Vacancy Chains Research Articles

Observation of water dissociation on nanometer-sized FeO islands grown on Pt(1 1 1)

Adsorption and dissociation of water on nanometer-sized FeO islands on Pt(1 1 1) substrate were investigated using scanning tunneling microscopy. It is observed that the adsorbed water molecules may spontaneously dissociate into hydroxyls on the step edges of the FeO(1 1 1) islands, even at 80 K, which can be attributed to the active sites of coordinatively unsaturated Fe atoms at the step edges. These hydroxyls are found to diffuse and form chains on the FeO terrace. The formation of the oxygen vacancy chains on FeO islands indicates that the hydroxyls may desorb from the surface as H 2 and H 2O at room temperature.

Electronic States of Sulfur Vacancies Formed on a MoS2 Surface

Sulfur vacancies formed on a MoS2 surface have been predicted to have electronic states at the Fermi level, and to work as conductive atomic scale structures. We made sulfur vacancies on a MoS2 surface by removing sulfur atoms using scanning tunneling microscopy (STM) induced field evaporation, and measured the current–voltage (I/V) characteristics of the vacancies. The I/V curve measured at the vacancies showed a linear increase at a zero bias region, indicating the existence of electronic states at the Fermi level. On the other hand, the I/V curve measured at a clean surface showed a gap of about 1 eV around the Fermi level, as was expected from the theoretical calculation. We also successfully carried out manipulation of Au nanoislands, which will be used as nanopads to be connected to a sulfur vacancy chain.

Vacancy chains and the degree of mobility in the housing market

Mobility among different states of the housing market (e.g., among tenure modes and price classes) not only generates liquidity but also carries a crucial effect on the price pattern in the market. Studying housing mobility may thus provide insights to the current developments in worldwide housing markets. We theoretically explore vacancy chains and mobility in the market where, among the various results, we show that, in comparing markets with different degrees of mobility, when the probability to transit from any given state to lower states is zero, the expected number of intra-state transitions (as opposed to inter-state transitions) is always smaller in the market that allows for the greater mobility. Also, we derive upper and lower bounds for the expected number of intra-state transitions in the market and specifically show that the “multiplier effect” of the vacancy chain represents a lower bound for this measure. Among the other results, we show that, for every given housing state i, the expected number of intra-state vacancy transitions is greater than the expected number of inter-state transitions for all states j ≠ i. Moreover, we find a condition under which we characterize a subgroup of markets whose expected number of intra-state transitions is greater than the expected number of inter-state transitions for each vacancy chain. Our analysis of mobility and vacancy chains, while specifically focusing on housing markets, may be generalized to other markets as well.

One-dimensional electronic states inGa2Se3on Si(001):As

Gallium selenide Ga2Se3 is a III-VI chalcogenide material whose intrinsic cation vacancy ordering into one dimensional chains has been predicted to result in a one dimensional band at the valence-band maximum VBM. The electronic structure of Ga2Se3 thin films on Si001:As is studied with angle-resolved photoemission spectroscopy. The integrated density of states and the dispersion exhibit good agreement with that predicted by published density-functional theory results. Consistent with a one dimensional state, the electronic states at the VBM show no dispersion perpendicular to the vacancy chains. The Se-Ga bond length from extended x-ray absorption fine structure is 2.34 A. Low-energy electron-diffraction results indicate that the surface is characterized by nanometer-scale 111 facets, consistent with previous scanning tunneling microscopy results.

Inter- and intraindustrial Job-to-Job Flows. A Linkage Analysis of Regional Vacancy Chains in Austria

Nine Austrian NUTS 2 inter- and intraindustrial job-to-job worker flows for 33 industries are analysed by means of input-output techniques, with these job-to-job flows being an intermediate input in the production of filled vacancies. A new dataset on individual labour market episodes allows for the tracing of individual careers. A linkage analysis of the Leontief multiplier shows that business services, wholesale&retail and the metal industry are ‘key’ industries in for- and backwarding employment, whereas construction and tourism exert an impact on other regional industries by absorbing workers upon aggregate external shocks. These findings can be incorporated in industrial employment forecasts or utilized in regional labour market impact studies.

Social context of shell acquisition in Coenobita clypeatus hermit crabs

Vacancy chains involve unique patterns of resource acquisition behaviors that determine how reusable resources are distributed through animal populations. Shell vacancy chains have been described for several hermit crab species, both terrestrial and marine, but little is known about the ecological and behavioral dynamics of shell choice in social versus solitary contexts. Here, we present a novel conceptual framework that differentiates 2 types of shell vacancy chain in hermit crabs and discuss fundamentally distinct predictions concerning the behavioral and ecological costs and benefits associated with synchronous versus asynchronous vacancy chains. In laboratory studies of the terrestrial hermit crab Coenobita clypeatus, we found support for the prediction that social context alters shell acquisition behaviors. Field observations demonstrated that both synchronous and asynchronous vacancy chains are common and revealed previously undescribed waiting and piggybacking behaviors that appear to facilitate synchronous vacancy chains. Additionally, simulation results from an agent-based model showed that population density and waiting behaviors can both influence the likelihood of synchronous vacancy chains. Together, these results indicate that better understanding of hermit crab resource acquisition requires studying social behaviors, including vacancy chain formation.

A Model of Vacancy Chains as a Mechanism for Resource Allocation

Vacancy chains can be tracked in any context where the availability of a desirable resource triggers a cascade of occupations through which the scarce resource flows through different owners. However, under certain conditions, vacancy chains, rather than markets or other forms of competition, determine the allocation of the resource. This article develops a formal and computational model of vacancy chains as a mechanism for resource allocation in order to find out their properties with respect to organizational forms. We find that hierarchies with few middle managers are particularly prone to make use of vacancy chains in order to allocate resources that originate at the top, such as employment positions. In fact, vacancy chains often disappear when information is widely available, because information is likely to attract applicants who engage in a competition. Thus, the many middle managers of a thick organization may compete for a resource that originates at the top. On the contrary, organizations that are thick at the bottom and at the top, but thin in the middle, are most likely to regulate resource allocation by means of vacancy chains.

Effects of Predation Threat on the Structure and Benefits from Vacancy Chains in the Hermit Crab Pagurus bernhardus

AbstractVacancy chains occur when individuals occupy discrete re‐useable resource units, which once abandoned by the current owner can then be occupied by a new owner. In order to enter the newly vacated resource the new owner must first vacate its current resource unit, such that a vacancy chain consists of a series of linked moves between resource units of different value, equivalent to different ‘strata’ in the chain. Vacancy chains may represent an important route by which resources are distributed through populations. Indeed, the arrival of a new resource has the potential to initiate a series of moves propagating beyond the individual that encounters the new resource unit. Thus, the chain participants as a whole may experience ‘aggregate benefits’ from the arrival of the new resource unit. The extent of these benefits, however, may not necessarily be evenly distributed between all chain participants; some individuals could receive greater than average benefits by moving through more than one stratum (‘skipping’) and some individuals could experience a reduction in resource value by moving to a resource unit of lower quality than that occupied initially (a ‘backward move’). Such moves represent deviations from the ‘ideal’ vacancy chains assumed by theory. Here we analyse the aggregate benefits and benefits to individuals participating in vacancy chains of empty gastropod shells in the hermit crab Pagurus bernhardus. We also investigate the effect of predation risk on these two levels of benefits and on chain structure. Adding a new shell at the top of the chain causes an overall increase in shell quality after 24 h but the distribution of benefits between strata in the chain varies with the presence and absence of the predator cue. Although there was significant concordance between chain structure in the presence and absence of the predator cue, the structure was significantly different from an ideal vacancy chain in the absence but not the presence of the predator cue.

A structure model of nano-sized domain in Gd-doped ceria

Nano-sized domains have been widely observed in rare-earth-doped ceria, but their structure model remains unclear. In this work, based on the study of high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) of Gd-doped ceria, the structure model of the domains was constructed by a combination of defect clusters with oxygen vacancy chains. Then, the SAED patterns were simulated using this structure model. The simulated results are consistent with the observed SAED patterns.

Study of diffusion and thermal stability of fluorine ions in GaN by Time‐of‐Flight Secondary Ion Mass Spectroscopy

AbstractThe diffusion properties of fluorine ions in GaN are investigated by means of Time‐of‐Flight secondary ion mass spectroscopy. Instead of incorporating fluorine ions close to the sample surface by plasma, fluorine ion implantation with an energy of 180 keV is utilized to implant fluorine ions deep into the GaN bulk, preventing the surface effects from affecting the data analysis. It is found that the diffusion of fluorine‐ions in GaN is a dynamic process, featuring a two‐step process. A defect‐assisted diffusion model is proposed to account for the experimental observations. Fluorine ions tend to occupy Ga vacancies induced by fluorine ion implantation and diffuse to vacancy rich regions. The fluorine ions become stable after continuous vacancy chains are significantly reduced or removed by thermal annealing. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Diffusion mechanism and the thermal stability of fluorine ions in GaN after ion implantation

The diffusion mechanisms of fluorine ions in GaN are investigated by means of time-of-flight secondary ion mass spectrometry. Instead of incorporating fluorine ions close to the sample surface by fluorine plasma treatment, fluorine ion implantation with an energy of 180 keV is utilized to implant fluorine ions deep into the GaN bulk, preventing the surface effects from affecting the data analysis. It is found that the diffusion of fluorine ions in GaN is a dynamic process featuring an initial out-diffusion followed by in- diffusion and the final stabilization. A vacancy-assisted diffusion model is proposed to account for the experimental observations, which is also consistent with results on molecular dynamic simulation. Fluorine ions tend to occupy Ga vacancies induced by ion implantation and diffuse to vacancy rich regions. The number of continuous vacancy chains can be significantly reduced by a dynamic thermal annealing process. As a result, strong local confinement and stabilization of fluorine ions can be obtained in GaN crystal, suggesting excellent thermal stability of fluorine ions for device applications.

Vacancy Chains Provide Aggregate Benefits To Coenobita clypeatus Hermit Crabs

AbstractVacancy chain theory describes a unique mechanism for the sequential distribution of animal resources across multiple individuals. This theory applies to any resources, such as shelters or nest sites, that are discrete, reusable, and limited in use to single individuals or groups at one time. Hermit crabs rely on gastropod shells for shelter, and a single vacant shell can initiate a chain of sequential shell switches that distributes new resources across many individuals. Using the terrestrial hermit crab Coenobita clypeatus, we examined the previously untested theoretical prediction that this process will yield trickle‐down resource benefits to vacancy chain participants (aggregate benefits). In laboratory experiments, we measured improvements in shell quality when a single vacant shell was provided to groups of eight crabs. We found that crabs participating in vacancy chains (averaging 3.2 individuals) gained significant reductions in their shell crowding. In addition, vacancy chains terminated early when experimental groups included a single crab occupying a damaged shell, because damaged vacancies always remained unoccupied. Hermit crabs in damaged shells were more likely to win resource contests for high quality shells against size‐matched hermit crabs in crowded shells. Finally, field additions of many new shells to an island population of C. clypeatus hermit crabs reduced average shell crowding for crabs of all sizes, possibly from propagation of benefits through vacancy chains. These results provide empirical support for the theoretical prediction that vacancy chains should provide benefits distributed across many vacancy chain participants. Since shelter‐based vacancy chains likely occur in other animals, additional studies of vacancy chain processes should provide new insights into resource acquisition behaviors in diverse animal groups.

Multi-robot task allocation through vacancy chain scheduling

Modeling the effects of robot interaction in multi-robot systems, i.e., the group dynamics, is difficult due to the complexity of such interactions. This article formalizes the concept of group dynamics in the framework of scheduling and presents a proof that multi-robot task allocation (MRTA), in systems with significant performance effects from group dynamics, is an N P -complete problem. As a way of dealing with this complexity we have developed vacancy chain scheduling (VCS), a new formal model of MRTA inspired by a resource distribution process commonly found in nature. VCS is also the foundation of a new MRTA algorithm which relies on optimal allocation patterns to emerge from the stigmergic effects of robot interactions. We present experimental evidence of the validity of the VCS model from high-fidelity simulations. The experimental results validate the VCS model by reliably producing the predicted allocation patterns in both homogeneous and heterogeneous groups of robots. The evidence also supports our claim that VCS is a feasible solution for a restricted class of MRTA problems.

A Model of Vacancy Chains as a Mechanism for Resource Allocation

Vacancy chains can be tracked in any context where a the availability of a desirable resource triggers a cascade of occupations through which the scarce resource flows through different owners. However, under certain conditions vacancy chains, rather than markets or other forms of competition, determine the allocation of the resource. This article develops a formal and computational model of vacancy chains as a mechanism for resource allocation in order to find out their properties with respect to organizational forms.We find that hierarchies with few middle managers are particularly prone to make use of vacancy chains in order to allocate resources that originate at the top, such as employment positions. In fact, vacancy chains often disappear when information is widely available, because information is likely to attract applicants who engage in a competition. Thus, the many middle managers of a thick organization may compete for a resource that originates at the top. On the contrary, organizations that are thick at the bottom and at the top, but thin in the middle, are most likely to regulate resource allocation by means of vacancy chains.

Who Gets What and Why? Vacancy Chains in Stockholm's Housing Market

Abstract The importance of physical resources in influencing life chances makes the study of resource allocation processes and rules especially pertinent and this leads naturally to the question: who gets what and why? This article focuses on the significance of housing construction for residential mobility and addresses the vital question: who will gain from new construction? It examines whether it is possible to build directly for well-resourced households and hope that it indirectly also supports lower income households. It also examines the possibility that changes in the way in which the housing market operates with market-driven construction, geared at a post-modern housing lifestyle, have changed the situation for less well resourced households, compared to traditional housing construction. The study is based on a unique longitudinal database that covers the total population in Sweden over the period 2000–2002. The data are analysed using a Markov chain model that provides a way of analysing the relationship between vacancies in the housing market and household mobility. Tentative answers to questions on the length of the vacancy chains that are created when different types of dwellings become vacant in Stockholm city and what type of households are involved and not involved, are given. These answers have important implications for urban planning.

Toward the jamming threshold of sphere packings: Tunneled crystals

We have discovered a family of three-dimensional crystal sphere packings that are strictly jammed (i.e., mechanically stable) and yet possess an anomalously low density. This family constitutes an uncountably infinite number of crystal packings that are subpackings of the densest crystal packings and are characterized by a high concentration of self-avoiding “tunnels” (chains of vacancies) that permeate the structures. The fundamental geometric characteristics of these tunneled crystals command interest in their own right and are described here in some detail. These include the lattice vectors (that specify the packing configurations), coordination structure, Voronoi cells, and density fluctuations. The tunneled crystals are not only candidate structures for achieving the jamming threshold (lowest-density rigid packing), but may have substantially broader significance for condensed matter physics and materials science.

Cross-sectional scanning tunneling microscopy study on II–VI multilayer structures

Cross-sectional scanning tunneling microscopy is used to study in the atomic scale the structural properties of ZnSeTe∕ZnTe multiple quantum wells and N:ZnTe delta-doped structures. Some peculiar effects are found on the cleaved (110) ZnTe surface plane, which have not been observed in III–V semiconductors. In particular, cleavage induced monatomic wide vacancy chains are always present on the Te sublattice. Furthermore, the semiconductor surface is manipulated when certain positive voltages are applied to the sample. Regarding the heterostructures, the ZnSeTe∕ZnTe quantum wells are found to have abrupt interfaces and the Se concentration is determined to be significantly larger than the nominal value.

On-the-Job Search Over the Business Cycle

On-the-Job Search is one of the most common and efficient ways to look for a new job: most of the time workers move directly from one employment position to another (E-to-E) without an intervening spell of unemployment. E-to-E transitions are a relevant component of total labour flows and have a definite cyclical pattern. This paper computes E-to-E worker flows through the development of a vacancy chain model. An iterative procedure is used to compute the successive reallocation runs, beginning from an autonomous vacancy and then to reconstruct the complete E-to-E transition process. The procedure is implemented and applied to a large micro-panel based on a highly industrialized Italian region from 1982 to 1996. E-to-E transitions are an increasingly large portion of worker flows in the labour market. They are clearly cyclic and the number of transitions increases over time as the labour market becomes tighter. These are the flows that explain labour market dynamics in upswings and recessions. Search models that look only at flows between employment, unemployment and outside of the labour force underestimate labour mobility and its cyclical pattern.

Evaluating Local Job Creation

This article introduces economic development planners to a new approach to evaluating local job creation efforts, an approach that explicitly considers the chains of employment vacancies that open up when new jobs are created. This “job chains” model is an analytic framework for assessing the employment impacts associated with economic development programs and the social value of those impacts. The approach focuses on measuring the wage gains to job changers and placing realistic values on jobs for those not previously employed in the area. It explicitly considers both efficiency and distributional effects of job creation. We discuss the simple mechanics of the technique and present an example relating to the establishment of a large auto plant in a major Midwestern city. We conclude with practical ground rules for planners carrying out a job chains analysis of an economic development effort.

Restricting access in a job chains model of local employment creation

The job chains model of local labor market change is a demand-driven analytic device for estimating the effects of new job creation. This paper explores the effects of restricting supply, i.e., limiting job access, on the model’s primary outcomes: vacancy chain multipliers, welfare effects, and distributional impacts. Major sources of labor supply are the local unemployed, out of the labor force and in-migrants. Three simulations are reported relating to (1) restricting new jobs to current local residents (i.e., no in-migrants), (2) restricting new jobs to current residents in the first round of hiring only, and (3) restricting hiring to local unemployed/out of labor force on the first round alone. The results are compared to the basic model that assumes no supply-side restrictions. In terms of chain length, welfare effects, distributional impacts, and policy palatability, first-round restrictions on in-migrants would seem to be the most plausible option. However, as an economic development strategy, well-targeted demand-side initiatives would still seem to be preferable.