arXiv subjects

M. van Schilfgaarde

Publications and source records attributed to M. van Schilfgaarde.

At least 19 recordsLinked to original sources

arXiv2025

Doping Topological Dirac Semimetal with magnetic impurities: electronic structure of Mn-doped Cd$_3$As$_2$

The prospect of transforming a Dirac topological semimetal (TSM) into a Weyl TSM phase, following doping by magnetic impurities, is central to TSM applications. The magnetic field from polarized $d$ levels of magnetic impurities produces a field with a sharp local structure. To what extent magnetic impurities act in the same manner as an applied field and what are the effects of such a field on the electronic structure of a Dirac TSM is the subject of this paper. We present electronic structure calculations of bulk Cd$_3$As$_2$ with substitutional doping of Mn impurities in the dilute alloy range. Quasi-particle $GW$ (QS$GW$) ab-initio electronic structure calculations are used in conjunction with $k \cdot p$ model Hamiltonian calculations. As expected, we observe the splitting of the Dirac points into pairs of Weyl points following the doping with Mn. We also show that the electronic structure of Mn-doped Cd$_3$As$_2$ can be emulated by the electronic structure of pristine Cd$_3$As$_2$ with an appropriate external magnetic field. Some properties of the conductivity of bulk Cd$_3$As$_2$ for different magnetic field orientations are also investigated. Our results inform future opportunities for unique device functionality based on band structure tuning not found in conventional magnetic Weyl TSM.

cond-mat.mtrl-sci↗

arXiv2024

Self-consistent quasi-particle $GW$ and hybrid functional calculations for Al/InAs/Al heterojunctions: band offset and spin-orbit coupling effects

The electronic structure of surfaces and interfaces plays a key role in the properties of quantum devices. Here, we study the electronic structure of realistic Al/InAs/Al heterojunctions using a combination of density functional theory (DFT) with hybrid functionals and state-of-the-art quasi-particle $GW$ (QS$GW$) calculations. We find a good agreement between QS$GW$ calculations and hybrid functional calculations which themselves compare favourably well with ARPES experiments. Our study confirm the need of well controlled quality of the interfaces to obtain the needed properties of InAs/Al heterojunctions. A detailed analysis of the effects of spin-orbit coupling on the spin-splitting of the electronic states show a linear scaling in $k$-space, related to the two-dimensional nature of some interface states. The good agreement by QS$GW$ and hybrid functional calculations open the door towards trust-able use of an effective approximation to QS$GW$ for studying very large heterojunctions.

cond-mat.mes-hall↗

arXiv2024

Tunneling magnetoresistance in MgO tunnel junctions with Fe-based leads in empirically corrected density functional theory

The minority-spin Fe/MgO interface states are at the Fermi level in density functional theory (DFT), but experimental evidence and GW calculations place them slightly higher in energy. This small shift can strongly influence tunneling magnetoresistance (TMR) in junctions with a thin MgO barrier and its dependence on the concentration of Co in the electrodes. Here, an empirical potential correction to DFT is introduced to shift the interface states up to match the tunnel spectroscopy data. With this shift, TMR in Fe/MgO/Fe junctions exceeds 800% and 3000% at 3 and 4 monolayers (ML) of MgO, respectively. We further consider the effect of alloying of the Fe electrodes with up to 30% Co or 10% V, treating them in the coherent potential approximation (CPA). Alloying with Co broadens the interface states and brings a large incoherent minority-spin spectral weight to the Fermi level. Alloying with V brings the minority-spin resonant states close to the Fermi level. However, in both cases the minority-spin spectral weight at the Fermi level resides primarily at the periphery of the Brillouin zone, which is favorable for spin filtering. Using convolutions of $\mathbf{k}_\parallel$-resolved barrier densities of states calculated in CPA, it is found that TMR is strongly reduced by alloying with Co or V but still remains above 500% at 4 ML of MgO up to 30% of Co or 5% V. At 5 ML, the TMR increases above 1000% in all systems considered. On the other hand, while TMR declines sharply with increasing bias up to 0.2 eV in the MTJ with pure Fe leads, it remains almost constant up to 0.5 eV if leads are alloyed with Co.

cond-mat.mtrl-sci↗

arXiv2023

Flat Bands at the Fermi Level in Unconventional Superconductor YFe2Ge2

We report heavy electron behavior in unconventional superconductor YFe$_2$Ge$_2$ ($T_C \,{=}\, 1.2$ K). We directly observe very heavy bands ($m_\mathrm{eff}\sim 25 m_e$) within $\sim$10 meV of the Fermi level $E_{F}$ using angle-resolved photoelectron spectroscopy (ARPES). The flat bands reside at the X points of the Brillouin zone and are composed principally of $d_{xz}$ and $d_{yz}$ orbitals. We utilize many-body perturbative theory, GW, to calculate the electronic structure of this material, obtaining excellent agreement with the ARPES data with relatively minor band renormalizations and band shifting required. We obtain further agreement at the Dynamical Mean Field Theory (DMFT) level, highlighting the emergence of the many-body physics at low energies (near $E_F$) and temperatures.

cond-mat.str-el↗

arXiv2023

Breakdown of the drift-diffusion model for transverse spin transport in a disordered Pt film

Spin accumulation and spin current profiles are calculated for a disordered Pt film subjected to an in-plane electric current within the nonequilibrium Green function approach. In the bulklike region of the sample, this approach captures the intrinsic spin Hall effect found in other calculations. Near the surfaces, the results reveal qualitative differences with the results of the widely used spin-diffusion model, even when the boundary conditions are modified to try to account for them. One difference is that the effective spin-diffusion length for transverse spin transport is significantly different from its longitudinal counterpart and is instead similar to the mean-free path. This feature may be generic for spin currents generated via the intrinsic spin-Hall mechanism because of the differences in transport mechanisms compared to longitudinal spin transport. Orbital accumulation in the Pt film is only significant in the immediate vicinity of the surfaces and has a small component penetrating into the bulk only in the presence of spin-orbit coupling, as a secondary effect induced by the spin accumulation.

cond-mat.mes-hall↗

arXiv2023

Ab initio transport calculations: from normal to superconducting current

Applying the Bogoliubov-de Gennes equations with density-functional theory, it is possible to formulate first-principles description of current-phase relationships in superconducting/normal (magnetic)/superconducting trilayers. Such structures are the basis for the superconducting analog of Magnetoresistive random access memory devices (JMRAM). In a recent paper [1] we presented results from the first attempt to formulate such a theory, applied to the Nb/Ni/Nb trilayers. In the present work we provide computational details, explaining how to construct key ingredient (scattering matrices $S_N$) in a framework of linear muffin-tin orbitals (LMTO).

cond-mat.mes-hall↗

arXiv2020

Generalized magnetoelectronic circuit theory and spin relaxation at interfaces in magnetic multilayers

Spin transport at metallic interfaces is an essential ingredient of various spintronic device concepts, such as giant magnetoresistance, spin-transfer torque, and spin pumping. Spin-orbit coupling plays an important role in many such devices. In particular, spin current is partially absorbed at the interface due to spin-orbit coupling. We develop a general magnetoelectronic circuit theory and generalize the concept of the spin mixing conductance, accounting for various mechanisms responsible for spin-flip scattering. For the special case when exchange interactions dominate, we give a simple expression for the spin mixing conductance in terms of the contributions responsible for spin relaxation (i.e., spin memory loss), spin torque, and spin precession. The spin-memory loss parameter $δ$ is related to spin-flip transmission and reflection probabilities. There is no straightforward relation between spin torque and spin memory loss. We calculate the spin-flip scattering rates for N|N, F|N, F|F interfaces using the Landauer-Büttiker method within the linear muffin-tin orbital method and determine the values of $δ$ using circuit theory.

cond-mat.mes-hall↗

arXiv2019

Interfacial contributions to spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers

The thickness dependence of spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers is studied using the first-principles non-equilibrium Green's function formalism combined with the Anderson disorder model. A systematic expansion in orthogonal vector spherical harmonics is used for the angular dependence of the torque. The damping-like torque in Co/Pt and Co/Au bilayers can be described as a sum of the spin-Hall contribution, which increases with thickness in agreement with the spin-diffusion model, and a comparable interfacial contribution. The magnetoconductance in the plane perpendicular to the current in Co/Pt bilayers is of the order of a conductance quantum per interfacial atom, exceeding the prediction of the spin-Hall model by more than an order of magnitude. This suggests that the "spin-Hall magnetoresistance," similarly to the damping-like torque, has a large interfacial contribution unrelated to the spin-Hall effect.

cond-mat.mes-hall↗

arXiv2019

First-principles calculation of spin-orbit torque in a Co/Pt bilayer

The angular dependence of spin-orbit torque in a disordered Co/Pt bilayer is calculated using a first-principles non-equilibrium Green's function formalism with an explicit supercell averaging over Anderson disorder. In addition to the usual dampinglike and fieldlike terms, the odd torque contains a sizeable planar Hall-like term $(\mathbf{m\cdot E})\mathbf{m}\times(\mathbf{z}\times\mathbf{m})$ whose contribution to current-induced damping is consistent with experimental observations. The dampinglike and planar Hall-like torquances depend weakly on disorder strength, while the fieldlike torquance declines with increasing disorder. The torques that contribute to damping are almost entirely due to spin-orbit coupling on the Pt atoms, but the fieldlike torque does not require it.

cond-mat.mtrl-sci↗

arXiv2017

Tunable dimensional crossover and magnetocrystalline anisotropy in Fe$_2$P-based alloys

Electronic structure calculations are used to examine the magnetic properties of Fe$_2$P-based alloys and the mechanisms through which the Curie temperature and magnetocrystalline anisotropy can be optimized for specific applications. It is found that at elevated temperatures the magnetic interaction in pure Fe$_2$P develops a pronounced two-dimensional character due to the suppression of the magnetization in one of the sublattices, but the interlayer coupling is very sensitive to band filling and structural distortions. This feature suggests a natural explanation of the observed sharp enhancement of the Curie temperature by alloying with multiple elements, such as Co, Ni, Si, and B. The magnetocrystalline anisotropy is also tunable by electron doping, reaching a maximum near the electron count of pure Fe$_2$P. These findings enable the optimization of the alloy content, suggesting co-alloying of Fe$_2$P with Co (or Ni) and Si as a strategy for maximizing the magnetocrystalline anisotropy at and above room temperature.

cond-mat.mtrl-sci↗

arXiv2016

Self-energies in itinerant magnets: A focus on Fe and Ni

We present a detailed study of local and non-local correlations in the electronic structure of elemental transition metals carried out by means of the Quasiparticle Self-consistent GW (QSGW ) and Dynamical Mean Field Theory (DMFT). Recent high resolution ARPES and Haas-van Alphen data of two typical transition metal systems (Fe and Ni) are used as case study. (i) We find that the properties of Fe are very well described by QSGW. Agreement with cyclotron and very clean ARPES measurements is excellent, provided that final-state scattering is taken into account. This establishes the exceptional reliability of QSGW also in metallic systems. (ii) Nonetheless QSGW alone is not able to provide an adequate description of the Ni ARPES data due to strong local spin fluctuations. We surmount this deficiency by combining nonlocal charge fluctuations in QSGW with local spin fluctuations in DMFT (QSGW + 'Magnetic DMFT'). (iii) Finally we show that the dynamics of the local fluctuations are actually not crucial. The addition of an external static field can lead to similarly good results if non-local correlations are included through QSGW.

cond-mat.str-el↗

arXiv2016

Theory of spin loss at metallic interfaces

Interfacial spin-flip scattering plays an important role in magnetoelectronic devices. Spin loss at metallic interfaces is usually quantified by matching the magnetoresistance data for multilayers to the Valet-Fert model, while treating each interface as a fictitious bulk layer whose thickness is $δ$ times the spin-diffusion length. By employing the properly generalized circuit theory and the scattering matrix approaches, we derive the relation of the parameter $δ$ to the spin-flip transmission and reflection probabilities at an individual interface. It is found that $δ$ is proportional to the square root of the probability of spin-flip scattering. We calculate the spin-flip transmission probability for flat and rough Cu/Pd interfaces using the Landauer-Büttiker method based on the first-principles electronic structure and find $δ$ in reasonable agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2016

High photon energy spectroscopy of NiO: experiment and theory

We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab-initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO.We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide.

cond-mat.str-el↗

arXiv2013

The Electronic Correlation Strength of Pu

An electronic quantity, the correlation strength, is defined as a necessary step for understanding the properties and trends in strongly correlated electronic materials. As a test case, this is applied to the different phases of elemental Pu. Within the GW approximation we have surprisingly found a "universal" scaling relationship, where the f-electron bandwidth reduction due to correlation effects is shown to depend only on the local density approximation bandwidth and is otherwise independent of crystal structure and lattice constant.

cond-mat.str-el↗

arXiv2012

Many-body effects in iron pnictides and chalcogenides -- non-local vs dynamic origin of effective masses

We apply the quasi-particle self-consistent GW (QSGW) approximation to some of the iron pnictide and chalcogenide superconductors. We compute Fermi surfaces and density of states, and find excellent agreement with experiment, substantially improving over standard band-structure methods. Analyzing the QSGW self-energy we discuss non-local and dynamic contributions to effective masses. We present evidence that the two contributions are mostly separable, since the quasi-particle weight is found to be essentially independent of momentum. The main effect of non locality is captured by the static but non-local QSGW effective potential. Moreover, these non-local self-energy corrections, absent in e.g. dynamical mean field theory (DMFT), can be relatively large. We show, on the other hand, that QSGW only partially accounts for dynamic renormalizations at low energies. These findings suggest that QSGW combined with DMFT will capture most of the many-body physics in the iron pnictides and chalcogenides.

cond-mat.str-el↗

arXiv2010

First-principles study of phase stability of Gd-doped EuO and EuS

Phase diagrams of isoelectronic Eu$_{1-x}$Gd$_x$O and Eu$_{1-x}$Gd$_{x}$S quasi-binary alloy systems are constructed using first-principles calculations combined with the standard cluster expansion approach and Monte-Carlo simulations. The oxide system has a wide miscibility gap on the Gd-rich side but forms ordered compounds on the Eu-rich side, exhibiting a deep asymmetric convex hull in the formation enthalpy diagram. The sulfide system has no stable compounds. The large difference in the formation enthalpies of the oxide and sulfide compounds is due to the contribution of local lattice relaxation, which is sensitive to the anion size. The solubility of Gd in both EuO and EuS is in the range of 10-20% at room temperature and quickly increases at higher temperatures, indicating that highly doped disordered solid solutions can be produced without the precipitation of secondary phases. We also predict that rocksalt GdO can be stabilized under appropriate experimental conditions.

cond-mat.mtrl-sci↗

arXiv2010

Low Energy, Coherent, Stoner-like Excitations in CaFe$_{2}$As$_{2}$

Using linear-response density-functional theory, magnetic excitations in the striped phase of CaFe$_{2}$As$_{2}$ are studied as a function of local moment amplitude. We find a new kind of excitation: sharp resonances of Stoner-like (itinerant) excitations at energies comparable to the N{é}el temperature, originating largely from a narrow band of Fe $d$ states near the Fermi level, and coexist with more conventional (localized) spin waves. Both kinds of excitations can show multiple branches, highlighting the inadequacy of a description based on a localized spin model.

cond-mat.mtrl-sci↗

arXiv2010

First-principles Theory of Nonlocal Screening in Graphene

Using the quasiparticle self-consistent GW (QSGW) and local-density (LD) approximations, we calculate the q-dependent static dielectric function, and derive an effective 2D dielectric function corresponding to screening of point charges. In the q-to-0 limit, the 2D function is found to scale approximately as the square root of the macroscopic dielectric function. Its value is ~4, a factor approximately 1.5 larger than predictions of Dirac model. Both kinds of dielectric functions depend strongly on q, in contrast with the Dirac model. The QSGW approximation is shown to describe QP levels very well, with small systematic errors analogous to bulk sp semiconductors. Local-field effects are rather more important in graphene than in bulk semiconductors.

cond-mat.mes-hall↗