arXiv subjects

Jaroslav Fabian

Publications and source records attributed to Jaroslav Fabian.

At least 91 records · Page 5Linked to original sources

arXiv2020

Giant proximity exchange and valley splitting in transition metal dichalcogenide/$h\mathrm{BN}$/(Co, Ni) heterostructures

We investigate the proximity-induced exchange coupling in transition-metal dichalcogenides (TMDCs), originating from spin injector geometries composed of hexagonal boron-nitride (hBN) and ferromagnetic (FM) cobalt (Co) or nickel (Ni), from first-principles. We employ a minimal tight-binding Hamiltonian that captures the low energy bands of the TMDCs around K and K' valleys, to extract orbital, spin-orbit, and exchange parameters. The TMDC/hBN/FM heterostructure calculations show that due to the hBN buffer layer, the band structure of the TMDC is preserved, with an additional proximity-induced exchange splitting in the bands. We extract proximity exchange parameters in the 1--10 meV range, depending on the FM. The combination of proximity-induced exchange and intrinsic spin-orbit coupling (SOC) of the TMDCs, leads to a valley polarization, translating into magnetic exchange fields of tens of Tesla. The extracted parameters are useful for subsequent exciton calculations of TMDCs in the presence of a hBN/FM spin injector. Our calculated absorption spectra show large splittings for the exciton peaks; in the case of MoS$_2$/hBN/Co we find a value of about 8 meV, corresponding to about 50 Tesla external magnetic field in bare TMDCs. The reason lies in the band structure, where a hybridization with Co $d$ orbitals causes a giant valence band exchange splitting of more than 10 meV. Structures with Ni do not show any $d$ level hybridization features, but still sizeable proximity exchange and exciton peak splittings of around 2 meV are present in the TMDCs.

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arXiv2019

Interfacial Spin-Orbit Coupling: New Platform for Superconducting Spintronics

Spin-orbit coupling (SOC) is a key interaction in spintronics, allowing an electrical control of spin or magnetization and, vice versa, a magnetic control of electrical current. However, recent advances have revealed much broader implications of SOC that is also central to the design of topological states, including topological insulators, skyrmions, and Majorana fermions, or to overcome the exclusion of two-dimensional ferro-magnetism expected from the Mermin-Wagner theorem. SOC and the resulting emergent interfacial spin-orbit fields are simply realized in junctions through structural inversion asymmetry, while the anisotropy in magnetoresistance (MR) allows for their experimental detection. Surprisingly, we demonstrate that an all-epitaxial ferromagnet/MgO/metal junction with only a negligible MR anisotropy undergoes a remarkable transformation below the superconducting transition temperature of the metal. The superconducting junction has a three orders of magnitude higher MR anisotropy and supports the formation of spin-triplet superconductivity, crucial for superconducting spintronics, and topologically-protected quantum computing. Our findings call for revisiting the role of SOC in other systems which, even when it seems negligible in the normal state, could have a profound influence on the superconducting response.

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arXiv2019

Strain-tunable orbital, spin-orbit, and optical properties of monolayer transition-metal dichalcogenides

When considering transition-metal dichalcogenides (TMDCs) in van der Waals (vdW) heterostructures for periodic ab-initio calculations, usually, lattice mismatch is present, and the TMDC needs to be strained. In this study we provide a systematic assessment of biaxial strain effects on the orbital, spin-orbit, and optical properties of the monolayer TMDCs using ab-initio calculations. We complement our analysis with a minimal tight-binding Hamiltonian that captures the low-energy bands of the TMDCs around K and K' valleys. We find characteristic trends of the orbital and spin-orbit parameters as a function of the biaxial strain. Specifically, the orbital gap decreases linearly, while the valence (conduction) band spin splitting increases (decreases) nonlinearly in magnitude when the lattice constant increases. Furthermore, employing the Bethe-Salpeter equation and the extracted parameters, we show the evolution of several exciton peaks, with biaxial strain, on different dielectric surroundings, which are particularly useful for interpreting experiments studying strain-tunable optical spectra of TMDCs.

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arXiv2019

Single and bilayer graphene on the topological insulator Bi$_2$Se$_3$: Electronic and spin-orbit properties from first principles

We present a detailed study of the electronic and spin-orbit properties of single and bilayer graphene in proximity to the topological insulator Bi$_2$Se$_3$. Our approach is based on first-principles calculations, combined with symmetry derived model Hamiltonians that capture the low-energy band properties. We consider single and bilayer graphene on 1--3 quintuple layers of Bi$_2$Se$_3$ and extract orbital and proximity induced spin-orbit coupling (SOC) parameters. We find that graphene gets significantly hole doped (350 meV), but the linear dispersion is preserved. The proximity induced SOC parameters are about 1 meV in magnitude, and are of valley-Zeeman type. The induced SOC depends weakly on the number of quintuple layers of Bi$_2$Se$_3$. We also study the effect of a transverse electric field, that is applied across heterostructures of single and bilayer graphene above 1 quintuple layer of Bi$_2$Se$_3$. Our results show that band offsets, as well as proximity induced SOC parameters can be tuned by the field. Most interesting is the case of bilayer graphene, in which the band gap, originating from the intrinsic dipole of the heterostructure, can be closed and reopened again, with inverted band character. The switching of the strong proximity SOC from the conduction to the valence band realizes a spin-orbit valve. Additonally, we find a giant increase of the proximity induced SOC of about 200%, when we decrease the interlayer distance between graphene and Bi$_2$Se$_3$ by only 10%. Finally, for a different substrate material Bi$_2$Te$_2$Se, band offsets are significantly different, with the graphene Dirac point located at the Fermi level, while the induced SOC strength stays similar in magnitude compared to the Bi$_2$Se$_3$ substrate.

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arXiv2019

Magnetic proximity in a van der Waals heterostructure of magnetic insulator and graphene

Engineering two-dimensional material heterostructures by combining the best of different materials in one ultimate unit can offer a plethora of opportunities in condensed matter physics. Here, in the van der Waals heterostructures of the ferromagnetic insulator Cr2Ge2Te6 and graphene, our observations indicate an out-of-plane proximity-induced ferromagnetic exchange interaction in graphene. The perpendicular magnetic anisotropy of Cr2Ge2Te6 results in significant modification of the spin transport and precession in graphene, which is ascribed to the proximity-induced exchange interaction. Furthermore, the observation of a larger lifetime for perpendicular spins in comparison to the in-plane counterpart suggests the creation of a proximity-induced anisotropic spin texture in graphene. Our experimental results and density functional theory calculations open up opportunities for the realization of proximity-induced magnetic interactions and spin filters in 2D material heterostructures and can form the basic building blocks for future spintronic and topological quantum devices.

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arXiv2019

First-principles theory of proximity spin-orbit torque on a two-dimensional magnet: Current-driven antiferromagnet-to-ferromagnet reversible transition in bilayer CrI$_3$

The recently discovered two-dimensional (2D) magnetic insulator CrI$_3$ is an intriguing case for basic research and spintronic applications since it is a ferromagnet in the bulk, but an antiferromagnet in bilayer form, with its magnetic ordering amenable to external manipulations. Using first-principles quantum transport approach, we predict that injecting unpolarized charge current parallel to the interface of bilayer-CrI$_3$/monolayer-TaSe$_2$ van der Waals heterostructure will induce spin-orbit torque (SOT) and thereby driven dynamics of magnetization on the first monolayer of CrI$_3$ in direct contact with TaSe$_2$. By combining calculated complex angular dependence of SOT with the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization, we demonstrate that current pulses can switch the direction of magnetization on the first monolayer to become parallel to that of the second monolayer, thereby converting CrI$_3$ from antiferromagnet to ferromagnet while not requiring any external magnetic field. We explain the mechanism of this reversible current-driven nonequilibrium phase transition by showing that first monolayer of CrI$_3$ carries current due to evanescent wavefunctions injected by metallic transition metal dichalcogenide TaSe$_2$, while concurrently acquiring strong spin-orbit coupling (SOC) via such proximity effect, whereas the second monolayer of CrI$_3$ remains insulating. The transition can be detected by passing vertical read current through the vdW heterostructure, encapsulated by bilayer of hexagonal boron nitride and sandwiched between graphite electrodes, where we find tunneling magnetoresistance of $\simeq 240$%.

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arXiv2019

$k \cdot p$ theory for phosphorene: Effective g-factors, Landau levels, and excitons

Phosphorene, a single layer of black phosphorus, is a direct-band gap two-dimensional semiconductor with promising charge and spin transport properties. The electronic band structure of phosphorene is strongly affected by the structural anisotropy of the underlying crystal lattice. We describe the relevant conduction and valence bands close to the $Γ$ point by four- and six-band (with spin) $k \cdot p$ models, including the previously overlooked interband spin-orbit coupling which is essential for studying anisotropic crystals. All the $k \cdot p$ parameters are obtained by a robust fit to {\it ab initio} data, by taking into account the nominal band structure and the $k$-dependence of the effective mass close to $Γ$-point. The inclusion of interband spin-orbit coupling allows us to determine dipole transitions along both armchair and zigzag directions. The interband coupling is also key to determine the effective g-factors and Zeeman splittings of the Landau levels. We predict the electron and hole g-factor correction of $\approx 0.03$ due to the intrinsic contributions in phosphorene, which lies within the existing range of experimental data. Furthermore, we investigate excitonic effects using the $k \cdot p$ models and find exciton binding energy (0.81 eV) and exciton diameters consistent with experiments and {\it ab initio} based calculations. The proposed $k \cdot p$ Hamiltonians should be useful for investigating magnetic, spin, transport, optical properties and many-body effects in phosphorene.

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arXiv2019

Skew Andreev reflection in ferromagnet/superconductor junctions

Andreev reflection (AR) in ferromagnet/superconductor junctions is an indispensable spectroscopic tool for measuring spin polarization. We study theoretically how the presence of a thin semiconducting interface in such junctions, inducing Rashba and Dresselhaus spin-orbit coupling, modifies AR processes. The interface gives rise to an effective momentum- and spin-dependent scattering potential, making the probability of AR strongly asymmetric with respect to the sign of the incident electrons' transverse momenta. This skew AR creates spatial charge carrier imbalances and transverse Hall currents flow in the ferromagnet. We show that the effect is giant, as compared to the normal regime. We provide a quantitative analysis and a qualitative picture of this phenomenon, and finally show that skew AR also leads to a widely tunable transverse supercurrent response in the superconductor.

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arXiv2019

Proximity exchange effects in MoSe$_2$ and WSe$_2$ heterostructures with CrI$_3$: twist angle, layer, and gate dependence

Proximity effects in two-dimensional (2D) van der Waals heterostructures offer controllable ways to tailor the electronic band structure of adjacent materials. Exchange proximity in particular is important for making materials magnetic without hosting magnetic ions. Such synthetic magnets could be used for studying magnetotransport in high-mobility 2D materials, or magneto-optics in highly absorptive nominally nonmagnetic semiconductors. Using first-principles calculations, we show that the proximity exchange in monolayer MoSe$_2$ and WSe$_2$ due to ferromagnetic monolayer CrI$_3$ can be tuned (even qualitatively) by twisting and gating. Remarkably, the proximity exchange remains the same when using antiferromagnetic CrI$_3$ bilayer, paving the way for optical and electrical detection of layered antiferromagnets. Interestingly, the proximity exchange is opposite to the exchange of the adjacent antiferromagnetic layer. Finally, we show that the exchange proximity is confined to the layer adjacent to CrI$_3$, and that adding a separating hBN barrier drastically reduces the proximity effect. We complement our it ab initio results with tight-binding modeling and solve the Bethe-Salpeter equation to provide experimentally verifiable optical signatures (in the exciton spectra) of the proximity exchange effects.

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arXiv2019

Common nonlinear features and spin-orbit coupling effects in the Zeeman splitting of novel wurtzite materials

The response of semiconductor materials to external magnetic fields is a reliable approach to probe intrinsic electronic and spin-dependent properties. In this study, we investigate the common Zeeman splitting features of novel wurtzite materials, namely InP, InAs, and GaAs. We present values for the effective g-factors of different energy bands and show that spin-orbit coupling effects, responsible for the spin splittings, also have noticeable contributions to the g-factors. Within the Landau level picture, we show that the nonlinear Zeeman splitting recently explained in magneto photoluminescence experiments for InP nanowires by Tedeschi et al. [Phys. Rev. B 99, 161204 (2019)] are also present in InAs, GaAs and even in the conventional GaN. Such nonlinear features stem from the peculiar coupling of the A and B valence bands, as a consequence of the interplay between the wurtzite crystal symmetry and the breaking of time-reversal symmetry by the external magnetic field. Moreover, we develop an analytical model to describe the experimental nonlinear Zeeman splitting and apply it to InP and GaAs data. Extrapolating our fitted results, we found that the Zeeman splitting of InP reaches a maximum value, which is a prediction that could be probed at higher magnetic fields.

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arXiv2019

Spin-orbit coupling in elemental two-dimensional materials

The fundamental spin-orbit coupling and spin mixing in graphene and rippled honeycomb lattice materials silicene, germanene, stanene, blue phosphorene, arsenene, antimonene, and bismuthene is investigated from first principles. The intrinsic spin-orbit coupling in graphene is revisited using multi-band $k\cdot p$ theory, showing the presence of non-zero spin mixing in graphene despite the mirror symmetry. However, the spin mixing itself does not lead to the the Elliott-Yafet spin relaxation mechanism, unless the mirror symmetry is broken by external factors. For other aforementioned elemental materials we present the spin-orbit splittings at relevant symmetry points, as well as the spin admixture $b^2$ as a function of energy close to the band extrema or Fermi levels. We find that spin-orbit coupling scales as the square of the atomic number Z, as expected for valence electrons in atoms. For isolated bands, it is found that $b^2\sim Z^4$. The spin-mixing parameter also exhibits giant anisotropy which, to a large extent, can be controlled by tuning the Fermi level. Our results for $b^2$ can be directly transferred to spin relaxation time due to the Elliott-Yafet mechanism, and therefore provide an estimate of the upper limit for spin lifetimes in materials with space inversion center.

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arXiv2019

Ultralong spin lifetimes in one-dimensional semiconductor nanowires

We experimentally demonstrate ultralong spin lifetimes of electrons in the one-dimensional (1D) quantum limit of semiconductor nanowires. Optically probing single wires of different diameters reveals an increase in the spin relaxation time by orders of magnitude as the electrons become increasingly confined until only a single 1D subband is populated. We find the observed spin lifetimes of more than $200\,\textrm{ns}$ to result from the robustness of 1D electrons against major spin relaxation mechanisms, highlighting the promising potential of these wires for long-range transport of coherent spin information.

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arXiv2019

Heterostructures of graphene and hBN: electronic, spin-orbit, and spin relaxation properties from first principles

We perform extensive first-principles calculations for heterostructures composed of monolayer graphene and hexagonal boron nitride (hBN). Employing a symmetry-derived minimal tight-binding model, we extract orbital and spin-orbit coupling (SOC) parameters for graphene on hBN, as well as for hBN encapsulated graphene. Our calculations show that the parameters depend on the specific stacking configuration of graphene on hBN. We also perform an interlayer distance study for the different graphene/hBN stacks to find the corresponding lowest energy distances. For very large interlayer distances, one can recover the pristine graphene properties, as we find from the dependence of the parameters on the interlayer distance. Furthermore, we find that orbital and SOC parameters, especially the Rashba one, depend strongly on an applied transverse electric field, giving a rich playground for spin physics. Armed with the model parameters, we employ the Dyakonov-Perel formalism to calculate the spin relaxation in graphene/hBN heterostructures. We find spin lifetimes in the nanosecond range, in agreement with recent measurements. The spin relaxation anisotropy, being the ratio of out-of-plane to in-plane spin lifetimes, is found to be giant close to the charge neutrality point, decreasing with increasing doping, and being highly tunable by an external transverse electric field. This is in contrast to bilayer graphene in which an external field saturates the spin relaxation anisotropy.

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arXiv2019

Spin relaxation in fluorinated single and bilayer graphene

We present a joint experiment-theory study on the role of fluorine adatoms in spin and momentum scattering of charge carriers in dilute fluorinated graphene and bilayer graphene. The experimental spin-flip and momentum scattering rates and their dependence on the density of fluorine and carrier doping are obtained through weak localization and conductivity measurements, respectively, and suggest the role of fluorine as resonant magnetic impurities. For the estimated fluorine concentration of a few 100 ppm, the observed spin lifetimes are in the range of 1-10\,ps. Theoretically, we established tight-binding electronic structures of fluorinated graphene and bilayer graphene by fitting to density functional supercell calculations and performed a comprehensive analysis of the spin-flip and momentum scattering rates within the same devices, aiming to develop a consistent description of both scattering channels. We find that resonant scattering in graphene is very sensitive to the precise position of the resonance level, as well as to the magnitude of the exchange coupling between itinerant carriers and localized spins. The experimental data point to the presence of weak spin-flip scatterers that, at the same time, relax the electron momentum strongly, nearly preserving the electron-hole symmetry. Such scatterers would exhibit resonance energies much closer to the neutrality point than what density functional theory predicts in the dilute limit. The inclusion of a magnetic moment on fluorine adatoms allowed us to qualitatively capture the carrier density dependence of the experimental rates but predicts a greater (weaker) spin (momentum) relaxation rate than the measurements. We discuss possible scenarios that may be responsible for the discrepancies. Our systematic study exposes the complexities involved in accurately capturing the behavior of adatoms on graphene.

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arXiv2019

Spin-orbit coupling and spin relaxation in phosphorene: Intrinsic versus extrinsic effects

First-principles calculations of the essential spin-orbit and spin relaxation properties of phosphorene are performed. Intrinsic spin-orbit coupling induces spin mixing with the probability of $b^2 \approx 10^{-4}$, exhibiting a large anisotropy, following the anisotropic crystalline structure of phosphorene. For realistic values of the momentum relaxation times, the intrinsic (Elliott--Yafet) spin relaxation times are hundreds of picoseconds to nanoseconds. Applying a transverse electric field (simulating gating and substrates) generates extrinsic $C_{2v}$ symmetric spin-orbit fields in phosphorene, which activate the D'yakonov--Perel' mechanism for spin relaxation. It is shown that this extrinsic spin relaxation also has a strong anisotropy, and can dominate over the Elliott-Yafet one for strong enough electric fields. Phosphorene on substrates can thus exhibit an interesting interplay of both spin relaxation mechanisms, whose individual roles could be deciphered using our results.

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arXiv2018

Interplay of resonant states and Landau levels in functionalized graphene

Adsorbates can drastically alter physical properties of graphene. Particularly important are adatoms and admolecules that induce resonances at the Dirac point. Such resonances limit electron mobilities and spin relaxation times. We present a systematic tight-binding as well as analytical modeling to investigate the properties of resonant states in the presence of a quantizing magnetic field. Landau levels are strongly influenced by the resonances, especially close to the Dirac point. Here the cyclotron motion of electrons around a defect leads to the formation of circulating local currents which are manifested by the appearance of side peaks around the zero-energy Landau level. Our study is based on realistic parameters for H, F, and Cu adatoms, each exhibiting distinct spectral features in the magnetic field. We also show that by observing a local density of states around an adatom in the presence of Landau levels useful microscopic model parameters can be extracted.

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arXiv2018

Connection between zero-energy Yu-Shiba-Rusinov states and 0-$π$ transitions in magnetic Josephson junctions

We study the bound state spectrum and 0-$π$ transitions in ballistic quasi-1D superconductor/ferromagnetic insulator/superconductor (S/FI/S) Josephson junctions. In addition to the Andreev bound states, stemming from the phase coherence, the magnetic barrier gives rise to qualitatively different Yu-Shiba-Rusinov (YSR) bound states with genuine spectral features and spin characteristics. We show that zero-energy YSR states are much more robust against scalar tunneling than their Andreev counterparts and also fingerprint a quantum phase transition from the junctions' 0 into the $π$ phase, connected to a measurable reversal of the Josephson current flow; this coincidence persists also in the presence of Rashba spin-orbit coupling.

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arXiv2018

Transport Spectroscopy of Sublattice-Resolved Resonant Scattering in Hydrogen-Doped Bilayer Graphene

We report the experimental observation of sublattice-resolved resonant scattering in bilayer graphene by performing simultaneous cryogenic atomic hydrogen doping and electron transport measurements in ultrahigh vacuum. This allows us to monitor the hydrogen adsorption on the different sublattices of bilayer graphene without atomic-scale microscopy. Specifically, we detect two distinct resonant scattering peaks in the gate-dependent resistance, which evolve as a function of atomic hydrogen dosage. Theoretical calculations show that one of the peaks originates from resonant scattering by hydrogen adatoms on the α-sublattice (dimer site) while the other originates from hydrogen adatoms on the \b{eta}-sublattice (non-dimer site), thereby enabling a method for characterizing the relative sublattice occupancy via transport measurements. Utilizing this new capability, we investigate the adsorption and thermal desorption of hydrogen adatoms via controlled annealing and conclude that hydrogen adsorption on the \b{eta}-sublattice is energetically favored. Through site-selective desorption from the α-sublattice, we realize hydrogen doping with adatoms primarily on a single sublattice, which is highly desired for generating ferromagnetism.

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