arXiv subjects

Benjamin Geisler

Publications and source records attributed to Benjamin Geisler.

26 records · Page 2Linked to original sources

arXiv2019

Adsorption and dissociation of iron phthalocyanine on H/Si(111): Impact of van-der-Waals interactions and perspectives for subsurface doping

The adsorption of iron phthalocyanine (FePc) on the passivated H/Si(111) surface is explored from first principles. We find that the organic molecule is predominantly physisorbed with a distance to the surface of $2.6 \pm 0.1$ Angstrom, but also exhibits sizable resonance with the underlying substrate. This establishes the present system as interesting mixed covalent-van-der-Waals-bound test case, which we use to compare the impact of different approaches to van-der-Waals interactions. (Spin-polarized) scanning tunneling microscopy (SP STM) images are simulated, selectively accessing different molecular orbitals via the applied bias voltage in the spirit of scanning tunneling spectroscopy. Comparison with experimental STM images reveals very good agreement. We find a significant magnetic contrast exceeding $\pm 1$ Angstrom in the SP STM images for $-2$ and $+1.5$ V. Binding energies of different (transition metal) atoms in the center of the Pc ring are presented, which particularly show that Fe is strongly bound in the molecule (about $9.6$ eV). Finally, we discuss different reactions for subsurface doping by room-temperature FePc deposition and point out two feasible reactions.

cond-mat.mtrl-sci↗

arXiv2019

Inducing $n$- and $p$-type thermoelectricity in oxide superlattices by strain tuning of orbital-selective transport resonances

By combining first-principles simulations including an on-site Coulomb repulsion term and Boltzmann theory, we demonstrate how the interplay of quantum confinement and epitaxial strain allows to selectively design $n$- and $p$-type thermoelectric response in (LaNiO$_3$)$_3$/(LaAlO$_3$)$_1(001)$ superlattices. In particular, varying strain from $-4.9$ to $+2.9\%$ tunes the Ni orbital polarization at the interfaces from $-6$ to $+3\%$. This is caused by an electron redistribution among Ni $3d_{x^2-y^2}$- and $3d_{z^2}$-derived quantum well states which respond differently to strain. Owing to this charge transfer, the position of emerging cross-plane transport resonances can be tuned relative to the Fermi energy. Already for moderate values of $1.5$ and $2.8\%$ compressive strain, the cross-plane Seebeck coefficient reaches $\sim -60$ and $+100$ $μ$V/K around room temperature, respectively. This provides a novel mechanism to tailor thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2018

Atomic-scale detection of magnetic impurity interactions in bulk semiconductors

We demonstrate on the basis of ab initio simulations how passivated semiconductor surfaces can be exploited to study bulklike interaction properties and wave functions of magnetic impurities on the atomic scale with conventional and spin-polarized scanning tunneling microscopy. By applying our approach to the case of $3d$ transition metal impurities close to the H/Si$(111)$ surface, we show exemplarily that their wave functions in Si are less extended than for Mn in GaAs, thus obstructing ferromagnetism in Si. Finally, we discuss possible applications of this method to other dilute magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2018

Spincaloric properties of epitaxial Co$_2$MnSi/MgO/Co$_2$MnSi magnetic tunnel junctions

The electronic transport and spincaloric properties of epitaxial magnetic tunnel junctions with half-metallic Co$_2$MnSi Heusler electrodes, MgO tunneling barriers, and different interface terminations are investigated by using first-principles calculations. A new approach to spincaloric properties is presented that circumvents the linear response approximation inherent in the Seebeck coefficient and compared to the method of Sivan and Imry. This approach supports two different temperatures in the two electrodes and provides the exact current and/or voltage response of the system. Moreover, it accounts for temperature-dependent chemical potentials in the electrodes and finite-bias effects. We find that especially the former are important for obtaining qualitatively correct results, even if the variations of the chemical potentials are small. It is shown how the spincaloric properties can be tailored by the choice of the growth conditions. We find a large effective and spin-dependent Seebeck coefficient of $-65$ $μ$V/K at room temperature for the purely Co-terminated interface. We suggest to use such interfaces in thermally operated magnetoresistive random access memory modules, which exploit the magneto-Seebeck effect, to maximize the thermally induced readout voltage.

cond-mat.mtrl-sci↗

arXiv2018

Confinement- and strain-induced enhancement of thermoelectric properties in LaNiO$_3$/LaAlO$_3(001)$ superlattices

By combining ab initio simulations including an on-site Coulomb repulsion term and Boltzmann theory, we explore the thermoelectric properties of (LaNiO$_3$)$_n$/(LaAlO$_3$)$_n$(001) superlattices ($n=1,3$) and identify a strong dependence on confinement, spacer thickness, and epitaxial strain. While the system with $n=3$ shows modest values of the Seebeck coefficient and power factor, the simultaneous reduction of the LaNiO$_3$ region and the LaAlO$_3$ spacer thickness to single layers results in a strong enhancement, in particular of the in-plane values. This effect can be further tuned by using epitaxial strain as control parameter: Under tensile strain corresponding to the lateral lattice constant of SrTiO$_3$ we predict in- and cross-plane Seebeck coefficients of $\pm 600$ $μ$V/K and an in-plane power factor of $11$ $μ$W/K$^2$cm for an estimated relaxation time of $τ= 4$ fs around room temperature. These values are comparable to some of the best performing oxide systems such as La-doped SrTiO$_3$ or layered cobaltates and are associated with the opening of a small gap ($0.29$ eV) induced by the concomitant effect of octahedral tilting and Ni-site disproportionation. This establishes oxide superlattices at the verge of a metal-to-insulator transition driven by confinement and strain as promising candidates for thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2017

Design of $n$- and $p$-type oxide thermoelectrics in LaNiO$_3$/SrTiO$_3(001)$ superlattices exploiting interface polarity

We investigate the structural, electronic, transport, and thermoelectric properties of LaNiO$_3$/SrTiO$_3(001)$ superlattices containing either exclusively $n$- or $p$-type interfaces or coupled interfaces of opposite polarity by using density functional theory calculations with an on-site Coulomb repulsion term. The results show that significant octahedral tilts are induced in the SrTiO$_3$ part of the superlattice. Moreover, the La-Sr distances and Ni-O out-of-plane bond lengths at the interfaces exhibit a distinct variation by about $7\,\%$ with the sign of the electrostatic doping. In contrast to the much studied LaAlO$_3$/SrTiO$_3$ system, the charge mismatch at the interfaces is exclusively accommodated within the LaNiO$_3$ layers, whereas the interface polarity leads to a band offset and to the formation of an electric field within the coupled superlattice. Features of the electronic structure indicate an orbital-selective quantization of quantum well states. The potential- and confinement-induced multiband splitting results in complex cylindrical Fermi surfaces with a tendency towards nesting that depends on the interface polarity. The analysis of the thermoelectric response reveals a particularly large positive Seebeck coefficient ($135~μ$V/K) and a high figure of merit ($0.35$) for room-temperature cross-plane transport in the $p$-type superlattice that is attributed to the participation of the SrTiO$_3$ valence band. Superlattices with either $n$- or $p$-type interfaces show cross-plane Seebeck coefficients of opposite sign and thus emerge as a platform to construct an oxide-based thermoelectric generator with structurally and electronically compatible $n$- and $p$-type oxide thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2014

Interplay of growth mode and thermally induced spin accumulation in epitaxial Al/Co$_2$TiSi/Al and Al/Co$_2$TiGe/Al contacts

The feasibility of thermally driven spin injectors built from half-metallic Heusler alloys inserted between aluminum leads was investigated by means of {\em ab initio} calculations of the thermodynamic equilibrium and electronic transport. We have focused on two main issues and found that: (i) the interface between Al and the closely lattice-matched Heusler alloys of type Co$_2$Ti$Z$ ($Z=$ Si or Ge) is stable under various growth conditions; and (ii) the conventional and spin-dependent Seebeck coefficients in such heterojunctions exhibit a strong dependence on both the spacer and the atomic composition of the Al/Heusler interface. The latter quantity gives a measure of the spin accumulation and varies between $+8$~$μ$V/K and $-3$~$μ$V/K near $300$~K, depending on whether a Ti-Ge or a Co-Co plane makes the contact between Al and Co$_2$TiGe in the trilayer. Our results show that it is in principle possible to tailor the spin-caloric effects by a targeted growth control of the samples.

cond-mat.mtrl-sci↗

arXiv2013

Electronic, magnetic and transport properties of full and half-metallic thin film Heusler alloys

The electronic and magnetic bulk properties of half-metallic Heusler alloys such as Co$_{2}$FeSi, Co$_{2}$FeAl, Co$_{2}$MnSi and Co$_{2}$MnAl are investigated by means of {\em ab initio} calculations in combination with Monte Carlo simulations. The electronic structure is analyzed using the plane wave code Quantum Espresso and magnetic exchange interactions are determined using the KKR method. From the magnetic exchange interactions the Curie temperature is obtained via Monte Carlo simulations. In addition, electronic transport properties of the trilayer systems consisting of two semi-infinite platinum leads and a Heusler layer in between are obtained from the fully relativistic KKR method by employing the Kubo-Greenwood formalism. The focus is on thermoelectric properties, namely the Seebeck effect and its spin dependence. It turns out that already thin Heusler layers provide highly polarized currents within the systems. This is attributed to the recovery of half-metallicity with increasing thickness. The absence of electronic states of the spin down electrons around the Fermi level suppresses the contribution of this spin channel to the total conductivity. This strongly influences the thermoelectric properties of such systems and results in polarized thermoelectric currents.

cond-mat.mtrl-sci↗