arXiv subjects

Lukas Cvitkovich

Publications and source records attributed to Lukas Cvitkovich.

11 recordsLinked to original sources

arXiv2026

Gate-tunable magnetic damping in van der Waals Heterostructures

Active control of magnetic damping is a crucial capability for the development of low-power, tunable spintronic devices. In this work, we demonstrate that the intrinsic magnetic damping of 2D materials is highly sensitive to the position of the Fermi level relative to spin-orbit driven anti-crossings, or ``spin hot spots.'' Starting from a minimal multi-band model evaluated via Kamberský's breathing Fermi surface theory, we elucidate the fundamental role that energy gaps and band alignment play in governing magnetic dissipation. We then translate these theoretical insights to van der Waals heterostructures, proposing a concrete mechanism for strongly tunable magnetic damping via electrostatic gating. This concept is validated from first principles via density functional theory calculations on a realistic Fe$_3$GeTe$_2$/graphene heterostructure. Our calculations reveal that an out-of-plane electric field selectively shifts the relevant energy bands, enabling the modulation of the heterostructure's magnetic damping over several orders of magnitude. These findings establish a robust theoretical framework and a promising materials platform for electrically tunable magnetization dynamics.

cond-mat.mtrl-sci↗

arXiv2026

Increasing valley splitting in Si/SiGe by practically achievable heterostructure profiles

Silicon spin qubits are marred by the valley degeneracy of the conduction band. In a nanodevice, the degeneracy is lifted by interfaces and alloy disorder, but the arising valley splitting is small, of order 100 $μ$eV in Si/SiGe quantum wells. Substantial efforts were invested both in theory and experiments to overcome the valley issue. Unfortunately, the existing recipes either rely on atomistic details of the interface that are beyond experimental control, or demand heterostructure profiles beyond current state-of-the-art heterostructure epitaxy. We revisit the valley splitting induced by non-trivial Ge profiles and advocate a novel view of the intervalley coupling as a backscattering on point-like impurities realized by crystal planes containing Ge atoms. This perspective reveals that enhancing the backscattering amplitude, which sets the valley splitting, requires constructive interference of multiple scatterers. % We arrive at a remarkable prediction, that the Ge content along the heterostructure growth direction does not have to have any specific periodicity, including the practically unreachable $2π/(2k_0)$ period, to significantly increase the valley splitting. This statement is corroborated with numerical evidence from tight-binding simulations and intuitive physical interpretations. We devise profiles that seem within the capabilities of current MBE growth techniques and boost the valley splitting beyond the 1\,meV scale.

cond-mat.mes-hall↗

arXiv2026

Valley physics in the two bands $\mathbf{k}\cdot\mathbf{p}$ model for SiGe heterostructures and spin qubits

We discuss the choice and implementation of inter-valley potentials in the so-called two bands $\mathbf{k}\cdot\mathbf{p}$ model for the opposite $X$, $Y$ or $Z$ valleys of silicon. We focus on the description of valley splittings in Si/SiGe heterostructures for spin qubits, with a particular attention to alloy disorder. We demonstrate that the two bands $\mathbf{k}\cdot\mathbf{p}$ model reproduces the valley splittings of atomistic tight-binding calculations in relevant heterostructures (SiGe spikes, wiggle wells...), yet at a much lower cost. We show that the model also captures the effects of valley-orbit mixing and yields the correct inter-valley dipole matrix elements that characterize manipulation, dephasing and relaxation in spin/valley qubits. We simulate a realistic Si/SiGe spin qubit device as an illustration, and discuss electron-phonon interactions in the two bands $\mathbf{k}\cdot\mathbf{p}$ model. Beyond spin qubits, this model enables efficient simulations of SiGe heterostructure devices where spin and valley physics are relevant.

cond-mat.mes-hall↗

arXiv2026

Resonant magnetic proximity hot spots in Co/hBN/graphene

Magnetic proximity effects in Co/hBN/graphene heterostructures are systematically analyzed via first-principles calculations, demonstrating a pronounced localized spatial variation of the induced spin polarization of graphene's Dirac states. The proximity-induced exchange coupling, magnetic moments, and tunneling spin polarization (TSP) are shown to depend sensitively on the atomic registry at the interfaces. We analyze more than twenty distinct stackings, including high- and low-symmetry configurations, and reveal that the spin splittings of graphene's Dirac bands span a wide range from 1 to 100 meV, depending on the local hybridization of Co $d_{z^2}$, hBN $p_z$, and graphene $p_z$ orbitals. The strongest proximity effects emerge at geometric resonances, or "proximity hot spots", where the three orbital states overlap maximally. The local spin polarization also depends sensitively on energy: Dirac states aligned with resonant Co orbitals experience the most pronounced exchange interaction. At these energies, the pseudospin Hamiltonian description of magnetic proximity effects breaks down. Outside these resonances, the pseudospin picture is restored. Our findings highlight the intrinsically local nature of proximity effects, governed by the spectral resonance and interlayer wavefunction overlap. We further quantify how additional hBN layers, interlayer twist, and multilayer graphene modify the proximity exchange and TSP, offering microscopic insight for designing spintronic van der Waals heterostructures with engineered interfaces and optimized spin transport.

cond-mat.mes-hall↗

arXiv2025

Valley Splittings in Si/SiGe Heterostructures from First Principles

We compute valley splittings in Si/SiGe superlattices using ab initio density functional theory (DFT). This first-principle approach is expected to provide an excellent description of interfaces, strains, and atomistic disorder without empirically fitted parameters. We benchmark atomistic tight-binding (TB) and the ``$2k_0$'' theory within the effective mass (EM) approximation against DFT. We show that DFT supports the main conclusions of the 2$k_0$ theory, but reveals some limitations of semi-empirical methods such as the EM and TB, in particular about the description of atomistic disorder. The DFT calculations also highlight the effects of strong valley-orbit mixing at large valley splittings. Nevertheless, TB and the 2$k_0$ theory shall provide reasonable valley splitting statistics in many heterostructures of interest for spin qubit devices.

cond-mat.mtrl-sci↗

arXiv2025

Machine Learning Prediction of Magnetic Proximity Effect in van der Waals Heterostructures: From Atoms to Moiré

We introduce a machine learning framework that efficiently predicts large-scale proximity-induced magnetism in van der Waals heterostructures, overcoming the high computational cost of density functional theory (DFT). We apply it to graphene/\CGT, which exhibits a previously unrecognized dichotomy. Unlike the spin polarization at the Fermi level, which follows the pseudospin, the proximity-induced magnetic moments vary across carbon atoms, defying analytical modeling. To address this, we develop an ensemble-based regression model trained on DFT data and employ local environment descriptors to map the local ($\sim 2$\,nm$^2$) atomic-scale geometry to the carbon magnetic moments. Besides demonstrating locality, the model reveals rich magnetic moiré textures. Crucially, this method can be broadly applied to orbital and spin proximity effects that are highly sensitive to local atomic environments and are beyond analytical description.

cond-mat.mtrl-sci↗

arXiv2025

Spin injection and detection in all-van der Waals 2D devices

In this work we report efficient out-of-plane spin injection and detection in an all-van der Waals based heterostructure using only exfoliated 2D materials. We demonstrate spin injection by measuring spin-valve and Hanle signals in non-local transport in a stack of Fe$_3$GeTe$_2$ (FGT), hexagonal boron nitride (hBN) and graphene layers. FGT flakes form the spin aligning electrodes necessary to inject and detect spins in the graphene channel. The hBN tunnel barrier provides a high-quality interface between the ferromagnetic electrodes and graphene, eliminating the conductivity mismatch problem, thus ensuring efficient spin injection and detection with spin injection efficiencies of up to $P=40$\%. Our results demonstrate that FGT/hBN/graphene heterostructures form a promising platform for realizing 2D van der Waals spintronic devices.

cond-mat.mes-hall↗

arXiv2024

Machine Learning Force Field for Thermal Oxidation of Silicon

Looking back at seven decades of highly extensive application in the semiconductor industry, silicon and its native oxide SiO$_2$ are still at the heart of several technological developments. Recently, the fabrication of ultra-thin oxide layers has become essential for keeping up with trends in down-scaling of nanoelectronic devices and for the realization of novel device technologies. With this comes a need for better understanding of the atomic configuration at the Si/SiO$_2$ interface. Classical force fields offer flexible application and relatively low computational costs, however, suffer from limited accuracy. Ab-initio methods give much better results but are extremely costly. Machine learning force fields (MLFF) offer the possibility to combine the benefits of both worlds. We train a MLFF for the simulation of the dry thermal oxidation process of a Si substrate. The training data is generated by density functional theory calculations. The obtained structures are in line with ab-initio simulations as well as with experimental observations. Compared to a classical force field, the most recent reactive force field (reaxFF), the resulting configurations are vastly improved.

cond-mat.mtrl-sci↗

arXiv2024

Coherence limit due to hyperfine interaction with nuclei in the barrier material of Si spin qubits

On the quest to understand and reduce environmental noise in Si spin qubits, hyperfine interactions between electron and nuclear spins impose a major challenge. Silicon is a promising host material because one can enhance the spin coherence time by removing spinful $^{29}$Si isotopes. As more experiments rely on isotopic purification of Si, the role of other spinful atoms in the device should be clarified. This is not a straightforward task, as the hyperfine interactions with atoms in the barrier layers are poorly understood. We utilize density functional theory to determine the hyperfine tensors of both Si and Ge in a crystalline epitaxial Si/SiGe quantum well as well as Si and O atoms in an amorphous Si/SiO$_2$ (MOS) interface structure. Based on these results, we estimate the dephasing time $T_2^*$ due to magnetic noise from the spin bath and show that the coherence is limited by interactions with non-Si barrier atoms to a few \textmu s in Si/SiGe (for non-purified Ge) and about 100\,\textmu s in Si-MOS. Expressing these numbers alternatively, in Si/SiGe the interactions with Ge dominate below 1000\,ppm of $^{29}$Si content, and, due to low natural concentration of the spinful oxygen isotopes, the interactions with oxygen in Si-MOS become significant only below 1\,ppm of $^{29}$Si content.

cond-mat.mes-hall↗

arXiv2022

Modeling the Initial Stages of Si(100) Thermal Oxidation: An Ab-initio Approach

Silicon together with its native oxide SiO$_2$ was recognized as an outstanding material system for the semiconductor industry in the 1950s. In state-of-the-art device technology, SiO$_2$ is widely used as an insulator in combination with high-$k$ dielectrics such as HfO$_2$, demanding fabrication of ultra-thin interfacial layers. The classical standard model derived by Deal and Grove accurately describes the oxidation of Si in a progressed stage, however, strongly underestimates growth rates for thin oxide layers. Recent studies report a variety of oxidation mechanisms during the growth of oxide films in the range of \SI{10}{\angstrom} with various details still under debate. This paper presents a first-principles based approach to theoretically assess the thermal oxidation process of the technologically relevant Si(100) surfaceduring this initial stage. Our investigations range from the chemisorption of single O$_2$ molecules onto the $p(2\times2)$ reconstructed Si surface to oxidized Si surface layers with a thickness of up to \SI{20}{\angstrom}. The initially observed enhanced growth rate is assigned to barrierless O$_2$ chemisorption events upon which the oxygen molecule dissociate. We present strong evidence for an immediate amorphization of the oxide layer from the onset of oxidation. Surface reactions dominate until the surface is saturated with oxygen and separated from the Si substrate by a \SI{5}{\angstrom} transition region. The saturated surface becomes inert to dissociative reactions and enables the diffusion of molecular oxygen to the \interface interface as assumed within the Deal-Grove model. Further oxidation of the Si substrate is then provided by O$_2$ dissociations at the interface due to the same charge transfer process responsible for the chemisorption at the surface.

cond-mat.mtrl-sci↗

arXiv2020

Anisotropic Physical Properties of the Kondo Semimetal CeCu$_{1.11}$As$_2$

The recently proposed novel materials class called Weyl-Kondo semimetal (WKSM) is a time reversal invariant but inversion symmetry broken Kondo semimetal in which Weyl nodes are pushed to the Fermi level by the Kondo interaction. Here we explore whether CeCu$_{1+x}$As$_2$ may be a new WKSM candidate. We report on its single-crystal growth, structure determination and physical properties investigation. Previously published studies on polycrystalline samples suggest that it is indeed a Kondo semimetal, which is confirmed by our investigations on single crystals. X-ray diffraction reveals that CeCu$_{1+x}$As$_2$ crystallizes in a tetragonal centrosymmetric structure, although the inversion symmetry could still be broken locally due to partially occupied Cu sites. Chemical analysis results in an average occupation $x$ = 0.11(1). The electrical resistivity increases logarithmically with decreasing temperature, and saturates below 10 K. A Kondo temperature $T_{\mathrm{K}}$ $\approx$ 4 K is extracted from entropy, estimated from the specific heat measurements. From Hall effect experiments, a charge carrier density of $8.8 \times 10^{20}$ cm$^{-3}$ is extracted, a value characteristic of a semimetal. The magnetization shows pronounced anisotropy, with no evidence of magnetic ordering down to 0.4 K. We thus classify CeCu$_{1.11}$As$_2$ as a tetragonal Kondo semimetal with anisotropic magnetic properties, with a possibly broken inversion symmetry, thus fulfilling the necessary conditions for a WKSM state.

cond-mat.str-el↗