Gravity model explained by the radiation model on a population landscape

Understanding the mechanisms behind human mobility patterns is crucial to improve our ability to optimize and predict traffic flows. Two representative mobility models, i.e., radiation and gravity models, have been extensively compared to each other against various empirical data sets, while their fundamental relation is far from being fully understood. In order to study such a relation, we first model the heterogeneous population landscape by generating a fractal geometry of sites and then by assigning to each site a population independently drawn from a power-law distribution. Then the radiation model on this population landscape, which we call the radiation-on-landscape (RoL) model, is compared to the gravity model to derive the distance exponent in the gravity model in terms of the properties of the population landscape, which is confirmed by the numerical simulations. Consequently, we provide a possible explanation for the origin of the distance exponent in terms of the properties of the heterogeneous population landscape, enabling us to better understand mobility patterns constrained by the travel distance.Comment: 14 pages, 4 figure

Nanoscale Memristive Devices for Memory and Logic Applications.

As the building block of semiconductor electronics, field effect transistor (FET), approaches the sub 100 nm regime, a number of fundamental and practical issues start to emerge such as short channel effects that prevent the FET from operating properly and sub-threshold slope non-scaling that leads to increased power dissipation. In terms of nonvolatile memory, it is generally believed that transistor based Flash memory will approach the end of scaling within about a decade. As a result, novel, non-FET based devices and architectures will likely be needed to satisfy the growing demands for high performance memory and logic electronics applications. In this thesis, we present studies on nanoscale resistance switching devices (memristive devices). The device shows excellent resistance switching properties such as fast switching time ( 10^6), good data retention (> 6 years) and programming endurance (> 10^5). The studies suggest that the nonvolatile resistance switching in a nanoscale a-Si resistive switch is caused by the formation of a single conductive filament within 10 nm range near the bottom electrode. New functionalities, such as multi-bit switching with partially formed filaments, can be obtained by controlling the resistance switching process through current programming. As digital memory devices, the devices are ideally suited in the crossbar architecture which offers ultra-high density and intrinsic defect tolerance capability. As an example, a high-density (2 Gbits/cm^2) 1kb crossbar memory was demonstrated with excellent uniformity, high yield (> 92%) and ON/OFF ratio (> 10^3), proving its promising aspects for memory and reconfigurable logic applications. Furthermore, we demonstrated that properly designed devices can exhibit controlled analog switching behavior and function as flux controlled memristor devices. The analog memristors can be used in biology-inspired neuromorphic circuits in which signal processing efficiency orders of magnitude higher than conventional digital computer systems can be reached. As a prototype illustration, we showed Spike Timing Dependent Plasticity (STDP), one of the key learning rules in biological system, can be realized by CMOS neurons and nanoscale memristor synapses.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/75835/1/josung_1.pd

Hierarchical burst model for complex bursty dynamics

Temporal inhomogeneities observed in various natural and social phenomena have often been characterized in terms of scaling behaviors in the autocorrelation function with a decaying exponent gamma, the interevent time distribution with a power-law exponent alpha, and the burst size distributions. Here the interevent time is defined as a time interval between two consecutive events in the event sequence, and the burst size denotes the number of events in a bursty train detected for a given time window. To understand such temporal scaling behaviors implying a hierarchical temporal structure, we devise a hierarchical burst model by assuming that each observed event might be a consequence of the multilevel causal or decision-making process. By studying our model analytically and numerically, we confirm the scaling relation alpha+gamma = 2, established for the uncorrelated interevent times, despite of the existence of correlations between interevent times. Such correlations between interevent times are supported by the stretched exponential burst size distributions, for which we provide an analytic argument. In addition, by imposing conditions for the ordering of events, we observe an additional feature of log-periodic behavior in the autocorrelation function. Our modeling approach for the hierarchical temporal structure can help us better understand the underlying mechanisms behind complex bursty dynamics showing temporal scaling behaviors.11Ysciescopu

Influence of ripening stages on phytochemical composition and bioavailability of ginseng berry (Panax ginseng C.A. Meyer)

The presence of large amounts of bioactive compounds such as saponins and flavonoids in ginseng (Panax ginseng) berry suggests its potential as a functional resource for the food and medical industries, despite the fact that been considered a useless by-products of P. ginseng. In this study, we examined the variations in the antioxidant and anti-melanogenic potential of ginseng berry during the ripening process. We found that fully ripe berry extracts (Go-S3) contained the highest level of antioxidant and anti-melanogenic activities. Phytochemical screening suggested that alterations in polyphenol contents correlated with the variation in bioactive principles of ginseng berry during the ripening process. Furthermore, results obtained by quantitative real-time PCR, western blot, tyrosinase inhibition assay and molecular docking analysis suggested that Go-S3 probably inhibits tyrosinase activity by interacting with copper-coordinating histidines and second shell residues of tyrosinase, resulting in the reduction of melanin production in α-MSH-stimulated B16F10 cells. Taken together, these finding suggest the potential of ginseng berry as a resource for functional applications in the cosmetic industries and demonstrate that fruit ripening stages have profound effects on the pharmaceutical value of ginseng berry

Low-Power Complementary Inverter Based on Graphene/Carbon-Nanotube and Graphene/MoS₂ Barristors

The recent report of a p-type graphene(Gr)/carbon-nanotube(CNT) barristor facilitates the application of graphene barristors in the fabrication of complementary logic devices. Here, a complementary inverter is presented that combines a p-type Gr/CNT barristor with a n-type Gr/MoS2 barristor, and its characteristics are reported. A sub-nW (~0.2 nW) low-power inverter is demonstrated with a moderate gain of 2.5 at an equivalent oxide thickness (EOT) of ~15 nm. Compared to inverters based on field-effect transistors, the sub-nW power consumption was achieved at a much larger EOT, which was attributed to the excellent switching characteristics of Gr barristors

Seawater-Mediated Solar-to-Sodium Conversion by Bismuth Vanadate Photoanode- Photovoltaic Tandem Cell: Solar Rechargeable Seawater Battery

Conversion of sunlight to chemical energy based on photoelectrochemical (PEC) processes has been considered as a promising strategy for solar energy harvesting. Here, we propose a novel platform that converts solar energy into sodium (Na) as a solid-state solar fuel via the PEC oxidation of natural seawater, for which a Na ion-selective ceramic membrane is employed together with photoelectrode (PE)-photovoltaic (PV) tandem cell. Using an elaborately modified bismuth vanadate-based PE in tandem with crystalline silicon PV, we demonstrate unassisted solar-to-Na conversion (equivalent to solar charge of seawater battery) with an unprecedentedly high efficiency of 8% (expected operating point under I sun) and measured operation efficiency of 5.7% (0.2 sun) and long-term stability, suggesting a new benchmark for low-cost, efficient, and scalable solid solar fuel production. The sodium turns easily into electricity on demand making the device a nature-friendly, monolithic solar rechargeable seawater battery

Dynamics of Helping Behavior and Networks in a Small World

To investigate an effect of social interaction on the bystanders' intervention in emergency situations a rescue model was introduced which includes the effects of the victim's acquaintance with bystanders and those among bystanders from a network perspective. This model reproduces the experimental result that the helping rate (success rate in our model) tends to decrease although the number of bystanders $k$ increases. And the interaction among homogeneous bystanders results in the emergence of hubs in a helping network. For more realistic consideration it is assumed that the agents are located on a one-dimensional lattice (ring), then the randomness $p \in [0,1]$ is introduced: the $kp$ random bystanders are randomly chosen from a whole population and the $k-kp$ near bystanders are chosen in the nearest order to the victim. We find that there appears another peak of the network density in the vicinity of $k=9$ and $p=0.3$ due to the cooperative and competitive interaction between the near and random bystanders.Comment: 13 pages, 8 figure

Enhanced Electrokinetic Transport of Sulfate in Saline Soil

The electrokinetic transport of sulfate was investigated as a means of treating and restoring a sulfate-accumulating saline soil. The electrokinetic treatment decreased the electrical conductivity of the soil, an indicator of soil salinity, to 58.6%, 73.1%, and 83.5% for 7, 14 and 21 days, respectively. More than 96% of the chloride and nitrate were removed within 7 days. However, the removal of sulfate was highly influenced by the anode material. An iron anode removed sulfate effectively, whereas, sulfate was hyper-accumulated in the anodic region when an inert anode was used. The iron anode was oxidized in a sacrificial anodic reaction, which competed with the electrolysis reaction of water at the anode, and finally the reaction prevented the severe acidification of the soil in the anodic region. However, the competing reactions produced hydrogen ions at the anode, and the ions were transported toward the cathode, which, in turn, acidified the soil, especially, in the anodic region. The acidification switched the surface charge of the soil from negative to positive, increasing the interaction between the soil surface and sulfate, and thus inhibiting the transport of sulfate under the electric field. The zeta potential analysis of the soil provided an explanation. The results indicate that preventing severe acidification is an important factor which influences the transport of anions and iron anode for the enhanced removal of anionic pollutants by electrokinetic remediation