Search Results: 1 - 10 of 225528 matches for " Mikael R?sander "
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Magnetism in Co$_{1-{\rm x}}$Fe$_{\rm x}$Sb$_{3}$ skutterudites from density functional theory
Mikael R?sander,Lars Bergqvist
Abstract: We have investigated the electronic and magnetic properties of Co$_{1-{\rm x}}$Fe$_{\rm x}$Sb$_{3}$ skutterudites from density functional theory and Monte Carlo simulations. We find that above a certain threshold in the Fe concentration, somewhere between x=0.125 and x=0.25, Co$_{1-{\rm x}}$Fe$_{\rm x}$Sb$_{3}$ is ferromagnetic with an atomic moment which increases asymptotically towards about 1 $\mu_{B}$/Fe and a non-zero Curie temperature which reaches 70 K for FeSb$_{3}$. Ferromagnetism is favored due to a Stoner instability in the electronic structure, where a large density of states at the Fermi-level makes it favorable to form the ferromagnetic ground state.
Density functional theory study of the electronic structure of fluorite Cu$_{2}$Se
Mikael R?sander,Lars Bergqvist,Anna Delin
Physics , 2012, DOI: 10.1088/0953-8984/25/12/125503
Abstract: We have investigated the electronic structure of fluorite Cu$_{2}$Se using density functional theory calculations within the LDA, PBE and AM05 approximations as well as with the non-local hybrid PBE0 and HSE approximations. Our results show that Cu$_{2}$Se is a zero gap semiconductor when using either a local or semi-local density functional approximation while there exists a gap when using the PBE0 functional. For the HSE approximation, we find that the presence of a gap depends on the range separation for the non-local exchange within the HSE approximation. For the occupied states we find that the LDA, PBE, AM05, PBE0 and HSE agrees when regarding the overall structure, however, the hybrid functionals are shifted towards lower energy values compared to the LDA, PBE and AM05. The valence bands obtained using the hybrid functionals are in good agreement with experimental valence band spectra. We also find that the PBE, PBE0 and HSE approximations give similar results regarding bulk properties, such as lattice constants and bulk modulus. In addition, we have investigated the localization of the Cu d-states and its effect on the band gap in the material using the LDA+U approach. We find that a gap is opened up by increasing the $U$, however, the $U$ values required for a gap opening is unrealistically high.
Electronic Structure and Lattice Dynamics in the FeSb$_{3}$ Skutterudite from Density Functional Theory
Physics , 2014, DOI: 10.1103/PhysRevB.91.014303
Abstract: We have performed density functional calculations of the electronic structure and lattice dynamics of the binary skutterudite FeSb$_{3}$. We find that the ground state of FeSb$_{3}$ is a near semi-metallic ferromagnet with $T_{c}=175$~K. Furthermore, we find that FeSb$_{3}$ is softer than CoSb$_{3}$ based on an analysis of the relation of the elastic constants and the shape of the phonon density of states in the two systems, which is in agreement with experimental observation. Based on these observations we find it plausible that FeSb$_{3}$ will have a lower thermal conductivity than CoSb$_{3}$. Additionally, our calculations indicate that FeSb$_{3}$ may be stable towards decomposition into FeSb$_{2}$ and Sb. Furthermore, for ferromagnetic FeSb$_{3}$ we obtain real-valued phonon frequencies and also a $c_{44}$ greater than zero, indicating that the system is mechanically as well as dynamically stable.
A first principles study of the stability and mobility of defects in titanium carbide
Mikael R?sander,Biplab Sanyal,Ulf Jansson,Olle Eriksson
Abstract: We have performed density functional calculations of the formation energies of substitutional transition metal (TM) defects, C vacancies, and C interstitial defects in TiC. In addition we have evaluated the migration energy barriers for C atoms in the presence of TM impurities. We find that the solubility of TM impurities taken from the 3d TM series is low and only Sc and V impurities can be dissolved into TiC at equilibrium. In addition, we find that the migration energy barriers of C in TiC are greatly affected by the presence of TM impurities: The migration barriers are generally lower in the presence of impurities compared to pure TiC and show a clear dependence on the atomic size of the TM impurities. We propose that the mobility of C in TiC will be the highest in the presence of TM impurities from the middle of the 3d TM series.
Carbon release by selective alloying of transition metal carbides
Mikael R?sander,Erik Lewin,Ola Wilhelmsson,Biplab Sanyal,Mattias Klintenberg,Olle Eriksson,Ulf Jansson
Abstract: We have performed first principles density functional theory calculations on TiC alloyed on the Ti sublattice with 3d transition metals ranging from Sc to Zn. The theory is accompanied with experimental investigations, both as regards materials synthesis as well as characterization. Our results show that by dissolving a metal with a weak ability to form carbides, the stability of the alloy is lowered and a driving force for the release of carbon from the carbide is created. During thin film growth of a metal carbide this effect will favor the formation of a nanocomposite with carbide grains in a carbon matrix. The choice of alloying elements as well as their concentrations will affect the relative amount of carbon in the carbide and in the carbon matrix. This can be used to design the structure of nanocomposites and their physical and chemical properties. One example of applications is as low-friction coatings. Of the materials studied, we suggest the late 3d transition metals as the most promising elements for this phenomenon, at least when alloying with TiC.
A novel wear-resistant magnetic thin film material based on a $Ti_{1-x}Fe_xC_{1-y}$ nanocomposite alloy
Stojanka Bijelovic,Mikael R?sander,Ola Wilhelmsson,Erik Lewin,Biplab Sanyal,Ulf Jansson,Olle Eriksson,Peter Svedlindh
Abstract: In this study we report on the film growth and characterization of thin (approximately 50 nm thick) Ti-Fe-C films deposited on amorphous quartz. The experimental studies have been complemented by first principles density functional theory (DFT) calculations. Upon annealing of as-prepared films, the composition of the metastable Ti-Fe-C film changes. An iron-rich phase is first formed close to the film surface, but with increasing annealing time this phase is gradually displaced toward the film-substrate interface where its position stabilizes. Both the magnetic ordering temperature and the saturation magnetization changes significantly upon annealing. The DFT calculations show that the critical temperature and the magnetic moment both increase with increasing Fe and C-vacancy concentration. The formation of the metastable iron-rich Ti-Fe-C compound is reflected in the strong increase of the magnetic ordering temperature. Eventually, after enough annealing time ($\geq 10$ minutes), nano-crystalline $\alpha$-Fe starts to precipitate and the amount and size of these precipitates can be controlled by the annealing procedure; after 20 minutes of annealing, the experimental results indicate a nano-crystalline iron-film embedded in a wear resistant TiC compound. This conclusion is further supported by transmission electron microscopy studies on epitaxial Ti-Fe-C films deposited on single crystalline MgO substrates where, upon annealing, an iron film embedded in TiC is formed. Our results suggest that annealing of metastable Ti-Fe-C films can be used as an efficient way of creating a wear-resistant magnetic thin film material.
Resistive Graphene Humidity Sensors with Rapid and Direct Electrical Readout
Anderson David Smith,Karim Elgammal,Frank Niklaus,Anna Delin,Andreas Fischer,Sam Vaziri,Fredrik Forsberg,Mikael R?sander,H?kan W. Hugosson,Lars Bergqvist,Stephan Schr?der,Satender Kataria,Mikael ?stling,Max C. Lemme
Physics , 2015, DOI: 10.1039/C5NR06038A
Abstract: We demonstrate humidity sensing using a change of electrical resistance of a single- layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N2), oxygen (O2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. Results from the interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO2 substrate, which in turn leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical read out and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.
It's Not Just the Money: Why Consumers Do Not Purchase Pharmacist-Provided Services [PDF]
Patricia R. Freeman, Mikael Jones, Karen Blumenschein
Pharmacology & Pharmacy (PP) , 2014, DOI: 10.4236/pp.2014.56067
The reimbursement model for pharmaceutical care remains a barrier to successful widespread implementation of pharmacist-provided services. In some instances, community pharmacists have been successful in obtaining direct compensation for services from patients; however, evidence suggests that lack of patient demand for pharmacist-services may ultimately undermine the campaign for widespread third-party payment. The purpose of this study is to conduct a secondary analysis of data indicating consumer/patients' rationale for not purchasing pharmacist-provided disease management services when offered the opportunity to do so in community pharmacies. Our review of the data indicates that while financial concerns are clearly important in consumer demand for pharmacist-provided services, other considerations exist. The consumer/patient belief that pharmacist-provided services are duplicative or that these services are not needed are significant barriers to overcome. Intensive education and marketing campaigns are needed to sway consumer opinion on the value of pharmacist-provided services.
A compilation of tropospheric measurements of gas-phase and aerosol chemistry in polar regions
R. Sander,J. Bottenheim
Earth System Science Data (ESSD) & Discussions (ESSDD) , 2012, DOI: 10.5194/essd-4-215-2012
Abstract: Measurements of atmospheric chemistry in polar regions have been made for more than half a century. Probably the first Antarctic ozone data were recorded in 1958 during the International Geophysical Year. Since then, many measurement campaigns followed, and the results are now spread over many publications in several journals. Here, we have compiled measurements of tropospheric gas-phase and aerosol chemistry made in the Arctic and the Antarctic. It is hoped that this data collection is worth more than the sum of its components and serves as a basis for future analyses of spatial and temporal trends in polar atmospheric chemistry.
Technical note: Simulating chemical systems in Fortran90 and Matlab with the Kinetic PreProcessor KPP-2.1
A. Sandu,R. Sander
Atmospheric Chemistry and Physics (ACP) & Discussions (ACPD) , 2006,
Abstract: This paper presents the new version 2.1 of the Kinetic PreProcessor (KPP). Taking a set of chemical reactions and their rate coefficients as input, KPP generates Fortran90, Fortran77, Matlab, or C code for the temporal integration of the kinetic system. Efficiency is obtained by carefully exploiting the sparsity structures of the Jacobian and of the Hessian. A comprehensive suite of stiff numerical integrators is also provided. Moreover, KPP can be used to generate the tangent linear model, as well as the continuous and discrete adjoint models of the chemical system.