Exchange-correlation potentials for inhomogeneous electron systems in two dimensions from exact diagonalization: comparison with the local-spin-density approximation

We consider electronic exchange and correlation effects in density-functional calculations of two-dimensional systems. Starting from wave function calculations of total energies and electron densities of inhomogeneous model systems, we derive corresponding exchange-correlation potentials and energies. We compare these with predictions of the local-spin-density approximation and discuss its accuracy. Our data will be useful as reference data in testing, comparing and parametrizing exchange and correlation functionals for two-dimensional electronic systems.Comment: Submitted to Physical Review B on January 3, 2012. Second revised version submitted on April 13, 201

Sleep duration and sleep difficulties as predictors of occupational injuries: a cohort study

STUDY OBJECTIVES: To examine the association between sleep duration and sleep difficulties with different types and causes of workplace and commuting injuries. METHODS: The data were derived from the Finnish Public Sector study including 89.543 participants (178.309 person-observations). Participants reported their sleep duration and sleep difficulties between 2000 and 2012. These were linked to occupational injury records from the national register maintained by the Federation of Accident Insurance Institutions. Risk of injuries was followed up 1 year after each study wave. Logistic regression analysis with generalised estimating equations (GEEs) was used to examine the association between sleep duration/difficulties and risk of injuries, and multinomial logistic regression with GEE was used to examine the association with injury types and causes. RESULTS: Both sleep duration and difficulties were associated with injuries. Employees with short sleep (≤6.5 hours) had 1.07-fold odds of workplace injuries (95% CI 1.00 to 1.14) and 1.14 times higher odds of commuting injuries (95% CI 1.04 to 1.26) compared with employees with normal sleep duration. For employees with disturbed sleep, the corresponding ORs were 1.09-fold (95% CI 1.02 to 1.17) and 1.14-fold (95% CI 1.04 to 1.26) compared with those without sleep difficulties, respectively. The risk of commuting injuries was higher among those who had difficulty in falling asleep (OR 1.29, 95% CI 1.07 to 1.55), woke up too early (OR 1.11, 95% CI 1.00 to 1.23) or had non-restorative sleep (OR 1.18, 95% CI 1.05 to 1.33). CONCLUSIONS: Short sleep duration and sleep difficulties are associated with slightly increased risk of workplace and commuting injuries

Scaling in the correlation energies of two-dimensional artificial atoms

We find an unexpected scaling in the correlation energy of artificial atoms, i.e., harmonically confined two-dimensional quantum dots. The scaling relation is found through extensive numerical examinations including Hartree-Fock, variational quantum Monte Carlo, density-functional, and full configuration-interaction calculations. We show that the correlation energy, i.e., the true ground-state total energy subtracted by the Hartree-Fock total energy, follows a simple function of the Coulomb energy, confimenent strength and, the number of electrons. We find an analytic expression for this function, as well as for the correlation energy per particle and for the ratio between the correlation and total energies. Our tests for independent diffusion Monte Carlo and coupled-cluster results for quantum dots -- including open-shell data -- confirm the generality of the obtained scaling. As the scaling is also well applicable to $\gtrsim$ 100 electrons, our results give interesting prospects for the development of correlation functionals within density-functional theory.Comment: Accepted to Journal of Physics: Condensed Matte

Energetics and structure of grain boundary triple junctions in graphene

© 2017 The Author(s). Grain boundary triple junctions are a key structural element in polycrystalline materials. They are involved in the formation of microstructures and can influence the mechanical and electronic properties of materials. In this work we study the structure and energetics of triple junctions in graphene using a multiscale modelling approach based on combining the phase field crystal approach with classical molecular dynamics simulations and quantum-mechanical density functional theory calculations. We focus on the atomic structure and formation energy of the triple junctions as a function of the misorientation between the adjacent grains. We find that the triple junctions in graphene consist mostly of five-fold and seven-fold carbon rings. Most importantly, in addition to positive triple junction formation energies we also find a significant number of orientations for which the formation energy is negative

Sleep duration and sleep difficulties as predictors of occupational injuries: a cohort study

Study objectives To examine the association between sleep duration and sleep difficulties with different types and causes of workplace and commuting injuries. Methods The data were derived from the Finnish Public Sector study including 89.543 participants (178.309 person-observations). Participants reported their sleep duration and sleep difficulties between 2000 and 2012. These were linked to occupational injury records from the national register maintained by the Federation of Accident Insurance Institutions. Risk of injuries was followed up 1 year after each study wave. Logistic regression analysis with generalised estimating equations (GEEs) was used to examine the association between sleep duration/difficulties and risk of injuries, and multinomial logistic regression with GEE was used to examine the association with injury types and causes. Results Both sleep duration and difficulties were associated with injuries. Employees with short sleep (Conclusions Short sleep duration and sleep difficulties are associated with slightly increased risk of workplace and commuting injuries.</p

Thermal conductivity decomposition in two-dimensional materials: Application to graphene

Two-dimensional materials have unusual phonon spectra due to the presence of flexural (out-of-plane) modes. Although molecular dynamics simulations have been extensively used to study heat transport in such materials, conventional formalisms treat the phonon dynamics isotropically. Here, we decompose the microscopic heat current in atomistic simulations into in-plane and out-of-plane components, corresponding to in-plane and out-of-plane phonon dynamics, respectively. This decomposition allows for direct computation of the corresponding thermal conductivity components in two-dimensional materials. We apply this decomposition to study heat transport in suspended graphene, using both equilibrium and nonequilibrium molecular dynamics simulations. We show that the flexural component is responsible for about two-thirds of the total thermal conductivity in unstrained graphene, and the acoustic flexural component is responsible for the logarithmic divergence of the conductivity when a sufficiently large tensile strain is applied

Multiscale modeling of polycrystalline graphene: A comparison of structure and defect energies of realistic samples from phase field crystal models

© 2016 American Physical Society. We extend the phase field crystal (PFC) framework to quantitative modeling of polycrystalline graphene. PFC modeling is a powerful multiscale method for finding the ground state configurations of large realistic samples that can be further used to study their mechanical, thermal, or electronic properties. By fitting to quantum-mechanical density functional theory (DFT) calculations, we show that the PFC approach is able to predict realistic formation energies and defect structures of grain boundaries. We provide an in-depth comparison of the formation energies between PFC, DFT, and molecular dynamics (MD) calculations. The DFT and MD calculations are initialized using atomic configurations extracted from PFC ground states. Finally, we use the PFC approach to explicitly construct large realistic polycrystalline samples and characterize their properties using MD relaxation to demonstrate their quality