We compute the orbital magnetization in real materials by evaluating a recently discovered formul... more We compute the orbital magnetization in real materials by evaluating a recently discovered formula for periodic systems, within density functional theory. We obtain improved values of the orbital magnetization in the ferromagnetic metals Fe, Co, and Ni, by taking into account the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the EPR $g$-tensor in molecules and solids. The present approach reproduces the $g$-tensor obtained by linear response (LR), when the spin-orbit can be treated as a perturbation. However, it can also be applied to radicals and defects with an orbital-degenerate ground-state or containing heavy atoms, that can not be properly described by LR.
The nearest-neighbor antisite pair defects in 4H-SiC, 6H-SiC, and 3C-SiC single crystals have bee... more The nearest-neighbor antisite pair defects in 4H-SiC, 6H-SiC, and 3C-SiC single crystals have been identified using electron paramagnetic resonance spectroscopy in combination with a nonperturbative ab initio scheme for the electronic g tensor. Based on the theoretical predictions, the positively charged defect has been found experimentally also in the cubic 3C-SiC polytype where it is characterized by spin 1/2 and highly anisotropic g values of g xx = 2.0030, g yy = 2.0241, and g zz = 2.0390 within C 1h symmetry. The exceptional large g values are explained by details of the spin-orbit coupling causing a strongly anisotropic quenching of the orbital angular momentum of the p-like unpaired electron.
Physical Review B Condensed Matter and Materials Physics, 2010
Within density-functional theory we compute the orbital magnetization for periodic systems evalua... more Within density-functional theory we compute the orbital magnetization for periodic systems evaluating a recently discovered Berry-phase formula. For the ferromagnetic metals Fe, Co, and Ni we explicitly calculate the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the electron paramagnetic resonance g tensor in paramagnetic systems. Here the method can also be applied in cases where linear-response theory fails, e.g., radicals and defects with an orbital-degenerate ground state or those containing heavy atoms.
The journal of physical chemistry. B, Jan 23, 2016
We study temperature-dependent hole transport in ideal crystal-phase poly(3-hexylthiophene) (P3HT... more We study temperature-dependent hole transport in ideal crystal-phase poly(3-hexylthiophene) (P3HT) with ab initio calculations, with the aim of estimating the maximum mobility in the limit of perfect order. To this end, the molecular transfer integrals, phonon frequencies, and electron-phonon coupling constants are obtained from density functional theory (DFT). This allows the determination of transport properties without fit parameters. The strong coupling between charge carriers and vibrations leads to strong scattering and polaronic effects that impact carrier transport. By providing an intrinsic mobility limit to ideal P3HT crystals, this work allows identification of the impact of disorder on the temperature-dependent transport in real samples. A detailed analysis of the transport-relevant phonon modes is provided that gives microscopic insight into the polaron effects and hints toward mobility optimization strategies.
In a recent paper [M. T. Bennebroek et al., Phys. Rev. B 54, 11 276 (1996)] shallow electron cent... more In a recent paper [M. T. Bennebroek et al., Phys. Rev. B 54, 11 276 (1996)] shallow electron centers in AgBr and AgCl have been investigated by pulsed ENDOR. From an analysis of these data in terms of the effective-mass approximation it has been concluded that the states at the bottom of the conduction band, commonly believed to be solely due to silver 5s states, must have a contribution from halogen s states that is too large to be accounted for by a one-electron theory. We have calculated the hyperfine interaction for these states using an ab initio calculation based on the local spin-density approximation of the density-functional theory. Our results which are in close agreement with the published experimental data show that the halogen s-like contributions to the conduction-band states are much more localized than the corresponding unoccupied atomic s states. Shallow electron centers in silver halides seem to be the first systems for which the ligand hyperfine interactions are quantitatively described within the effective-mass approximation.
ABSTRACT We investigated radiation-induced defects in neutron-irradiated and subsequently anneale... more ABSTRACT We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S = 1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite-carbon vacancy pair (C-Si-V-C) in the doubly positive charge state can explain all experimental findings. We propose that the (C-Si-V-C) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy.
We compute the orbital magnetization in real materials by evaluating a recently discovered formul... more We compute the orbital magnetization in real materials by evaluating a recently discovered formula for periodic systems, within density functional theory. We obtain improved values of the orbital magnetization in the ferromagnetic metals Fe, Co, and Ni, by taking into account the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the EPR $g$-tensor in molecules and solids. The present approach reproduces the $g$-tensor obtained by linear response (LR), when the spin-orbit can be treated as a perturbation. However, it can also be applied to radicals and defects with an orbital-degenerate ground-state or containing heavy atoms, that can not be properly described by LR.
Physical Review B Condensed Matter and Materials Physics, May 1, 2002
We have calculated the electronic structure of the lattice vacancy in silicon in the negative cha... more We have calculated the electronic structure of the lattice vacancy in silicon in the negative charge state V- using the self-consistent charge density-functional theory based tight-binding scheme for the computation of large supercells containing up to 512 atoms in combination with the linear muffin-tin orbitals method in the atomic-spheres approximation. Many-body effects are treated in the local spin density approximation of the density functional theory (LSDA-DFT). We find the ground state of the V- to be the low-spin 2B1 state of the group C2v, which is lower in energy by 0.09 eV than the 4A2 high-spin state of the group Td. We have also calculated the hyperfine interactions with 18 shells containing 46 29Si ligand atoms. We find the largest HF interactions in the (11bar0) plane in agreement with experimental data. The HF interactions with nuclei in the (110) plane, which are about two orders of magnitude smaller than those with nuclei in the (11bar0) plane, also agree with the experimental data. We conclude that the LSDA-DFT describes the magnetization density of the V- well. It is therefore not necessary to include configuration interactions as has been proposed by M. Lannoo [Phys. Rev. B 28, 2403 (1983)].
We investigate the validity of the Makov–Payne correction if applied to supercell calculations fo... more We investigate the validity of the Makov–Payne correction if applied to supercell calculations for charged defects by comparing the results of a supercell calculations with those obtained using a Green's function approach. For several defects in 3C–SiC the observed energy differences between both calculations are not bridged by the corrections of Makov–Payne. Whereas a correction proportional to the defect charge
By combining scanning tunneling microscopy with density functional theory it is shown that the Bi... more By combining scanning tunneling microscopy with density functional theory it is shown that the Bi(111) surface provides a well-defined incorporation site in the first bilayer that traps highly coordinating atoms such as transition metals (TMs) or noble metals. All deposited atoms assume exactly the same specific sevenfold coordinated subsurface interstitial site while the surface topography remains nearly unchanged. Notably, 3d TMs show a barrier-free incorporation. The observed surface modification by barrier-free subsorption helps to suppress aggregation in clusters. It allows a tuning of the electronic properties not only for the pure Bi surface, but may also be observed for topological insulators formed by substrate-stabilized Bi bilayers.
We compute the orbital magnetization in real materials by evaluating a recently discovered formul... more We compute the orbital magnetization in real materials by evaluating a recently discovered formula for periodic systems, within density functional theory. We obtain improved values of the orbital magnetization in the ferromagnetic metals Fe, Co, and Ni, by taking into account the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the EPR $g$-tensor in molecules and solids. The present approach reproduces the $g$-tensor obtained by linear response (LR), when the spin-orbit can be treated as a perturbation. However, it can also be applied to radicals and defects with an orbital-degenerate ground-state or containing heavy atoms, that can not be properly described by LR.
The nearest-neighbor antisite pair defects in 4H-SiC, 6H-SiC, and 3C-SiC single crystals have bee... more The nearest-neighbor antisite pair defects in 4H-SiC, 6H-SiC, and 3C-SiC single crystals have been identified using electron paramagnetic resonance spectroscopy in combination with a nonperturbative ab initio scheme for the electronic g tensor. Based on the theoretical predictions, the positively charged defect has been found experimentally also in the cubic 3C-SiC polytype where it is characterized by spin 1/2 and highly anisotropic g values of g xx = 2.0030, g yy = 2.0241, and g zz = 2.0390 within C 1h symmetry. The exceptional large g values are explained by details of the spin-orbit coupling causing a strongly anisotropic quenching of the orbital angular momentum of the p-like unpaired electron.
Physical Review B Condensed Matter and Materials Physics, 2010
Within density-functional theory we compute the orbital magnetization for periodic systems evalua... more Within density-functional theory we compute the orbital magnetization for periodic systems evaluating a recently discovered Berry-phase formula. For the ferromagnetic metals Fe, Co, and Ni we explicitly calculate the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the electron paramagnetic resonance g tensor in paramagnetic systems. Here the method can also be applied in cases where linear-response theory fails, e.g., radicals and defects with an orbital-degenerate ground state or those containing heavy atoms.
The journal of physical chemistry. B, Jan 23, 2016
We study temperature-dependent hole transport in ideal crystal-phase poly(3-hexylthiophene) (P3HT... more We study temperature-dependent hole transport in ideal crystal-phase poly(3-hexylthiophene) (P3HT) with ab initio calculations, with the aim of estimating the maximum mobility in the limit of perfect order. To this end, the molecular transfer integrals, phonon frequencies, and electron-phonon coupling constants are obtained from density functional theory (DFT). This allows the determination of transport properties without fit parameters. The strong coupling between charge carriers and vibrations leads to strong scattering and polaronic effects that impact carrier transport. By providing an intrinsic mobility limit to ideal P3HT crystals, this work allows identification of the impact of disorder on the temperature-dependent transport in real samples. A detailed analysis of the transport-relevant phonon modes is provided that gives microscopic insight into the polaron effects and hints toward mobility optimization strategies.
In a recent paper [M. T. Bennebroek et al., Phys. Rev. B 54, 11 276 (1996)] shallow electron cent... more In a recent paper [M. T. Bennebroek et al., Phys. Rev. B 54, 11 276 (1996)] shallow electron centers in AgBr and AgCl have been investigated by pulsed ENDOR. From an analysis of these data in terms of the effective-mass approximation it has been concluded that the states at the bottom of the conduction band, commonly believed to be solely due to silver 5s states, must have a contribution from halogen s states that is too large to be accounted for by a one-electron theory. We have calculated the hyperfine interaction for these states using an ab initio calculation based on the local spin-density approximation of the density-functional theory. Our results which are in close agreement with the published experimental data show that the halogen s-like contributions to the conduction-band states are much more localized than the corresponding unoccupied atomic s states. Shallow electron centers in silver halides seem to be the first systems for which the ligand hyperfine interactions are quantitatively described within the effective-mass approximation.
ABSTRACT We investigated radiation-induced defects in neutron-irradiated and subsequently anneale... more ABSTRACT We investigated radiation-induced defects in neutron-irradiated and subsequently annealed 6H-silicon carbide (SiC) with electron paramagnetic resonance (EPR), the magnetic circular dichroism of the absorption (MCDA), and MCDA-detected EPR (MCDA-EPR). In samples annealed beyond the annealing temperature of the isolated silicon vacancy we observed photoinduced EPR spectra of spin S = 1 centers that occur in orientations expected for nearest neighbor pair defects. EPR spectra of the defect on the three inequivalent lattice sites were resolved and attributed to optical transitions between photon energies of 999 and 1075 meV by MCDA-EPR. The resolved hyperfine structure indicates the presence of one single carbon nucleus and several silicon ligand nuclei. These experimental findings are interpreted with help of total energy and spin density data obtained from the standard local-spin density approximation of the density-functional theory, using relaxed defect geometries obtained from the self-consistent charge density-functional theory based tight binding scheme. We have checked several defect models of which only the photoexcited spin triplet state of the carbon antisite-carbon vacancy pair (C-Si-V-C) in the doubly positive charge state can explain all experimental findings. We propose that the (C-Si-V-C) defect is formed from the isolated silicon vacancy as an annealing product by the movement of a carbon neighbor into the vacancy.
We compute the orbital magnetization in real materials by evaluating a recently discovered formul... more We compute the orbital magnetization in real materials by evaluating a recently discovered formula for periodic systems, within density functional theory. We obtain improved values of the orbital magnetization in the ferromagnetic metals Fe, Co, and Ni, by taking into account the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the EPR $g$-tensor in molecules and solids. The present approach reproduces the $g$-tensor obtained by linear response (LR), when the spin-orbit can be treated as a perturbation. However, it can also be applied to radicals and defects with an orbital-degenerate ground-state or containing heavy atoms, that can not be properly described by LR.
Physical Review B Condensed Matter and Materials Physics, May 1, 2002
We have calculated the electronic structure of the lattice vacancy in silicon in the negative cha... more We have calculated the electronic structure of the lattice vacancy in silicon in the negative charge state V- using the self-consistent charge density-functional theory based tight-binding scheme for the computation of large supercells containing up to 512 atoms in combination with the linear muffin-tin orbitals method in the atomic-spheres approximation. Many-body effects are treated in the local spin density approximation of the density functional theory (LSDA-DFT). We find the ground state of the V- to be the low-spin 2B1 state of the group C2v, which is lower in energy by 0.09 eV than the 4A2 high-spin state of the group Td. We have also calculated the hyperfine interactions with 18 shells containing 46 29Si ligand atoms. We find the largest HF interactions in the (11bar0) plane in agreement with experimental data. The HF interactions with nuclei in the (110) plane, which are about two orders of magnitude smaller than those with nuclei in the (11bar0) plane, also agree with the experimental data. We conclude that the LSDA-DFT describes the magnetization density of the V- well. It is therefore not necessary to include configuration interactions as has been proposed by M. Lannoo [Phys. Rev. B 28, 2403 (1983)].
We investigate the validity of the Makov–Payne correction if applied to supercell calculations fo... more We investigate the validity of the Makov–Payne correction if applied to supercell calculations for charged defects by comparing the results of a supercell calculations with those obtained using a Green's function approach. For several defects in 3C–SiC the observed energy differences between both calculations are not bridged by the corrections of Makov–Payne. Whereas a correction proportional to the defect charge
By combining scanning tunneling microscopy with density functional theory it is shown that the Bi... more By combining scanning tunneling microscopy with density functional theory it is shown that the Bi(111) surface provides a well-defined incorporation site in the first bilayer that traps highly coordinating atoms such as transition metals (TMs) or noble metals. All deposited atoms assume exactly the same specific sevenfold coordinated subsurface interstitial site while the surface topography remains nearly unchanged. Notably, 3d TMs show a barrier-free incorporation. The observed surface modification by barrier-free subsorption helps to suppress aggregation in clusters. It allows a tuning of the electronic properties not only for the pure Bi surface, but may also be observed for topological insulators formed by substrate-stabilized Bi bilayers.
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Papers by Uwe Gerstmann