Stefan Bluegel - Academia.edu (original) (raw)

Papers by Stefan Bluegel

Computer Physics Communications, Aug 1, 2012

Density Functional Theory (DFT) is one of the most used ab initio theoretical frameworks in mater... more Density Functional Theory (DFT) is one of the most used ab initio theoretical frameworks in materials science. It derives the ground state properties of a multi-atomic ensemble directly from the computation of its one-particle density n(r). In DFT-based simulations the solution is calculated through a chain of successive self-consistent cycles; in each cycle a series of coupled equations (Kohn-Sham) translates to a large number of generalized eigenvalue problems whose eigenpairs are the principal means for expressing n(r). A simulation ends when n(r) has converged to the solution within the required numerical accuracy. This usually happens after several cycles, resulting in a process calling for the solution of many sequences of eigenproblems. In this paper, the authors report evidence showing unexpected correlations between adjacent eigenproblems within each sequence. By investigating the numerical properties of the sequences of generalized eigenproblems it is shown that the eigenvectors undergo an "evolution" process. At the same time it is shown that the Hamiltonian matrices exhibit a similar evolution and manifest a specific pattern in the information they carry. Correlation between eigenproblems within a sequence is of capital importance: information extracted from the simulation at one step of the sequence could be used to compute the solution at the next step. Although they are not explored in this work, the implications could be manifold: from increasing the performance of material simulations, to the development of an improved iterative solver, to modifying the mathematical foundations of the DFT computational paradigm in use, thus opening the way to the investigation of new materials.

Physical review, Oct 6, 2022

We describe a density-functional method which aims at computing the ground state electron density... more We describe a density-functional method which aims at computing the ground state electron density and the spectral function at the same time. One basic ingredient of our method is the construction of the spectral function from the first four spectral moment matrices. The second basic ingredient is the construction of the spectral moment matrices from density functionals. We call our method moment-functional based spectral density-functional theory (MFbSDFT), because it is based on density-functionals for the spectral moments and because it allows us to compute the spectral function. If it is implemented in second variation our method consumes only a fraction more computer time than a standard DFT calculation with the PBE functional. We show that MFbSDFT captures correlation effects such as the valence-band satellite in Ni and the formation of lower and upper Hubbard bands in SrVO3. For the purpose of constructing the spectral function from the first four N × N spectral moment matrices we describe an efficient algorithm based on the diagonalization of one hermitean 2N × 2N matrix.

Physical review, Jan 12, 2022

In our previous Letter [Phys. Rev. B 103, L140408 (2021)], we presented a discussion of the funda... more In our previous Letter [Phys. Rev. B 103, L140408 (2021)], we presented a discussion of the fundamental physical properties of the interactions parameterizing atomistic spin models in connection to first-principles approaches that enable their calculation for a given material. This explained how some of those approaches can apparently lead to magnetic interactions that do not comply with the expected physical properties, such as Dzyaloshinskii-Moriya interactions which are non-chiral and independent of the spin-orbit interaction, and which we consequently termed 'improper'. In the preceding Comment [Phys. Rev. B 105, 026401], the authors present arguments based on the distinction between global and local approaches to the mapping of the magnetic energy using firstprinciples calculations to support their proposed non-chiral Dzyaloshinskii-Moriya interactions and their dismissal of our distinction between 'proper' and 'improper' magnetic interactions. In this Reply, we identify the missing step in the local approach to the mapping and explain how ignoring this step leads to the identification of magnetic interactions which do not comply with established physical principles and that we have previously termed 'improper'.

Bulletin of the American Physical Society, Mar 15, 2021

Frontiers in Physics, 2021

We present a general method for solving the modified Helmholtz equation without shape approximati... more We present a general method for solving the modified Helmholtz equation without shape approximation for an arbitrary periodic charge distribution, whose solution is known as the Yukawa potential or the screened Coulomb potential. The method is an extension of Weinert’s pseudo-charge method [Weinert M, J Math Phys, 1981, 22:2433–2439] for solving the Poisson equation for the same class of charge density distributions. The inherent differences between the Poisson and the modified Helmholtz equation are in their respective radial solutions. These are polynomial functions, for the Poisson equation, and modified spherical Bessel functions, for the modified Helmholtz equation. This leads to a definition of a modified pseudo-charge density and modified multipole moments. We have shown that Weinert’s convergence analysis of an absolutely and uniformly convergent Fourier series of the pseudo-charge density is transferred to the modified pseudo-charge density. We conclude by illustrating the ...

Bulletin of the American Physical Society, 2018

Physical Review B, 2021

Atomistic spin models are of great value for predicting and understanding the magnetic properties... more Atomistic spin models are of great value for predicting and understanding the magnetic properties of real materials, and extensions of the existing models open routes to new physics and potential applications. The Dzyaloshinskii-Moriya interaction is the prototype for chiral magnetic interactions, and several recent works have uncovered or proposed various types of generalized chiral interactions. However, in some cases the proposed interactions or their interpretation do not comply with basic principles such as being independent of the magnetic configuration from which they are evaluated, or even obeying time-reversal invariance. In this brief contribution, we present a simple explanation for the origin of these puzzling findings, and point out how to resolve them.

Bulletin of the American Physical Society, 2018

Bulletin of the American Physical Society, 2017

and JARA, Germany, YUKIKO YAMADA-TAKAMURA, Japan Advanced Institute of Science and Technology (JA... more and JARA, Germany, YUKIKO YAMADA-TAKAMURA, Japan Advanced Institute of Science and Technology (JAIST), Japan-To achieve the goal of scalable molecular electronics, it will be necessary to retain the functionality of molecular components even when the molecules are strongly bound to a surface. The structural and electronic properties of two-dimensional (2D) materials have already proven useful in templating molecules at the nanoscale. However, hybridization between substrate and molecule can often destroy the single molecule functionality essential for use in electronic devices. Here we use scanning tunneling microscopy (STM) and spectroscopy coupled with density functional theory (DFT) studies to show how the domain boundary structure of the 2D material silicene on ZrB 2 can be used to linearly template iron phthalocyanine (FePc) molecules, even at room temperature, while retaining sharp, molecular-like electronic states that are indicative of the isolated molecule. These results highlight the important role of the interface between molecules and 2D materials as well as their edges in controlling the properties of the combined system, and in determining its usefulness in future device applications.

Physical Review B, 2020

We present a novel approach to understanding the extraordinary diversity of magnetic skyrmion sol... more We present a novel approach to understanding the extraordinary diversity of magnetic skyrmion solutions. Our approach combines a new classification scheme with efficient analytical and numerical methods. We introduce the concept of chiral kinks to account for regions of disfavoured chirality in spin textures, and classify two-dimensional magnetic skyrmions in terms of closed domain walls carrying such chiral kinks. In particular, we show that the topological charge of magnetic skyrmions can be expressed in terms of the constituent closed domain walls and chiral kinks. Guided by our classification scheme, we propose a method for creating hitherto unknown magnetic skyrmions which involves initial spin configurations formulated in terms of holomorphic functions and subsequent numerical energy minimization. We numerically study the stability of the resulting magnetic skyrmions for a range of external fields and anisotropy parameters, and provide quantitative estimates of the stability range for the whole variety of skyrmions with kinks. We show that the parameters limiting this range can be well described in terms of the relative energies of particular skyrmion solutions and isolated stripes with and without chiral kinks.

New Journal of Physics, 2020

In this study, we investigate the electron transport properties of a B-doped armchair graphene na... more In this study, we investigate the electron transport properties of a B-doped armchair graphene nanoribbon (AGNR) suspended between graphene electrodes based on first-principles calculations. Our calculations reveal that one of the electron transmission channels of a pristine AGNR junction is closed by the B-doping. We then proceed to explore the effect of the B-doping on the spin-polarized electron transport behavior of a Fe-functionalized AGNR junction. As a result, transmission channels for majority-spin electrons are closed and the spin polarization of the electron transmission is enhanced from 0.60 for the Fe-functionalized AGNR junction to 0.96 for the B- and Fe-codoped one. This observation implies that the codoped AGNR junction can be employed as a spin filter. In addition, we investigate the electronic nature of the transmission suppression caused by the B-doping. A detailed analysis of the scattering wave functions clarifies that a mode modulation of an incident wave arises...

Journal of Physics: Condensed Matter, 2019

Wannier90 is an open-source computer program for calculating maximally-localised Wannier function... more Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spi...

Graphene is an interesting new material, which consists of carbon atoms forming a hexagonal latti... more Graphene is an interesting new material, which consists of carbon atoms forming a hexagonal lattice. Within graphene, carbon atoms are connected by strong chemical bonds but when graphene sheets bind to something else different binding mechanisms take place. For example, when graphene sheets bind among themselves forming graphite, bonding between them is exclusively of van der Waals type i.e. there

Skyrmions are topologically protected field configurations with particle-like properties which we... more Skyrmions are topologically protected field configurations with particle-like properties which were believed to crystallize only under non-equilibrium conditions. Recently, it has been predicted that they can also become ground states in magnetic systems with broken inversion symmetry [1]. We give theoretical evidence of a two-dimensional lattice of atomic-scale skyrmions as the magnetic ground state of a monolayer Fe on Ir(111),

Physical Review a, 2013

In a recent paper [Phys. Rev. A 85, 052508 (2012)], Gidopoulos and Lathiotakis discuss difficulti... more In a recent paper [Phys. Rev. A 85, 052508 (2012)], Gidopoulos and Lathiotakis discuss difficulties in solving the optimized effective potential (OEP) equation in a finite basis. We show that these difficulties are resolved by choosing basis sets that fulfill a balance condition. In particular, the nonanalyticity of the finite-basis OEP equation disappears in basis-set convergence. In connection with the basis-set balance, we also discuss the role of small eigenvalues. The approximate treatment of them with a common energy denominator suggested by the authors requires care in order to avoid spurious oscillations in the potential. We briefly discuss an alternative scheme that overcomes the aforementioned difficulties and enables OEP calculations with a minimal orbital basis.

Physical Review B, 2011

Fe3Si is a ferromagnetic material with possible applications in magnetic tunnel junctions. When d... more Fe3Si is a ferromagnetic material with possible applications in magnetic tunnel junctions. When doped with Mn, the material shows a complex magnetic behavior, as suggested by older experiments. We employed the Korringa-Kohn-Rostoker (KKR) Green function method within density-functional theory (DFT) in order to study the alloy Fe3−xMnxSi, with 0 ≤ x ≤ 1. Chemical disorder is described within the coherent potential approximation (CPA). In agreement with experiment, we find that the Mn atoms align ferromagnetically to the Fe atoms, and that the magnetization and Curie temperature drop with increasing Mn-concentration x. The calculated spin polarization P at the Fermi level varies strongly with x, from P = −0.3 at x = 0 (ordered Fe3Si) through P = 0 at x = 0.28, to P = +1 for x > 0.75; i.e., at high Mn concentrations the system is half-metallic. We discuss the origin of the trends of magnetic moments, exchange interactions, Curie temperature and the spin polarization.

Physical Review B, 2009

The magnetic state of Nitrogen-doped MgO, with N substituting O at concentrations between 1% and ... more The magnetic state of Nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be magnetic with a moment of 1 µB per Nitrogen atom and to interact ferromagnetically via the double exchange mechanism. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The Curie temperature TC increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single Nitrogen atoms, also interstitial Nitrogen atoms, clusters of Nitrogen atoms and their structural relaxation on the magnetism are discussed. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase of TC is dicult to achieve, requiring a particular attention to the choice of chemistry.

Computer Physics Communications, Aug 1, 2012

Density Functional Theory (DFT) is one of the most used ab initio theoretical frameworks in mater... more Density Functional Theory (DFT) is one of the most used ab initio theoretical frameworks in materials science. It derives the ground state properties of a multi-atomic ensemble directly from the computation of its one-particle density n(r). In DFT-based simulations the solution is calculated through a chain of successive self-consistent cycles; in each cycle a series of coupled equations (Kohn-Sham) translates to a large number of generalized eigenvalue problems whose eigenpairs are the principal means for expressing n(r). A simulation ends when n(r) has converged to the solution within the required numerical accuracy. This usually happens after several cycles, resulting in a process calling for the solution of many sequences of eigenproblems. In this paper, the authors report evidence showing unexpected correlations between adjacent eigenproblems within each sequence. By investigating the numerical properties of the sequences of generalized eigenproblems it is shown that the eigenvectors undergo an "evolution" process. At the same time it is shown that the Hamiltonian matrices exhibit a similar evolution and manifest a specific pattern in the information they carry. Correlation between eigenproblems within a sequence is of capital importance: information extracted from the simulation at one step of the sequence could be used to compute the solution at the next step. Although they are not explored in this work, the implications could be manifold: from increasing the performance of material simulations, to the development of an improved iterative solver, to modifying the mathematical foundations of the DFT computational paradigm in use, thus opening the way to the investigation of new materials.

Physical review, Oct 6, 2022

We describe a density-functional method which aims at computing the ground state electron density... more We describe a density-functional method which aims at computing the ground state electron density and the spectral function at the same time. One basic ingredient of our method is the construction of the spectral function from the first four spectral moment matrices. The second basic ingredient is the construction of the spectral moment matrices from density functionals. We call our method moment-functional based spectral density-functional theory (MFbSDFT), because it is based on density-functionals for the spectral moments and because it allows us to compute the spectral function. If it is implemented in second variation our method consumes only a fraction more computer time than a standard DFT calculation with the PBE functional. We show that MFbSDFT captures correlation effects such as the valence-band satellite in Ni and the formation of lower and upper Hubbard bands in SrVO3. For the purpose of constructing the spectral function from the first four N × N spectral moment matrices we describe an efficient algorithm based on the diagonalization of one hermitean 2N × 2N matrix.

Physical review, Jan 12, 2022

In our previous Letter [Phys. Rev. B 103, L140408 (2021)], we presented a discussion of the funda... more In our previous Letter [Phys. Rev. B 103, L140408 (2021)], we presented a discussion of the fundamental physical properties of the interactions parameterizing atomistic spin models in connection to first-principles approaches that enable their calculation for a given material. This explained how some of those approaches can apparently lead to magnetic interactions that do not comply with the expected physical properties, such as Dzyaloshinskii-Moriya interactions which are non-chiral and independent of the spin-orbit interaction, and which we consequently termed 'improper'. In the preceding Comment [Phys. Rev. B 105, 026401], the authors present arguments based on the distinction between global and local approaches to the mapping of the magnetic energy using firstprinciples calculations to support their proposed non-chiral Dzyaloshinskii-Moriya interactions and their dismissal of our distinction between 'proper' and 'improper' magnetic interactions. In this Reply, we identify the missing step in the local approach to the mapping and explain how ignoring this step leads to the identification of magnetic interactions which do not comply with established physical principles and that we have previously termed 'improper'.

Bulletin of the American Physical Society, Mar 15, 2021

Frontiers in Physics, 2021

We present a general method for solving the modified Helmholtz equation without shape approximati... more We present a general method for solving the modified Helmholtz equation without shape approximation for an arbitrary periodic charge distribution, whose solution is known as the Yukawa potential or the screened Coulomb potential. The method is an extension of Weinert’s pseudo-charge method [Weinert M, J Math Phys, 1981, 22:2433–2439] for solving the Poisson equation for the same class of charge density distributions. The inherent differences between the Poisson and the modified Helmholtz equation are in their respective radial solutions. These are polynomial functions, for the Poisson equation, and modified spherical Bessel functions, for the modified Helmholtz equation. This leads to a definition of a modified pseudo-charge density and modified multipole moments. We have shown that Weinert’s convergence analysis of an absolutely and uniformly convergent Fourier series of the pseudo-charge density is transferred to the modified pseudo-charge density. We conclude by illustrating the ...

Bulletin of the American Physical Society, 2018

Physical Review B, 2021

Atomistic spin models are of great value for predicting and understanding the magnetic properties... more Atomistic spin models are of great value for predicting and understanding the magnetic properties of real materials, and extensions of the existing models open routes to new physics and potential applications. The Dzyaloshinskii-Moriya interaction is the prototype for chiral magnetic interactions, and several recent works have uncovered or proposed various types of generalized chiral interactions. However, in some cases the proposed interactions or their interpretation do not comply with basic principles such as being independent of the magnetic configuration from which they are evaluated, or even obeying time-reversal invariance. In this brief contribution, we present a simple explanation for the origin of these puzzling findings, and point out how to resolve them.

Bulletin of the American Physical Society, 2018

Bulletin of the American Physical Society, 2017

and JARA, Germany, YUKIKO YAMADA-TAKAMURA, Japan Advanced Institute of Science and Technology (JA... more and JARA, Germany, YUKIKO YAMADA-TAKAMURA, Japan Advanced Institute of Science and Technology (JAIST), Japan-To achieve the goal of scalable molecular electronics, it will be necessary to retain the functionality of molecular components even when the molecules are strongly bound to a surface. The structural and electronic properties of two-dimensional (2D) materials have already proven useful in templating molecules at the nanoscale. However, hybridization between substrate and molecule can often destroy the single molecule functionality essential for use in electronic devices. Here we use scanning tunneling microscopy (STM) and spectroscopy coupled with density functional theory (DFT) studies to show how the domain boundary structure of the 2D material silicene on ZrB 2 can be used to linearly template iron phthalocyanine (FePc) molecules, even at room temperature, while retaining sharp, molecular-like electronic states that are indicative of the isolated molecule. These results highlight the important role of the interface between molecules and 2D materials as well as their edges in controlling the properties of the combined system, and in determining its usefulness in future device applications.

Physical Review B, 2020

We present a novel approach to understanding the extraordinary diversity of magnetic skyrmion sol... more We present a novel approach to understanding the extraordinary diversity of magnetic skyrmion solutions. Our approach combines a new classification scheme with efficient analytical and numerical methods. We introduce the concept of chiral kinks to account for regions of disfavoured chirality in spin textures, and classify two-dimensional magnetic skyrmions in terms of closed domain walls carrying such chiral kinks. In particular, we show that the topological charge of magnetic skyrmions can be expressed in terms of the constituent closed domain walls and chiral kinks. Guided by our classification scheme, we propose a method for creating hitherto unknown magnetic skyrmions which involves initial spin configurations formulated in terms of holomorphic functions and subsequent numerical energy minimization. We numerically study the stability of the resulting magnetic skyrmions for a range of external fields and anisotropy parameters, and provide quantitative estimates of the stability range for the whole variety of skyrmions with kinks. We show that the parameters limiting this range can be well described in terms of the relative energies of particular skyrmion solutions and isolated stripes with and without chiral kinks.

New Journal of Physics, 2020

In this study, we investigate the electron transport properties of a B-doped armchair graphene na... more In this study, we investigate the electron transport properties of a B-doped armchair graphene nanoribbon (AGNR) suspended between graphene electrodes based on first-principles calculations. Our calculations reveal that one of the electron transmission channels of a pristine AGNR junction is closed by the B-doping. We then proceed to explore the effect of the B-doping on the spin-polarized electron transport behavior of a Fe-functionalized AGNR junction. As a result, transmission channels for majority-spin electrons are closed and the spin polarization of the electron transmission is enhanced from 0.60 for the Fe-functionalized AGNR junction to 0.96 for the B- and Fe-codoped one. This observation implies that the codoped AGNR junction can be employed as a spin filter. In addition, we investigate the electronic nature of the transmission suppression caused by the B-doping. A detailed analysis of the scattering wave functions clarifies that a mode modulation of an incident wave arises...

Journal of Physics: Condensed Matter, 2019

Wannier90 is an open-source computer program for calculating maximally-localised Wannier function... more Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spi...

Graphene is an interesting new material, which consists of carbon atoms forming a hexagonal latti... more Graphene is an interesting new material, which consists of carbon atoms forming a hexagonal lattice. Within graphene, carbon atoms are connected by strong chemical bonds but when graphene sheets bind to something else different binding mechanisms take place. For example, when graphene sheets bind among themselves forming graphite, bonding between them is exclusively of van der Waals type i.e. there

Skyrmions are topologically protected field configurations with particle-like properties which we... more Skyrmions are topologically protected field configurations with particle-like properties which were believed to crystallize only under non-equilibrium conditions. Recently, it has been predicted that they can also become ground states in magnetic systems with broken inversion symmetry [1]. We give theoretical evidence of a two-dimensional lattice of atomic-scale skyrmions as the magnetic ground state of a monolayer Fe on Ir(111),

Physical Review a, 2013

In a recent paper [Phys. Rev. A 85, 052508 (2012)], Gidopoulos and Lathiotakis discuss difficulti... more In a recent paper [Phys. Rev. A 85, 052508 (2012)], Gidopoulos and Lathiotakis discuss difficulties in solving the optimized effective potential (OEP) equation in a finite basis. We show that these difficulties are resolved by choosing basis sets that fulfill a balance condition. In particular, the nonanalyticity of the finite-basis OEP equation disappears in basis-set convergence. In connection with the basis-set balance, we also discuss the role of small eigenvalues. The approximate treatment of them with a common energy denominator suggested by the authors requires care in order to avoid spurious oscillations in the potential. We briefly discuss an alternative scheme that overcomes the aforementioned difficulties and enables OEP calculations with a minimal orbital basis.

Physical Review B, 2011

Fe3Si is a ferromagnetic material with possible applications in magnetic tunnel junctions. When d... more Fe3Si is a ferromagnetic material with possible applications in magnetic tunnel junctions. When doped with Mn, the material shows a complex magnetic behavior, as suggested by older experiments. We employed the Korringa-Kohn-Rostoker (KKR) Green function method within density-functional theory (DFT) in order to study the alloy Fe3−xMnxSi, with 0 ≤ x ≤ 1. Chemical disorder is described within the coherent potential approximation (CPA). In agreement with experiment, we find that the Mn atoms align ferromagnetically to the Fe atoms, and that the magnetization and Curie temperature drop with increasing Mn-concentration x. The calculated spin polarization P at the Fermi level varies strongly with x, from P = −0.3 at x = 0 (ordered Fe3Si) through P = 0 at x = 0.28, to P = +1 for x > 0.75; i.e., at high Mn concentrations the system is half-metallic. We discuss the origin of the trends of magnetic moments, exchange interactions, Curie temperature and the spin polarization.

Physical Review B, 2009

The magnetic state of Nitrogen-doped MgO, with N substituting O at concentrations between 1% and ... more The magnetic state of Nitrogen-doped MgO, with N substituting O at concentrations between 1% and the concentrated limit, is calculated with density-functional methods. The N atoms are found to be magnetic with a moment of 1 µB per Nitrogen atom and to interact ferromagnetically via the double exchange mechanism. The long-range magnetic order is established above a finite concentration of about 1.5% when the percolation threshold is reached. The Curie temperature TC increases linearly with the concentration, and is found to be about 30 K for 10% concentration. Besides the substitution of single Nitrogen atoms, also interstitial Nitrogen atoms, clusters of Nitrogen atoms and their structural relaxation on the magnetism are discussed. Possible scenarios of engineering a higher Curie temperature are analyzed, with the conclusion that an increase of TC is dicult to achieve, requiring a particular attention to the choice of chemistry.