arXiv subjects

Hadar Steinberg

Publications and source records attributed to Hadar Steinberg.

At least 19 recordsLinked to original sources

arXiv2025

PyPAS -- Python package for Positron Annihilation Spectroscopy Doppler Broadening Analysis

Doppler Broadening (DB) of annihilation radiation is a well-established technique within Positron Annihilation Spectroscopy (PAS), used for probing the electronic structure of materials. The analysis of DB experimental data relies on gamma spectroscopy analysis tools, while depth profiling using variable-energy slow positron beams depends on solving the positron diffusion equation. Traditional Variable Energy Doppler Broadening (VEDB) analysis tools, such as VEPFIT and ROYPROF, often present limitations due to outdated interfaces and lack of integration with comprehensive spectroscopy analysis platforms. Addressing these challenges, an open-source Python package for PAS analysis, PyPAS, is introduced. PyPAS offers functionalities including Coincidence Doppler Broadening (CDB) filtering, two-dimensional CDB analysis with DB and resolution extraction, and computation of lineshape parameters (S and W). Furthermore, it integrates modules for generating thermal positron implantation profiles based on established models, solving positron diffusion equations using finite-difference methods and optimizing diffusion length. This work presents the architecture of the PyPAS package and the validation results and demonstrates the application of the package through case studies.

cond-mat.mtrl-sci↗

arXiv2025

Quantum Imaging of Ferromagnetic van der Waals Magnetic Domain Structures at Ambient Conditions

Recently discovered 2D van der Waals magnetic materials, and specifically Iron-Germanium-Telluride ($\rm Fe_{5}GeTe_{2}$), have attracted significant attention both from a fundamental perspective and for potential applications. Key open questions concern their domain structure and magnetic phase transition temperature as a function of sample thickness and external field, as well as implications for integration into devices such as magnetic memories and logic. Here we address key questions using a nitrogen-vacancy center based quantum magnetic microscope, enabling direct imaging of the magnetization of $\rm Fe_{5}GeTe_{2}$ at sub-micron spatial resolution as a function of temperature, magnetic field, and thickness. We employ spatially resolved measures, including magnetization variance and cross-correlation, and find a significant spread in transition temperature yet with no clear dependence on thickness down to 15 nm. We also identify previously unknown stripe features in the optical as well as magnetic images, which we attribute to modulations of the constituting elements during crystal synthesis and subsequent oxidation. Our results suggest that the magnetic anisotropy in this material does not play a crucial role in their magnetic properties, leading to a magnetic phase transition of $\rm Fe_{5}GeTe_{2}$ which is largely thickness-independent down to 15 nm. Our findings could be significant in designing future spintronic devices, magnetic memories and logic with 2D van der Waals magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Thickness Dependence of the Vortex Pearl Length in Few-Layer NbSe2

The coexistence of multiple types of orders is a common thread in condensed matter physics and unconventional superconductors. The nature of superconducting orders may be unveiled by analyzing local perturbations such as vortices. For thin films, the vortex magnetic profile is characterized by the Pearl-length {\Lambda}, which is inversely proportional to the 2D superfluid density; hence, normally, also inversely proportional to the film thickness, d. Here we employ the scanning SQUID-on-tip microscopy to measure {\Lambda} in NbSe2 flakes with thicknesses ranging from N=3 to 53 layers. For N>10, we find the expected dependence {\Lambda}{\varpropto}1/d. However, six-layer films show a sharp increase of {\Lambda} deviating by a factor of three from the expected value. This value remains fixed for N=3 to 6. This unexpected behavior suggests the competition between two orders; one residing only on the first and last layers of the film while the other prevails in all layers.

cond-mat.supr-con↗

arXiv2024

Field-induced antiferromagnetic correlations in a nanopatterned van der Waals ferromagnet: a potential artificial spin ice

Nano-patterned magnetic materials have opened new venues on the investigation of strongly correlated phenomena including artificial spin-ice systems, geometric frustration, magnetic monopoles, for technologically important applications such as reconfigurable ferromagnetism. With the advent of atomically thin two-dimensional (2D) van der Waals (vdW) magnets a pertinent question is whether such compounds could make their way into this realm where interactions can be tailored so that unconventional states of matter could be assessed. Here we show that square islands of CrGeTe3 vdW ferromagnets distributed in a grid manifest antiferromagnetic correlations, essential to enable frustration resulting in an artificial spin-ice. By using a combination of SQUID-on-tip microscopy, focused ion beam lithography, and atomistic spin dynamic simulations, we show that pristine, isolated CGT flakes as small as 150*150*60 nm3 have tunable dipole-dipole interactions, which can be precisely controlled by their lateral spacing. There is a crossover between non-interacting islands and significant inter-island anticorrelation depending how they are spatially distributed allowing the creation of complex magnetic patterns not observable at the isolated flakes. Our findings suggest that the cross-talk between the nano-patterned magnets can be explored in the generation of even more complex spin configurations where exotic interactions may be manipulated in an unprecedent way.

cond-mat.mtrl-sci↗

arXiv2024

Anomalous size dependence of the coercivity of nanopatterned CrGeTe3

The coercivity of single-domain magnetic nanoparticles typically decreases with the nanoparticle size and reaches zero when thermal fluctuations overcome the magnetic anisotropy. Here, we used SQUID-on-tip microscopy to investigate the coercivity of square-shaped CrGeTe3 nanoislands with a wide range of sizes and width-to-thickness aspect ratios. The results reveal an anomalous size-dependent coercivity, with smaller islands exhibiting higher coercivity. The nonconventional scaling of the coercivity in CrGeTe3 nanoislands was found to be inversely proportional to the island width and thickness (1 over wd). This scaling implies that the nanoisland magnetic anisotropy is proportional to the perimeter rather than the volume, suggesting a magnetic edge state. In addition, we observe that 1600 nm wide islands display multi-domain structures with zero net remnant field, corresponding to the magnetic properties of pristine CrGeTe3 flakes. Our findings highlight the significant influence of edge states on the magnetic properties of CrGeTe3 and deepen our understanding of low-dimensional magnetic systems.

cond-mat.mtrl-sci↗

arXiv2024

The transition-metal-dichalcogenide family as a superconductor tuned by charge density wave strength

Metallic transition metal dichalcogenides (TMDs), consisting of H-NbSe$_2$, H-NbS$_2$, H-TaSe$_2$ and H-TaS$_2$, remain superconducting down to a thickness of a single layer. In these materials, thickness affects a variety of properties, including Ising protection, two-band superconductivity, and the critical temperature $T_C$, which decreases for the Nb-based, and increases for the Ta-based materials. This contradicting trend is puzzling, and has precluded the development of a unified theory. We approach the question of thickness-evolution of $T_C$ and the superconducting gap $Δ$ by measuring high-resolution tunneling spectra in TaS$_2$-based stacked devices. Our measurements allow for simultaneous evaluation of $Δ$, $T_C$, and the upper critical field $H_{C2}$. The latter, we find, is strongly enhanced towards the single-layer limit, following a $H_{C2} \propto Δ^2$ proportionality ratio. Our main finding is that the same ratio holds for the entire family of metallic TMDs: TaS$_2$ and NbSe$_2$ of all thicknesses, bulk TaSe$_2$ and bulk NbS$_2$, extending over 4 orders of magnitude in $H_{C2}$ and covering both clean and dirty limits. We propose that this tunability across the TMD family is controlled by the competing charge density wave (CDW) phase. Using Gor'kov's theory, we calculate how a CDW order affects the quasiparticle density of states and the resulting $T_C$ and $H_{C2}$. Our results suggest that CDW is the key determinant factor limiting $T_C$ in the TMD family. They also show that $H_{C2}$ is universally enhanced by a factor of two orders of magnitude above the expected value, an effect that remains an open question.

cond-mat.supr-con↗

arXiv2024

Signature of long-ranged spin triplets across a two-dimensional superconductor/helimagnet van der Waals interface

The combination of a superconductor with a magnetically inhomogeneous material has been established as an efficient mechanism for the generation of long-ranged spin-polarized (spin-triplet) Cooper pairs. Evidence for this mechanism, however, has been established based on studies done on three-dimensional systems, where the strong bonds existing at the interface between the superconductor and the magnetic material should in principle enhance proximity effects and strengthen any electronic correlations. Here, we fabricate devices based on van der Waals stacks of flakes of the two-dimensional superconductor $NbS_2$ combined with flakes of $Cr_{1/3}NbS_2$, which has a built-in magnetic inhomogeneity due to its helimagnetic spin texture at low temperatures. We find that the critical temperature of these vdW bilayers is strongly dependent on the magnetic state of $Cr_{1/3}NbS_2$, whose degree of magnetic inhomogeneity can be controlled via an applied magnetic field. Our results demonstrate evidence for the generation of long-ranged spin-triplet pairs across the $Cr_{1/3}NbS_2$/$NbS_2$ vdW interface.

cond-mat.supr-con↗

arXiv2023

Nano-Patterned Magnetic Edges in CrGeTe3 for Quasi 1-D Spintronic Devices

The synthesis of two-dimensional van der Waals magnets has paved the way for both technological applications and fundamental research on magnetism confined to ultra-small length scales. Edge magnetic moments in ferromagnets are expected to be less magnetized than in the sample interior because of the reduced amount of neighboring ferromagnetic spins at the sample edge. We recently demonstrated that CrGeTe3 (CGT) flakes thinner than 10 nm are hard ferromagnets; i.e., they exhibit an open hysteresis loop. In contrast, thicker flakes exhibit zero net remnant field in the interior, with hard ferromagnetism present only at the cleaved edges. This experimental observation suggests that a nontrivial interaction exists between the sample edge and the interior. Here, we demonstrate that artificial edges fabricated by focus ion beam etching also display hard ferromagnetism. This enables us to write magnetic nanowires in CGT directly and use this method to characterize the magnetic interaction between the interior and edge. The results indicate that the interior saturation and depolarization fields depend on the lateral dimensions of the sample. Most notably, the interior region between the edges of a sample narrower than 300 nm becomes a hard ferromagnet, suggesting an enhancement of the magnetic exchange induced by the proximity of the edges. Last, we find that the CGT regions amorphized by the gallium beam are nonmagnetic, which introduces a novel method to tune the local magnetic properties of CGT films, potentially enabling integration into spintronic devices.

cond-mat.mes-hall↗

arXiv2022

Chip-Integrated Vortex Manipulation

Abrikosov Vortices have long been considered as means to encode classical information in low-temperature logic circuits (1) and memory devices (2-4). Although it is possible to control individual vortices using local probes (5-11), scalability towards the control of of multiple vortices remains challenging. Vortex logic devices require means to shuttle selected vortices reliably over long distances between engineered pinning potentials. Concomitantly, all other vortices should remains fixed to their precise locations. Here we demonstrate such capabilities using Nb loops patterned below a NbSe$_2$ layer. SQUID-on-Tip (SOT) microscopy reveals that the loops can position vortices in sites designated to a precision better than 100 nm; they can realize "push" and "pull" operations of vortices as far as 3$μ$m. Successive application of such operations shuttles a vortex between adjacent loops. Our results may be used as means to integrate vortices in future quantum circuitry. Strikingly, we are able to demonstrate a winding operation. Such winding, if realized in topological superconductors, is considered an essential part of future topological quantum information processing (12-17).

cond-mat.supr-con↗

arXiv2022

Kondo Effect in Defect-bound Quantum Dots Coupled to NbSe$_2$

We report the fabrication of a van der Waals tunneling device hosting a defect-bound quantum dot coupled to NbSe$_2$. We find that upon application of magnetic field, the device exhibits a zero-bias conductance peak. The peak, which splits at higher fields, is associated with a Kondo effect. At the same time, the junction retains conventional quasiparticle tunneling features at finite bias. Such coexistence of a superconducting gap and a Kondo effect are unusual, and are explained by noting the two-gap nature of the superconducting state of NbSe$_2$, where a magnetic field suppresses the low energy gap associated with the Se band. Our data shows that van der Waals architectures, and defect-bound dots in them, can serve as a novel and effective platform for investigating the interplay of Kondo screening and superconducting pairing in unconventional superconductors.

cond-mat.mes-hall↗

arXiv2022

Thickness mapping and layer number identification of exfoliated van der Waals materials by Fourier imaging micro-ellipsometry

As properties of mono- to few layers of exfoliated van der Waals heterostructures are heavily dependent on their thicknesses, accurate thickness measurement becomes imperative in their study. Commonly used atomic force microscopy and Raman spectroscopy techniques may be invasive and produce inconclusive results. Alternatively, spectroscopic ellipsometry is limited by tens-of-microns lateral resolution and/or low data acquisition rates, inhibiting its utilization for micro-scale exfoliated flakes. In this work, we demonstrate a Fourier imaging spectroscopic micro-ellipsometer with sub-5 microns lateral resolution along with fast data acquisition rate and present angstrom-level accurate and consistent thickness mapping on mono-, bi- and trilayers of graphene, hexagonal boron nitride and transition metal dichalcogenide (MoS2, WS2, MoSe2, WSe2) flakes. We show that the optical microscope integrated ellipsometer can also map minute thickness variations over a micro-scale flake. In addition, our system addresses the pertinent issue of identifying monolayer thick hBN.

physics.optics↗

arXiv2022

High magnetic field stability in a planar graphene-NbSe$_2$ SQUID

Thin NbSe$_2$ retains superconductivity at high in-plane magnetic field up to 30 T. In this work we construct an atomically thin, all van der Waals SQUID, in which current flows between NbSe$_2$ contacts through two parallel graphene weak links. This fully planar device remains uniquely stable at high in-plane field. This enables tracing the evolution of the critical current interference patterns as a function of the field up to 4.5 T, allowing nm-scale sensitivity to deviations from a perfect atomic plane. We present numerical methods to retrieve asymmetric current distributions J$_0$ from measured interference maps, and suggest a new application of the dual junction geometry to probe the current density in the absence of phase information. The interference maps exhibit a striking field-driven transition, indicating a redistribution of supercurrents to narrow channels. Our results suggest the existence of a preferred conductance channel with an exceptional stability to in-plane magnetic field.

cond-mat.supr-con↗

arXiv2022

Interior and edge magnetization in thin exfoliated CrGeTe3 films

CrGeTe3 (CGT) is a semiconducting vdW ferromagnet shown to possess magnetism down to a two-layer thick sample. Although CGT is one of the leading candidates for spintronics devices, a comprehensive analysis of CGT thickness dependent magnetization is currently lacking. In this work, we employ scanning SQUID-on-tip (SOT) microscopy to resolve the magnetic properties of exfoliated CGT flakes at 4.2 K. Combining transport measurements of CGT/NbSe2 samples with SOT images, we present the magnetic texture and hysteretic magnetism of CGT, thereby matching the global behavior of CGT to the domain structure extracted from local SOT magnetic imaging. Using this method, we provide a thickness dependent magnetization state diagram of bare CGT films. No zero-field magnetic memory was found for films thicker than 10 nm and hard ferromagnetism was found below that critical thickness. Using scanning SOT microscopy, we identify a unique edge magnetism, contrasting the results attained in the CGT interior.

cond-mat.mes-hall↗

arXiv2021

Planar graphene-NbSe$_2$ Josephson junctions in a parallel magnetic field

Thin transition metal dichalcogenides sustain superconductivity at large in-plane magnetic fields due to Ising spin-orbit protection, which locks their spins in an out-of-plane orientation. Here we use thin NbSe$_2$ as superconducting electrodes laterally coupled to graphene, making a planar, all van der Waals two-dimensional Josephson junction (2DJJ). We map out the behavior of these novel devices with respect to temperature, gate voltage, and both out-of-plane and in-plane magnetic fields. Notably, the 2DJJs sustain supercurrent up to $H_\parallel$ as high as 8.5 T, where the Zeeman energy $E_Z$ rivals the Thouless energy $E_{Th}$, a regime hitherto inaccessible in graphene. As the parallel magnetic field $H_\parallel$ increases, the 2DJJ's critical current is suppressed and in a few cases undergoes suppression and recovery. We explore the behavior in $H_\parallel$ by considering theoretically two effects: a 0-$π$ transition induced by tuning of the Zeeman energy and the unique effect of ripples in an atomically thin layer which create a small spatially varying perpendicular component of the field. The 2DJJs have potential utility as flexible probes for two-dimensional superconductivity in a variety of materials and introduce high $H_\parallel$ as a newly accessible experimental knob.

cond-mat.supr-con↗

arXiv2021

Spectroscopy of $NbSe_2$ using Energy-Tunable Defect-Embedded Quantum Dots

Quantum dots have sharply defined energy levels, which can be used for high resolution energy spectroscopy when integrated in tunneling circuitry. Here we report dot-assisted spectroscopy measurements of the superconductor $NbSe_2$, using a van der Waals device consisting of a vertical stack of $graphene-MoS_2-NbSe_2$. The $MoS_2$ tunnel barriers host naturally occurring defects which function as quantum dots, allowing transport via resonant tunneling. The dot energies are tuned by an electric field exerted by a back-gate, which penetrates the graphene source electrode. Scanning the dot potential across the superconductor Fermi energy, we reproduce the $NbSe_2$ density of states which exhibits a well-resolved two-gap spectrum. Surprisingly, we find that the dot-assisted current is dominated by the lower energy feature of the two $NbSe_2$ gaps, possibly due to a selection rule which favors coupling between the dots and the orbitals which exhibit this gap.

cond-mat.supr-con↗

arXiv2021

Hidden spin-texture at topological domain walls drive exchange bias in a Weyl semimetal

Exchange bias is a phenomenon critical to solid-state technologies that require spin valves or non-volatile magnetic memory. The phenomenon is usually studied in the context of magnetic interfaces between antiferromagnets and ferromagnets, where the exchange field of the former acts as a means to pin the polarization of the latter. In the present study, we report an unusual instance of this phenomenon in the topological Weyl semimetal Co3Sn2S2, where the magnetic interfaces associated with domain walls suffice to bias the entire ferromagnetic bulk. Remarkably, our data suggests the presence of a hidden order parameter whose behavior can be independently tuned by applied magnetic fields. For micron-size samples, the domain walls are absent, and the exchange bias vanishes, suggesting the boundaries are a source of pinned uncompensated moment arising from the hidden order. The novelty of this mechanism suggests exciting opportunities lie ahead for the application of topological materials in spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2020

Combined Zeeman and orbital effect on the Josephson effect in rippled graphene

The two-dimensional nature of graphene Josephson junctions offers the possibility of creating effective superconductor-ferromagnet-superconductor junctions with tunable Zeeman splitting caused by an in-plane magnetic field. Such junctions would be able to alternate between a conventional superconducting ground state and a ground state with an intrinsic phase difference, making them controllable $0-π$ Josephson junctions. However, in addition to the Zeeman splitting, an in-plane magnetic field will in general also produce an orbital effect because of height variations in graphene, colloquially known as ripples. Both the Zeeman and orbital effect will thus affect the critical current, so to be able to identify $0-π$ transitions it is necessary to understand their combined effect. From both analytical and numerical solutions of the Usadel equation we find that ripples can in fact produce a current response similar to that which is characteristic of a $0-π$ transition. Hence, additional analysis is required in order to reveal the presence of a $0-π$ transition caused by spin-splitting in graphene with ripples. We provide a closed form analytical expression for the critical current in the presence of exchange field and ripple effects as well as an expression for the scaling of critical current zeroes with junction parameters.

cond-mat.supr-con↗

arXiv2020

Quantum-Dot Assisted Spectroscopy of Degeneracy-Lifted Landau Levels in Graphene

Energy spectroscopy of strongly interacting phases requires probes which minimize screening while retaining spectral resolution and local sensitivity. Here we demonstrate that such probes can be realized using atomic sized quantum dots bound to defects in hexagonal Boron Nitride tunnel barriers, placed at nanometric distance from graphene. With dot energies capacitively tuned by a planar graphite electrode, dot-assisted tunneling becomes highly sensitive to the graphene excitation spectrum. The spectra track the onset of degeneracy lifting with magnetic field at the ground state, and at unoccupied exited states, revealing symmetry-broken gaps which develop steeply with magnetic field - corresponding to Landé $g$ factors as high as 160. Measured up to $B = 33$ T, spectra exhibit a primary energy split between spin-polarized excited states, and a secondary spin-dependent valley-split. Our results show that defect dots probe the spectra while minimizing local screening, and are thus exceptionally sensitive to interacting states.

cond-mat.mes-hall↗