Vacancy Chains Research Articles

Effect of electron-phonon interaction on the formation of one-dimensional electronic states in coupled Cl vacancies

The formation of extended electron states in one-dimensional nanostructures is of key importance for the function of molecular electronics devices. Here we study the effects of strong electron-phonon interaction on the formation of extended electronic states in intentionally created Cl vacancy pairs and chains in a NaCl bilayer on Cu(111). The interaction between the vacancies was tailored by fabricating vacancy pairs and chains of different orientation and separation with atomic precision using vertical manipulation. Small separation of divacancies led to the formation of symmetric and antisymmetric vacancy states and localized interface-states. By scanning tunneling spectroscopy (STS) we measured their energy splitting and broadening as a function of the inter-vacancy separation. Unexpectedly, the energy splitting between the vacancy states is enlarged by level repulsion resulting from phonon dressing of the electronic states, as evidenced by theory. Already for a few coupled vacancies we observe an emerging band structure of the defect band.

First-Principles Study of Carbon and Vacancy Structures in Niobium

The interstitial chemical impurities hydrogen, oxygen, nitrogen, and carbon are important for niobium metal production and particularly for the optimization of niobium SRF technology. These atoms are present in refined sheets and can be absorbed into niobium during processing treatments, resulting in changes to the residual resistance and the performance of SRF cavities. A first-principles approach is taken to study the properties of carbon in niobium, and the results are compared and contrasted with the properties of the other interstitial impurities. The results indicate that C will likely form precipitates or atmospheres around defects rather than strongly bound complexes with other impurities. On the basis of the analysis of carbon and hydrogen near niobium lattice vacancies and small vacancy chains and clusters, the formation of extended carbon chains and hydrocarbons is not likely to occur. Association of carbon with hydrogen atoms can, however, occur through the strain fields created by interstitia...

Vacancy chains and the business cycle. Stringing together job-to-job transitions in micro data

Purpose– The purpose of this paper is to examine the dynamics of labour market flows over the business cycle through a vacancy chain model. It provides a direct computation of vacancy chains using micro data, empirically investigates the relationship between chain length and the characteristics of jobs and workers initiating the chain, and finally assesses the wage progression of workers moving along the chain.Design/methodology/approach– The paper draws on a longitudinal matched employer-employee database covering all employees in manufacturing in a large region of Italy. A transparent algorithm for vacancy chain computation is developed and standard econometric techniques are employed to analyze job-to-job transitions within identified chains.Findings– Vacancy chains account on average for more than one-third of total hires, and both the number and the length of chains are clearly pro-cyclical. Chains set in motion by women workers, young, old, blue collars, or employed by small firms tend to be shorter. There is a well-defined wage progression from the tail to the head of the chain, revealing that workers are sorted along chains according to skill and/or bargaining power.Research limitations/implications– There is a limited possibility of identifying separately individual ability and bargaining power.Practical implications– The vacancy chain methodology can increase the ability of policy makers to produce detailed maps of the labour market and identify worker profiles associated with poor outcomes and hence deserving special attention.Originality/value– For the first time, this paper operationalizes the vacancy chain approach on a large scale, at a very high level of detail, and over a long-time span.

Charge-Induced Defect Formation in LixCoO2 Battery Cathodes: XRD and PA Spectroscopy Study

Lithium-ion batteries have developed into most advanced battery systems, e.g. laptops and mobile phones. LiCoO2 is a typical intercalation battery cathode material. However, reversible charge-discharge cycling of LiCoO2 is only possible down to 50% of the available Li-ions since further removal of Li-ions drastically reduces the capacity and cycle stability. The formation of vacancy-type defects during the charging process in LixCoO2 battery cathodes was investigated by XRD and position life-time spectroscopy and Doppler broadening of positron-electron annihilation (PA) radiation as defect specific techniques [1]. Li+-extraction, which in a battery mode corresponds to charging, was performed at 293 K under electrochemical control in a 3-electrode test-cell with a Maccor Series 4000 battery tester. The composition of the lithium-ion electrode material used was: 88wt.% LiCoO2 particles, 7 wt.% carbon black as conducting agent, 5 wt.% binder material (polyvinylidene difluoride hexafluoropropylene copolymer). Structural analysis of the electrode samples was performed by means of X-Ray diffraction using a Bruker D8 Advance diffractometer in Bragg-Brentano geometry with Cu-Kα radiation. Diffractograms were measured in the 2-Theta angle range from 150to 1300and were analysed by Rietveld refinement with the programs FULLPROF [2] and X'PertHighScorePlus (Panalytical). For positron annihilation measurements a positron source (22NaCl) was sandwiched between two identical LiCoO2 electrode samples. Positron lifetime measurements were performed with a fast-fast spectrometer with a time resolution of 221 ps. The spectra were analysed by using the program pfposfit [3]. Doppler broadening (DB) measurements were performed in a coincidence setup with two high purity Ge detectors.with energy resolution for the 511 keV annihilation γ-line in the detector system corresponds to ca. 0.88keV (FWHM). Both the Doppler broadening S parameter as well as the positron lifetime component τ1 exhibit a characteristic variation with increasing amount of Li+-extraction; the S-parameter and τ1 first increases upon decreasing x from 1 to 0.6. Further Li+-extraction causes a decrease of S and τ1 (x = 0.55), followed by a re-increase for x<0.55. Conclusions: The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from the two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Energy band modulation of graphane by hydrogen-vacancy chains: A first-principles study

We investigated a variety of configurations of hydrogen-vacancy chains in graphane by first-principles density functional calculation. We found that graphane with two zigzag H-vacancy chains segregated by one or more H chain is generally a nonmagnetic conductor or has a negligible band gap. However, the same structure is turned into a semiconductor and generates a magnetic moment if either one or both of the vacancy chains are blocked by isolated H atoms. If H-vacancy chains are continuously distributed, the structure is similar to a zigzag graphene nanoribbon embedded in graphane. It was also found that the embedded zigzag graphene nanoribbon is antiferromagnetic, and isolated H atoms left in the 2-chain nanoribbon can tune the band gap and generate net magnetic moments. Similar effects are also obtained if bare carbon atoms are present outside the nanoribbon. These results are useful for designing graphene-based nanoelectronic circuits.

Effects of the spin–orbit coupling on the vacancy-induced magnetism on the honeycomb lattice

The local magnetism induced by vacancies in the presence of the spin–orbit interaction is investigated based on the half-filled Kane–Mele–Hubbard model on the honeycomb lattice. Using a self-consistent mean-field theory, we find that the spin–orbit coupling will enhance the localization of the spin moments near a single vacancy. We further study the magnetic structures along the zigzag edges formed by a chain of vacancies. We find that the spin–orbit coupling tends to suppress the counter-polarized ferrimagnetic order on the upper and lower edges, because of the open of the spin–orbit gap. As a result, in the case of the balance number of sublattices, it will suppress completely this kind of ferrimagnetic order. But, for the imbalance case, a ferrimagnetic order along both edges exists because additional zero modes will not be affected by the spin–orbit coupling. • Spin–orbit coupling (SOC) localizes a vacancy-induced magnetism on honeycomb lattice. • SOC suppresses counter-polarized magnetism on both inner edges around vacancy chain. • Zero mode protects magnetism around vacancy chain with odd number of sites from SOC.

The Influence of Personality on a Group‐Level Process: Shy Hermit Crabs Make Longer Vacancy Chains

AbstractAnimal personality has been investigated in a variety of contexts but few studies have investigated the effects of individual‐level traits on processes that occur at the group or population level. Vacancy chains describe sequences of linked vacancy moves in systems where individuals use discrete and reuseable resource units. When a new resource unit becomes available, this can cause a cascade of moves between resource units, beyond the first individual that encounters the new unit. Thus, vacancy chains can drive the distribution of resources through a population. Here, I investigate the effect of variation in boldness on movements between vacant gastropod shells in the hermit crab Pagurus bernhardus. Boldness was quantified using the duration of a startle response, which varied consistently between individuals. In groups of five crabs, introduction of a single new shell led to aggregate benefits, in terms of improving shell quality, across all group members. Boldness did not influence the extent of these benefits or the extent to which they were distributed between crabs of different sizes. Initially, vacancy chains were longer, indicating that more individuals moved between shells, for bold groups compared with shy groups. After 24 h, however, the chain length was longer for shy groups. Thus, although participation in a vacancy chain may lead to similar benefits, these could be more widespread for groups comprising shy individuals that show long startle responses, low inquisitiveness and low aggression. Similar effects on resource acquisition could be present in other examples of animals that utilise reusable resource units and show consistent between‐individual differences in behaviour.

Interaction of binary cuprates with oxygen and water vapor at temperatures of 200–400°C

Interaction of binary cuprates with oxygen and water vapor at T = 200–400°C has been studied. It has been established that only compounds containing oxygen vacancy chains in their structure can absorb oxygen and moisture from annealing atmosphere. Absorption of oxygen brings about decrease in the lattice parameters while embedding of OH− groups leads to their growth. In contrast to YBa2Cu3Oy, binary cuprates do not undergo phase transitions in interaction with the atmosphere. Saturation with water and formation of oxyhydroxides is followed by their hydrolytic decomposition involving formation of simpler oxides and hydroxides.

One-dimensional surface damage at grazing projectile incidence: linear vacancy chains on channeled planes

The onset of erosion pattern formation through exposure of the pristine {211} and {321} tungsten surface with 5keV He atoms at grazing incidence has been investigated at 21K by field ion microscopy. We demonstrate that the <111> linear chains of surface vacancies related to irradiation are created in addition to expected individual vacancies and two-dimensional vacancy clusters. The vacancy chains were ideally straight and did not displayed atomic-scale kinks. The asymmetry of the radiation adatom production on the two lateral sides of the <111> vacancy chains was revealed on the W {321} plane. Possible origins for the asymmetry of damage production with respect to the vacancy chains were analyzed in terms of the details of the energy landscape of interatomic interactions on this surface.

Charging-induced defect formation in LixCoO2 battery cathodes studied by positron annihilation spectroscopy

Charging-induced formation of vacancy-type defects in LixCoO2 battery cathodes was studied by the defect-specific techniques of positron lifetime spectroscopy and Doppler broadening of positron–electron annihilation radiation. The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Testing vacancy chain predictions in Pagurus longicarpus hermit crabs: Does ecological gain and behavioral motivation match environmental context?

Both ecological and social contexts are recognized as key variables in hermit crab shell switching behavior, but the impact of site variability on social shell choice has not yet been explored. Pagurus longicarpus marine hermit crabs are common inhabitants of highly variable intertidal habitats where they compete for gastropod shell resources. The goal of this study is two-fold: (1) to assess natural resource (shell) quality and use in the field; (2) to determine how differences in resource quality and ecological context affect social behavioral decisions in the lab. We used vacancy chain theory to analyze the impacts, benefits and consequences of shell choice on the population. Specifically, we quantified movement of shells through a group and described hermit crab shell decision patterns from two different intertidal populations: Beverly (rocky shore) and Nahant, MA (mudflat). Shell damage and shell fit are both known to negatively impact hermit crab fitness and survival; we found that Nahant crabs inhabited poorer quality shells compared to Beverly crabs. Similarly, Nahant crabs were more crowded and spent more time investigating shells. Despite the clear motivational and behavioral differences in populations, vacancy chain lengths were extremely similar between Nahant and Beverly populations, and chains always terminated when the vacant shell was highly damaged, suggesting that chain length is mediated most by shell quality. These results suggest that relative (rather than absolute) differences in resource quality may dictate the dynamics of social vacancy chain outcomes in this species.

First-principles study of hydrogen ordering in lanthanum hydride and its effect on the metal-insulator transition

We discuss the structural details and the ordering of hydrogen in LaH${}_{x}$ for $2\ensuremath{\leqslant}x\ensuremath{\leqslant}3$. To this end, we combine first-principles calculations with the cluster-expansion method. This approach allows us to follow the H occupation of the interstitial sites within the face-centered cubic matrix of La atoms. We find that LaH${}_{x}$ clearly favors the fluorite structure at $x=2$ and adds excess H atoms at the octahedral interstitial sites. The ground-state behavior of the system is discussed and is found in agreement with experimentally observed structures at compositions LaH${}_{2.25}$ and LaH${}_{2.50}$; an additional ground state at composition LaH${}_{2.71}$ is predicted. The cluster expansion also permits an extensive scan of LaH${}_{x}$ structures with two octahedral vacancies per unit cell. For energetically favorable configurations, this scan yields a vacancy percolation threshold at LaH${}_{2.75}$ that possibly drives the concentration-dependent metal-insulator transition: The band gap calculated for isolated vacancy pairs disappears for percolating vacancy chains. This transition from metallic to insulating state is also experimentally observed near to the composition LaH${}_{2.8}$ and gives rise to the ``switchable mirror'' phenomenon.

First principles calculations of oxygen vacancy-ordering effects in resistance change memory materials incorporating binary transition metal oxides

Resistance change random access memories based on transition metal oxides had been recently proposed as promising candidates for the next generation of memory devices, due to their simplicity in composition and scaling capability. The resistance change phenomena had been observed in various materials, however the fundamental understanding of the switching mechanism and of its physical origin has not been agreed upon. We have employed first principles simulations based on density functional theory to elucidate the effect of oxygen vacancies on the electronic structure of rutile TiO2 and NiO using the local density and generalized gradient approximations with correction of on-site Coulomb interactions (LDA + U for TiO2 and GGA + U for NiO). We find that an ordered oxygen vacancy filament induces several defect states within the band gap of both materials, and can lead to the defect-assisted electron transport. This state may account for the “ON”-state low resistance conduction observed experimentally in rutile TiO2 and NiO. As the filament structure is perturbed by oxygen ions moving into the ordered chain of vacancies under applied electric field, charges are trapped and the conductivity can be significantly reduced. We predict this partially disordered arrangement of vacancies may correspond to the “OFF”-state of the resistance change memories.

Oxygen Vacancy Chain Formation in TiO2 under External Strain for Resistive Switching Memory

The electronic structure and vacancy formation energy of rutile TiO2 with ordered oxygen vacancies were calculated using the density functional theory with on-site Coulomb corrections between Ti 3d orbital and O 2p orbital (LDA+Ud+Up). The calculated band gaps are about 3 eV, using LDA+Ud+Up, and a hybrid functional proposed by Heyd–Scuseria–Ernzerhog. The ordered oxygen vacancies were introduced along the [001] direction within a 3×3×4 supercell of rutile TiO2-x that consisted of 72 Ti and 136 O atoms. Biaxial strain was induced in the rutile TiO2 along the x- and y-directions up to ±5%. The lowest formation energy of ordered oxygen vacancies was found in 5% compressive strain and deemed as a thermodynamically favorable structure.

Oxygen Vacancy Chain Formation in TiO2under External Strain for Resistive Switching Memory

Oxygen Vacancy Chain Formation in TiO₂under External Strain for Resistive Switching Memory

Transport properties of HfO2−xbased resistive-switching memories

Transport measurements of both the dc and the low-frequency ac are performed on Pt/HfO2−x/TiN resistiveswitching memory cells at various temperatures. The conductance of the pristine cells has a power law ω S T N relationship with temperature and frequency. To account for the much larger conductance of both the high resistance states (HRSs) and the low resistance states (LRSs), an additional conductance term associated with oxygen vacancy filaments is added, which is independent of the cross-sectional area of the memory cell. This additional component of conductance in a HRS is frequency independent but temperature dependent, showing the small polaron originated transport, with an activation energy of 50 (2.1) meV at temperatures above (below) half of the Debye temperature, which agrees with the analysis of the electric field dependence data. The frequencyand temperature-dependent conduction of HRSs indicate the existence of polarization centers which facilitate the transport and make HfO2−x highly polarizable. However, the additional conductance term associated with filaments in LRS, of an order of ∼10 5 Sm −1 , exhibits a weak metallic behavior in temperature-dependent measurements. Properties of aligned oxygen vacancy chains on the (¯ surface are calculated byfirst-principles simulation. Through analysis of the partial density of states and spatial distribution of the wave function of impurity states generated by oxygen vacancies, this weak metallic behavior is attributed to the delocalization of the impurity band associated with aligned oxygen vacancies.

Homes for hermits: temporal, spatial and structural dynamics as transportable homes are incorporated into a population

AbstractUnderstanding the process by which limiting resources are incorporated into populations is a major goal of ecology. While many studies have examined this dynamic process using essential resources like homes, few of these studies have involved homes that can be transported by their occupants. This study introduced over a thousand transportable homes into a population of terrestrial hermit crabs Coenobita compressus, animals that carry their homes with them wherever they travel. These new homes were tracked between years to test key predictions about the temporal dynamics the homes would generate, and the spatial and structural changes the homes would undergo as they were used by the population. When moving into new homes, crabs dropped off their old homes directly at the exchange site, and the number of such traded‐in homes peaked rapidly in time. Traded‐in homes were under half the diameter of new homes, a difference apparently magnified by social formations involving vacancy chains. After crabs moved into new homes, they carried the homes away from the exchange site. The following year, these homes were displaced a distance four orders of magnitude times their diameter, thus penetrating extensively through the population. Between years, crabs also remodeled the internal architecture of the homes, creating homes that were more spacious and less of a burden to carry. These results suggest that transportable homes generate novel ecological dynamics along temporal, spatial and structural dimensions, which are a direct consequence of their transportability.

Memory and Threshold Resistance Switching in Ni/NiO Core–Shell Nanowires

We report on the first controlled alternation between memory and threshold resistance switching (RS) in single Ni/NiO core-shell nanowires by setting the compliance current (I(CC)) at room temperature. The memory RS is triggered by a high I(CC), while the threshold RS appears by setting a low I(CC), and the Reset process is achieved without setting a I(CC). In combination with first-principles calculations, the physical mechanisms for the memory and threshold RS are fully discussed and attributed to the formation of an oxygen vacancy (Vo) chain conductive filament and the electrical field induced breakdown without forming a conductive filament, respectively. Migration of oxygen vacancies can be activated by appropriate Joule heating, and it is energetically favorable to form conductive chains rather than random distributions due to the Vo-Vo interaction, which results in the nonvolatile switching from the off- to the on-state. For the Reset process, large Joule heating reorders the oxygen vacancies by breaking the Vo-Vo interactions and thus rupturing the conductive filaments, which are responsible for the switching from on- to off-states. This deeper understanding of the driving mechanisms responsible for the threshold and memory RS provides guidelines for the scaling, reliability, and reproducibility of NiO-based nonvolatile memory devices.

High-velocity mass transfer in FCC-metals containing chains of vacancies and interstitial atoms

Using the method of molecular dynamics simulations, the dynamics of structure rearrangement in the course of relaxation in a three-dimensional aluminum is investigated via introduction of one-dimensional chains containing equal number of vacancies and interstitial atoms and located in close-packed positions along the directions. This model represents a starting material structure possessing regions with differing mass densities: m+ and m–. The process of relaxation is shown to proceed via a number of phases: generation of shock waves, nucleation of vortex displacements of atoms, transformation of shock waves into acoustic waves, and correlated high-velocity collective displacements of atoms from interstitial into vacancy positions. The latter displacements are developed at velocities much higher than acoustic velocity.

Conceptualising Social Housing within the Wider Housing Market: A Vacancy Chain Model

There have been increasing calls for studies of housing systems that are more integrative and pluralistic in nature. Understanding the relationship of social housing systems to the wider housing market remains a key challenge. The mobility of households and the structural configuration of supply are both of importance, demanding methods able to reconcile both. This paper propounds vacancy chain models as offering significant potential in this regard, allowing policy analysis and options appraisal to be built on more dynamic conceptualisations of housing systems. The theoretical basis for vacancy chain models is developed before an account is given of a model developed of the Bradford (UK) social rented sector. The results suggest that social renting is very closely linked to the wider housing market and consequently the impacts of policy and investment may be felt beyond the sector. Observations on the future development of vacancy chain models are offered.