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- Dziekan, Thomas, 1975-
(författare)
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Electronic Transport in Strained Materials
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis the conductivity of strained materials has been investigated using density functional theory and a semiclassical transport theory based on the Boltzmann equation.In transition metals trends are reproduced without adjustable parameters. The introduction of one temperature dependent cross section allowed the reproduction of resistivity trends between 10 and 1000K.The effect of strain on transition metals in bcc and fcc structure was studied deforming the unit cell along the tetragonal deformation path. The anisotropy of the conductivity varied on wide range of the c/a-ratio. The orbitals at the Fermi level determined the principal behavior. Pairs of elements with permutated number of electrons and holes in the 4d band showed similar behavior. The concept of the tetragonal deformation was also applied on semiconductors.The deformation of Vanadium in X/V superlattices (X=Cr,~Fe,~Mo) due to Hydrogen loading depends on the properties of X. It was found that counteracting effects due to the presence of Hydrogen influence the conductivity.It is shown that a small magnetic moment of the V host reduces the hydrogen solubility. Depending on the magnitude of the tetragonal distortion of V, the hydrogen dissolution becomes favored for larger moments.Finally, extra charge filling of the bandstructure of Cr and Mo decreases the Fermi velocity and increases the density of states at the Fermi energy.
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- Dziekan, Thomas, et al.
(författare)
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Influence of vanadium spin-polarization on the dissolution of hydrogen in vanadium
- 2009
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Ingår i: PHYSICAL REVIEW B. - 1098-0121 .- 1550-235X. ; 79:1, s. 012402-
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Tidskriftsartikel (refereegranskat)abstract
- Transition-metal (TM)/V superlattices (TM=Cr,Fe,Mo) show certain peculiarities under hydrogen uptake. Here we investigate the influence of an induced magnetization of the V layers on the hydrogen dissolution by means of first-principles calculations. We find that below a certain value for the magnetic moment of the V host the hydrogen solubility is slightly reduced, whereas for larger moments the hydrogen dissolution becomes favored. The actual position of this transition depends on the tetragonal distortion of the V layers.
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- Dziekan, Thomas, et al.
(författare)
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Theoretical calculations of mobility enhancement in strained silicon
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195213-
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Tidskriftsartikel (refereegranskat)abstract
- The conductivity enhancement of n -doped Si due to strain has been calculated by solving the Boltzmann equation within the relaxation-time approximation. For T=0 , we determined the Fermi surface from first principles, whereas for T>0 , we modeled the Fermi surface using the effective-mass approximation. We explain the saturation of the conductivity enhancement with increasing strain in terms of the topology of the Fermi surface. We find in accordance with experiment a decreasing enhancement with increasing doping concentration.
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- Grossmann, Igor, et al.
(författare)
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Insights into the accuracy of social scientists' forecasts of societal change
- 2023
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Ingår i: Nature Human Behaviour. - : Springer Nature. - 2397-3374. ; 7, s. 484-501
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Tidskriftsartikel (refereegranskat)abstract
- How well can social scientists predict societal change, and what processes underlie their predictions? To answer these questions, we ran two forecasting tournaments testing the accuracy of predictions of societal change in domains commonly studied in the social sciences: ideological preferences, political polarization, life satisfaction, sentiment on social media, and gender-career and racial bias. After we provided them with historical trend data on the relevant domain, social scientists submitted pre-registered monthly forecasts for a year (Tournament 1; N = 86 teams and 359 forecasts), with an opportunity to update forecasts on the basis of new data six months later (Tournament 2; N = 120 teams and 546 forecasts). Benchmarking forecasting accuracy revealed that social scientists' forecasts were on average no more accurate than those of simple statistical models (historical means, random walks or linear regressions) or the aggregate forecasts of a sample from the general public (N = 802). However, scientists were more accurate if they had scientific expertise in a prediction domain, were interdisciplinary, used simpler models and based predictions on prior data. How accurate are social scientists in predicting societal change, and what processes underlie their predictions? Grossmann et al. report the findings of two forecasting tournaments. Social scientists' forecasts were on average no more accurate than those of simple statistical models.
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- Meded, Velimir, et al.
(författare)
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Anomalies in transition metal conductivity : Strong evidence for Fermi-velocity dominance
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:15, s. 155123-
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Tidskriftsartikel (refereegranskat)abstract
- We have calculated the conductivities of the 3d, 4d, and 5d transition metals by using a quasiclassical Boltzmann approach which utilizes self-consistent energy bands. Electron scattering is characterized by a constant scattering strength which treats all sources of resistivity on an equal footing. This results in a single parameter theory with which we are able to reproduce, qualitatively, trends and anomalies in conductivity throughout the transition metal series. By the choice of an appropriate scattering strength at each temperature, we are able to reproduce these trends over the experimentally observable temperature range of 10-1000 K. In particular, the well known dip in conductivity at the middle of the series, especially pronounced for Mn, is understood in terms of the host electronic structure.
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