Brownian Motion and Entropic Torque Driven Motion of Domain-Wall in Antiferromagnets

We study the spin dynamics in antiferromagnetic nanowire under an applied temperature gradient using micromagnetic simulations on a classical spin model with a uniaxial anisotropy. The entropic torque driven domain-wall motion and the Brownian motion are discussed in detail, and their competition determines the antiferromagnetic wall motion towards the hotter or colder region. Furthermore, the spin dynamics in an antiferromagnet can be well tuned by the anisotropy and the temperature gradient. Thus, this paper not only strengthens the main conclusions obtained in earlier works [Kim et al., Phys. Rev. B 92, 020402(R) (2015); Selzer et al., Phys. Rev. Lett. 117, 107201 (2016)], but more importantly gives the concrete conditions under which these conclusions apply, respectively. Our results may provide useful information on the antiferromagnetic spintronics for future experiments and storage device design.Comment: 6 pages, 7 figures, published in Physical Review

An Event-Based Neurobiological Recognition System with Orientation Detector for Objects in Multiple Orientations

A new multiple orientation event-based neurobiological recognition system is proposed by integrating recognition and tracking function in this paper, which is used for asynchronous address-event representation (AER) image sensors. The characteristic of this system has been enriched to recognize the objects in multiple orientations with only training samples moving in a single orientation. The system extracts multi-scale and multi-orientation line features inspired by models of the primate visual cortex. An orientation detector based on modified Gaussian blob tracking algorithm is introduced for object tracking and orientation detection. The orientation detector and feature extraction block work in simultaneous mode, without any increase in categorization time. An addresses lookup table (addresses LUT) is also presented to adjust the feature maps by addresses mapping and reordering, and they are categorized in the trained spiking neural network. This recognition system is evaluated with the MNIST dataset which have played important roles in the development of computer vision, and the accuracy is increase owing to the use of both ON and OFF events. AER data acquired by a DVS are also tested on the system, such as moving digits, pokers, and vehicles. The experimental results show that the proposed system can realize event-based multi-orientation recognition.The work presented in this paper makes a number of contributions to the event-based vision processing system for multi-orientation object recognition. It develops a new tracking-recognition architecture to feedforward categorization system and an address reorder approach to classify multi-orientation objects using event-based data. It provides a new way to recognize multiple orientation objects with only samples in single orientation

Mercury redox chemistry in natural systems: the role of atmospheric waters and the marine cryosphere

Mercury is a contaminant of global concern. It is present widely in global ecosystems and its methylated species is a known developmental neurotoxin to humans. The fate and behaviour of mercury in the environment are largely affected by its speciation, especially among different oxidation states controlled by redox chemistry. Mercury redox chemistry is the key chemical mechanism mediating the transport of the contaminant, yet it has not been well represented in global and regional mercury transport models. Most models rely heavily on computational data to parameterize mercury redox chemistry, and large discrepancies have been reported between the model parameters and experimental results. In this thesis, several critically important mercury redox processes in the atmosphere and marine cryosphere are investigated experimentally in laboratory, mesocosm and field studies. In the atmosphere, in-cloud mercury photoreduction is found to occur at rates that are much slower than those currently used in models, questioning the presumed dominance of the aqueous-phase reduction in the atmosphere. Mercury redox reactions are also studied in an outdoor sea ice mesocosm. At the atmosphere-sea ice interface, saline surfaces of experimental sea ice are shown to support heterogenous photochemical reactions causing the depletion of gaseous elemental mercury in the atmospheric boundary layer; whereas at the sea ice-seawater interface, cryo-photochemical processes could cause the decrease in the concentrations of dissolved gaseous mercury during the formation of sea ice. Overall, the results from this thesis research provide new and important concepts and experimental datasets to the mercury redox mechanism during its geochemical cycle. The results will advance model parametrizations on mercury redox chemistry and improve future projections of mercury cycling in the atmosphere and the Arctic marine cryosphere, which are especially important under a rapidly changing environment. Furthermore, the results also validate the mesocosm approach on studying cryo-photochemical processes in the sea ice environment, which opens up a new platform to study the geochemistry of other contaminants in the marine cryosphere.October 202

Tuning ice nucleation with counterions on polyelectrolyte brush surfaces

Heterogeneous ice nucleation (HIN) on ionic surfaces is ubiquitous in a wide range of atmospheric aerosols and at biological interfaces. Despite its great importance in cirrus cloud formation and cryopreservation of cells, organs, and tissues, it remains unclear whether the ion-specific effect on ice nucleation exists. Benefiting from the fact that ions at the polyelectrolyte brush (PB)/water interface can be reversibly exchanged, we report the effect of ions on HIN on the PB surface, and we discover that the distinct efficiency of ions in tuning HIN follows the Hofmeister series. Moreover, a large HIN temperature window of up to 7.8°C is demonstrated. By establishing a correlation between the fraction of ice-like water molecules and the kinetics of structural transformation from liquid- to ice-like water molecules at the PB/water interface with different counterions, we show that our molecular dynamics simulation analysis is consistent with the experimental observation of the ion-specific effect on HIN