| 1. |
- Snezhkova, Olesia, et al.
(författare)
-
Nature of the bias-dependent symmetry reduction of iron phthalocyanine on Cu(111)
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:7
-
Tidskriftsartikel (refereegranskat)abstract
- Subtle changes in the geometric and electronic properties of supported molecules, with a potential impact on the functioning of molecular devices, can typically be imaged by scanning probe microscopy, but their exact origin and nature often remain unclear. Here we show explicitly that the symmetry reduction of iron phthalocyanine upon adsorption on Cu(111) can be observed not only in scanning tunneling microscopy, but also in core-level spectroscopy, and that it is related to nonisotropic charge transfer into the two principal molecular axes, but in combination with topographic influences.
|
|
| 2. |
- Barthel, J., et al.
(författare)
-
Understanding electron magnetic circular dichroism in a transition potential approach
- 2018
-
Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 97:14
-
Tidskriftsartikel (refereegranskat)abstract
- This paper introduces an approach based on transition potentials for inelastic scattering to understand the underlying physics of electron magnetic circular dichroism (EMCD). The transition potentials are sufficiently localized to permit atomic-scale EMCD. Two-beam and three-beam systematic row cases are discussed in detail in terms of transition potentials for conventional transmission electron microscopy, and the basic symmetries which arise in the three-beam case are confirmed experimentally. Atomic-scale EMCD in scanning transmission electron microscopy (STEM), using both a standard STEM probe and vortex beams, is discussed.
|
|
| 3. |
- Cedervall, Johan, et al.
(författare)
-
Influence of cobalt substitution on the magnetic properties of Fe5PB2
- 2018
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 57:2, s. 777-784
-
Tidskriftsartikel (refereegranskat)abstract
- In this study the effects of cobalt substitutions in Fe5PB2 have been studied. An increased cobalt content reduces the magnetic exchange interactions. This has been concluded from a large, linear decrease in both the Curie temperature as well as the saturated magnetic moment. At high cobalt concentrations, cobalt prefers to order at the M(2) position in the crystal structure. A tunable Curie transition like this shows some prerequisites for magnetic cooling applications.The substitutional effects of cobalt in (Fe1–xCox)5PB2 have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (Fe1–xCox)5PB2 is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (TC) with increasing cobalt content, from 622 to 152 K for Fe5PB2 and (Fe0.3Co0.7)5PB2, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune TC to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.
|
|
| 4. |
- Duan, Yu Xia, et al.
(författare)
-
Crystal electric field splitting and f -electron hybridization in heavy-fermion CePt2In7
- 2019
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9969 .- 2469-9950. ; 100:8
-
Tidskriftsartikel (refereegranskat)abstract
- We use high-resolution angle-resolved photoemission spectroscopy to investigate the electronic structure of the antiferromagnetic heavy fermion compound CePt2In7, which is a member of the CeIn3-derived heavy fermion material family. Weak hybridization among 4f electron states and conduction bands was identified in CePt2In7 at low temperature much weaker than that in the other heavy fermion compounds like CeIrIn5 and CeRhIn5. The Ce 4f spectrum shows fine structures near the Fermi energy, reflecting the crystal electric field splitting of the 4f5/21 and 4f7/21 states. Also, we find that the Fermi surface has a strongly three-dimensional topology, in agreement with density-functional theory calculations. © 2019 American Physical Society.
|
|
| 5. |
- Edstrom, Alexander, et al.
(författare)
-
Quantum mechanical treatment of atomic-resolution differential phase contrast imaging of magnetic materials
- 2019
-
Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 99:17
-
Tidskriftsartikel (refereegranskat)abstract
- Utilizing the Pauli equation based multislice method, introduced in a previous paper [A. Edstrom, A. Lubk, and J. Rusz, Phys. Rev. Lett. 116, 127203 (2016)], we study the atomic-resolution differential phase contrast (DPC) imaging on an example of a hard magnet FePt with in-plane magnetization. Simulated center-of-mass pattern in a scanning transmission electron microscopy experiment carries information about both electric and magnetic fields. The momentum transfer remains curl free, which has consequences for interpretation of the integrated DPC technique. The extracted magnetic component of the pattern is compared to the expected projected microscopic magnetic field as obtained by density functional theory calculation. Qualitative agreement is obtained for low sample thicknesses and a suitable range of collection angles.
|
|
| 6. |
- Edström, Alexander, et al.
(författare)
-
Elastic Scattering of Electron Vortex Beams in Magnetic Matter.
- 2016
-
Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 116:12
-
Tidskriftsartikel (refereegranskat)abstract
- Elastic scattering of electron vortex beams on magnetic materials leads to a weak magnetic contrast due to Zeeman interaction of orbital angular momentum of the beam with magnetic fields in the sample. The magnetic signal manifests itself as a redistribution of intensity in diffraction patterns due to a change of sign of the orbital angular momentum of the electron vortex beam. While in the atomic resolution regime the magnetic signal is most likely under the detection limits of present transmission electron microscopes, for electron probes with high orbital angular momenta, and correspondingly larger spatial extent, its detection is predicted to be feasible.
|
|
| 7. |
- Edström, Alexander, et al.
(författare)
-
Magnetic Effects in the Paraxial Regime of Elastic Electron Scattering
- 2016
-
Ingår i: Physical review B: covering condensed matter and materials physics. - 2469-9950. ; 94:17
-
Tidskriftsartikel (refereegranskat)abstract
- Motivated by a recent claim [Phys.Rev.Lett. 116, 127203 ( 2016)] that electron vortex beams can be used to image magnetism at the nanoscale in elastic scattering experiments, using transmission electron microscopy, a comprehensive computational study is performed to study magnetic effects in the paraxial regime of elastic electron scattering in magnetic solids. Magnetic interactions from electron vortex beams, spin polarized electron beams, and beams with phase aberrations are considered, as they pass through ferromagnetic FePt or antiferromagnetic LaMnAsO. The magnetic signals are obtained by comparing the intensity over a disk in the diffraction plane for beams with opposite angular momentum or aberrations. The strongest magnetic signals are obtained from vortex beams with large orbital angular momentum, where relative magnetic signals above 10(-3) are indicated for 10 (h) over bar orbital angular momentum, meaning that relative signals of one percent could be expected with the even larger orbital angular momenta, which have been produced in experimental setups. All results indicate that beams with low acceleration voltage and small convergence angles yield stronger magnetic signals, which is unfortunately problematic for the possibility of high spatial resolution imaging. Nevertheless, under atomic resolution conditions, relative magnetic signals in the order of 10(-4) are demonstrated, corresponding to an increase with one order of magnitude compared to previous work.
|
|
| 8. |
- Edström, Alexander, et al.
(författare)
-
Magnetic properties of (Fe1-xCox)(2)B alloys and the effect of doping by 5d elements
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:17
-
Tidskriftsartikel (refereegranskat)abstract
- We have explored, computationally and experimentally, the magnetic properties of (Fe1-xCox)(2)B alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing Co2B, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between x = 0.1 and x = 0.5. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated nonmonotonic behavior is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutionally doping the transition metal sublattice by the whole range of 5d transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5 at.% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50% compared to its parent (Fe0.7Co0.3)(2)B compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.
|
|
| 9. |
- Edström, Alexander, 1988-
(författare)
-
Theoretical and Computational Studies on the Physics of Applied Magnetism : Magnetocrystalline Anisotropy of Transition Metal Magnets and Magnetic Effects in Elastic Electron Scattering
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, two selected topics in magnetism are studied using theoretical modelling and computational methods. The first of these is the magnetocrystalline anisotropy energy (MAE) of transition metal based magnets. In particular, ways of finding 3d transition metal based materials with large MAE are considered. This is motivated by the need for new permanent magnet materials, not containing rare-earth elements, but is also of interest for other technological applications, where the MAE is a key quantity. The mechanisms of the MAE in the relevant materials are reviewed and approaches to increasing this quantity are discussed. Computational methods, largely based on density functional theory (DFT), are applied to guide the search for relevant materials. The computational work suggests that the MAE of Fe1-xCox alloys can be significantly enhanced by introducing a tetragonality with interstitial B or C impurities. This is also experimentally corroborated. Alloying is considered as a method of tuning the electronic structure around the Fermi energy and thus also the MAE, for example in the tetragonal compound (Fe1-xCox)2B. Additionally, it is shown that small amounts (2.5-5 at.%) of various 5d dopants on the Fe/Co-site can enhance the MAE of this material with as much as 70%. The magnetic properties of several technologically interesting, chemically ordered, L10 structured binary compounds, tetragonal Fe5Si1-xPxB2 and Hexagonal Laves phase Fe2Ta1-xWx are also investigated. The second topic studied is that of magnetic effects on the elastic scattering of fast electrons, in the context of transmission electron microscopy (TEM). A multislice solution is implemented for a paraxial version of the Pauli equation. Simulations require the magnetic fields in the sample as input. A realistic description of magnetism in a solid, for this purpose, is derived in a scheme starting from a DFT calculation of the spin density or density matrix. Calculations are performed for electron vortex beams passing through magnetic solids and a magnetic signal, defined as a difference in intensity for opposite orbital angular momentum beams, integrated over a disk in the diffraction plane, is observed. For nanometer sized electron vortex beams carrying orbital angular momentum of a few tens of ħ, a relative magnetic signal of order 10-3 is found. This is considered realistic to be observed in experiments. In addition to electron vortex beams, spin polarised and phase aberrated electron beams are considered and also for these a magnetic signal, albeit weaker than that of the vortex beams, can be obtained.
|
|
| 10. |
- Gang, Seung-gi, et al.
(författare)
-
Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation
- 2018
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 97:6
-
Tidskriftsartikel (refereegranskat)abstract
- We studied femtosecond spin dynamics in NixPd1-x magnetic thin films by optically pumping the system with infrared (1.55 eV) laser pulses and subsequently recording the reflectivity of extreme ultraviolet (XUV) pulses synchronized with the pump pulse train. XUV light in the energy range from 20 to 72 eV was produced by laser high-harmonic generation. The reflectivity of XUV radiation at characteristic resonant energies allowed separate detection of the spin dynamics in the elemental subsystems at the M-2,M-3 absorption edges of Ni (68.0 and 66.2 eV) and N-2,N-3 edges of Pd (55.7 and 50.9 eV). The measurements were performed in transversal magneto-optical Kerr effect geometry. In static measurements, we observed a magnetic signature of the Pd subsystem due to an induced magnetization. Calculated magneto-optical asymmetries based on density functional theory show close agreement with the measured results. Femtosecond spin dynamics measured at the Ni absorption edges indicates that increasing the Pd concentration, which causes a decrease in the Curie temperature T-C, results in a drop of the demagnetization time tau(M), contrary to the tau(M) similar to 1/T-C scaling expected for single-species materials. This observation is ascribed to the increase of the Pd-mediated spin-orbit coupling in the alloy.
|
|
| 11. |
- Hedlund, Daniel, et al.
(författare)
-
Magnetic properties of the Fe5SiB2−Fe5PB2 system
- 2017
-
Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 96:9
-
Tidskriftsartikel (refereegranskat)abstract
- The magnetic properties of the compound Fe5Si1−xPxB2 have been studied, with a focus on the Curie temperature TC, saturation magnetization MS, and magnetocrystalline anisotropy. Field and temperature dependent magnetization measurements were used to determine TC(x) and MS(x). The saturation magnetization at 10 K (300 K) is found to monotonically decrease from 1.11MA/m (1.03MA/m) to 0.97MA/m (0.87MA/m), as x increases from 0 to 1. The Curie temperature is determined to be 810 and 615 K in Fe5SiB2 and Fe5PB2, respectively. The highest TC is observed for x=0.1, while it decreases monotonically for larger x. The Curie temperatures have also been theoretically determined to be 700 and 660 K for Fe5SiB2 and Fe5PB2, respectively, using a combination of density functional theory and Monte Carlo simulations. The magnitude of the effective magnetocrystalline anisotropy was extracted using the law of approach to saturation, revealing an increase with increasing phosphorus concentration. Low-field magnetization vs temperature results for x=0,0.1,0.2 indicate that there is a transition from easy-axis to easy-plane anisotropy with decreasing temperature.
|
|
| 12. |
|
|
| 13. |
- Idrobo, Juan Carlos, et al.
(författare)
-
Detecting magnetic ordering with atomic size electron probes
- 2016
-
Ingår i: ADVANCED STRUCTURAL AND CHEMICAL IMAGING. - : SPRINGER HEIDELBERG. - 2198-0926. ; 2
-
Tidskriftsartikel (refereegranskat)abstract
- Although magnetism originates at the atomic scale, the existing spectroscopic techniques sensitive to magnetic signals only produce spectra with spatial resolution on a larger scale. However, recently, it has been theoretically argued that atomic size electron probes with customized phase distributions can detect magnetic circular dichroism. Here, we report a direct experimental real-space detection of magnetic circular dichroism in aberration-corrected scanning transmission electron microscopy (STEM). Using an atomic size-aberrated electron probe with a customized phase distribution, we reveal the checkerboard antiferromagnetic ordering of Mn moments in LaMnAsO by observing a dichroic signal in the Mn L-edge. The novel experimental setup presented here, which can easily be implemented in aberration-corrected STEM, opens new paths for probing dichroic signals in materials with unprecedented spatial resolution.
|
|
| 14. |
- Kislitsyn, Dmitry A., et al.
(författare)
-
Communication : Visualization and spectroscopy of defects induced by dehydrogenation in individual silicon nanocrystals
- 2016
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 144:24
-
Tidskriftsartikel (refereegranskat)abstract
- We present results of a scanning tunneling spectroscopy (STS) study of the impact of dehydrogenation on the electronic structures of hydrogen-passivated silicon nanocrystals (SiNCs) supported on the Au(111) surface. Gradual dehydrogenation is achieved by injecting high-energy electrons into individual SiNCs, which results, initially, in reduction of the electronic bandgap, and eventually produces midgap electronic states. We use theoretical calculations to show that the STS spectra of midgap states are consistent with the presence of silicon dangling bonds, which are found in different charge states. Our calculations also suggest that the observed initial reduction of the electronic bandgap is attributable to the SiNC surface reconstruction induced by conversion of surface dihydrides to monohydrides due to hydrogen desorption. Our results thus provide the first visualization of the SiNC electronic structure evolution induced by dehydrogenation and provide direct evidence for the existence of diverse dangling bond states on the SiNC surfaces. Published by AIP Publishing.
|
|
| 15. |
- Kislitsyn, Dmitry A, et al.
(författare)
-
Mapping of Defects in Individual Silicon Nanocrystals Using Real-Space Spectroscopy.
- 2016
-
Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:6, s. 1047-1054
-
Tidskriftsartikel (refereegranskat)abstract
- The photophysical properties of silicon semiconductor nanocrystals (SiNCs) are extremely sensitive to the presence of surface chemical defects, many of which are easily produced by oxidation under ambient conditions. The diversity of chemical structures of such defects and the lack of tools capable of probing individual defects continue to impede understanding of the roles of these defects in SiNC photophysics. We use scanning tunneling spectroscopy to study the impact of surface defects on the electronic structures of hydrogen-passivated SiNCs supported on the Au(111) surface. Spatial maps of the local electronic density of states (LDOS) produced by our measurements allowed us to identify locally enhanced defect-induced states as well as quantum-confined states delocalized throughout the SiNC volume. We use theoretical calculations to show that the LDOS spectra associated with the observed defects are attributable to Si-O-Si bridged oxygen or Si-OH surface defects.
|
|
| 16. |
- Kocevski, Vancho, 1984-, et al.
(författare)
-
Band alignment switching and the interaction between neighbouring silicon nanocrystals embedded in a SiC matrix
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91, s. 165429-165435
-
Tidskriftsartikel (refereegranskat)abstract
- We present results from density functional theory of the electronic properties of silicon nanocrystals (Si NCs) embedded in a silicon carbide (SiC) matrix, considering different combinations of various NCs and host matrix sizes. We show that the NC and the host matrix form a type-II band alignment, with the states at the top of the valence band being in the Si NC and the states at the bottom of the conduction band in the host matrix. Moreover, this band alignment can be interchanged with introducing oxygen at the interface. We demonstrate that the charge densities of some valence band states can overlap with the charge densities of the neighbouring NCs. We also demonstrate that this leakage of states is significant when the distance between the neighbouring NCs is less than ~1.6 nm.
|
|
| 17. |
- Kocevski, Vancho, 1984-, et al.
(författare)
-
First-principles study of the influence of different interfaces and core types on the properties of CdSe/CdS core-shell nanocrystals
- 2015
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 5
-
Tidskriftsartikel (refereegranskat)abstract
- With the expanding field of nanoengineering and the production of nanocrystals (NCs) with higher quality and tunable size, having reliable theoretical calculations to complement the experimental results is very important. Here we present such a study of CdSe/CdS core-shell NCs using density functional theory, where we focus on dependence of the properties of these NCs on core types and interfaces between the core and the shell, as well as on the core/shell ratio. We show that the density of states and the absorption indices depend rather weakly on the type of interface and core type. We demonstrate that the HOMO wavefunction is mainly localised in the core of the nanocrystal, depending primarily on the core/shell ratio. On the other hand the LUMO wavefunction spreads more into the shell of the nanocrystal, where its confinement in the core is almost the same in each of the studied structural models. Furthermore, we show that the radiative lifetimes decrease with increasing core sizes due to changes in the dipolar overlap integral of the HOMO and LUMO wavefunctions. In addition, the electron-hole Coulomb interaction energies follow a similar pattern as the localisation of the wavefunctions, with the smaller NCs having higher Coulomb interaction energies.
|
|
| 18. |
- Kocevski, Vancho, 1984-, et al.
(författare)
-
Influence of dimensionality and interface type on optical and electronic properties of CdS/ZnS core-shell nanocrystals : a first-principles study
- 2015
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 143:16
-
Tidskriftsartikel (refereegranskat)abstract
- Semiconducting nanocrystals (NCs) have become one of the leading materials in a variety of applications, mainly due to their size tunable band gap and high intensity emission. Their photoluminescence properties can be notably improved by capping the nanocrystals with a shell of another semiconductor, making core-shell structures. We focus our study on the CdS/ZnS core-shell nanocrystals, that are closely related to extensively studied CdSe/CdS NCs, albeit exhibiting rather different photoluminescence properties. We employ density functional theory to investigate the changes in the electronic and optical properties of these nanocrystals with size, core/shell ratio and interface structure between the core and the shell. We show that the band gap depends on the size of the NCs and the core/shell ratio. We suggest that the differences in the density of states and absorption are mainly governed by the core/shell ratio. We present that both the LUMO and the HOMO wavefunctions are localised in the core of the NCs, with the distribution of the LUMO wavefunction being more sensitive to the size and the core/shell ratio. We also demonstrate that the Coulomb interaction energies closely follow the behaviour of the localisation of the HOMO and LUMO wavefunctions, and are decreasing with increasing NC size. Furthermore, we investigated the electronic and optical properties of the NCs with different interfaces between the core and the shell, and different core types. We find that the different interfaces and core types have rather small influence on the band gaps and the absorption indices, as well as on the confinement of the HOMO and LUMO wavefunctions. In addition, we compare these results with the previous results for CdSe/CdS NCs, reflecting the different PL properties of these two types of NCs. We argue that the difference in their Coulomb interaction energies is one of the main reasons for their distinct PL properties.
|
|
| 19. |
- Kocevski, Vancho, 1984-, et al.
(författare)
-
Size dependence of the stability, electronic structure, and optical properties of silicon nanocrystals with various surface impurities
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:12
-
Tidskriftsartikel (refereegranskat)abstract
- We present a comprehensive, ground-state density functional theory study of the size dependence of the optical and electronic properties and the stability of spherical silicon nanocrystals (NCs) with different impurities on the surface. We vary the size of the NCs from 1.0 to 3.5 nm, considering single-bonded (CH3, F, Cl, OH) and double-bonded (O, S) impurities and bridged oxygen. We show that the density of states (DOS) and absorption indices of the NCs with single-bonded impurities are very similar to each other and the fully hydrogenated NCs, except for the 1.0-nm NCs, where a slight difference is present. In the case of the NCs with double-bonded impurities, the DOS and absorption indices exhibit a significant difference, compared to the fully hydrogenated NCs, for sizes up to 2.5 nm. We argue that this difference arises from the difference in the contribution from the impurity to the states around the gap, which can considerably change the character of the states. We demonstrate that the double-bonded impurities contribute significantly to the states around the gap, compared to the single-bonded impurities, causing changes in the symmetry of these states. This observation was further supported by analyzing the changes of the Fourier transform of the charge densities of the highest occupied and lowest unoccupied eigenstate. We also show that the formation energies of NCs with bridged oxygen and fluorine are the lowest, regardless of the size. Furthermore, we show that high hydrogen concentration can be used to suppress the addition of oxygen and fluorine on the surface of the Si NCs.
|
|
| 20. |
- Kocevski, Vancho, 1984-
(författare)
-
Theory and Modelling of Functional Materials
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The diverse field of material research has been steadily expanding with a great help from computational physics, especially in the investigation of the fundamental properties of materials. This has driven the computational physics to become one of the main branches of physics, allowing for density functional theory (DFT) to develop as one of the cornerstones of material research. Nowdays, DFT is the method of choice in a great variety of studies, from fundamental properties, to materials modelling and searching for new materials. In this thesis, DFT is employed for the study of a small part of this vast pool of applications. Specifically, the microscopic characteristics of Zn1-xCdxS alloys are studied by looking into the evolution of the local structure. In addition, the way to model the growth of graphene on Fe(110) surface is discussed. The structural stability of silicon nanocrystals with various shapes is analysed in detail, as well.DFT is further used in studying different properties of semiconductor nanocrystals. The size evolution of the character of the band gap in silicon nanocrystals is investigated in terms of changes in the character of the states around the band gap. The influence of various surface impurities on the band gap, as well as on the electronic and optical properties of silicon nanocrystals is further studied. In addition, the future use of silicon nanocrystals in photovoltaic devices is examined by studying the band alignment and the charge densities of silicon nanocrystals embedded in a silicon carbide matrix. Furthermore, the electronic and optical properties of different semiconductor nanocrystals is also investigated. In the case of the CdSe/CdS and CdS/ZnS core-shell nanocrystals the influence of the nanocrystal size and different structural models on their properties is analysed. For silicon nanocrystal capped with organic ligands, the changes in the optical properties and lifetimes is thoroughly examined with changes in the type of organic ligand.
|
|
| 21. |
|
|
| 22. |
- Kuz'min, M. D., et al.
(författare)
-
Magnetic anisotropy of La2Co7
- 2015
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 118:5
-
Tidskriftsartikel (refereegranskat)abstract
- A magnetization study of a La2Co7 single crystal has obtained the following anisotropy constants: K-1 = 1.4 MJ/m(3) and K-2 = 0.02 MJ/m(3) (at room temperature). The corresponding anisotropy field is 6.7 T; an earlier report of a much higher value (17 T) has not been confirmed. A significant (10%) magnetization anisotropy has been observed. Density-functional calculations are in qualitative agreement with the new data. (C) 2015 AIP Publishing LLC.
|
|
| 23. |
- Lin, J., et al.
(författare)
-
Detection of magnetic circular dichroism in amorphous materials utilizing a single-crystalline overlayer
- 2017
-
Ingår i: Physical Review Materials. - 2475-9953. ; 1:7
-
Tidskriftsartikel (refereegranskat)abstract
- Electron energy-loss magnetic chiral dichroism (EMCD) is a novel technique that allows magnetic information determination down to the nanoscale. However, constrained by the predefined diffraction geometry in regular EMCD experiments, it has not yet been feasible to obtain EMCD signals from amorphous materials, due to the lack of long-range ordering. Here we propose a protocol for EMCD detection in amorphous materials utilizing a single-crystalline overlayer acting as a two-beam splitter. Phase locking of the EMCD signals is observed and explained by two conceivable scenarios. Both experimental results and theoretical calculations demonstrate significant EMCD signals of amorphous materials.
|
|
| 24. |
- Lubk, A., et al.
(författare)
-
Jacob's ladder of approximations to paraxial dynamic electron scattering
- 2015
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 92:23
-
Tidskriftsartikel (refereegranskat)abstract
- Dynamical scattering theory describes the dominant scattering process of beam electrons at targets in the transmission electron microscope (TEM). Hence, practically every quantitative TEM study has to consider its ramifications, typically by some approximate modeling. Here, we elaborate on a hierarchy within the various approximations focusing on the two principal approaches used in practice, Bloch wave and multislice. We reveal characteristic differences in the capability of these methods to reproduce the correct local propagation of the wave function, while convergent results are obtained over larger propagation distances. We investigate the dependency of local variations of the wave function on the atomic number of the atomic scatterers and discuss their significance for, e.g., inelastic scattering.
|
|
| 25. |
- Löfgren, André, et al.
(författare)
-
Influence of nuclear quantum effects on frozen phonon simulations of electron vortex beam HAADF-STEM images.
- 2016
-
Ingår i: Ultramicroscopy. - : Elsevier BV. - 0304-3991 .- 1879-2723. ; 164, s. 62-69
-
Tidskriftsartikel (refereegranskat)abstract
- We have evaluated atomic resolution high-angle annular dark field images with ordinary beams and electron vortex beams for thin crystals of bcc iron, explicitly considering the atomic vibrations using molecular dynamics. The shape of the image representing an atomic column depends on the orbital angular momentum, sample thickness and temperature. For electron vortex beams we observe characteristic doughnut-shaped images of atomic columns. It is shown how the thermal diffuse scattering reduces the depth of their central minima, which get further smeared by finite source size effects. In addition, it is shown that in calculations of HAADF-STEM images at low temperatures one has to explicitly consider the nuclear quantum effects (zero point vibrations), otherwise the effect of atomic vibrations is strongly underestimated.
|
|
