arXiv subjects

Endre Tóvári

Publications and source records attributed to Endre Tóvári.

At least 19 recordsLinked to original sources

arXiv2026

Efficient non-volatile electric control of magnetosensing in CrSBr

In two-dimensional crystals, electrostatic gating can modulate the charge carrier density, thereby tuning their electrical properties. In magnetic materials such as CrSBr, this approach can also enable direct electrical control of magnetoresistance. Non-volatile gating is particularly attractive since it allows the carrier density to be maintained at a desired level without the need for a continuous gate bias, offering significant advantages for technological applications. Here, we demonstrate a simple device architecture in which a solution-processed ferroelectric P(VDF-TrFE) layer is integrated directly onto a CrSBr channel, enabling non-volatile control of its magnetoresistance. We investigate the magnetotransport response over a broad temperature range and demonstrate a reproducible and robust gate dependence. Notably, the ferroelectric gate achieves a gating efficiency approximately one order of magnitude higher than that of a conventional SiO$_2$ dielectric, highlighting the potential of ferroelectric gating for electrical control of 2D magnetic materials.

cond-mat.mes-hall↗

arXiv2026

Effect of pressure on the magnetic properties of (Co$_{0.5}$Fe$_{0.5}$)$_5$GeTe$_2$

Cobalt-doped Fe$_5$GeTe$_2$ possesses a rich magnetic phase diagram as a function of Co concentration. The nature of magnetic order in (Co$_{0.5}$Fe$_{0.5}$)$_5$GeTe$_2$ is especially interesting, as it has been shown to exhibit ferromagnetic order, A-type antiferromagnetic (AFM) order, or potentially both at the same time. Here we present magnetoresistance measurements on antiferromagnetic (Co$_{0.5}$Fe$_{0.5}$)$_5$GeTe$_2$ at a series of pressures and extract the anisotropy and interlayer exchange fields using the two-sublattice model. We show a 50 % increase of the interlayer exchange at 2 GPa, highlighting the sensitivity of magnetic properties to interlayer distance. In addition, we find that the sharp hysteretic transitions observed within the AFM state can be qualitatively described by a linear chain model, which suggests an even-odd effect as a function of layer number instead of a coexisting ferromagnetic phase.

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arXiv2026

Impact of current-induced magnons on spin-orbit torque analysis

The second-harmonic Hall technique is a widely used, sensitive method for studying the spin-orbit torques generated by charge current. It exploits the dependence of the Hall resistance on the magnetization direction, although thermal phenomena also contribute. Historically, deviations from the expected magnetic field dependence have usually been neglected. Based on our studies on permalloy/platinum bilayers, we show that a counterpart to the magnon-related spin-flip unidirectional magnetoresistance - known to appear in the second-harmonic longitudinal resistance - appears in the Hall data, and that describing the results in a wide field range with these contributions is essential to accurately estimate the torques.

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arXiv2025

Increasing the proximity induced spin-orbit coupling in bilayer graphene/WSe$_2$ heterostructures with pressure

Combining graphene with transition metal dichalcogenides (TMDs) leads to enhanced spin-orbit coupling (SOC) in the graphene. The induced SOC has a large effect on the low-energy part of the band structure leading to or stabilizing novel phases such as topological phases or superconductivity. Here, the pressure dependence of the SOC strength is investigated in bilayer graphene/WSe$_2$ heterostructures. We performed magnetoconductance studies, such as weak localization, quantum Hall, and Shubnikov-de Haas oscillation measurements to extract the different SOC terms which determine the low-energy band structure of BLG. We find the proximity-induced SOC strengths increased by more than 50\% as a result of applying 2\,GPa hydrostatic pressure. Our studies highlight the opportunity to increase the SOC coupling strength with pressure, which can be important for correlated phases or spin qubits in BLG/WSe$_2$ heterostructures.

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arXiv2024

Signature of pressure-induced topological phase transition in ZrTe$_5$

The layered van der Waals material ZrTe$_5$ is known as a candidate topological insulator (TI), however its topological phase and the relation with other properties such as an apparent Dirac semimetallic state is still a subject of debate. We employ a semiclassical multicarrier transport (MCT) model to analyze the magnetotransport of ZrTe$_5$ nanodevices at hydrostatic pressures up to 2 GPa. The temperature dependence of the MCT results between 10 and 300 K is assessed in the context of thermal activation, and we obtain the positions of conduction and valence band edges in the vicinity of the chemical potential. We find evidence of the closing and re-opening of the band gap with increasing pressure, which is consistent with a phase transition from weak to strong TI. This matches expectations from ab initio band structure calculations, as well as previous observations that CVT-grown ZrTe$_5$ is a weak TI in ambient conditions.

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arXiv2024

Stabilizing the inverted phase of a WSe$_2$/BLG/WSe$_2$ heterostructure via hydrostatic pressure

Bilayer graphene (BLG) was recently shown to host a band-inverted phase with unconventional topology emerging from the Ising-type spin--orbit interaction (SOI) induced by the proximity of transition metal dichalcogenides with large intrinsic SOI. Here, we report the stabilization of this band-inverted phase in BLG symmetrically encapsulated in tungsten-diselenide (WSe$_2$) via hydrostatic pressure. Our observations from low temperature transport measurements are consistent with a single particle model with induced Ising SOI of opposite sign on the two graphene layers. To confirm the strengthening of the inverted phase, we present thermal activation measurements and show that the SOI-induced band gap increases by more than 100% due to the applied pressure. Finally, the investigation of Landau level spectra reveals the dependence of the level-crossings on the applied magnetic field, which further confirms the enhancement of SOI with pressure.

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arXiv2024

Determination of the current-phase relation of an InAs 2DEG Josephson junction with a microwave resonator

Semiconductor-superconductor hybrid nanocircuits are of high interest due to their potential applications in quantum computing. Semiconductors with a strong spin-orbit coupling and large $g$-factor are particularly attractive since they are the basic building blocks of novel qubit architectures. However, for the engineering of these complex circuits, the building blocks must be characterized in detail. We have investigated a Josephson junction where the weak link is a two-dimensional electron gas (2DEG) hosted in an InAs/InGaAs heterostructure grown on a GaAs substrate. We employed the in-situ epitaxially grown Al layer as superconducting contacts to form an rf SQUID, and also to create a microwave resonator for sensing the Josephson inductance. We determined the gate-dependent current-phase relation, and observed supercurrent interference in out-of-plane magnetic fields. With the application of an in-plane magnetic field, we induced asymmetry in the interference pattern, which was found to be anisotropic in the device plane.

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arXiv2024

Ultimate charge transport regimes in doping-controlled graphene laminates: phonon-assisted processes revealed by the linear magnetoresistance

Understanding and controlling the electrical properties of solution-processed 2D materials is key to further printed electronics progress. Here we demonstrate that the thermolysis of the aromatic intercalants utilized in nanosheet exfoliation for graphene laminates opens the route to achieving high intrinsic mobility and simultaneously controlling doping type ($n$- and $p$-) and concentration over a wide range. We establish that the intra-flake mobility is high by observing a linear magnetoresistance of such solution-processed graphene laminates and using it to devolve the inter-flake tunneling and intra-layer magnetotransport. Consequently, we determine the temperature dependences of the inter- and intra-layer characteristics, which both appear to be dominated by phonon-assisted processes at temperature $T>$20 Kelvin. In particular, we identify the efficiency of phonon-assisted tunneling as the main limiting factor for electrical conductivity in graphene laminates at room temperature. We also demonstrate a thermoelectric sensitivity of around 50 $μ$V K$^{-1}$ in a solution-processed metal-free graphene-based thermocouple.

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arXiv2023

Thermopower in hBN/graphene/hBN superlattices

Thermoelectric effects are highly sensitive to the asymmetry in the density of states around the Fermi energy and can be exploited as probes of the electronic structure. We experimentally study thermopower in high-quality monolayer graphene, within heterostructures consisting of complete hBN encapsulation and 1D edge contacts, where the graphene and hBN lattices are aligned. When graphene is aligned to one of the hBN layers, we demonstrate the presence of additional sign reversals in the thermopower as a function of carrier density, directly evidencing the presence of the moiré superlattice. We show that the temperature dependence of the thermopower enables the assessment of the role of built-in strain variation and van Hove singularities and hints at the presence of Umklapp electron-electron scattering processes. As the thermopower peaks around the neutrality point, this allows to probe the energy spectrum degeneracy. Further, when graphene is double-aligned with the top and bottom hBN crystals, the thermopower exhibits features evidencing multiple cloned Dirac points caused by the differential super-moiré lattice. For both cases we evaluate how well the thermopower agrees with Mott's equation. Finally, we show the same superlattice device can exhibit a temperature-driven thermopower reversal from positive to negative and vice versa, by controlling the carrier density. The study of thermopower provides an alternative approach to study the electronic structure of 2D superlattices, whilst offering opportunities to engineer the thermoelectric response on these heterostructures.

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arXiv2023

Near-surface InAs 2DEG on a GaAs substrate: characterization and superconducting proximity effect

We have studied a near-surface two-dimensional electron gas based on an InAs quantum well on a GaAs substrate. In devices without a dielectric layer we estimated large electron mobilities on the order of $10^5$ cm$^2$/Vs. We have observed quantized conductance in a quantum point contact, and determined the g-factor. Using samples with an epitaxial Al layer, we defined multiple Josephson junctions and found the critical current to be gate tunable. Based on multiple Andreev reflections the semiconductor-superconductor interface is transparent, with an induced gap of 125 $μ$eV. Our results demonstrate the viability of this platform for hybrid topological superconductor devices.

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arXiv2023

Revealing the band structure of ZrTe$_5$ using Multicarrier Transport

The layered material ZrTe$_5$ appears to exhibit several exotic behaviors which resulted in significant interest recently, although the exact properties are still highly debated. Among these we find a Dirac/Weyl semimetallic behavior, nontrivial spin textures revealed by low temperature transport, and a potential weak or strong topological phase. The anomalous behavior of resistivity has been recently elucidated as originating from band shifting in the electronic structure. Our work examines magnetotransport behavior in ZrTe$_5$ samples in the context of multicarrier transport. The results, in conjunction with ab-initio band structure calculations, indicate that many of the transport features of ZrTe$_5$ across the majority of the temperature range can be adequately explained by the semiclassical multicarrier transport model originating from a complex Fermi surface.

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arXiv2021

Tailoring the band structure of twisted double bilayer graphene with pressure

Twisted two-dimensional structures open new possibilities in band structure engineering. At magic twist angles, flat bands emerge, which give a new drive to the field of strongly correlated physics. In twisted double bilayer graphene dual gating allows changing the Fermi level and hence the electron density and also allows tuning the interlayer potential, giving further control over band gaps. Here, we demonstrate that by applying hydrostatic pressure, an additional control of the band structure becomes possible due to the change of tunnel couplings between the layers. We find that the flat bands and the gaps separating them can be drastically changed by pressures up to 2 GPa, in good agreement with our theoretical simulations. Furthermore, our measurements suggest that in finite magnetic field due to pressure a topologically non-trivial band gap opens at the charge neutrality point at zero displacement field.

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arXiv2021

Transport measurements on van der Waals heterostructures under pressure

The interlayer coupling, which has a strong influence on the properties of van der Waals heterostructures, strongly depends on the interlayer distance. Although considerable theoretical interest has been demonstrated, experiments exploiting a variable interlayer coupling on nanocircuits are scarce due to the experimental difficulties. Here, we demonstrate a novel method to tune the interlayer coupling using hydrostatic pressure by incorporating van der Waals heterostructure based nanocircuits in piston-cylinder hydrostatic pressure cells with a dedicated sample holder design. This technique opens the way to conduct transport measurements on nanodevices under pressure using up to 12 contacts without constraints on the sample at fabrication level. Using transport measurements, we demonstrate that hexagonal boron nitride capping layer provides a good protection of van der Waals heterostructures from the influence of the pressure medium, and we show experimental evidence of the influence of pressure on the interlayer coupling using weak localization measurements on a TMDC/graphene heterostructure.

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arXiv2021

Boosting proximity spin orbit coupling in graphene/WSe$_2$ heterostructures via hydrostatic pressure

Van der Waals heterostructures composed of multiple few layer crystals allow the engineering of novel materials with predefined properties. As an example, coupling graphene weakly to materials with large spin orbit coupling (SOC) allows to engineer a sizeable SOC in graphene via proximity effects. The strength of the proximity effect depends on the overlap of the atomic orbitals, therefore, changing the interlayer distance via hydrostatic pressure can be utilized to enhance the interlayer coupling between the layers. In this work, we report measurements on a graphene/WSe$_2$ heterostructure exposed to increasing hydrostatic pressure. A clear transition from weak localization to weak anti-localization is visible as the pressure increases, demonstrating the increase of induced SOC in graphene.

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arXiv2020

In situ tuning of symmetry-breaking induced non-reciprocity in giant-Rashba semiconductor BiTeBr

Non-reciprocal transport, where the left to right flowing current differs from the right to left flowing one, is an unexpected phenomenon in bulk crystals. BiTeBr is a non-centrosymmetric material, with a giant Rashba spin-orbit coupling which presents this unusual effect when placed in an in-plane magnetic field. It has been shown that this effect depends strongly on the carrier density, however, in-situ tuning has not yet been demonstrated. We developed a method where thin BiTeBr flakes are gate tuned via ionic-liquid gating through a thin protective hBN layer. Tuning the carrier density allows a more than \SI{400}{\percent} variation of the non-reciprocal response. Our study serves as a milestone on how a few-atomic-layer-thin van der Waals protection layer allows ionic gating of chemically sensitive, exotic nanocrystals.

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arXiv2018

Gate-Defined Quantum Confinement in InSe-based van der Waals Heterostructures

Indium selenide, a post-transition metal chalcogenide, is a novel two-dimensional (2D) semiconductor with interesting electronic properties. Its tunable band gap and high electron mobility have already attracted considerable research interest. Here we demonstrate strong quantum confinement and manipulation of single electrons in devices made from few-layer crystals of InSe using electrostatic gating. We report on gate-controlled quantum dots in the Coulomb blockade regime as well as one-dimensional quantization in point contacts, revealing multiple plateaus. The work represents an important milestone in the development of quality devices based on 2D materials and makes InSe a prime candidate for relevant electronic and optoelectronic applications.

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arXiv2017

Exfoliation of single layer BiTeI flakes

Spin orbit interaction can be strongly boosted when a heavy element is embedded into an inversion asymmetric crystal field. A simple structure to realize this concept in a 2D crystal contains three atomic layers, a middle one built up from heavy elements generating strong atomic spin-orbit interaction and two neighboring atomic layers with different electron negativity. BiTeI is a promising candidate for such a 2D crystal, since it contains heavy Bi layer between Te and I layers. Recently the bulk form of BiTeI attracted considerable attention due to its giant Rashba interaction, however, 2D form of this crystal was not yet created. In this work we report the first exfoliation of single layer BiTeI using a recently developed exfoliation technique on stripped gold. Our combined scanning probe studies and first principles calculations show that SL BiTeI flakes with sizes of 100 $μ$m were achieved which are stable at ambient conditions. The giant Rashba splitting and spin-momentum locking of this new member of 2D crystals open the way towards novel spintronic applications and synthetic topological heterostructures.

cond-mat.mes-hall↗

arXiv2016

Gate-controlled conductance enhancement from quantum Hall channels along graphene p-n junctions

The formation of quantum Hall channels inside the bulk of graphene is studied using various contact and gate geometries. p-n junctions are created along the longitudinal direction of samples, and enhanced conductance is observed in the case of bipolar doping due to new conducting channels forming in the bulk, whose position, propagating direction and, in one geometry, coupling to electrodes are determined by the gate-controlled filling factor across the device. This effect could be exploited to probe the behavior and interaction of quantum Hall channels protected against uncontrolled scattering at the edges.

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