Electonic transport properties of nitrate-doped carbon nanotube networks

The conductivity of carbon nanotube (CNT) networks can be improved markedly by doping with nitric acid. In the present work, CNTs and junctions of CNTs functionalized with NO$_3$ molecules are investigated to understand the microscopic mechanism of nitric acid doping. According to our density functional theory band structure calculations, there is charge transfer from the CNT to adsorbed molecules indicating p-type doping. The average doping efficiency of the NO$_3$ molecules is higher if the NO$_3$ molecules form complexes with water molecules. In addition to electron transport along individual CNTs, we have also studied electron transport between different types (metallic, semiconducting) of CNTs. Reflecting the differences in the electronic structures of semiconducting and metallic CNTs, we have found that besides turning semiconducting CNTs metallic, doping further increases electron transport most efficiently along semiconducting CNTs as well as through a junction between them.Comment: 13 pages, 12 figure

Equilibrium shape and dislocation nucleation in strained epitaxial nanoislands

We study numerically the equilibrium shapes, shape transitions and dislocation nucleation of small strained epitaxial islands with a two-dimensional atomistic model, using simple interatomic pair potentials. We first map out the phase diagram for the equilibrium island shapes as a function of island size (up to N = 105 atoms) and lattice misfit with the substrate and show that nanoscopic islands have four generic equilibrium shapes, in contrast with predictions from the continuum theory of elasticity. For increasing substrate-adsorbate attraction, we find islands that form on top of a finite wetting layer as observed in Stranski-Krastanow growth. We also investigate energy barriers and transition paths for transitions between different shapes of the islands and for dislocation nucleation in initially coherent islands. In particular, we find that dislocations nucleate spontaneously at the edges of the adsorbate-substrate interface above a critical size or lattice misfit.Comment: 4 pages, 3 figures, uses wrapfig.sty and epsfig.st

Stress release mechanisms for Cu on Pd(111) in the submonolayer and monolayer regimes

We study the strain relaxation mechanisms of Cu on Pd(111) up to the monolayer regime using two different computational methodologies, basin-hopping global optimization and energy minimization with a repulsive bias potential. Our numerical results are consistent with experimentally observed layer-by-layer growth mode. However, we find that the structure of the Cu layer is not fully pseudomorphic even at low coverages. Instead, the Cu adsorbates forms fcc and hcp stacking domains, separated by partial misfit dislocations. We also estimate the minimum energy path and energy barriers for transitions from the ideal epitaxial state to the fcc-hcp domain pattern.Comment: 4 pages, 4 figure

A chain-of-states acceleration method for the efficient location of minimum energy paths

We describe a robust and efficient chain-of-states method for computing Minimum Energy Paths (MEPs) associated to barrier-crossing events in poly-atomic systems, which we call the acceleration method. The path is parametrized in terms of a continuous variable t ∈ [0, 1] that plays the role of time. In contrast to previous chain-of-states algorithms such as the nudged elastic band or string methods, where the positions of the states in the chain are taken as variational parameters in the search for the MEP, our strategy is to formulate the problem in terms of the second derivatives of the coordinates with respect to t, i.e., the state accelerations. We show this to result in a very simple and efficient method for determining the MEP. We describe the application of the method to a series of test cases, including two low-dimensional problems and the Stone-Wales transformation in C60This work has been supported by the Spanish Research and Innovation Office through Project Nos. FIS2012-31713 and FIS2012-3754

Epidemiology of Spinal Cord Injury in Iceland from 1975 to 2014

Efst á síðunni er hægt að nálgast greinina í heild sinni með því að smella á hlekkinnInngangur: Mænuskaði er meðal alvarlegustu afleiðinga slysa og þar sem ekki hefur tekist að finna lækningu við mænuskaða hefur áhersla verið lögð á forvarnarstarf. Nýgengi mænuskaða er breytilegt milli landa og ýmsar breytingar hafa orðið á faraldsfræði mænuskaða í áranna rás. Markmið þessarar rannsóknar var að kanna faraldsfræði mænuskaða af völdum slysa á Íslandi og leita áhættuþátta sem nýta mætti í forvarnarskyni. Efniviður og aðferðir: Farið var afturskyggnt yfir sjúkraskrár allra sem greindust með mænuskaða samkvæmt ICD-9/ICD-10 á Landspítala á árunum 1975-2014. Upplýsinga var aflað um nýgengi, aldur, kynjaskiptingu og orsakir. Notaður var skalinn American Spinal Injury Association Impairment Scale (AIS) við mat á alvarleika mænuskaða. Niðurstöður: Á rannsóknartímabilinu hlutu 233 einstaklingar mænuskaða af völdum áverka, eða 26 á hverja milljón íbúa á ári að meðaltali. Karlmenn voru 73% og meðalaldurinn 39 ár. Umferðarslys voru algengasta orsök mænuskaða. Oftast var um að ræða bílveltur í dreifbýli og í að minnsta kosti helmingi tilfella voru bílbelti ekki notuð. Fall var næstalgengasta orsök mænuskaða en í þeim orsakaflokki var meðalaldurinn hæstur. Reiðmennsku- og vetraríþróttaslys voru algengust íþrótta- og tómstundaslysa. Í um þriðjungi mænuskaðatilfella var um að ræða alskaða á mænu. Við útskrift höfðu 9% náð fullum bata. Ályktun: Mikilvægt er að efla enn frekar forvarnir og áróður í tengslum við bílbeltanotkun og öryggi á vegum landsins. Einnig þarf að leita leiða til að fækka mænuskaðatilfellum vegna falls, svo sem með því að kanna nánar ástæður falla hjá eldra fólki og bæta öryggisreglur á vinnustöðum. Hugsanlega mætti fækka íþrótta- og tómstundaslysum með forvarnaraðgerðum og bættum öryggisbúnaði.Introduction: Traumatic spinal cord injury (TSCI) is serious and often has long-term consequences. Since no cure has been found the emphasis has been on preventive measures. The incidence of TSCI varies between countries and the epidemiology has been changing. The aim of this study was to gather epidemiological data on patients with TSCI in Iceland and search for risk factors. Material and methods: Hospital records of everyone diagnosed with TSCI in 1975-2014 admitted to Landspitali University Hospital were reviewed and information gathered on incidence, age, gender and causes of injury. The American Spinal Injury Association Impairment Scale (AIS) was used to assess the extent of TSCI. Results: A total of 233 patients were diagnosed with TSCI during the study period or 26 per million annually on average. Males were 73% and the mean age was 39 years. Traffic accidents were the most common cause of TSCI. The majority were car rollovers in rural areas. Around 50% did not use a seatbelt. The second most common cause of TSCI were falls. The most common sport/leisure accidents were those related to horseback-riding and winter sports. A third of patients had a complete SCI. At discharge 9% had gained full recovery. Conclusions: Safe roads and good traffic culture are essential factors in the prevention of serious traffic accidents. Strict safety regulations in the work place and an investigation of causes of falls amongst the elderly could decrease SCIs due to falls. Further preventive measures and protective equipment could possibly be of use in sport- or leisure-related activities

Global transition path search for dislocation formation in Ge on Si(001)

Global optimization of transition paths in complex atomic scale systems is addressed in the context of misfit dislocation formation in a strained Ge film on Si(001). Such paths contain multiple intermediate minima connected by minimum energy paths on the energy surface emerging from the atomic interactions in the system. The challenge is to find which intermediate states to include and to construct a path going through these intermediates in such a way that the overall activation energy for the transition is minimal. In the numerical approach presented here, intermediate minima are constructed by heredity transformations of known minimum energy structures and by identifying local minima in minimum energy paths calculated using a modified version of the nudged elastic band method. Several mechanisms for the formation of a 90{\deg} misfit dislocation at the Ge-Si interface are identified when this method is used to construct transition paths connecting a homogeneously strained Ge film and a film containing a misfit dislocation. One of these mechanisms which has not been reported in the literature is detailed. The activation energy for this path is calculated to be 26% smaller than the activation energy for half loop formation of a full, isolated 60{\deg} dislocation. An extension of the common neighbor analysis method involving characterization of the geometrical arrangement of second nearest neighbors is used to identify and visualize the dislocations and stacking faults

Orbital-optimized Density Functional Calculations of Molecular Rydberg Excited States with Real Space Grid Representation and Self-Interaction Correction

Density functional calculations of Rydberg excited states up to high energy are carried out for several molecules using an approach where the orbitals are variationally optimized by converging on saddle points on the electronic energy surface within a real space grid representation. Remarkably good agreement with experimental estimates of the excitation energy is obtained using the generalized gradient approximation (GGA) functional of Perdew, Burke and Ernzerhof (PBE) when Perdew-Zunger self-interaction correction is applied in combination with complex-valued orbitals. Even without the correction, the PBE functional gives quite good results despite the fact that corresponding Rydberg virtual orbitals have positive energy in the ground state calculation. Results obtained using the TPSS and r2SCAN meta-GGA functionals are also presented, but they do not provide a systematic improvement over the results from the uncorrected PBE functional. The grid representation combined with the projector augmented-wave approach gives a simpler and better representation of the diffuse Rydberg orbitals than a linear combination of atomic orbitals with commonly used basis sets, the latter leading to an overestimation of the excitation energy due to confinement of the excited states.Comment: 34 pages, 8 figure

Meteorological effects of the solar eclipse of 20 March 2015: analysis of UK Met Office automatic weather station data and comparison with automatic weather station data from the Faroes and Iceland.

Here, we analyse high-frequency (1 min) surface air temperature, mean sea-level pressure (MSLP), wind speed and direction and cloud-cover data acquired during the solar eclipse of 20 March 2015 from 76 UK Met Office weather stations, and compare the results with those from 30 weather stations in the Faroe Islands and 148 stations in Iceland. There was a statistically significant mean UK temperature drop of 0.83±0.63°C, which occurred over 39 min on average, and the minimum temperature lagged the peak of the eclipse by about 10 min. For a subset of 14 (16) relatively clear (cloudy) stations, the mean temperature drop was 0.91±0.78 (0.31±0.40)°C but the mean temperature drops for relatively calm and windy stations were almost identical. Mean wind speed dropped significantly by 9% on average during the first half of the eclipse. There was no discernible effect of the eclipse on the wind-direction or MSLP time series, and therefore we can discount any localized eclipse cyclone effect over Britain during this event. Similar changes in air temperature and wind speed are observed for Iceland, where conditions were generally clearer, but here too there was no evidence of an eclipse cyclone; in the Faroes, there was a much more muted meteorological signature.This article is part of the themed issue 'Atmospheric effects of solar eclipses stimulated by the 2015 UK eclipse'

Structure and variability of the shelfbreak East Greenland Current north of Denmark Strait

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 47 (2017): 2631-2646, doi:10.1175/JPO-D-17-0062.1.Data from a mooring array deployed north of Denmark Strait from September 2011 to August 2012 are used to investigate the structure and variability of the shelfbreak East Greenland Current (EGC). The shelfbreak EGC is a surface-intensified current situated just offshore of the east Greenland shelf break flowing southward through Denmark Strait. This study identified two dominant spatial modes of variability within the current: a pulsing mode and a meandering mode, both of which were most pronounced in fall and winter. A particularly energetic event in November 2011 was related to a reversal of the current for nearly a month. In addition to the seasonal signal, the current was associated with periods of enhanced eddy kinetic energy and increased variability on shorter time scales. The data indicate that the current is, for the most part, barotropically stable but subject to baroclinic instability from September to March. By contrast, in summer the current is mainly confined to the shelf break with decreased eddy kinetic energy and minimal baroclinic conversion. No other region of the Nordic Seas displays higher levels of eddy kinetic energy than the shelfbreak EGC north of Denmark Strait during fall. This appears to be due to the large velocity variability on mesoscale time scales generated by the instabilities. The mesoscale variability documented here may be a source of the variability observed at the Denmark Strait sill.Support for this work was provided by the Norwegian Research Council under Grant Agreement 231647 (LH and KV) and the Bergen Research Foundation under Grant BFS2016REK01 (KV). Additional funding was provided by the National Science Foundation under Grants OCE-0959381 and OCE-1558742 (RP)

Force-matched embedded-atom method potential for niobium

Large-scale simulations of plastic deformation and phase transformations in alloys require reliable classical interatomic potentials. We construct an embedded-atom method potential for niobium as the first step in alloy potential development. Optimization of the potential parameters to a well-converged set of density-functional theory (DFT) forces, energies, and stresses produces a reliable and transferable potential for molecular dynamics simulations. The potential accurately describes properties related to the fitting data, and also produces excellent results for quantities outside the fitting range. Structural and elastic properties, defect energetics, and thermal behavior compare well with DFT results and experimental data, e.g., DFT surface energies are reproduced with less than 4% error, generalized stacking-fault energies differ from DFT values by less than 15%, and the melting temperature is within 2% of the experimental value.Comment: 17 pages, 13 figures, 7 table