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Phase velocity of drifting spin wave packets in semiconductor two-dimensional electron gas
摘要： We investigated the drift and diffusion dynamics of spin wave packets under spin–orbit effective magnetic fields in two-dimensional electron systems. A simplified model with a spin drift-diffusion equation predicted that the spin phase velocity will change over time from a large negative value to a small positive value. Kerr rotation microscopy revealed the trend we expected for the phase velocity in spin wave packets in a GaAs quantum well. Monte-Carlo simulations agreed with the experiment and showed that the phase velocity can be simply characterized by the spin–orbit parameters and the size ratio between the instantaneous and initial wave packets.
关键词： two-dimensional electron systems,spin wave packets,Kerr rotation microscopy,Monte-Carlo simulations,spin–orbit effective magnetic fields
更新于2025-09-09 09:28:46
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Organ doses evaluation for chest computed tomography procedures with TL dosimeters: Comparison with Monte Carlo simulations
摘要： Purpose: To evaluate organ doses in routine and low‐dose chest computed tomography (CT) protocols using an experimental methodology. To compare experimental results with results obtained by the National Cancer Institute dosimetry system for CT (NCICT) organ dose calculator. To address the differences on organ dose measurements using tube current modulation (TCM) and fixed tube current protocols. Methods: An experimental approach to evaluate organ doses in pediatric and adult anthropomorphic phantoms using thermoluminescent dosimeters (TLDs) was employed in this study. Several analyses were performed in order to establish the best way to achieve the main results in this investigation. The protocols used in this study were selected after an analysis of patient data collected from the Institute of Radiology of the School of Medicine of the University of S?o Paulo (InRad). The image quality was evaluated by a radiologist from this institution. Six chest adult protocols and four chest pediatric protocols were evaluated. Lung doses were evaluated for the adult phantom and lung and thyroid doses were evaluated for the pediatric phantom. The irradiations were performed using both a GE and a Philips CT scanner. Finally, organ doses measured with dosimeters were compared with Monte Carlo simulations performed with NCICT. Results: After analyzing the data collected from all CT examinations performed during a period of 3 yr, the authors identified that adult and pediatric chest CT are among the most applied protocol in patients in that clinical institution, demonstrating the relevance on evaluating organ doses due to these examinations. With regards to the scan parameters adopted, the authors identified that using 80 kV instead of 120 kV for a pediatric chest routine CT, with TCM in both situations, can lead up to a 28.7% decrease on the absorbed dose. Moreover, in comparison to the standard adult protocol, which is performed with fixed mAs, TCM, and ultra low‐dose protocols resulted in dose reductions of up to 35.0% and 90.0%, respectively. Finally, the percent differences found between experimental and Monte Carlo simulated organ doses were within a 20% interval. Conclusions: The results obtained in this study measured the impact on the absorbed dose in routine chest CT by changing several scan parameters while the image quality could be potentially preserved.
关键词： Monte Carlo simulations,computed tomography,dosimetry/exposure assessment,organ dose,image quality
更新于2025-09-04 15:30:14
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Study of Sensitivity and Resolution for Full Ring PET Prototypes based on Continuous Crystals and analytical modeling of the light distribution
摘要： Sensitivity and spatial resolution are the main parameters to maximize in the performance of a PET scanner. For this purpose, detectors consisting of a combination of continuous crystals optically coupled to segmented photodetectors have been employed. With the use of continuous crystals the sensitivity is increased with respect to the pixelated crystals. In addition, spatial resolution is no longer limited to the crystal size. The main drawback is the difficulty in determining the interaction position. In this work, we present the characterization of the performance of a full ring based on cuboid continuous crystals coupled to SiPMs. To this end, we have employed the simulations developed in a previous work for our experimental detector head. Sensitivity could be further enhanced by using tapered crystals. This enhancement is obtained by increasing the solid angle coverage, reducing the wedge-shaped gaps between contiguous detectors. The performance of the scanners based on both crystal geometries was characterized following NEMA NU 4-2008 standardized protocol in order to compare them. An average sensitivity gain over the entire axial field of view of 13.63% has been obtained with tapered geometry while similar performance of the spatial resolution has been proven with both scanners. The activity at which NECR and True peak occur is smaller and the peak value is greater for tapered crystals than for cuboid crystals. Moreover, a higher degree of homogeneity was obtained in the sensitivity map due to the tighter packing of the crystals, which reduces the gaps and results in a better recovery of homogeneous regions than for the cuboid configuration. Some of the results obtained, such as spatial resolution, depend on the interaction position estimation and may vary if other method is employed.
关键词： NEMA NU 4-2008,Monte Carlo simulations,image reconstruction,continuous crystals,depth of interaction,positron emission tomography (PET)
更新于2025-09-04 15:30:14
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Solid-state order and charge mobility in [5]-[12] cycloparaphenylenes
摘要： We report a computational study of mesoscale morphology and charge transport properties of radially π-conjugated cycloparaphenylenes [n]CPPs of various ring sizes (n = 5-12, where n is the number of repeating phenyl units). These molecules are considered as structural constituents of fullerenes and carbon nanotubes. [n]CPP molecules are nested in a unique fashion in the solid state. Molecular dynamics simulations show that while intramolecular structural stability (order) increases with system size, intermolecular structural stability reduces. Density functional calculations reveal that reorganization energy, an important parameter in charge transfer, decreases as n is increased. Intermolecular charge-transfer electronic couplings in the solid state are relatively weak (due to curved π-conjugation and loose intermolecular contacts) and are on the same order of magnitude (i.e., ~10 meV) for each system. Intrinsic charge-carrier mobilities were simulated from kinetic Monte Carlo simulations; hole mobilities increased with system size and scaled as ~n4. We predict that disordered [n]CPPs exhibit hole mobilities as high as 2 cm2/Vs. A strong correlation between reorganization energy and hole mobility, i.e. μ~λ?4, was computed. Quantum mechanical calculations were performed on co-facially stacked molecular pairs for varying phenyl units and revealed that orbital delocalization is responsible for both decreasing reorganization energies and electronic couplings as n is increased.
关键词： kinetic Monte Carlo simulations,charge transport,density functional calculations,molecular dynamics,cycloparaphenylenes
更新于2025-09-04 15:30:14
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The Sommerfeld ground-wave limit for a molecule adsorbed at a surface
摘要： Using a mid-infrared emission spectrometer based on a superconducting nanowire single-photon detector (SNSPD), we observe the dynamics of vibrational energy pooling of CO adsorbed at the surface of a NaCl crystal. After exciting a majority of the CO molecules to their first vibrationally excited state (v = 1), we observe infrared emission from states up to v = 27. Kinetic Monte Carlo simulations show that vibrational energy collects in a few CO molecules at the expense of those up to eight lattice sites away by selective excitation of NaCl’s transverse phonons. The vibrating CO molecules behave like classical oscillating dipoles, losing their energy to NaCl lattice-vibrations via the electromagnetic near-field. This is analogous to Sommerfeld’s description of the Earth’s influence on radio transmission by ground waves.
关键词： vibrational energy pooling,NaCl crystal,superconducting nanowire single-photon detector,electromagnetic near-field,Kinetic Monte Carlo simulations,Sommerfeld ground-wave limit,CO adsorbed
更新于2025-09-04 15:30:14