arXiv subjects

Ingrid Mertig

Publications and source records attributed to Ingrid Mertig.

At least 91 records · Page 5Linked to original sources

arXiv2015

Magnetic properties of defect-free and oxygen-deficient cubic SrCoO$_{3-δ}$

We investigated theoretically electronic and magnetic properties of the perovskite material SrCoO$_{3-δ}$ with $δ\leq 0.15$ using a projector-augmented plane-wave method and a Green's function method. This material is known from various experiments to be ferromagnetic with a Curie temperature of 260$\,$K to 305$\,$K and a magnetic moment of 1.5${\,μ_\text{B}}$ to 3.0${\,μ_\text{B}}$. Applying the magnetic force theorem as it is formulated within Green's function method, we calculated for SrCoO$_{3-δ}$ the magnetic exchange parameters and estimated the Curie temperature. Including correlation effects by an effective $U$ parameter within the GGA$+U$ approach and verifying this by hybrid functional calculations, we obtained the Curie temperatures in dependence of the oxygen deficiency close to the experimental values.

cond-mat.mtrl-sci↗

arXiv2015

Numerical solution of the relativistic single-site scattering problem for the Coulomb and the Mathieu potential

For a reliable fully-relativistic Korringa-Kohn-Rostoker Green function method, an accurate solution of the underlying single-site scattering problem is necessary. We present an extensive discussion on numerical solutions of the related differential equations by means of standard methods for a direct solution and by means of integral equations. Our implementation is tested and exemplarily demonstrated for a spherically symmetric treatment of a Coulomb potential and for a Mathieu potential to cover the full-potential implementation. For the Coulomb potential we include an analytic discussion of the asymptotic behaviour of irregular scattering solutions close to the origin ($r\ll1$).

cond-mat.mtrl-sci↗

arXiv2015

Spin-dependent transport in a multiferroic tunnel junction: Theory for Co/PbTiO$_{3}$/Co

Spin-dependent electronic transport through multiferroic Co/PbTiO$_{3}$/Co tunnel junctions is studied theoretically. Conductances calculated within the Landauer-Büttiker formalism yield both a large tunnel magnetoresistance (TMR) and a large tunnel electroresistance (TER). On top of this, we establish a four-conductance state. The conductances depend crucially on the details of the electronic structure at the interfaces. In particular, the spin polarization of the tunneling electronic states is affected by the hybridization of orbitals and the associated charge transfer at both interfaces. Digital doping of the PbTiO$_{3}$ barrier with Zr impurities at the TiO$_{2}$/Co$_{2}$ interface significantly enhances the TMR\@. In addition, it removes the metalization of the barrier.

cond-mat.mtrl-sci↗

arXiv2015

Impact of the Topological Surface State on the Thermoelectric Transport in Sb$_2$Te$_3$ Thin Films

Ab initio electronic structure calculations based on density functional theory and tight-binding methods for the thermoelectric properties of p-type Sb$_2$Te$_3$ films are presented. The thickness-dependent electrical conductivity and the ther- mopower are computed in the diffusive limit of transport based on the Boltzmann equation. Contributions of the bulk and the surface to the transport coefficients are separated which enables to identify a clear impact of the topological surface state on the thermoelectric properties. By tuning the charge carrier concentration, a crossover between a surface-state-dominant and a Fuchs-Sondheimer transport regime is achieved. The calculations are corroborated by thermoelectric transport measurements on Sb$_2$Te$_3$ films grown by atomic layer deposition.

cond-mat.mtrl-sci↗

arXiv2015

Skew scattering in dilute ferromagnetic alloys

The challenging problem of skew scattering for Hall effects in dilute ferromagnetic alloys, with intertwined effects of spin-orbit coupling, magnetism and impurity scattering, is studied here from first principles. Our main aim is to identify chemical trends and work out simple rules for large skew scattering in terms of the impurity and host states at the Fermi surface, with particular emphasis on the interplay of the spin and anomalous Hall effects in one and the same system. The predicted trends are benchmarked by referring to three different \emph{ab initio} methods based on different approximations with respect to the electronic structure and transport properties.

cond-mat.mtrl-sci↗

arXiv2014

Thermal String Excitations in Artificial Spin-Ice Dipolar Arrays

We report on a theoretical investigation of artificial spin-ice dipolar arrays, using a nanoisland geometry adopted from recent experiments [A. Farhan \textit{et al.}, Nature Phys.\ \textbf{9} (2013) 375]. The number of thermal magnetic string excitations in the square lattice is drastically increased by a vertical displacement of rows and columns. We find large increments especially for low temperatures and for string excitations with quasi-monopoles of charges $\pm 4$. By kinetic Monte Carlo simulations we address the thermal stability of such excitations, thereby providing time scales for their experimental observation.

cond-mat.mes-hall↗

arXiv2013

Colossal Spin Hall Effect in Ultrathin Metallic Films

We predict spin Hall angles up to 80% for ultrathin noble metal films with substitutional Bi impurities. The colossal spin Hall effect is caused by enhancement of the spin Hall conductivity in reduced sample dimension and a strong reduction of the charge conductivity by resonant impurity scattering. These findings can be exploited to create materials with high efficiency of charge to spin current conversion by strain engineering.

cond-mat.mes-hall↗

arXiv2013

Insight into the skew-scattering mechanism of the spin Hall effect: potential scattering versus spin-orbit scattering

We present a detailed analysis of the skew-scattering contribution to the spin Hall conductivity using an extended version of the resonant scattering model of Fert and Levy [Phys. Rev. Lett. {\bf 106}, 157208 (2011)]. For $5d$ impurities in a Cu host, the proposed phase shift model reproduces the corresponding first-principles calculations. Crucial for that agreement is the consideration of two scattering channels related to $p$ and $d$ impurity states, since the discussed mechanism is governed by a subtle interplay between the spin-orbit and potential scattering in both angular-momentum channels. It is shown that the potential scattering strength plays a decisive role for the magnitude of the spin Hall conductivity.

cond-mat.mes-hall↗

arXiv2013

On the origin of the giant SHE in Cu(Bi) alloys

Two years after the prediction of a giant spin Hall effect for the dilute Cu(Bi) alloy [Gradhand et al., Phys. Rev. B 81, 245109 (2010)], a comparably strong effect was measured in thin films of Cu(Bi) alloys by Niimi et al. [Phys. Rev. Lett. 109, 156602 (2012)]. Both theory and experiment consider the skew-scattering mechanism to be responsible, however they obtain opposite sign for the spin Hall angle. Based on a detailed analysis of existing theoretical results, we explore differences between theory and experiment.

cond-mat.mes-hall↗

arXiv2011

Effect of strain on the thermoelectric properties of silicon: An ab initio study

On the basis of detailed first-principles calculations the anisotropic thermoelectric transport properties of biaxially strained silicon were studied with focus on a possible enhancement of the powerfactor. Electron as well as hole doping were examined in a broad doping and temperature range. In the low-temperature and low-doping regime an enhancement of the powerfactor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. In the thermoelectrically more important high-temperature and high- doping regime a slight enhancement of the powerfactor was only found for the hole-doped case under small biaxial tensile strain. The results are discussed in terms of band-structure effects. An analytical model is presented to understand the fact that the thermopower decreases if degenerate bands are energetically lifted due to a strain- induced redistribution of states.

cond-mat.mtrl-sci↗

arXiv2011

$Bi_2Te_3$: Implications of the rhombohedral k-space texture on the evaluation of the in-plane/out-of-plane conductivity anisotropy

Different computational scheme for calculating surface integrals in anisotropic Brillouin zones are compared. The example of the transport distribution function (plasma frequency) of the thermoelectric Material \BiTe near the band edges will be discussed. The layered structure of the material together with the rhombohedral symmetry causes a strong anisotropy of the transport distribution function for the directions in the basal (in-plane) and perpendicular to the basal plane (out-of-plane). It is shown that a thorough reciprocal space integration is necessary to reproduce the in-plane/out-of-plane anisotropy. A quantitative comparison can be made at the band edges, where the transport anisotropy is given in terms of the anisotropic mass tensor.

cond-mat.mtrl-sci↗

arXiv2011

Tailoring magnetoresistance through rotating Ni particles

We present \textit{ab initio} studies for different Ni nanocontacts and show changes in the conductance of such constrictions due to atomic rearrangements in the contact. In particular we consider a Ni particle and show that the magnetoresistance can change from a few to 50% and can even reverse sign as a function of the contact area formed between the particle and the leads.

cond-mat.mes-hall↗

arXiv2011

Evaluation of conduction eigenchannels of an adatom probed by an STM tip

Ballistic conductance through a single atom adsorbed on a metallic surface and probed by a scanning tunneling microscope (STM) tip can be decomposed into eigenchannel contributions, which can be potentially obtained from shot noise measurements. Our density functional theory calculations provide evidence that transmission probabilities of these eigenchannels encode information on the modifications of the adatom's local density of states caused by its interaction with the STM tip. In the case of open shell atoms, this can be revealed in nonmonotonic behavior of the eigenchannel's transmissions as a function of the tip-adatom separation.

cond-mat.mes-hall↗

arXiv2010

Spin Hall effect of alloys: Extrinsic and intrinsic contribution

A fully relativistic description of the spin-orbit induced spin Hall effect is presented that is based on Kubo's linear response formalism. Using an appropriate operator for the spin current density a Kubo-Středa-like equation for the spin Hall conductivity (SHC) is obtained. An implementation using the Korringa-Kohn-Rostoker (KKR) band structure method in combination with the Coherent Potential Approximation (CPA) allow detailed investigations on various alloy systems. A decomposition of the SHC into intrinsic and extrinsic contributions is suggested. Accompanying calculations for the skew-scattering contribution of the SHC using the Boltzmann equation demonstrate the equivalence to the Kubo formalism in the dilute alloy regime and support the suggested decomposition scheme.

cond-mat.mtrl-sci↗

arXiv2010

Theory of real space imaging of Fermi surfaces

A scanning tunneling microscope can be used to visualize in real space Fermi surfaces with buried impurities far below substrates acting as local probes. A theory describing this feature is developed based on the stationary phase approximation. It is demonstrated how a Fermi surface of a material acts as a mirror focusing electrons that scatter at hidden impurities.

cond-mat.mes-hall↗

arXiv2010

Effect of interfacial Cr on magnetoelectricity of Fe2/CrO2/BaTiO3(001)

On the basis of first-principles calculations we study the effect of interfacial Cr on the magnetoelectric properties of a composite multiferroic Fe_L/BaTiO_3(001), with the Fe thickness L=1,2 monolayers. The use of the CrO_2-terminated interface instead of TiO_2 may significantly enhance magnetoelectricity in the system, showing an unexpected change in magnetization induced by the electric polarization reversal. In the case of L=2, for instance, the magnetic order of the Fe bilayer can be switched from nearly zero ferrimagnetic to ferromagnetic upon polarization reversal.

cond-mat.mtrl-sci↗

arXiv2008

Implementation of a non-equilibrium Green's function method to calculate spin transfer torque

We present an implementation of the steady state Keldysh approach in a Green's function multiple scattering scheme to calculate the non-equilibrium spin density. This density is used to obtain the spin transfer torque in junctions showing the magnetoresistance effect. We use our implementation to study the spin transfer torque in metallic Co/Cu/Co junctions.

cond-mat.mes-hall↗

arXiv2007

Conduction eigenchannels of atomic-sized contacts: Ab initio KKR Green's function formalism

We develop a formalism for the evaluation of conduction eigenchannels of atomic-sized contacts from first-principles. The multiple scattering Korringa-Kohn-Rostoker (KKR) Green's function method is combined with the Kubo linear response theory. Solutions of the eigenvalue problem for the transmission matrix are proven to be identical to eigenchannels introduced by Landauer and B{ü}ttiker. Applications of the method are presented by studying ballistic electron transport through Cu, Pd, Ni and Co single-atom contacts. We show in detail how the eigenchannels are classified in terms of irreducible representations of the symmetry group of the system as well as by orbital contributions when the channels wave functions are projected on the contact atom.

cond-mat.mtrl-sci↗