arXiv subjects

Olle Eriksson

Publications and source records attributed to Olle Eriksson.

178 records · Page 10Linked to original sources

arXiv2009

Molecular adsorption in graphene with divacancy defects

We have investigated theoretically the adsorption of molecules onto graphene with divacancy defects. Using ab-initio density functional calculations, we have found that O2, CO, N2, B2 and H2O molecules all interact strongly with a divacancy in a graphene layer. Along with a complex geometry of the molecule-graphene bonding, metallic behavior of the graphene layer in presence of CO and N2 molecules have been found with a large density of states in the vicinity of the Fermi level suggesting an increase in the conductivity. The adsorption of N2 is particularly interesting since the N atoms dissociate in the vicinity of the defects, and take the place where the missing C atoms of the divacancy used to sit. In this way, the defected graphene structure is healed geometrically, and at the same time doped with electron states.

cond-mat.mtrl-sci↗

arXiv2009

Conductivity engineering of graphene by defect formation

Transport measurements have revealed several exotic electronic properties of graphene. The possibility to influence the electronic structure and hence control the conductivity by adsorption or doping with adatoms is crucial in view of electronics applications. Here, we show that in contrast to expectation, the conductivity of graphene increases with increasing concentration of vacancy defects, by more than one order of magnitude. We obtain a pronounced enhancement of the conductivity after insertion of defects by both quantum mechanical transport calculations as well as experimental studies of carbon nano-sheets. Our finding is attributed to the defect induced mid-gap states, which create a region exhibiting metallic behavior around the vacancy defects. The modification of the conductivity of graphene by the implementation of stable defects is crucial for the creation of electronic junctions in graphene-based electronics devices.

cond-mat.mtrl-sci↗

arXiv2009

Order--disorder induced magnetic structures of FeMnP$_{0.75}$Si$_{0.25}$

We report on the synthesis and structural characterization of the magnetocaloric FeMnP$_{0.75}$Si$_{0.25}$ compound. Two types of samples (as quenched and annealed) were synthesized and characterized structurally and magnetically. We have found that minute changes in the degree of crystallographic order causes a large change in the magnetic properties. The annealed sample, with higher degree of order is antiferromagnetic with a zero net moment. The as-quenched sample has a net moment of 1.26 $μ_B$/f.u. and ferrimagnetic-like behavior. Theoretical calculations give rather large values for the Fe and Mn magnetic moments, both when occupied on the tetrahedral and pyramidal lattice site. The largest being the Mn moment for the pyramidal site reaches values as high as 2.8 $μ_B$/atom.

cond-mat.mtrl-sci↗

arXiv2009

Evolving properties of two dimensional materials, from graphene to graphite

We have studied theoretically, using density functional theory, several materials properties when going from one C layer in graphene to two and three g raphene layers and on to graphite. The properties we have focused on are the elastic constants, electronic structure (energy bands and density of state s), and the dielectric properties. For any of the properties we have investigated the modification due to an increase in the number of graphene layers is within a few percent. Our results are in agreement with the analysis presented recently by Kopelevich and Esquinazi (unpublished).

cond-mat.mtrl-sci↗

arXiv2009

Quantitative magnetic information from reciprocal space maps in transmission electron microscopy

One of the most challenging issues in the characterization of magnetic materials is to obtain quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope (TEM) can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of $m_L/m_S=0.08 \pm 0.01$ is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative element specific technique for magnetic studies, using a widely available instrument with superior spatial resolution.

cond-mat.mtrl-sci↗

arXiv2007

Sum-rules for electron energy-loss near-edge spectra

We derive four sum-rule expressions for spectra measured in electron energy-loss near edge structure experiments. These sum-rules permit the determination spin and orbital magnetic moments, spin-orbit interaction and number of states, analogously to the sum rules of x-ray magnetic circular dichroism. The derivation of the sum-rules is based on dynamical electron diffraction theory and the properties of the mixed dynamic form-factor. The accuracy of the sum-rules is tested by a complete evaluation of the thickness dependent electron energy-loss spectra for iron, cobalt, and nickel crystals. We find that the sum-rules reproduce both spin and orbital moments with very good accuracy. Our results provide a foundation for the use of the energy loss magnetic chiral dichroism technique as a quantitative probe of element specific magnetic properties.

cond-mat.mtrl-sci↗

arXiv2007

Electronic, magnetic and optical properties of random Fe-Cr alloys

In this communication we have studied the electronic structure, magnetic and optical properties of bcc \fecr alloys in the ferromagnetic phase. We have used the augmented space recursion technique coupled with tight-binding linearized muffin-tin orbital technique (TB-LMTO-ASR) as well as the coherent-potential approximation based on the Korringa-Kohn-Rostocker method (KKR-CPA). Also the plane wave projector augmented wave (PAW) method has been used with the disorder simulated by the special quasi-random structure method f or configuration averaging (SQS). This was to provide a comparison between the different methods in common use for random alloys. Moreover, using the self-consistent potential parameters from TB-LMTO-ASR ca lculations we obtained the spin resolved optical conductivity using the generalized recursion technique proposed by Müller and Vishwanathan.

cond-mat.dis-nn↗

arXiv2007

Magnetic structure of Fe, Mn, and Cr clusters supported on Cu(111)

The magnetic structures of small clusters of Fe, Mn, and Cr supported on a Cu(111) surface have been studied with non-collinear first principles theory. Different geometries such as triangles, pyramids and wires are considered and the cluster sizes have been varied between two to ten atoms. The calculations have been performed using a real space linear muffin-tin orbital method. The Fe clusters are found to order ferromagnetically regardless of the cluster geometry. For Mn and Cr clusters, antiferromagnetic exchange interactions between nearest-neighbours are found to cause collinear antiferromagnetic ordering when the geometry allows it. If the antiferromagnetism is frustrated by the cluster geometry non-collinear ordering is found. A comparison between the calculated structures and ground states obtained from simplified Heisenberg Hamiltonians show that the exchange interaction varies for different atoms in the clusters as a result of the different local structure.

cond-mat.mtrl-sci↗

arXiv2006

Theory of quasiparticle spectra for Fe, Co, and Ni: bulk and surface

The correlated electronic structure of iron, cobalt and nickel is investigated within the dynamical mean-field theory formalism, using the newly developed full-potential LMTO-based LDA+DMFT code. Detailed analysis of the calculated electron self-energy, density of states and the spectral density are presented for these metals. It has been found that all these elements show strong correlation effects for majority spin electrons, such as strong damping of quasiparticles and formation of a density of states satellite at about -7 eV below the Fermi level. The LDA+DMFT data for fcc nickel and cobalt (111) surfaces and bcc iron (001) surface is also presented. The electron self energy is found to depend strongly on the number of nearest neighbors, and it practically reaches the bulk value already in the second layer from the surface. The dependence of correlation effects on the dimensionality of the problem is also discussed.

cond-mat.str-el↗

arXiv2005

Magnetic interactions of supported magnetic clusters

It is demonstrated that the magnetic interactions can be drastically different for nano-sized systems compared to those of bulk or surfaces. Using a real-space formalism we have developed a method to calculate non-collinear magnetization structures and hence exchange interactions. Our results for magnetic clusters supported on a Cu(111) surface show that the magnetic ordering as a rule is non-collinear and can not always be described using a simple Heisenberg Hamiltonian. We suggest that ab initio calculations allowing for non-collinear coupling between atomic spins is the best tool for analyzing nano-sized magnets.

cond-mat.mtrl-sci↗

arXiv2005

Polar magneto-optical Kerr effect for low-symmetric ferromagnets

The polar magneto-optical Kerr effect (MOKE) for low-symmetric ferromagnetic crystals is investigated theoretically based on first-principle calculations of optical conductivities and a transfer matrix approach for the electrodynamics part of the problem. Exact average magneto-optical properties of polycrystals are described, taking into account realistic models for the distribution of domain orientations. It is shown that for low-symmetric ferromagnetic single crystals the MOKE is determined by an interplay of crystallographic birefringence and magnetic effects. Calculations for single and bi-crystal of hcp 11-20 Co and for a polycrystal of CrO_2 are performed, with results being in good agreement with experimental data.

cond-mat.other↗

arXiv2002

Modeling the actinides with disordered local moments

A first-principles disordered local moment (DLM) picture within the local-spin-density and coherent potential approximations (LSDA+CPA) of the actinides is presented. The parameter free theory gives an accurate description of bond lengths and bulk modulus. The case of $δ$-Pu is studied in particular and the calculated density of states is compared to data from photo-electron spectroscopy. The relation between the DLM description, the dynamical mean field approach and spin-polarized magnetically ordered modeling is discussed.

cond-mat.mtrl-sci↗

arXiv2001

A novel method of self interaction corrections in density functional calculations

It is demonstrated that the commonly applied self interaction correction (SIC) used in density functional theory does not remove all self interaction. We present as an alternative a novel method which, by construction, is totally free from self interaction. The method has the correct asymptotic 1/r dependence. We apply the new theory to localized $f$-electrons in praseodymium and compare with the old version of SIC, the local density approximation (LDA) and with an atomic Hartree-Fock calculation. The results show a lowering of the $f$ level, a contraction of the $f$ electron cloud and a lowering of the total energy by 13 eV per 4\f electron compared to LDA. The equilibrium volume of the new SIC method is close to the ones given by LDA and the older SIC method, and is in good agreement with experiment. The experimental cohesive energy is in better agreement using the new SIC method, both compared to LDA and another SIC method.

cond-mat.mtrl-sci↗

arXiv2000

Theory of strongly correlated f and d-electron systems. I. Exact Hamiltonian, Hubbard-Anderson models and perturbation theory near atomic limit within non-orthogonal basis set

The theory of correlated electron systems is formulated in a form which allows to use as a reference point an ab initio band structure theory (AIBST). The theory is constructed in two steps. As a first step the total Hamiltonian is transformed into a correlated form. In order to elucidate the microscopical origin of the parameters of the periodical Hubbard-Anderson model (PHAM) the terms of the full Hamiltonian which have the operator structure of PHAM are separated. It is found that the matrix element of mixing interaction includes ion-configuration and number-of-particles dependent contributions from the Coulomb interaction. In a second step the diagram technique (DT) is developed by means of generalization of the Baym-Kadanoff method for correlated systems.

cond-mat.str-el↗

arXiv2000

Reflectivity Anisotropy Spectra of Cu- and Ag- (110) surfaces from {\it ab initio} theory

We are able to disentagle the effects of the intraband and interband parts of the bulk dielectric function on the bare dielectric anisotropy of the surface. We show how the position, sign and amplitude of the structures observed in such spectra depend on the above quantities. The lineshape of all the calculated structures agree very well with the ones observed experimentally for samples treated by suitable surface cleaning. In particular, we reproduce the observed single peak structure of Ag at high energy, found to represent a state of the clean surface different from the one giving the originally observed double peak structure. This results is not reproduced by the 'local field' model.

cond-mat.mtrl-sci↗

arXiv1999

Full-Potential LMTO: Total Energy and Force Calculations

The essential features of a full potential electronic structure method using Linear Muffin-Tin Orbitals (LMTOs) are presented. The electron density and potential in the this method are represented with no inherent geometrical approximation. This method allows the calculation of total energies and forces with arbitrary accuracy while sacrificing much of the efficiency and physical content of approximate methods such as the LMTO-ASA method.

cond-mat.mtrl-sci↗