arXiv subjects

N. Binggeli

Publications and source records attributed to N. Binggeli.

At least 19 recordsLinked to original sources

arXiv2019

Structural and magnetic response of CrI3 monolayer to electric field

A recent theoretical study reported large effects of perpendicular electric fields on the atomic structure of a monolayer CrI3, which could be related to the microscopic origin of the technologically promising and experimentally observed electrical switching of magnetization in bilayer CrI3. However, those theoretical results are not in line with a previous theoretical finding of only slight changes under a strong electric field in CrI3. Given the important consequences that the presence of large structural distortions in an electric field might have, we investigated the effects of external electric fields on the CrI3 monolayer using density functional theory for a wide range of field strengths. Conclusively, we find that the structural response of CrI3 to the applied perpendicular electric field is extremely small due to a very efficient electronic screening of the electric field within the monolayer. Therefore it cannot be the origin of the observed electrical switching of magnetization in the bilayer CrI3. Furthermore, we find that the very small linear dependence of the structural changes on the electric field persists up to a field value of 0.45 V/Angstrom, while the Cr magnetic moment remains constant for the same strengths of electric field.

cond-mat.mtrl-sci↗

arXiv2013

Self-organization in Pd/W(110): interplay between surface structure and stress

It has recently been shown that submonolayer Pd on W(110) forms highly-ordered linear mesoscopic stripes at high temperatures. The stripes display an internal Pd superstructure with a nano-scale periodicity along the direction perpendicular to the periodicity of the stripes. The same type of superstructure is also observed in a wide range of temperatures below the stripe formation temperature. We present a combined experimental and theoretical study of this superstructure of Pd on W(110) and investigate its influence on the appearance of the linear mesoscopic stripes. By means of low-energy electron diffraction and low-energy-electron microscopy we show that it has a far more peculiar dependence on temperature and coverage than expected from a regular surface reconstruction. Using density-functional theory, we model the Pd superstructures as periodic vacancy-line type of configurations and investigate their energetics and elastic properties. From our calculated surface stresses and anisotropies for the vacancy-line type of configurations, and based on the continuum elasticity theory, we demonstrate quantitatively that the vacancy-line type of structure is a prerequisite for the formation of the linear mesoscopic stripes. Moreover, we show that the physics driving the formation of the internal superstructure is very similar to the one at play in forming the mesoscopic stripes themselves.

cond-mat.mtrl-sci↗

arXiv2013

Optimal interface doping at La_2/3Sr_1/3MnO_3/SrTiO_3(001) heterojunctions for spintronic applications

We examine, by means of ab initio pseudopotential calculations, La_2/3Sr_1/3MnO_3/SrTiO_3 (LSMO/STO) heterojunctions in which one unit layer of La_(1-x)Sr_xMnO_3 (with 0<x<1) is inserted at the interface. The optimal interlayer doping x for a robust interface ferromagnetism is investigated by considering the energy differences between antiferromagnetic and ferromagnetic alignment of the MnO_2-interface layer relative to bulk LSMO. The optimal doping is found to be close to x=1/3, which corresponds to an abrupt TiO_2 (001)-layer termination of STO. This is also the composition which gives the largest p-type Schottky barrier height in our calculations.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic properties of La(0.67)Sr(0.33)MnO3/BiFeO3(001) heterojunctions: chemically abrupt versus atomic intermixed interface

Using first-principles density-functional calculations, we address the magnetic properties of the ferromagnet/antiferromagnet La(0.67)Sr(0.33)MnO3/BiFeO3(001) heterojunctions, and investigate possible driving mechanisms for a ferromagnetic (FM) interfacial ordering of the Fe spins recently observed experimentally. We find that the chemically abrupt defect-free La(0.67)Sr(0.33)MnO3/BiFeO3(001) heterojunction displays, as ground state, an ordering with compensated Fe spins. Cation Fe/Mn intermixing at the interface tends to favour, instead, a FM interfacial order of the Fe spins, coupled antiferromagnetically to the bulk La(0.67)Sr(0.33)MnO3 spins, as observed experimentally. Such trends are understood based on a model description of the energetics of the exchange interactions.

cond-mat.mtrl-sci↗

arXiv2011

Quantum-size effects on chemisorption properties: CO on Cu ultrathin films

We address, by means of ab-initio calculations, the origin of the correlation that has been observed experimentally between the chemisorption energy of CO on nanoscale Cu(001) supported films and quantum-size effects. The calculated chemisorption energy shows systematic oscillations, as a function of film thickness, with a periodicity corresponding to that of quantum-well states at the surface-Brillouin-zone center crossing the Fermi energy. We explain this trend based on the oscillations, with film thickness, of the decay length on the vacuum side of the quantum-well states at the Fermi energy. Contrary to previous suggestions, we find that the actual oscillations with film thickness of the density of states per atom of the film at the Fermi energy cannot account for the observed trend in the chemisorption energy.

cond-mat.mes-hall↗

arXiv2011

Stress engineering at the nanometer scale: Two-component adlayer stripes

Spontaneously formed equilibrium nanopatterns with long-range order are widely observed in a variety of systems, but their pronounced temperature dependence remains an impediment to maintain such patterns away from the temperature of formation. Here, we report on a highly ordered stress-induced stripe pattern in a two-component, Pd-O, adsorbate monolayer on W(110), produced at high temperature and identically preserved at lower temperatures. The pattern shows a tunable period (down to 16 nm) and orientation, as predicted by a continuum model theory along with the surface stress and its anisotropy found in our DFT calculations. The control over thermal fluctuations in the stripe formation process is based on the breaking/restoring of ergodicity in a high-density lattice gas with long-range interactions upon turning off/on particle exchange with a heat bath.

cond-mat.mes-hall↗

arXiv2010

Temperature dependence of surface stress across an order-disorder transition: p(1x2)O/W(110)

Strain relaxations of a p(1x2) ordered oxygen layer on W(110) are measured as a function of temperature across the disordering transition using low-energy electron diffraction. The measured strains approach values of 0.027 in the [1-10] and -0.053 in the [001] direction. On the basis of the measured strain relaxations, we give quantitative information on temperature-dependent surface stress using the results of ab initio calculations. From the surface formation energy for different strains, determined by first-principles calculations, we estimate that surface stress changes from -1.1 for the ordered phase to -0.2N/m for the disordered one along [1-10], and from 5.1 to 3.4 N/m along [001]. Moreover, our observation that the strains scale inversely with domain size confirms that the strain relaxation takes place at the domain boundaries.

cond-mat.mtrl-sci↗

arXiv2009

Computation of correlation-induced atomic displacements and structural transformations in paramagnetic KCuF3 and LaMnO3

We present a computational scheme for ab initio total-energy calculations of materials with strongly interacting electrons using a plane-wave basis set. It combines ab initio band structure and dynamical mean-field theory and is implemented in terms of plane-wave pseudopotentials. The present approach allows us to investigate complex materials with strongly interacting electrons and is able to treat atomic displacements, and hence structural transformations, caused by electronic correlations. Here it is employed to investigate two prototypical Jahn-Teller materials, KCuF3 and LaMnO3, in their paramagnetic phases. The computed equilibrium Jahn-Teller distortion and antiferro-orbital order agree well with experiment, and the structural optimization performed for paramagnetic KCuF3 yields the correct lattice constant, equilibrium Jahn-Teller distortion and tetragonal compression of the unit cell. Most importantly, the present approach is able to determine correlation-induced structural transformations, equilibrium atomic positions and lattice structure in both strongly and weakly correlated solids in their paramagnetic phases as well as in phases with long-range magnetic order.

cond-mat.str-el↗

arXiv2008

Structural relaxation due to electronic correlations in the paramagnetic insulator KCuF3

A computational scheme for the investigation of complex materials with strongly interacting electrons is formulated which is able to treat atomic displacements, and hence structural relaxation, caused by electronic correlations. It combines ab initio band structure and dynamical mean-field theory and is implemented in terms of plane-wave pseudopotentials. The equilibrium Jahn-Teller distortion and antiferro-orbital order found for paramagnetic KCuF3 agree well with experiment.

cond-mat.str-el↗

arXiv2008

Construction and solution of a Wannier-functions based Hamiltonian in the pseudopotential plane-wave framework for strongly correlated materials

Ab initio determination of model Hamiltonian parameters for strongly correlated materials is a key issue in applying many-particle theoretical tools to real narrow-band materials. We propose a self-contained calculation scheme to construct, with an ab initio approach, and solve such a Hamiltonian. The scheme uses a Wannier-function-basis set, with the Coulomb interaction parameter U obtained specifically for these Wannier functions via constrained Density functional theory (DFT) calculations. The Hamiltonian is solved by Dynamical Mean-Field Theory (DMFT) with the effective impurity problem treated by the Quantum Monte Carlo (QMC) method. Our scheme is based on the pseudopotential plane-wave method, which makes it suitable for developments addressing the challenging problem of crystal structural relaxations and transformations due to correlation effects. We have applied our scheme to the "charge transfer insulator" material nickel oxide and demonstrate a good agreement with the experimental photoemission spectra.

cond-mat.str-el↗

arXiv2008

Strain relaxation in small adsorbate islands: O on W(110)

The stress-induced lattice changes in a p(1x2) ordered oxygen layer on W(110) are measured by low-energy electron diffraction. We have observed that small oxygen islands show a mismatch with the underlying lattice. Our results indicate that along [1-10] the average mismatch scales inversely with the island size as 1/L for all oxygen coverages up to 0.5 ML, while along [001] it is significant only for the smallest oxygen islands and scales as a higher power of the inverse island size. The behaviour along [1-10] is described by a one-dimensional finite-size Frenkel-Kontorova model. Using this model, together with calculated force constants, we make a quantitative estimate for the change of surface-stress upon oxygen adsorption. The result is consistent with our ab-initio calculations, which give a relative compressive stress of -4.72 N/m along [1-10] and a minute relative tensile stress of 0.15 N/m along [001]. The scaling along [001] is qualitatively explained as an effect induced by the lattice relaxation in the [1-10] direction.

cond-mat.mtrl-sci↗

arXiv2008

LDA+DMFT implemented with the pseudopotential plane-wave approach

We present a joint implementation of dynamical-mean-field theory (DMFT) with the pseudopotential plane-wave approach, via Wannier functions, for the determination of the electronic properties of strongly correlated materials. The scheme uses, as input for the DMFT calculations, a tight-binding Hamiltonian obtained from the plane-wave calculations by projecting onto atomic-centered symmetry-constrained Wannier functions for the correlated orbitals. We apply this scheme to two prototype systems: a paramagnetic correlated metal, SrVO3, and a paramagnetic correlated system, V2O3, which exhibits a metal-insulator transition. Comparison with available Linear-Muffin-Tin-Orbital (LMTO) plus DMFT calculations demonstrate the suitability of the joint DMFT pseudopotential-plane-wave approach to describe the electronic properties of strongly correlated materials. This opens the way to future developments using the pseudopotential-plane-wave DMFT approach to address also total-energy properties, such as structural properties.

cond-mat.str-el↗

arXiv2006

Evidence of Orbital Ordering in Jahn-Teller Undistorted LaSr$_{2}$Mn$_{2}$O$_{7}$

Resonant soft x-ray diffraction has been used to probe the temperature dependent orbital and magnetic structure of $\mathrm{LaSr_{2}Mn_{2}O_7}$. Previous crystallographic studies have shown that this material has almost no MnO$_{6}$ oxygen displacement due to Jahn-Teller distortions at low temperatures. Within the low-temperature A-type antiferromagnetic phase, we found strong intensity at the $({1/4},{1/4},0)$ orbital and $LaSr_{2}Mn_{2}O_7$ magnetic reflections. This shows that even in the near absence of Jahn-Teller distortion, this compound is strongly orbitally ordered. A fit to the Mn $L$-edge resonance spectra demonstrates the presence of orbital ordering of the Mn$^{3+}$ ions with virtually no Jahn-Teller crystal field in addition to possible Mn$^{3+}$ and Mn$^{2+}$~like valence fluctuations.

cond-mat.str-el↗

arXiv2006

Surface magnetism of Rh(001) from LDA+U calculations

We report calculations indicating the presence of a surface magnetic moment for Rh(001), motivated by the detection of a finite moment by magnetic linear dichroism experiments. We show that, while the density functional with the local density or generalized gradient approximations (LDA and GGA) for exchange and correlation yields a non-magnetic ground state, the application of the GGA plus on-site Coulomb interaction U method predicts surface magnetism, thus offering a solution to the long-standing discrepancy between experiment and theory. The calculated moment on the outermost Rh atom increases with the strength of the effective on-site parameter Ueff=U-J, for Ueff>=1.2 eV, and is as large as 1.24 mu_B for Ueff=2.5 eV.

cond-mat.mtrl-sci↗

arXiv2006

Surface Reactivity and Quantum-Size effects on the Electronic Density Decay Length of ultrathin Metal Films

The origin of the correlation between surface reactivity and quantum-size effects, observed in recent experiments on the oxidation of ultrathin magnesium films, is addressed by means of ab-initio calculations and model predictions. We show that the decay length in vacuum of the electronic local density of states at the Fermi energy exhibits systematic oscillations with film thickness, with local maxima induced when a quantum well state at k// = 0 crosses the Fermi energy. The predicted changes in the decay length are expected to have a major impact on the electron transfer rate by tunneling, which has been proposed to control the initial sticking of O2 in the oxidation process.

cond-mat.mtrl-sci↗

arXiv2005

Mn L$_{2,3}$ edge resonant x-ray scattering in manganites: Influence of the magnetic state

We present an analysis of the dependence of the resonant orbital order and magnetic scattering spectra on the spin configuration. We consider an arbitrary spin direction with respect to the local crystal field axis, thus lowering significantly the local symmetry. To evaluate the atomic scattering in this case, we generalized the Hannon-Trammel formula and implemented it inside the framework of atomic multiplet calculations in a crystal field. For an illustration, we calculate the magnetic and orbital scattering in the CE phase of \lsmo in the cases when the spins are aligned with the crystal lattice vector ${\vec a}$ (or equivalently ${\vec b}$) and when they are rotated in the $ab$-plane by 45$^{\circ}$ with respect to this axis. Magnetic spectra differ for the two cases. For the orbital scattering, we show that for the former configuration there is a non negligible $σ\to σ'$ ($π\to π'$) scattering component, which vanishes in the 45$^\circ$ case, while the $σ\to π'$ ($π\to σ'$) components are similar in the two cases. From the consideration of two 90$^\circ$ spin canted structures, we conclude there is a significant dependence of the orbital scattering spectra on the spin arrangement. Recent experiments detected a sudden decrease of the orbital scattering intensity upon increasing the temperature above the N\' eel temperature in \lsmo. We discuss this behavior considering the effect of different types of misorientations of the spins on the orbital scattering spectrum.

cond-mat.str-el↗

arXiv2005

One-dimensional Fe surface states confined by self-assembled carbon chains at the Fe(001) surface

A c($3\sqrt{2} \times \sqrt{2}$) reconstructed structure of the Fe(001) surface with self-assembled C zigzag chains has been recently observed experimentally. Using ab initio density-functional calculations, we address the effect of the C zigzag chains on the electronic structure of this surface. We find that the formation of the C chains produces one-dimensional surface states localized along the zigzag chains. These states are spin-polarized and derive from preexisting two-dimensional Fe $d_{3z^{2}-r^{2}}$-like surface states of the clean Fe(001) surface. The simulation of the STM image allows us to assign the chain-like structure, observed in STM experiments, to the one-dimensional Fe surface states laterally confined within the C zigzag stripes.

cond-mat.mtrl-sci↗

arXiv2005

Heterovalent interlayers and interface states: an ab initio study of GaAs/Si/GaAs (110) and (100) heterostructures

We have investigated ab initio the existence of localized states and resonances in abrupt GaAs/Si/GaAs (110)- and (100)-oriented heterostructures incorporating 1 or 2 monolayers (MLs) of Si, as well as in the fully developed Si/GaAs (110) heterojunction. In (100)-oriented structures, we find both valence- and conduction-band related near-band edge states localized at the Si/GaAs interface. In the (110) systems, instead, interface states occur deeper in the valence band; the highest valence-related resonances being about 1 eV below the GaAs valence-band maximum. Using their characteristic bonding properties and atomic character, we are able to follow the evolution of the localized states and resonances from the fully developed Si/GaAs binary junction to the ternary GaAs/Si/GaAs (110) systems incorporating 2 or 1 ML of Si. This approach also allows us to show the link between the interface states of the (110) and (100) systems. Finally, the conditions for the existence of localized states at the Si/GaAs (110) interface are discussed based on a Koster-Slater model developed for the interface-state problem.

cond-mat.mtrl-sci↗