arXiv subjects

Ingrid Mertig

Publications and source records attributed to Ingrid Mertig.

At least 37 records · Page 2Linked to original sources

arXiv2024

Spin and orbital Edelstein effect in a bilayer system with Rashba interaction

The spin Edelstein effect has proven to be a promising phenomenon to generate spin polarization from a charge current in systems without inversion symmetry. In recent years, a current-induced orbital magnetization, called orbital Edelstein effect, has been predicted for various systems with broken inversion symmetry, using the atom-centered approximation and the modern theory of orbital magnetization. In this work, we study the current-induced spin and orbital magnetization for a bilayer system with Rashba interaction, using the modern theory of orbital magnetization and Boltzmann transport theory in relaxation-time approximation. We found that the orbital effect can be significantly larger than the spin effect, depending on the model parameters. Furthermore, the Edelstein response can be enhanced, suppressed, and even reversed, depending on the relation of the effective Rashba parameters of each layer. A sign change of the orbital polarization is related to an interchange of the corresponding layer localization of the states.

cond-mat.mes-hall↗

arXiv2023

Electrical Activity of Topological Chiral Edge Magnons

Topological magnon insulators support chiral edge excitations, whose lack of electric charge makes them notoriously difficult to detect experimentally. We show that relativistic magnetoelectric coupling universally renders chiral edge magnons electrically active, thereby facilitating electrical probes of magnon topology. Considering a two-dimensional out-of-plane magnetized topological magnon insulator, we predict a fluctuation-activated electric polarization perpendicular to the sample edges. Furthermore, the chiral topological electromagnons give rise to a unique in-gap signal in electrical absorption experiments. These results suggest THz spectroscopy as a promising probe for topological magnons.

cond-mat.mes-hall↗

arXiv2023

First-principles prediction of energy band gaps in 18-valence electron semiconducting half-Heusler compounds: Exploring the role of exchange and correlation

The choice of exchange functional is a critical factor in determining the energy bandgap of semiconductors. Ab initio calculations using different exchange functionals, including the conventional generalized-gradient approximation (GGA) functionals, meta-GGA functionals, and hybrid functionals, show significant differences in the calculated energy bandgap for semiconducting half-Heusler compounds. These compounds, which have 18 valence electrons per unit cell, are of great interest due to their thermoelectric properties, making them suitable for energy conversion applications. In addition, accounting for electronic correlations using the GW method also affects the calculated energy bandgaps compared to standard GGA calculations. The variations in calculated energy bandgaps are specific to each material when using different functionals. Hence, a detailed investigation of the electronic properties of each compound is necessary to determine the most appropriate functional for an accurate description of the electronic properties. Our results indicate that no general rules can be established and a comparison with experimental results is required to determine the most appropriate functional.

cond-mat.mtrl-sci↗

arXiv2023

Electronic bandstructure of superconducting KTaO3 (111) interfaces

Two-dimensional electron gases(2DEGs)based on KTaO3 are emerging as a promising platform for spin-orbitronics due to their high Rashba spin-orbit coupling (SOC) and gate-voltage tunability. The recent discovery of a superconducting state in KTaO3 2DEGs now expands their potential towards topological superconductivity. Although the band structure of KTaO3 surfaces of various crystallographic orientations has already been mapped using angle-resolved photoemission spectroscopy(ARPES), this is not the case for superconducting KTaO3 2DEGs. Here, we reveal the electronic structure of superconducting 2DEGs based on KTaO3 (111) single crystals through ARPES measurements. We fit the data with a tight-binding model and compute the associated spin textures to bring insight into the SOC-driven physics of this fascinating system.

cond-mat.mtrl-sci↗

arXiv2023

Boosting the Edelstein effect of two-dimensional electron gases by ferromagnetic exchange

Strontium titanate (SrTiO$_3$) two-dimensional electron gases (2DEGs) have broken spatial inversion symmetry and possess a finite Rashba spin-orbit coupling. This enables the interconversion of charge and spin currents through the direct and inverse Edelstein effects, with record efficiencies at low temperature, but more modest effects at room temperature. Here, we show that making these 2DEGs ferromagnetic enhances the conversion efficiency by nearly one order of magnitude. Starting from the experimental band structure of non-magnetic SrTiO$_3$ 2DEGs, we mimic magnetic exchange coupling by introducing an out-of-plane Zeeman term in a tight-binding model. We then calculate the band structure and spin textures for increasing internal magnetic fields and compute the Edelstein effect using a semiclassical Boltzmann approach. We find that the conversion efficiency first increases strongly with increasing magnetic field, then shows a maximum and finally decreases. This field dependence is caused by the competition of the exchange coupling with the effective Rashba interaction. While enhancing the splitting of band pairs amplifies the Edelstein effect, weakening the in-plane Rashba-type spin texture reduces it.

cond-mat.mes-hall↗

arXiv2023

Orbital Hall effect and orbital edge states caused by s electrons

An orbital current can be generated whenever an object has a translational and rotational degree of freedom. In condensed matter physics, intra-atomic contributions to the transverse orbital transport, labeled orbital Hall effect, rely on propagating wave packets that must consist of hybridized atomic orbitals. However, inter-atomic contributions have to be considered as well because they give rise to a new mechanism for generating orbital currents. As we show, even wave packets consisting purely of s electrons can transport orbital angular momentum if they move on a cycloid trajectory. We introduce the kagome lattice with a single s orbital per atom as the minimal model for the orbital Hall effect and observe the cycloid motion of the electrons in the surface states.

cond-mat.mes-hall↗

arXiv2023

Skyrmion motion in magnetic anisotropy gradients: Acceleration caused by deformation

Magnetic skyrmions are nano-sized topologically non-trivial spin textures that can be moved by external stimuli such as spin currents and internal stimuli such as spatial gradients of a material parameter. Since the total energy of a skyrmion depends linearly on most of these parameters, like the perpendicular magnetic anisotropy, the exchange constant, or the Dzyaloshinskii-Moriya interaction strength, a skyrmion will move uniformly in a weak parameter gradient. In this paper, we show that the linear behavior changes once the gradients are strong enough so that the magnetic profile of a skyrmion is significantly altered throughout the propagation. In that case, the skyrmion experiences acceleration and moves along a curved trajectory. Furthermore, we show that when spin-orbit torques and material parameter gradients trigger a skyrmion motion, it can move on a straight path along the current or gradient direction. We discuss the significance of suppressing the skyrmion Hall effect for spintronic and neuromorphic applications of skyrmions. Lastly, we extend our discussion and compare it to a gradient generated by the Dzyaloshinskii-Moriya interaction.

cond-mat.mes-hall↗

arXiv2023

Ultrafast Orbital Hall Effect in Metallic Nanoribbons

The orbital Hall effect can generate currents of angular momentum more efficiently than the spin Hall effect in most metals. However, so far, it has only been understood as a steady state phenomenon. In this theoretical study, the orbital Hall effect is extended into the time domain. We investigate the orbital angular momenta and their currents induced by a femtosecond laser pulse in a Cu nanoribbon. Our numerical simulations provide detailed insights into the laser-driven electron dynamics on ultrashort timescales with atomic resolution. The ultrafast orbital Hall effect described in this work is consistent with the familiar pictorial representation of the static orbital Hall effect, but we also find pronounced differences between physical quantities that carry orbital angular momentum and those that carry charge. For example, there are deviations in the time series of the respective currents. This study lays the foundations for investigating ultrafast Hall effects in confined metallic systems.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast Dynamics of Orbital Angular Momentum of Electrons Induced by Femtosecond Laser Pulses: Generation and Transfer Across Interfaces

The orbital angular momenta (OAM) of electrons play an increasingly important role in ultrafast electron and magnetization dynamics. In this theoretical study, we investigate the electron dynamics induced by femtosecond laser pulses in a normal metal, a ferromagnet, and a ferromagnet/normal metal heterostructure. We analyze the spatio-temporal distributions of the laser-induced OAM and their respective currents. Our findings demonstrate that a circularly polarized laser pulse can induce a sizable and long-lasting OAM component in a normal metal. Furthermore, an interface between a ferromagnet and a normal metal facilitates the demagnetization of the magnet by the OAM contribution to the total magnetization. Finally, to transfer OAM from a ferromagnet into a normal metal, it is advantageous to use a laser setup that induces the desired OAM component in the ferromagnet, but not in the normal metal.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast dynamics of electrons excited by femtosecond laser pulses: spin polarization and spin-polarized currents

Laser radiation incident on a ferromagnetic sample produces excited electrons and currents whose spin polarization must not be aligned with the magnetization -- an effect due to spin-orbit coupling that is ubiquitous in spin- and angle-resolved photoemission. In this Paper, we report on a systematic investigation of the dynamics of spin polarization and spin-polarized currents produced by femtosecond laser pulses, modeled within our theoretical framework EVOLVE. The spin polarization depends strongly on the properties of the laser pulse and on the sample composition, as is shown by comparing results for Cu(100), Co(100), and a Co/Cu heterostructure. We find a transition from coherence before the laser pulse's maximum to incoherence thereafter. Moreover, the time dependence of the spin-polarization components induced by spin-orbit coupling differ significantly in Cu and Co: in Cu, we find long-period oscillations with tiny rapid modulations, whereas in Co prominent rapid oscillations with long period ones are superimposed. The pronounced spatial dependencies of the signals underline the importance of inhomogeneities, in particular magnetic/non-magnetic interfaces `act as source' for ultrafast spin-polarization effects. Our investigation provides detailed insight into electron dynamics during and shortly after a femtosecond laser excitation.

cond-mat.mtrl-sci↗

arXiv2023

Spin-polarized two-dimensional electron/hole gas at the interface of non-magnetic semiconducting half-Heusler compounds: Modified Slater-Pauling rule for half-metallicity at the interface

Half-Heusler compounds with 18 valence electrons per unit cell are well-known non-magnetic semiconductors. Employing first-principles electronic band structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas (2DEG) or hole gas (2DHG) at the interface. We qualitatively discuss the origin of the spin polarization at the interfaces on the basis of the Stoner model. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures and magnetic anisotropy energies for magnetic interfaces. Our study, combined with the recent experimental evidence for the presence of 2DEG at CoTiSb/NiTiSn heterojunctions might motivate future efforts and studies toward the experimental realization of devices using the proposed heterojunctions.

cond-mat.mtrl-sci↗

arXiv2022

Generation of out-of-plane polarized spin current by spin swapping

The generation of spin currents and their application to the manipulation of magnetic states is fundamental to spintronics. Of particular interest are chiral antiferromagnets that exhibit properties typical of ferromagnetic materials even though they have negligible magnetization. Here, we report the generation of a robust spin current with both in-plane and out-of-plane spin polarization in epitaxial thin films of the chiral antiferromagnet Mn3Sn in proximity to permalloy thin layers. By employing temperature-dependent spin-torque ferromagnetic resonance, we find that the chiral antiferromagnetic structure of Mn3Sn is responsible for an in-plane polarized spin current that is generated from the interior of the Mn3Sn layer and whose temperature dependence follows that of this layer's antiferromagnetic order. On the other hand, the out-of-plane spin polarized spin current is unrelated to the chiral antiferromagnetic structure and is instead the result of scattering from the Mn3Sn/permalloy interface. We substantiate the later conclusion by performing studies with several other non-magnetic metals all of which are found to exhibit out-of-plane polarized spin currents arising from the spin swapping effect.

cond-mat.mtrl-sci↗

arXiv2022

Biskyrmion-based artificial neuron with refractory period

Magnetic skyrmions are nanoscale magnetic whirls that are highly stable and can be moved by currents which has led to the prediction of a skyrmion-based artificial neuron device with leak-integrate-fire functionality. However, so far, these devices lack a refractory process, estimated to be crucial for neuronal dynamics. Here we demonstrate that a biskyrmion-based artificial neuron overcomes this insufficiency. When driven by spin-orbit torques, a single biskyrmion splits into two subskyrmions that move towards a designated location and can be detected electrically, resembling the excitation process of a neuron that fires ultimately. The attractive interaction of the two skyrmions leads to a unique trajectory: Once they reach the detector area, they automatically return to the center to reform the biskyrmion but on a different path. During this reset period, the neuron cannot fire again. Our suggested device resembles a biological neuron with the leak, integrate, fire and refractory characteristics better than all existing artificial devices so far.

cond-mat.dis-nn↗

arXiv2022

Proposal for semiconductor-free negative differential resistance tunnel diode with ultra-high peak-to-valley current ratio

The negative differential resistance (NDR) tunnel diodes are promising alternative devices for beyond-CMOS computing as they offer several potential applications when integrated with transistors. We propose a novel semiconductor-free NDR tunnel diode concept that exhibits an ultra-high peak-to-valley current ratio (PVCR) value. Our proposed NDR diode consists of two cold metal electrodes separated by a thin insulating tunnel barrier. The NDR effect stems from the unique electronic band structure of the cold metal electrodes, i.e., the width of the isolated metallic bands around the Fermi level as well as the energy gaps separating higher- and lower-lying bands determine the current-voltage ($I$-$V$) characteristics and the PVCR value of the tunnel diode. By proper choice of the cold metal electrode materials, either a conventional N-type or $Λ$-type NDR effect can be obtained. Two-dimensional (2D) materials offer a unique platform for the realization of proposed NDR tunnel diodes. To demonstrate the proof of concept we employ the nonequilibrium Green function method combined with density functional theory to calculate the $I$-$V$ characteristic of the lateral (AlI$_2$/MgI$_2$/AlI$_2$) and vertical (NbS$_2$/h-BN/NbS$_2$) heterojunction tunnel diodes based on 2D cold metals. For the lateral tunnel diode, we obtain a $Λ$-type NDR effect with an ultra-high PVCR value of 10$^{16}$ at room temperature, while the vertical tunnel diode exhibits a conventional N-type NDR effect with a smaller PVCR value of about 10$^4$. The proposed concept provides a semiconductor-free solution for NDR devices to achieve desired $I$-$V$ characteristics with ultra-high PVCR values for memory and logic applications.

cond-mat.mes-hall↗

arXiv2022

Direct visualization of Rashba-split bands and spin/orbital-charge interconversion at KTaO$_3$ interfaces

Rashba interfaces have emerged as promising platforms for spin-charge interconversion through the direct and inverse Edelstein effects. Notably, oxide-based two-dimensional electron gases (2DEGs) display a large and gate-tunable conversion efficiency, as determined by transport measurements. However, a direct visualization of the Rashba-split bands in oxide 2DEGs is lacking, which hampers an advanced understanding of their rich spin-orbit physics. Here, we investigate KTaO$_3$-2DEGs and evidence their Rashba-split bands using angle resolved photoemission spectroscopy. Fitting the bands with a tight-binding Hamiltonian, we extract the effective Rashba coefficient and bring insight into the complex multiorbital nature of the band structure. Our calculations reveal unconventional spin and orbital textures, showing compensation effects from quasi-degenerate band pairs which strongly depend on in-plane anisotropy. We compute the band-resolved spin and orbital Edelstein effects, and predict interconversion efficiencies exceeding those of other oxide 2DEGs. Finally, we suggest design rules for Rashba systems to optimize spin-charge interconversion performance.

cond-mat.mes-hall↗

arXiv2022

Ultrafast spin dynamics: complementing theoretical analyses by quantum-information measures

Theoretical analyses of ultrafast spin dynamics commonly address and discuss simulated phenomena by means of observables, whereas in quantum information theory one often utilizes measures of quantum states. In this Paper we report on possible benefits of quantum information measures in simulations of ultrafast spin dynamics. For Co/Cu heterostructures illuminated by femtosecond laser pulses, we discuss the general behaviour of quantum information measures, in particular distances in Hilbert space and degrees of mixing in the density matrix. The measures are in particular sensitive to variations of the polarization of a laser pulse and the sample composition. Moreover, they are closely related to magnetization and number of excited electrons.

cond-mat.mes-hall↗

arXiv2022

Gate-tuneable and chirality-dependent charge-to-spin conversion in Tellurium nanowires

Chiral materials are the ideal playground for exploring the relation between symmetry, relativistic effects, and electronic transport. For instance, chiral organic molecules have been intensively studied to electrically generate spin-polarized currents in the last decade, but their poor electronic conductivity limits their potential for applications. Conversely, chiral inorganic materials such as Tellurium are excellent electrical transport materials, but have not been explored to enable the electrical control of spin polarization in devices. Here, we demonstrate the all-electrical generation, manipulation, and detection of spin polarization in chiral single-crystalline Tellurium nanowires. By recording a large (up to 7%) and chirality-dependent unidirectional magnetoresistance, we show that the orientation of the electrically generated spin polarization is determined by the nanowire handedness and uniquely follows the current direction, while its magnitude can be manipulated by an electrostatic gate. Our results pave the way for the development of magnet-free chirality-based spintronic devices.

cond-mat.mes-hall↗

arXiv2021

Externally controlled and switchable 2D electron gas at the Rashba interface between ferroelectrics and heavy $d$ metals

Strong spin-orbit coupling in noncentrosymmetric materials and interfaces results in remarkable physical phenomena, such as nontrivial spin textures, which may exhibit Rashba, Dresselhaus, and other intricated configurations. This provides a promising basis for nonvolatile spintronic devices and further implications. Here, we simulate from first principles a two-dimensional electron gas in ultrathin platinum and palladium layers grown on ferroelectric PbTiO$_3$(001). The latter allows, in principle, to switch and control the spin-to-charge conversion by the polarization reversal. We show how the band structure and its Rashba splitting differ in the Pt and Pd overlayers and how these electronic features change with increasing the overlayer thickness and upon reversal of polarization. Besides, for both overlayers, we simulated their current-voltage ($I-V$) characteristics, the resistance of which upon the polarization reversal changes between 20% and several hundred percent. The reported findings can be used to model directly the Rashba-Edelstein effect.

cond-mat.mes-hall↗