized model and assumes harmonic time dependence. Remarkably, when accounting for the 180-degree phase shift in this harmonic approximation, I found that the phase shifts correspond to reversals in the scattering direction: in the magnon populations' equations of motion, the phase shift switches the sign of the scattering terms such that the scattering is now driving the FMR mode instead of damping it. These reversals explain the observed transient oscillations after turn-off: even without the excitation field, the FMR population may still oscillate via reversals between the forward and backward scattering process. These experimental and theoretical developments further the state of the art of the investigation of magnon scattering processes. The findings of my investigation provide a more comprehensive understanding of the transient behavior of this scattering process, and reveal the nontrivial interplay between three-magnon scattering and the magnons' phases.