arXiv subjects

Jaroslav Fabian

Publications and source records attributed to Jaroslav Fabian.

At least 37 records · Page 2Linked to original sources

arXiv2024

Quantum Spin Hall Effect in Magnetic Graphene

A promising approach to attain long-distance coherent spin propagation is accessing topological spin-polarized edge states in graphene. Achieving this without external magnetic fields necessitates engineering graphene band structure, obtainable through proximity effects in van der Waals heterostructures. In particular, proximity-induced staggered potentials and spin-orbit coupling are expected to form a topological bulk gap in graphene with gapless helical edge states that are robust against disorder. In this work, we detect the spin-polarized helical edge transport in graphene at zero external magnetic field, allowed by the proximity of an interlayer antiferromagnet, CrPS$_4$. We show the coexistence of the quantum spin Hall (QSH) states and magnetism in graphene, where the induced spin-orbit and exchange couplings also give rise to a large anomalous Hall (AH) effect. The detection of the QSH states at zero external magnetic field, together with the AH signal that persists up to room temperature, opens the route for practical applications of magnetic graphene in quantum spintronic circuitries.

cond-mat.mes-hall↗

arXiv2024

Electrical manipulation of intervalley trions in twisted MoSe$_2$ homobilayers at room temperature

The impressive physics and applications of intra- and interlayer excitons in a transition metal dichalcogenide twisted-bilayer make these systems compelling platforms for exploring the manipulation of their optoelectronic properties through electrical fields. This work studies the electrical control of excitonic complexes in twisted MoSe$_2$ homobilayer devices at room temperature. Gate-dependent micro-photoluminescence spectroscopy reveals an energy tunability of several meVs originating from the emission of excitonic complexes. Furthermore, our study investigates the twist-angle dependence of valley properties by fabricating devices with stacking angles of $θ\sim1\degree$, $θ\sim4\degree$ and $θ\sim18\degree$. Strengthened by density functional theory calculations, the results suggest that, depending on the twist angle, the conduction band minima and hybridized states at the \textbf{Q}-point promote the formation of intervalley hybrid trions involving the \textbf{Q}-and \textbf{K}-points in the conduction band and the \textbf{K}-point in the valence band. By revealing the gate control of exciton species in twisted homobilayers, our findings open new avenues for engineering multifunctional optoelectronic devices based on ultrathin semiconducting systems.

cond-mat.mtrl-sci↗

arXiv2024

Excitonic signatures of ferroelectric order in parallel-stacked MoS$_2$

Interfacial ferroelectricity, prevalent in various parallel-stacked layered materials, allows switching of out-of-plane ferroelectric order by in-plane sliding of adjacent layers. Its resilience against doping potentially enables next-generation storage and logic devices. However, studies have been limited to indirect sensing or visualization of ferroelectricity. For transition metal dichalcogenides, there is little knowledge about the influence of ferroelectric order on their intrinsic valley and excitonic properties. Here, we report direct probing of ferroelectricity in few-layer 3R-MoS$_2$ using reflectance contrast spectroscopy. Contrary to a simple electrostatic perception, layer-hybridized excitons with out-of-plane electric dipole moment remain decoupled from ferroelectric ordering, while intralayer excitons with in-plane dipole orientation are sensitive to it. Ab initio calculations identify stacking-specific interlayer hybridization leading to this asymmetric response. Exploiting this sensitivity, we demonstrate optical readout and control of multi-state polarization with hysteretic switching in a field-effect device. Time-resolved Kerr ellipticity reveals a direct correspondence between spin-valley dynamics and stacking order.

cond-mat.mtrl-sci↗

arXiv2024

Giant asymmetric proximity-induced spin-orbit coupling in twisted graphene/SnTe heterostructure

We analyze the spin-orbit coupling effects in a three-degree twisted bilayer heterostructure made of graphene and an in-plane ferroelectric SnTe, with the goal of transferring the spin-orbit coupling from SnTe to graphene, via the proximity effect. Our results indicate that the point-symmetry breaking due to the incompatible mutual symmetry of the twisted monolayers and a strong hybridization has a massive impact on the spin splitting in graphene close to the Dirac point, with the spin splitting values greater than 20 meV. The band structure and spin expectation values of graphene close to the Dirac point can be described using a symmetry-free model, triggering different types of interaction with respect to the threefold symmetric graphene/transition-metal dichalcogenide heterostructure. We show that the strong hybridization of the Dirac cone's right movers with the SnTe band gives rise to a large asymmetric spin splitting in the momentum space. Furthermore, we discover that the ferroelectricity-induced Rashba spin-orbit coupling in graphene is the dominant contribution to the overall Rashba field, with the effective in-plane electric field that is almost aligned with the (in-plane) ferroelectricity direction of the SnTe monolayer. We also predict an anisotropy of the in-plane spin relaxation rates. Our results demonstrate that the group-IV monochalcogenides MX (M=Sn, Ge; X=S, Se, Te) are a viable alternative to transition-metal dichalcogenides for inducing strong spin-orbit coupling in graphene.

cond-mat.mes-hall↗

arXiv2024

Tuning proximity spin-orbit coupling in graphene/NbSe$_2$ heterostructures via twist angle

We investigate the effect of the twist angle on the proximity spin-orbit coupling (SOC) in graphene/NbSe$_2$ heterostructures from first principles. The low-energy Dirac bands of several different commensurate twisted supercells are fitted to a model Hamiltonian, allowing us to study the twist-angle dependency of the SOC in detail. We predict that the magnitude of the Rashba SOC can triple, when going from $Θ=0^\circ$ to $Θ=30^\circ$ twist angle. Furthermore, at a twist angle of $Θ\approx23^\circ$ the in-plane spin texture acquires a large radial component, corresponding to a Rashba angle of up to $Φ=25^\circ$. The twist-angle dependence of the extracted proximity SOC is explained by analyzing the orbital decomposition of the Dirac states to reveal with which NbSe$_2$ bands they hybridize strongest. Finally, we employ a Kubo formula to evaluate the efficiency of conventional and unconventional charge-to-spin conversion in the studied heterostructures.

cond-mat.mes-hall↗

arXiv2024

Layer-selective spin-orbit coupling and strong correlation in bilayer graphene

Spin-orbit coupling (SOC) and electron-electron interaction can mutually influence each other and give rise to a plethora of intriguing phenomena in condensed matter systems. In pristine bilayer graphene, which has weak SOC, intrinsic Lifshitz transitions and concomitant van-Hove singularities lead to the emergence of many-body correlated phases. Layer-selective SOC can be proximity induced by adding a layer of tungsten diselenide (WSe2) on its one side. By applying an electric displacement field, the system can be tuned across a spectrum wherein electronic correlation, SOC, or a combination of both dominates. Our investigations reveal an intricate phase diagram of proximity-induced SOC-selective bilayer graphene. Not only does this phase diagram include those correlated phases reminiscent of SOC-free doped bilayer graphene, but it also hosts unique SOC-induced states allowing a compelling measurement of valley g-factor and a seemingly impossible correlated insulator at charge neutrality, thereby showcasing the remarkable tunability of the interplay between interaction and SOC in WSe2 enriched bilayer graphene.

cond-mat.mes-hall↗

arXiv2024

Proximity-enabled control of spin-orbit coupling in phosphorene symmetrically and asymmetrically encapsulated by WSe$_2$ monolayers

We analyze, using first-principles calculations and the method of invariants, the spin-orbit proximity effects in trilayer heterostructures comprising phosphorene and encapsulating WSe$_2$ monolayers. We focus on four different configurations, in which the top/bottom WSe$_2$ monolayer is twisted by 0 or 60 degrees with respect to phosphorene, and analyze the spin splitting of phosphorene hole bands around the $Γ$ point. Our results show that the spin texture of phosphorene hole bands can be dramatically modified by different encapsulations of phosphorene monolayer. For a symmetrically encapsulated phosphorene, the momentum-dependent spin-orbit field has the out-of-plane component only, simulating the spin texture of phosphorene-like group-IV monochalcogenide ferroelectrics. Furthermore, we reveal that the direction of the out-of-plane spin-orbit field can be controlled by switching the twist angle from 0 to 60 degrees. Finally, we show that the spin texture in asymmetrically encapsulated phosphorene has the dominant in-plane component of the spin-orbit field, comparable to the Rashba effect in phosphorene with an applied sizable external electric field. Our results confirm that the significant modification and control of the spin texture is possible in low common-symmetry heterostructures, paving the way for using different substrates to modify spin properties in materials important for spintronics.

cond-mat.mes-hall↗

arXiv2024

Emergence of radial Rashba spin-orbit fields in twisted van der Waals heterostructures

Rashba spin-orbit coupling is a quintessential spin interaction appearing in virtually any electronic heterostructure. Its paradigmatic spin texture in the momentum space forms a tangential vector field. Using first-principles investigations, we demonstrate that in twisted homobilayers and hetero-multilayers, the Rashba coupling can be predominantly radial, parallel to the momentum. Specifically, we study four experimentally relevant structures: twisted bilayer graphene (Gr), twisted bilayer WSe$_2$, and twisted multilayers WSe$_2$/Gr/WSe$_2$ and WSe$_2$/Gr/Gr/WSe$_2$. We show, that the Rashba spin-orbit field texture in such structures can be controlled by an electric field, allowing to tune it from radial to tangential. Such spin-orbit engineering should be useful for designing novel spin-charge conversion and spin-orbit torque schemes, as well as for controlling correlated phases and superconductivity in van der Waals materials.

cond-mat.mes-hall↗

arXiv2023

Amplification of interlayer exciton emission in twisted WSe$_2$/WSe$_2$/MoSe$_2$ heterotrilayers

Transition metal dichalcogenide (TMDC) heterostructures have unique properties that depend on the twisting angle and stacking order of two or more monolayers. However, their practical applications are limited by the low photoluminescence yield of interlayer excitons. This limits the use of layered 2D materials as a versatile platform for developing innovative optoelectronic and spintronic devices. In this study, we report on the emission enhancement of interlayer excitons in multilayered-stacked monolayers through the fabrication of heterotrilayers consisting of WSe$_2$/WSe$_2$/MoSe$_2$ with differing twist angles. Our results show that an additional WSe$_2$ monolayer introduces new absorption pathways, leading to an improvement in the emission of interlayer excitons by more than an order of magnitude. The emission boost is affected by the twist angle, and we observe a tenfold increase in the heterotrilayer area when there is a 44$^\circ$ angle between the WSe$_2$ and MoSe$_2$ materials, as opposed to their heterobilayer counterparts. Furthermore, using density functional theory, we identify the emergence of new carrier transfer pathways in the three-layer sample which extends the current understanding of 2D semiconducting heterostructures. In addition, our research provides a viable way to significantly enhance the emission of interlayer excitons. The emission enhancement of interlayer excitons is significant not only for studying the fundamental properties of interlayer excitons, but also for enabling optoelectronic applications that utilize engineered 2D quantum materials with high luminescence yield.

cond-mat.mes-hall↗

arXiv2023

Twist- and gate-tunable proximity spin-orbit coupling, spin relaxation anisotropy, and charge-to-spin conversion in heterostructures of graphene and transition-metal dichalcogenides

We present a DFT-based investigation of the twist-angle dependent proximity spin-orbit coupling (SOC) in graphene/TMDC structures. We find that for Mo-based TMDCs the proximity valley-Zeeman SOC exhibits a maximum at around 15--20°, and vanishes at 30°, while for W-based TMDCs we find an almost linear decrease of proximity valley-Zeeman SOC when twisting from 0° to 30°. The induced Rashba SOC is rather insensitive to twisting, while acquiring a nonzero Rashba phase angle, $φ\in [-20;40]$°, for twist angles different from 0° and 30°. This finding contradicts earlier tight-binding predictions that the Rashba angle can be 90° in the studied systems. In addition, we study the influence of several tunability knobs on the proximity SOC for selected twist angles. By applying a transverse electric field in the limits of $\pm 2$ V/nm, mainly the Rashba SOC can be tuned by about 50\%. The interlayer distance provides a giant tunability, since the proximity SOC can be increased by a factor of 2--3, when reducing the distance by about 10\%. Encapsulating graphene between two TMDCs, both twist angles are important to control the interference of the individual proximity SOCs, allowing to precisely tailor the valley-Zeeman SOC in graphene, while the Rashba SOC becomes suppressed. Finally, based on our effective Hamiltonians with fitted parameters, we calculate experimentally measurable quantities such as spin lifetime anisotropy and charge-to-spin conversion efficiencies. The spin lifetime anisotropy can become giant, up to $10^4$, in encapsulated structures. The charge-to-spin conversion, which is due to spin-Hall and Rashba-Edelstein effects, can lead to twist-tunable non-equilibrium spin-density polarizations that are perpendicular and parallel to the applied charge current.

cond-mat.mes-hall↗

arXiv2023

Electronic and Spin-Orbit Properties of hBN Encapsulated Bilayer Graphene

Van der Waals (vdW) heterostructures consisting of Bernal bilayer graphene (BLG) and hexagonal boron nitride (hBN) are investigated. By performing first-principles calculations we capture the essential BLG band structure features for several stacking and encapsulation scenarios. A low-energy model Hamiltonian, comprising orbital and spin-orbit coupling (SOC) terms, is employed to reproduce the hBN-modified BLG dispersion, spin splittings, and spin expectation values. Most important, the hBN layers open an orbital gap in the BLG spectrum, which can range from zero to tens of meV, depending on the precise stacking arrangement of the individual atoms. Therefore, large local band gap variations may arise in experimentally relevant moiré structures. Moreover, the SOC parameters are small (few to tens of $μ$eV), just as in bare BLG, but are markedly proximity modified by the hBN layers. Especially when BLG is encapsulated by monolayers of hBN, such that inversion symmetry is restored, the orbital gap and spin splittings of the bands vanish. In addition, we show that a transverse electric field mainly modifies the potential difference between the graphene layers, which perfectly correlates with the orbital gap for fields up to about 1~V/nm. Moreover, the layer-resolved Rashba couplings are tunable by $\sim 5~μ$eV per V/nm. Finally, by investigating twisted BLG/hBN structures, with twist angles between 6$^{\circ}$ -- 20$^{\circ}$, we find that the global band gap increases linearly with the twist angle. The extrapolated $0^{\circ}$ band gap is about 23~meV and results roughly from the average of the stacking-dependent local band gaps. Our investigations give new insights into proximity spin physics of hBN/BLG heterostructures, which should be useful for interpreting experiments on extended as well as confined (quantum dot) systems.

cond-mat.mes-hall↗

arXiv2023

Proximity-induced spin-orbit coupling in phosphorene on a WSe$_2$ monolayer

We investigate, using first-principles methods and effective-model simulations, the spin-orbit coupling proximity effects in a bilayer heterostructure comprising phosphorene and WSe$_2$ monolayers. We specifically analyze holes in phosphorene around the $Γ$ point, at which we find a significant increase of the spin-orbit coupling that can be attributed to the strong hybridization of phosphorene with the WSe$_2$ bands. We also propose an effective spin-orbit model based on the ${\bf C}_{1{\rm v}}$ symmetry of the studied heterostructure. The corresponding spin-orbit field can be divided into two parts: the in-plane field, present due to the broken nonsymmorphic horizontal glide mirror plane symmetry, and the dominant out-of-plane field triggered by breaking the out-of-plane rotational symmetry of the phosphorene monolayer. Furthermore, we also demonstrate that a heterostructure with 60$^\circ$ twist angle exhibits an opposite out-of-plane spin-orbit field, indicating that the coupling can effectively be tuned by twisting. The studied phosphorene/WSe$_2$ bilayer is a prototypical low common-symmetry heterostructure in which the proximity effect can be used to engineer the spin texture of the desired material.

cond-mat.mes-hall↗

arXiv2023

Magneto-optical anisotropies of 2D antiferromagnetic MPX$_3$ from first principles

Here we systematically investigate the impact of the spin direction on the electronic and optical properties of transition metal phosphorus trichalcogenides (MPX$_3$, M=Mn, Ni, Fe; X=S, Se) exhibiting various antiferromagnetic arrangement within the 2D limit. Our analysis based on the density functional theory and versatile formalism of Bethe-Salpeter equation reveals larger exciton binding energies for MPS$_3$ (up to 1.1 eV in air) than MPSe$_3$(up to 0.8 eV in air), exceeding the values of transition metal dichalcogenides (TMDs). For the (Mn,Fe)PX$_3$ we determine the optically active band edge transitions, revealing that they are sensitive to in-plane magnetic order, irrespective of the type of chalcogen atom. We predict the anistropic effective masses and the type of linear polarization as an important fingerprints for sensing the type of magnetic AFM arrangements. Furthermore, we identify the spin-orientation-dependent features such as the valley splitting, the effective mass of holes, and the exciton binding energy. In particular, we demonstrate that for MnPX$_3$ (X=S, Se) a pair of non equivalent K+ and K- points exists yielding the valley splittings that strongly depend on the direction of AFM aligned spins. Notably, for the out-of-plane direction of spins, two distinct peaks are expected to be visible below the absorption onset, whereas one peak should emerge for the in-plane configuration of spins. These spin-dependent features provide an insight into spin flop transitions of 2D materials. Finally, we propose a strategy how the spin valley polarization can be realized in 2D AFM within honeycomb lattice.

cond-mat.mtrl-sci↗

arXiv2023

Microscopic study of the Josephson supercurrent diode effect in Josephson junctions based on two-dimensional electron gas

Superconducting systems that simultaneously lack space-inversion and time-reversal symmetries have recently been the subject of a flurry of experimental and theoretical research activities. Their ability to carry supercurrents with magnitudes depending on the polarity (current direction) - termed supercurrent diode effect - might be practically exploited to design dissipationless counterparts of contemporary semiconductor-based diodes. Magnetic Josephson junctions realized in the two-dimensional electron gas (2DEG) within a narrow quantum well through proximity to conventional superconductors perhaps belong to the most striking and versatile platforms for such supercurrent rectifiers. Starting from the Bogoliubov-de Gennes approach, we provide a minimal theoretical model to explore the impact of the spin-orbit coupling and magnetic exchange inside the 2DEG on the Andreev bound states and Josephson current-phase relations. Assuming realistic junction parameters, we evaluate the polarity-dependent critical currents to quantify the efficiency of these Josephson junctions as supercurrent diodes, and discuss the tunability of the Josephson supercurrent diode effect in terms of spin-orbit coupling, magnetic exchange, and transparency of the nonsuperconducting weak link. Furthermore, we demonstrate that the junctions might undergo current-reversing $ 0 $-$ π$-like phase transitions at large enough magnetic exchange, which appear as sharp peaks followed by a sudden suppression in the supercurrent-diode-effect efficiency. The characteristics of the Josephson supercurrent diode effect obtained from our model convincingly reproduce many unique features observed in recent experiments, validating its robustness and suitability for further studies.

cond-mat.supr-con↗

arXiv2023

Emergent Trion-Phonon Coupling in Atomically-Reconstructed MoSe$_2$-WSe$_2$ Heterobilayers

In low-temperature resonant Raman experiments on MoSe$_2$-WSe$_2$ heterobilayers, we identify a hybrid interlayer shear mode (HSM) with an energy, close to the interlayer shear mode (SM) of the heterobilayers, but with a much broader, asymmetric lineshape. The HSM shows a pronounced resonance with the intralayer hybrid trions (HX$^-$) of the MoSe$_2$ and WSe$_2$ layers, only. No resonance with the neutral intralayer excitons is found. First-principles calculations reveal a strong coupling of Q-valley states, which are delocalized over both layers and participate in the HX$^-$, with the SM. This emerging trion-phonon coupling may be relevant for experiments on gate-controlled heterobilayers.

cond-mat.mes-hall↗

arXiv2023

Link between supercurrent diode and anomalous Josephson effect revealed by gate-controlled interferometry

In Josephson diodes the asymmetry between positive and negative current branch of the current-phase relation leads to a polarity-dependent critical current and Josephson inductance. The supercurrent nonreciprocity can be described as a consequence of the anomalous Josephson effect -- a $φ_0$-shift of the current-phase relation -- in multichannel ballistic junctions with strong spin-orbit interaction. In this work, we simultaneously investigate $φ_0$-shift and supercurrent diode efficiency on the same Josephson junction by means of a superconducting quantum interferometer. By electrostatic gating, we reveal a direct link between $φ_0$-shift and diode effect. Our findings show that the supercurrent diode effect mainly results from magnetochiral anisotropy induced by spin-orbit interaction in combination with a Zeeman field.

cond-mat.supr-con↗

arXiv2023

Twist-angle dependent proximity induced spin-orbit coupling in graphene/transition-metal dichalcogenide heterostructures

We investigate the proximity-induced spin-orbit coupling in heterostructures of twisted graphene and monolayers of transition-metal dichalcogenides (TMDCs) MoS$_2$, WS$_2$, MoSe$_2$, and WSe$_2$ from first principles. We identify strain, which is necessary to define commensurate supercells, as the key factor affecting the band offsets and thus magnitudes of the proximity couplings. We establish that for biaxially strained graphene the band offsets between the Dirac point and conduction (valence) TMDC bands vary linearly with strain, regardless of the twist angle. This relation allows to identify the apparent zero-strain band offsets and find a compensating transverse electric field correcting for the strain. The resulting corrected band structure is then fitted around the Dirac point to an established spin-orbit Hamiltonian. This procedure yields the dominant, valley-Zeeman and Rashba spin-orbit couplings. The magnitudes of these couplings do not vary much with the twist angle, although the valley-Zeeman coupling vanishes for 30$^{\circ}$ and Mo-based heterostructures exhibit a maximum of the coupling at around 20$^{\circ}$. The maximum for W-based stacks is at 0$^{\circ}$. The Rashba coupling is in general weaker than the valley-Zeeman coupling, except at angles close to 30$^{\circ}$. We also identify the Rashba phase angle which measures the deviation of the in-plane spin texture from tangential, and find that this angle is very sensitive to the applied transverse electric field. We further discuss the reliability of the supercell approach with respect to atomic relaxation (rippling of graphene), relative lateral shifts of the atomic layers, and transverse electric field.

cond-mat.mes-hall↗

arXiv2023

Twist-angle dependent proximity induced spin-orbit coupling in graphene/topological insulator heterostructures

The proximity-induced spin-orbit coupling (SOC) in heterostructures of twisted graphene and topological insulators (TIs) Bi$_2$Se$_3$ and Bi$_2$Te$_3$ is investigated from first principles. To build commensurate supercells, we strain graphene and correct thus resulting band offsets by applying a transverse electric field. We then fit the low-energy electronic spectrum to an effective Hamiltonian that comprises orbital and spin-orbit terms. For twist angles 0$^\circ\leqΘ\lessapprox 20^\circ$, we find the dominant spin-orbit couplings to be of the valley-Zeeman and Rashba types, both a few meV strong. We also observe a sign change in the induced valley-Zeeman SOC at $Θ\approx 10^\circ$. Additionally, the in-plane spin structure resulting from the Rashba SOC acquires a non-zero radial component, except at $0^\circ$ or $30^\circ$. At $30^\circ$ the graphene Dirac cone interacts directly with the TI surface state. We therefore explore this twist angle in more detail, studying the effects of gating, TI thicknesses, and lateral shifts on the SOC parameters. We find, in agreement with previous results, the emergence of the proximitized Kane-Mele SOC, with a change in sign possible by electrically tuning the Dirac cone within the TI bulk band gap.

cond-mat.mes-hall↗