arXiv subjects

Jaroslav Fabian

Publications and source records attributed to Jaroslav Fabian.

At least 55 records · Page 3Linked to original sources

arXiv2023

Emergent correlated phases in rhombohedral trilayer graphene induced by proximity spin-orbit and exchange coupling

The impact of proximity-induced spin-orbit and exchange coupling on the correlated phase diagram of rhombohedral trilayer graphene (RTG) is investigated theoretically. By employing \emph{ab initio}-fitted effective models of RTG encapsulated by transition metal dichalcogenides (spin-orbit proximity effect) and ferromagnetic Cr$_2$Ge$_2$Te$_6$ (exchange proximity effect), we incorporate the Coulomb interactions within the random-phase approximation to explore potential correlated phases at different displacement field and doping. We find a rich spectrum of spin-valley resolved Stoner and intervalley coherence instabilities induced by the spin-orbit proximity effects, such as the emergence of a \textit{spin-valley-coherent} phase due to the presence of valley-Zeeman coupling. Similarly, proximity exchange removes the phase degeneracies by biasing the spin direction, enabling a magneto-correlation effect -- strong sensitivity of the correlated phases to the relative magnetization orientations (parallel or antiparallel) of the encapsulating ferromagnetic layers.

cond-mat.str-el↗

arXiv2023

Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe$_2$/MoSe$_2$ heterobilayers

We identify an optical excitation mechanism that transfers a valley polarization from photo-excited electron-hole pairs to free charge carriers in twisted WSe$_2$/MoSe$_2$ heterobilayers. For small twist angles, the valley lifetimes of the charge carriers are surprisingly short, despite the occurrence of interlayer excitons with their presumably long recombination and polarization lifetimes. For large twist angles, we measure an increase in both the valley polarization and its respective lifetime by more than two orders of magnitude. Interestingly, in such heterobilayers we observe an interlayer transfer of valley polarization from the WSe$_2$ layer into the MoSe$_2$ layer. This mechanism enables the creation of a photo-induced valley polarization of free charge carriers in MoSe$_2$, which amplitude scales with the gate-induced charge carrier density. This is in contrast to monolayer MoSe$_2$, where such a gate-tunable valley polarization cannot be achieved. By combining time-resolved Kerr rotation, photoluminesence and angle-resolved photoemission spectroscopy measurements with first principles calculations, we show that these findings can be explained by twist angle dependent interlayer scattering mechanisms involving the Q- and $Γ$-valleys.

cond-mat.mtrl-sci↗

arXiv2023

Signatures of electric field and layer separation effects on the spin-valley physics of MoSe$_2$/WSe$_2$ heterobilayers: from energy bands to dipolar excitons

We investigate the spin-valley physics (SVP) in MoSe$_2$/WSe$_2$ heterobilayers under external electric field (EF) and changes of the interlayer distance (ID). We analyze the spin ($S_z$) and orbital ($L_z$) degrees of freedom, and the symmetry properties of relevant band edges (at K, Q, and $Γ$ points) in high-symmetry stackings at 0 (R-type) and 60 (H-type) degree angles, the important building blocks of moiré or atomically reconstructed structures. We reveal distinct hybridization signatures of $S_z$ and $L_z$ in low-energy bands due to the wave function mixing between the layers, which are stacking-dependent and can be further modified by EF and ID. The H-type stackings favor large changes in the g-factors under EF, e. g. from $-5$ to $3$ in the valence bands of the H$^h_h$ stacking, due to the opposite orientation of $S_z$ and $L_z$ in the individual monolayers. For the low-energy dipolar excitons (DEs), direct and indirect in $k$-space, we quantify the electric dipole moments and polarizabilities, reflecting the layer delocalization of the constituent bands. We found that direct DEs carry a robust valley Zeeman effect nearly independent of the EF but tunable by the ID, which can be experimentally accessible via applied external pressure. For the momentum-indirect DEs, our symmetry analysis indicates that phonon-mediated optical processes can easily take place. For the indirect DEs with conduction bands at the Q point for H-type stackings, we found marked variations of the valley Zeeman ($\sim 4$) as a function of the EF that notably stand out from the other DE species. Stronger signatures of the coupled SVP are favored in H-type stackings, which can be experimentally investigated in $\sim 60^\text{o}$ samples. Our study provides fundamental insights into the SVP of van der Waals heterostructures, relevant to understand the valley Zeeman of DEs and intralayer excitons.

cond-mat.mes-hall↗

arXiv2023

Strain control of exciton and trion spin-valley dynamics in monolayer transition metal dichalcogenides

The electron-hole exchange interaction is a fundamental mechanism that drives valley depolarization via intervalley exciton hopping in semiconductor multi-valley systems. Here, we report polarization-resolved photoluminescence spectroscopy of neutral excitons and negatively charged trions in monolayer MoSe$_2$ and WSe$_2$ under biaxial strain. We observe a marked enhancement(reduction) on the WSe$_2$ triplet trion valley polarization with compressive(tensile) strain while the trion in MoSe$_2$ is unaffected. The origin of this effect is shown to be a strain dependent tuning of the electron-hole exchange interaction. A combined analysis of the strain dependent polarization degree using ab initio calculations and rate equations shows that strain affects intervalley scattering beyond what is expected from strain dependent bandgap modulations. The results evidence how strain can be used to tune valley physics in energetically degenerate multi-valley systems.

cond-mat.mes-hall↗

arXiv2023

Proximity-enhanced valley Zeeman splitting at the WS$_2$/graphene interface

The valley Zeeman physics of excitons in monolayer transition metal dichalcogenides provides valuable insight into the spin and orbital degrees of freedom inherent to these materials. Being atomically-thin materials, these degrees of freedom can be influenced by the presence of adjacent layers, due to proximity interactions that arise from wave function overlap across the 2D interface. Here, we report 60 T magnetoreflection spectroscopy of the A- and B- excitons in monolayer WS$_2$, systematically encapsulated in monolayer graphene. While the observed variations of the valley Zeeman effect for the A- exciton are qualitatively in accord with expectations from the bandgap reduction and modification of the exciton binding energy due to the graphene-induced dielectric screening, the valley Zeeman effect for the B- exciton behaves markedly different. We investigate prototypical WS$_2$/graphene stacks employing first-principles calculations and find that the lower conduction band of WS$_2$ at the $K/K'$ valleys (the $CB^-$ band) is strongly influenced by the graphene layer on the orbital level. This leads to variations in the valley Zeeman physics of the B- exciton, consistent with the experimental observations. Our detailed microscopic analysis reveals that the conduction band at the $Q$ point of WS$_2$ mediates the coupling between $CB^-$ and graphene due to resonant energy conditions and strong coupling to the Dirac cone. Our results therefore expand the consequences of proximity effects in multilayer semiconductor stacks, showing that wave function hybridization can be a multi-step process with different bands mediating the interlayer interactions. Such effects can be exploited to resonantly engineer the spin-valley degrees of freedom in van der Waals and moiré heterostructures.

cond-mat.mes-hall↗

arXiv2023

Strong manipulation of the valley splitting upon twisting and gating in MoSe$_2$/CrI$_3$ and WSe$_2$/CrI$_3$ van der Waals heterostructures

We investigate the twist-angle and gate dependence of the proximity-induced exchange coupling in the monolayer transition-metal dichalcogenides (TMDCs) MoSe$_2$ and WSe$_2$ due to the vdW coupling to the ferromagnetic semiconductor CrI$_3$, from first-principles calculations. A model Hamiltonian, that captures the relevant band edges at the $K/K^{\prime}$ valleys of the proximitized TMDCs, is employed to quantify the proximity-induced exchange. Upon twisting from 0° to 30°, we find a transition of the TMDC valence band (VB) edge exchange splitting from about $-2$ to $2$ meV, while the conduction band (CB) edge exchange splitting remains nearly unchanged at around $-3$ meV. For the VB of WSe$_2$ (MoSe$_2$) on CrI$_3$, the exchange coupling changes sign at around 8° (16°). We find that even at the angles with almost zero spin splittings of the VB, the real-space spin polarization profile of holes at the band edge is highly non-uniform, with alternating spin up and spin down orbitals. Furthermore, a giant tunability of the proximity-induced exchange coupling is provided by a transverse electric field of a few V/nm. We complement our \textit{ab initio} results by calculating the excitonic valley splitting to provide experimentally verifiable optical signatures of the proximity exchange. Specifically, we predict that the valley splitting increases almost linearly as a function of the twist angle. Furthermore, the proximity exchange is highly tunable by gating, allowing to tailor the valley splitting in the range of 0 to 12 meV in WSe$_2$/CrI$_3$, which is equivalent to external magnetic fields of up to about 60 Tesla. Our results highlight the important impact of the twist angle and gating when employing magnetic vdW heterostructures in experimental geometries.

cond-mat.mes-hall↗

arXiv2022

Sign reversal of the AC and DC supercurrent diode effect and 0-$π$-like transitions in ballistic Josephson junctions

The recent discovery of intrinsic supercurrent diode effect, and its prompt observation in a rich variety of systems, has shown that nonreciprocal supercurrents naturally emerge when both space- and time-inversion symmetries are broken. In Josephson junctions, nonreciprocal supercurrent can be conveniently described in terms of spin-split Andreev states. Here, we demonstrate a sign reversal of the supercurrent diode effect, in both its AC and DC manifestations. In particular, the AC diode effect -- i.e., the asymmetry of the Josephson inductance as a function of the supercurrent -- allows us to probe the current-phase relation near equilibrium. Using a minimal theoretical model, we can then link the sign reversal of the AC diode effect to the so-called 0-$π$-like transition, a predicted, but still elusive feature of multi-channel junctions. Our results demonstrate the potential of inductance measurements as sensitive probes of the fundamental properties of unconventional Josephson junctions.

cond-mat.supr-con↗

arXiv2022

Proximity effects in graphene on monolayers of transition-metal phosphorus trichalcogenides MPX$_3$

We investigate the electronic band structure of graphene on a series of two-dimensional magnetic transition-metal phosphorus trichalcogenide monolayers, MPX$_3$ with M={Mn,Fe,Ni,Co} and X={S,Se}, with first-principles calculations. A symmetry-based model Hamiltonian is employed to extract orbital parameters and sublattice resolved proximity-induced exchange couplings ($λ_{\textrm{ex}}^\textrm{A}$ and $λ_{\textrm{ex}}^\textrm{B}$) from the low-energy Dirac bands of the proximitized graphene. Depending on the magnetic phase of the MPX$_3$ layer (ferromagnetic and three antiferromagnetic ones), completely different Dirac dispersions can be realized with exchange splittings ranging from 0 to 10~meV. Surprisingly, not only the magnitude of the exchange couplings depends on the magnetic phase, but also the global sign and the type. Important, one can realize uniform ($λ_{\textrm{ex}}^\textrm{A} \approx λ_{\textrm{ex}}^\textrm{B}$) and staggered ($λ_{\textrm{ex}}^\textrm{A} \approx -λ_{\textrm{ex}}^\textrm{B}$) exchange couplings in graphene. From selected cases, we find that the interlayer distance, as well as a transverse electric field are efficient tuning knobs for the exchange splittings of the Dirac bands. More specifically, decreasing the interlayer distance by only about 10\%, a giant 5-fold enhancement of proximity exchange is found, while applying few V/nm of electric field, provides tunability of proximity exchange by tens of percent. We have also studied the dependence on the Hubbard $U$ parameter and find it to be weak. Moreover, we find that the effect of SOC on the proximitized Dirac dispersion is negligible compared to the exchange coupling.

cond-mat.mes-hall↗

arXiv2022

Ultrafast pseudospin quantum beats in multilayer WSe$_2$ and MoSe$_2$

Layered van-der-Waals materials with hexagonal symmetry offer an extra degree of freedom to their electrons, the so called valley index or valley pseudospin. This quantity behaves conceptually like the electron spin and the term valleytronics has been coined. In this context, the group of semiconducting transition-metal dichalcogenides (TMDC) are particularly appealing, due to large spin-orbit interactions and a direct bandgap at the K points of the hexagonal Brillouin zone. In this work, we present investigations of excitonic transitions in mono- and multilayer WSe$_2$ and MoSe$_2$ materials by time-resolved Faraday ellipticity (TRFE) with in-plane magnetic fields, $B_{\parallel}$, of up to 9 T. In monolayer samples, the measured TRFE time traces are almost independent of $B_{\parallel}$, which confirms a close to zero in-plane exciton $g$ factor $g_\parallel$, consistent with first-principles calculations. In stark contrast, we observe pronounced temporal oscillations in multilayer samples for $B_{\parallel}>0$. Remarkably, the extracted in-plane $g_\parallel$ are very close to reported out-of-plane exciton $g$ factors of the materials, namely $|g_{\parallel 1s}|=3.1\pm 0.2$ and $2.5\pm0.2$ for the 1s A excitons in WSe$_2$ and MoSe$_2$ multilayers, respectively. Our first-principles calculations nicely confirm the presence of a non-zero $g_{\parallel}$ for the multilayer samples. We propose that the oscillatory TRFE signal in the multilayer samples is caused by pseudospin quantum beats of excitons, which is a manifestation of spin- and pseudospin layer locking in the multilayer samples. Our results demonstrate ultrafast pseudospin rotations in the GHz- to THz frequency range, which pave the way towards ultrafast pseudospin manipulation in multilayer TMDC samples.

cond-mat.mes-hall↗

arXiv2022

Wurtzite quantum wires with strong spatial confinement: polarization anisotropies in single wire spectroscopy

We report GaAs/AlGaAs nanowires in the one-dimensional (1D) quantum limit. The ultrathin wurtzite GaAs cores between 20-40\,nm induce large confinement energies of several tens of meV, allowing us to experimentally resolve up to four well separated subband excitations in microphotoluminescence spectroscopy. Our detailed experimental and theoretical polarization-resolved study reveals a strong diameter-dependent anisotropy of these transitions: We demonstrate that the polarization of the detected photoluminescence is governed by the symmetry of the wurtzite 1D quantum wire subbands on the one hand, but also by the dielectric mismatch of the wires with the surrounding material on the other hand. The latter effect leads to a strong attenuation of perpendicularly polarized light in thin dielectric wires, making the thickness of the AlGaAs shell an important factor in the observed polarization behavior. Including the dielectric mismatch to our k.p-based simulated polarization-resolved spectra of purely wurtzite GaAs quantum wires, we find an excellent agreement between experiment and theory.

cond-mat.mes-hall↗

arXiv2022

Strong substrate strain effects in multilayered WS2 revealed by high-pressure optical measurements

The optical properties of two-dimensional materials can be effectively tuned by strain induced from a deformable substrate. In the present work we combine first-principles calculations based on density functional theory and the effective Bethe-Salpeter equation with high-pressure optical measurements in order to thoroughly describe the effect of strain and dielectric environment onto the electronic band structure and optical properties of a few-layered transition metal dichalcogenide. Our results show that WS2 remains fully adhered to the substrate at least up to a -0.6% in-plane compressive strain for a wide range of substrate materials. We provide a useful model to describe effect of strain on the optical gap energy. The corresponding experimentally-determined out-of-plane and in-plane stress gauge factors for WS2 monolayers are -8 and 24 meV/GPa, respectively. The exceptionally large in-plane gauge factor confirm transition metal dichalcogenides as very promising candidates for flexible functionalities. Finally, we discuss the pressure evolution of an optical transition closely-lying to the A exciton for bulk WS2 as well as the direct-to-indirect transition of the monolayer upon compression.

cond-mat.mtrl-sci↗

arXiv2022

Signatures of superconducting triplet pairing in Ni--Ga-bilayer junctions

Ni-Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al2O3(/EuS)/Ni-Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)-Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films' superconducting gaps, thinner Ni (1.6 nm)-Ga (30 nm) bilayers entail completely different physics, and give rise to novel large-bias (when compared to the superconducting gap of the thin Al film as a reference) conductance-peak subseries that we term conductance shoulders. These conductance shoulders might attract considerable attention also in similar magnetic superconducting bilayer junctions, as we predict them to offer an experimentally well-accessible transport signature of superconducting triplet pairings that are induced around the interface of the Ni-Ga bilayer. We further substantiate this claim performing complementary polarized neutron reflectometry measurements on the bilayers, from which we deduce (1) a nonuniform magnetization structure in Ga in a several nanometer-thick area around the Ni-Ga boundary and can simultaneously (2) satisfactorily fit the obtained data only considering the paramagnetic Meissner response scenario. While the latter provides independent experimental evidence of induced triplet superconductivity inside the Ni-Ga bilayer, the former might serve as the first experimental hint of its potential microscopic physical origin.

cond-mat.supr-con↗

arXiv2022

Counterintuitive gate dependence of weak antilocalization in bilayer graphene/WSe$_2$ heterostructures

Strong gate control of proximity-induced spin-orbit coupling was recently predicted in bilayer graphene/transition metal dichalcogenides (BLG/TMDC) heterostructures, as charge carriers can easily be shifted between the two graphene layers, and only one of them is in close contact to the TMDC. The presence of spin-orbit coupling can be probed by weak antilocalization (WAL) in low field magnetotransport measurements. When the spin-orbit splitting in such a heterostructure increases with the out of plane electric displacement field $\bar D$, one intuitively expects a concomitant increase of WAL visibility. Our experiments show that this is not the case. Instead, we observe a maximum of WAL visibility around $\bar D=0$. This counterintuitive behaviour originates in the intricate dependence of WAL in graphene on symmetric and antisymmetric spin lifetimes, caused by the valley-Zeeman and Rashba terms, respectively. Our observations are confirmed by calculating spin precession and spin lifetimes from an $8\times 8$ model Hamiltonian of BLG/TMDC.

cond-mat.mes-hall↗

arXiv2022

Engineering Proximity Exchange by Twisting: Reversal of Ferromagnetic and Emergence of Antiferromagnetic Dirac Bands in Graphene/Cr$_2$Ge$_2$Te$_6$

We investigate the twist-angle and gate dependence of the proximity exchange coupling in twisted graphene on monolayer Cr$_2$Ge$_2$Te$_6$ from first principles. The proximitized Dirac band dispersions of graphene are fitted to a model Hamiltonian, yielding effective sublattice-resolved proximity-induced exchange parameters ($λ_{\textrm{ex}}^\textrm{A}$ and $λ_{\textrm{ex}}^\textrm{B}$) for a series of twist angles between 0$^{\circ}$ and 30$^{\circ}$. For aligned layers (0$^{\circ}$ twist angle), the exchange coupling of graphene is the same on both sublattices, $λ_{\textrm{ex}}^\textrm{A} \approx λ_{\textrm{ex}}^\textrm{B} \approx 4$ meV, while the coupling is reversed at 30$^{\circ}$ (with $λ_{\textrm{ex}}^\textrm{A} \approx λ_{\textrm{ex}}^\textrm{B} \approx -4$ meV). Remarkably, at 19.1$^{\circ}$ the induced exchange coupling becomes antiferromagnetic: $λ_{\textrm{ex}}^\textrm{A} < 0, λ_{\textrm{ex}}^\textrm{B} > 0$. Further tuning is provided by a transverse electric field and the interlayer distance. The predicted proximity magnetization reversal and emergence of an antiferromagnetic Dirac dispersion make twisted graphene/Cr$_2$Ge$_2$Te$_6$ bilayers a versatile platform for realizing topological phases and for spintronics applications.

cond-mat.mes-hall↗

arXiv2022

Proximity spin-orbit and exchange coupling in ABA and ABC trilayer graphene van der Waals heterostructures

We investigate the proximity spin-orbit and exchange couplings in ABA and ABC trilayer graphene encapsulated within monolayers of semiconducting transition-metal dichalcogenides and the ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$. Employing first-principles calculations we obtain the electronic structures of the multilayer stacks and extract the relevant proximity-induced orbital and spin interaction parameters by fitting the low-energy bands to model Hamiltonians. We also demonstrate the tunability of the proximity effects by a transverse electric field. Using the model Hamiltonians we also study mixed spin-orbit/exchange coupling encapsulation, which allows to tailor the spin interactions very efficiently by the applied field. We also summarize the spin-orbit physics of bare ABA, ABC, and ABB trilayers, and provide, along with the first-principles results of the electronic band structures, density of states, spin splittings, and electric-field tunabilities of the bands, qualitative understanding of the observed behavior and realistic model parameters as a resource for model simulations of transport and correlation physics in trilayer graphene.

cond-mat.mes-hall↗

arXiv2022

Effect of Rashba and Dresselhaus spin-orbit coupling on supercurrent rectification and magnetochiral anisotropy of ballistic Josephson junctions

Simultaneous breaking of inversion- and time-reversal symmetry in Josephson junction leads to a possible violation of the $I(φ)=-I(-φ)$ equality for the current-phase relation. This is known as anomalous Josephson effect and it produces a phase shift $φ_0$ in sinusoidal current-phase relations. In ballistic Josephson junctions with non-sinusoidal current phase relation the observed phenomenology is much richer, including the supercurrent diode effect and the magnetochiral anisotropy of Josephson inductance. In this work, we present measurements of both effects on arrays of Josephson junctions defined on epitaxial Al/InAs heterostructures. We show that the orientation of the current with respect to the lattice affects the magnetochiral anisotropy, possibly as the result of a finite Dresselhaus component. In addition, we show that the two-fold symmetry of the Josephson inductance reflects in the activation energy for phase slips.

cond-mat.supr-con↗

arXiv2021

Edge states in proximitized graphene ribbons and flakes in a perpendicular magnetic field: emergence of lone pseudohelical pairs and pure spin-current states

We investigate the formation of edge states in graphene ribbons and flakes with proximity induced valley-Zeeman and Rashba spin-orbit couplings in the presence of a perpendicular magnetic field $B$. Two types of edges states appear in the spin-orbit gap at the Fermi level at zero field: strongly localized pseudohelical (intervalley) states and weakly localized intravalley states. We show that if the magnetic field is stronger than a crossover field $B_c$, which is a few mT for realistic systems such as graphene/WSe$_2$, only the pseudohelical edge states remain in zigzag graphene ribbons; the intravalley states disappear. The crossover is directly related to the closing and reopening of the bulk gap formed between nonzero Landau levels. Remarkably, in finite flakes the pseudohelical states undergo perfect reflection at the armchair edges if $B > B_c$, forming standing waves at the zigzag edges. These standing waves comprise two counterpropagating pseudohelical states, so while they carry no charge current, they do carry (pure) spin current.

cond-mat.mes-hall↗

arXiv2021

Superconducting triplet pairings and anisotropic magnetoresistance effects in ferromagnet/superconductor/ferromagnet double-barrier junctions

Ferromagnetic spin valves offer the key building blocks to integrate giant- and tunneling-magnetoresistance effects into spintronics devices. Starting from a generalized Blonder-Tinkham-Klapwijk approach, we theoretically investigate the impact of interfacial Rashba and Dresselhaus spin-orbit couplings on the tunneling conductance, and thereby the magnetoresistance characteristics, of ferromagnet/superconductor/ferromagnet spin-valve junctions embedding thin superconducting spacers between the either parallel or antiparallel magnetized ferromagnets. We focus on the unique interplay between usual electron tunnelings-that fully determine the magnetoresistance in the normal-conducting state-and the peculiar Andreev reflections in the superconducting state. In the presence of interfacial spin-orbit couplings, special attention needs to be paid to the spin-flip ("unconventional") Andreev-reflection process that is expected to induce superconducting triplet correlations in proximitized regions. As a transport signature of these triplet pairings, we detect conductance double peaks around the singlet-gap energy, reflecting the competition between the singlet and an additionally emerging triplet gap; the latter is an effective superconducting gap that can be ascribed to the formation of triplet Cooper pairs through interfacial spin-flip scatterings (i.e., to the generation of an effective triplet-pairing term in the order parameter). We thoroughly analyze the Andreev reflections' role in connection with superconducting magnetoresistance phenomena, and eventually unravel huge conductance and magnetoresistance magnetoanisotropies-easily exceeding their normal-state counterparts by several orders of magnitude-as another experimentally accessible fingerprint of unconventional Andreev reflections.

cond-mat.supr-con↗