arXiv subjects

A. Marty

Publications and source records attributed to A. Marty.

At least 19 recordsLinked to original sources

arXiv2025

Dzyaloshinskii-Moriya interaction in Fe5GeTe2 epitaxial thin films

Van der Waals ferromagnets, such as Fe5GeTe2, offer a promising platform for spintronic devices based on chiral magnetic textures, provided a significant Dzyaloshinskii-Moriya interaction (DMI) can be induced to stabilise the textures. Here, we directly measure DMI in epitaxial Fe5GeTe2 thin films using Brillouin light scattering spectroscopy and observe a consistent DMI (D=0.04 mJ/m2) across various thicknesses. Its weak thickness dependence, combined with the nominally symmetric film interfaces, suggests a bulk origin. Although we do not determine the microscopic mechanism, our findings are compatible with ab initio calculations linking DMI to partial ordering of Fe split sites. Additionally, we find a low magnetic dissipation (alpha<0.02). The observed DMI, which could be further enhanced by optimising the Fe site ordering, combined with low dissipation, makes Fe5GeTe2 a strong candidate for exploring the dynamics of chiral magnetic textures in two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2025

Multi-functional Wafer-Scale Van der Waals Heterostructures and Polymorphs

Van der Waals heterostructures have promised the realisation of artificial materials with multiple physical phenomena such as giant optical nonlinearities, spin-to-charge interconversion in spintronics and topological carrier protection, in a single layered device through an infinitely diverse set of quantum materials. However, most efforts have only focused on exfoliated material that inherently limits both the dimensions of the materials and the scalability for applications. Here, we show the epitaxial growth of large area heterostructures of topological insulators (Bi2Se3), transition metal dichalcogenides (TMDs, WSe2) and ferromagnets (Co), resulting in the combination of functionalities including tuneable optical nonlinearities, spin-to-charge conversion and magnetic proximity effects. This is demonstrated through coherent phase resolved terahertz currents, bringing novel functionalities beyond those achievable in simple homostructures. In particular, we show the role of different TMD polymorphs, with the simple change of one atomic monolayer of the artificial material stack entirely changing its optical, electrical and magnetic properties. This epitaxial integration of diverse two-dimensional materials offers foundational steps towards diverse perspectives in quantum material engineering, where the material polymorph can be controlled at technological relevant scales for coupling applications in, for example, van der Waals nonlinear optics, optoelectronics, spintronics, multiferroics and coherent current control.

cond-mat.mtrl-sci↗

arXiv2025

Monolayer control of spin-charge conversion in van der Waals heterostructures

The diversity of 2D materials and their van der Waals (vdW) stacking presents a fertile ground for engineering novel multifunctional materials and quantum states of matter. This permits unique opportunities to tailor the electronic properties of vdW heterostructures by the insertion of only a single 2D material layer. However, such vdW materials engineering at the atomic scale has yet to be investigated for spin-charge interconversion phenomena. Here, we report on the control of these effects at the monolayer level, where drastic increase in intensity and change in sign of THz spintronic emission are demonstrated by inserting a single layer of MoSe$_2$ between PtSe$_2$ and graphene in a fully epitaxial, large area stacked structure. By using a combination of spin and angle resolved photoemission and density functional theory to reveal the electronic and spin structures, we illustrate two different mechanisms relying on charge transfer and electronic hybridization for the formation of Rashba states, which are responsible for spin-charge conversion and hence the THz spintronic emission. These findings open new pathways to design, at the atomic scale, efficient THz spintronic emitters made of 2D materials and other spintronic devices based on spin-charge interconversion phenomena.

cond-mat.mtrl-sci↗

arXiv2024

Enhanced optical properties of MoSe$_2$ grown by molecular beam epitaxy on hexagonal boron nitride

Transition metal dichalcogenides (TMD) like MoSe$_2$ exhibit remarkable optical properties such as intense photoluminescence (PL) in the monolayer form. To date, narrow-linewidth PL is only achieved in micrometer-sized exfoliated TMD flakes encapsulated in hexagonal boron nitride (hBN). In this work, we develop a growth strategy to prepare monolayer MoSe$_2$ on hBN flakes by molecular beam epitaxy in the van der Waals regime. It constitutes the first step towards the development of large area single crystalline TMDs encapsulated in hBN for potential integration in electronic or opto-electronic devices. For this purpose, we define a two-step growth strategy to achieve monolayer-thick MoSe$_2$ grains on hBN flakes. The high quality of MoSe$_2$ allows us to detect very narrow PL linewidth down to 5.5 meV at 13 K, comparable to the one of encapsulated exfoliated MoSe$_2$ flakes. Moreover, sizeable PL can be detected at room temperature as well as clear reflectivity signatures of A, B and charged excitons.

cond-mat.mtrl-sci↗

arXiv2023

Angular dependence of the interlayer coupling at the interface between two dimensional materials 1T-PtSe$_2$ and graphene

We present a study by Scanning Tunneling Microscopy, supported by ab initio calculations, of the interaction between graphene and monolayer (semiconducting) PtSe$_2$ as a function of the twist angle $θ$ between the two layers. We analyze the PtSe$_2$ contribution to the hybrid interface states that develop within the bandgap of the semiconductor to probe the interaction. The experimental data indicate that the interlayer coupling increases markedly with the value of $θ$, which is confirmed by ab initio calculations. The moiré patterns observed within the gap are consistent with a momentum conservation rule between hybridized states, and the strength of the hybridization can be qualitatively described by a perturbative model.

cond-mat.mes-hall↗

arXiv2023

Atomic scale imaging of the negative charge induced by a single vanadium dopant atom in monolayer WSe$_2$ using 4D-STEM

There has been extensive activity exploring the doping of semiconducting two-dimensional (2D) transition metal dichalcogenides in order to tune their electronic and magnetic properties. The outcome of doping depends on various factors, including the intrinsic properties of the host material, the nature of the dopants used, their spatial distribution as well as their interactions with other types of defects. A thorough atomic-level analysis is essential to fully understand these mechanisms. In this work, vanadium doped WSe$_2$ monolayer grown by molecular beam epitaxy is investigated using four-dimensional scanning transmission electron microscopy (4D-STEM). Through center of mass-based reconstruction, atomic scale maps are produced, allowing the visualization of both the electric field and the electrostatic potential around individual V atoms. To provide quantitative insights, these results are successfully compared with multislice image simulations based on ab initio calculations, accounting for lens aberrations. Finally, a negative charge around the V dopants is detected as a drop in the electrostatic potential, unambiguously demonstrating that 4D-STEM can be used to detect and to accurately analyze single dopant charge states in semiconducting 2D materials.

cond-mat.mtrl-sci↗

arXiv2023

Electrical measurement of the Spin Hall Effect isotropy in a ferromagnet

The spin-dependent transport properties of paramagnetic metals are roughly invariant under rotation. By contrast, in ferromagnetic materials the magnetization breaks the rotational symmetry, and thus the spin Hall effect is expected to become anisotropic. Here, using a specific design of lateral spin valves, we measure electrically the spin Hall Effect anisotropy in NiCu and NiPd, both in their ferromagnetic and paramagnetic phases. We show that the appearance of the ferromagnetic order does not lead to a sizeable anisotropy of the spin charge interconversion in these materials.

cond-mat.mtrl-sci↗

arXiv2023

Atomic-layer controlled THz Spintronic emission from Epitaxially grown Two dimensional PtSe$_2$/ferromagnet heterostructures

Terahertz (THz) Spintronic emitters based on ferromagnetic/metal junctions have become an important technology for the THz range, offering powerful and ultra-large spectral bandwidths. These developments have driven recent investigations of two-dimensional (2D) materials for new THz spintronic concepts. 2D materials, such as transition metal dichalcogenides (TMDs), are ideal platforms for SCC as they possess strong spin-orbit coupling (SOC) and reduced crystal symmetries. Moreover, SCC and the resulting THz emission can be tuned with the number of layers, electric field or strain. Here, epitaxially grown 1T-PtSe$_2$ and sputtered Ferromagnet (FM) heterostructures are presented as a novel THz emitter where the 1T crystal symmetry and strong SOC favor SCC. High quality of as-grown PtSe$_2$ layers is demonstrated and further FM deposition leaves the PtSe$_2$ unaffected, as evidenced with extensive characterization. Through this atomic growth control, the unique thickness dependent electronic structure of PtSe$_2$ allows the control of the THz emission by SCC. Indeed, we demonstrate the transition from the inverse Rashba-Edelstein effect in one monolayer to the inverse spin Hall effect in multilayers. This band structure flexibility makes PtSe$_2$ an ideal candidate as a THz spintronic 2D material and to explore the underlying mechanisms and engineering of the SCC for THz emission.

cond-mat.mtrl-sci↗

arXiv2022

Alloying 2D VSe2 with Pt: from a charge density wave state to a disordered insulator

We have analyzed by means of scanning tunneling microscopy and spectroscopy the atomic and electronic structure of monolayers of 1T-VxPt1-xSe2 alloys grown by molecular beam epitaxy on epitaxial graphene substrates. We have focused on the composition range (0.1<x<0.35) where ferromagnetic behaviour has recently been demonstrated. For low Pt concentration, (x=0.07 and x=0.21), small domains (a few nanometres in diameter) exhibiting the characteristic superstructure of the charge density wave (CDW) state of pristine VSe2 monolayer remain visible on most of the sample surface. Thus alloying preserves the short range order of the CDW phase, although it destroys its long range order. For higher Pt concentration (x=0.35) a disordered alloy forms. It presents a fully developped gap (a few tens meV in width) at the Fermi level and is thus a disordered insulator. This gap exhibits strong variations at the nanometer scale, reflecting the local fluctuations in the composition. An unexpectedly large interaction of the TMD layer with the graphene substrate sets in for this composition range.

cond-mat.mtrl-sci↗

arXiv2021

Coexistence of ferromagnetism and spin-orbit coupling by incorporation of platinum in two-dimensional VSe$_2$

We report on a novel material, namely two-dimensional (2D) V$_{1-x}$Pt$_x$Se$_2$ alloy, exhibiting simultaneously ferromagnetic order and Rashba spin-orbit coupling. While ferromagnetism is absent in 1T-VSe$_2$ due to the competition with the charge density wave phase, we demonstrate theoretically and experimentally that the substitution of vanadium by platinum in VSe$_2$ (10-50 %) to form an homogeneous 2D alloy restores ferromagnetic order with Curie temperatures of 6 K for 5 monolayers and 25 K for one monolayer of V$_{0.65}$Pt$_{0.35}$Se$_2$. Moreover, the presence of platinum atoms gives rise to Rashba spin-orbit coupling in (V,Pt)Se$_2$ providing an original platform to study the interplay between ferromagnetism and spin-orbit coupling in the 2D limit.

cond-mat.mtrl-sci↗

arXiv2020

Large Rashba unidirectional magnetoresistance in the Fe/Ge(111) interface states

The structure inversion asymmetry at surfaces and interfaces give rise to the Rashba spin-orbit interaction (SOI), that breaks the spin degeneracy of surface or interface states. Hence, when an electric current runs through a surface or interface, this Rashba effect generates an effective magnetic field acting on the electron spin. This provides an additional tool to manipulate the spin state in materials such as Si and Ge that, in their bulk form, possess inversion symmetry (or lack structural inersion asymmetry). The existence of Rashba states could be demonstrated by photoemission spectroscopy at the interface between different metals and Ge(111) and by spin-charge conversion experiments at the Fe/Ge(111) interface even though made of two light elements. In this work, we identify the fingerprint of the Rashba states at the Fe/Ge(111) interface by magnetotransport measurements in the form of a large unidirectional magnetoresistance of up to 0.1 \%. From its temperature dependence, we find that the Rashba energy splitting is larger than in pure Ge(111) subsurface states.

cond-mat.mtrl-sci↗

arXiv2020

Synthesis of epitaxial monolayer Janus SPtSe

Janus single-layer transition metal dichalcogenides, in which the two chalcogen layers have a different chemical nature, push chemical composition control beyond what is usually achievable with van der Waals heterostructures. Here we report such a novel Janus compound, SPtSe, which is predicted to exhibit strong Rashba spin-orbit coupling. We synthetized it by conversion of a single-layer of PtSe$_2$ on Pt(111) via sulphurization under H$_2$S atmosphere. Our in situ and operando structural analysis with grazing incidence synchrotron X-ray diffraction reveals the process by which the Janus alloy forms. The crystalline long-range order of the as-grown PtSe$_2$ monolayer is first lost due to thermal annealing. A subsequent recrystallization in presence of a source of sulphur yields a highly ordered SPtSe alloy, which is iso-structural to the pristine PtSe$_2$. The chemical composition is resolved, layer-by-layer, using angle-resolved X-ray photoelectron spectroscopy, demonstrating that Se-by-S substitution occurs selectively in the topmost chalcogen layer.

cond-mat.mtrl-sci↗

arXiv2020

Electrical detection of magnetic circular dichroism: application to magnetic microscopy in ultra-thin ferromagnetic films

Imaging the magnetic configuration of thin-films has been a long-standing area of research. Since a few years, the emergence of two-dimensional ferromagnetic materials calls for innovation in the field of magnetic imaging. As the magnetic moments are extremely small, standard techniques like SQUID, torque magnetometry, magnetic force microscopy and Kerr effect microscopy are challenging and often lead to the detection of parasitic magnetic contributions or spurious effects. In this work, we report a new magnetic microscopy technique based on the combination of magnetic circular dichroism and Seebeck effect in semiconductor/ferromagnet bilayers. We implement this method with perpendicularly magnetized (Co/Pt) multilayers sputtered on Ge (111). We further show that the electrical detection of MCD is more sensitive than the Kerr magnetometry, especially in the ultra-thin film regime, which makes it particularly promising for the study of emergent two-dimensional ferromagnetic materials.

cond-mat.mtrl-sci↗

arXiv2019

Spin-orbitronics at a topological insulator-semiconductor interface

Topological insulators (TIs) hold great promises for new spin-related phenomena and applications thanks to the spin texture of their surface states. However, a versatile platform allowing for the exploitation of these assets is still lacking due to the difficult integration of these materials with the mainstream Si-based technology. Here, we exploit germanium as a substrate for the growth of Bi$_2$Se$_3$, a prototypical TI. We probe the spin properties of the Bi$_2$Se$_3$/Ge pristine interface by investigating the spin-to-charge conversion taking place in the interface states by means of a non-local detection method. The spin population is generated by optical orientation in Ge, and diffuses towards the Bi$_2$Se$_3$ which acts as a spin detector. We compare the spin-to-charge conversion in Bi$_2$Se$_3$/Ge with the one taking place in Pt in the same experimental conditions. Notably, the sign of the spin-to-charge conversion given by the TI detector is reversed compared to the Pt one, while the efficiency is comparable. By exploiting first-principles calculations, we ascribe the sign reversal to the hybridization of the topological surface states of Bi$_2$Se$_3$ with the Ge bands. These results pave the way for the implementation of highly efficient spin detection in TI-based architectures compatible with semiconductor-based platforms.

cond-mat.mes-hall↗

arXiv2019

Van der Waals epitaxy of Mn-doped MoSe$_2$ on mica

The magnetic order associated with the degree of freedom of spin in two-dimensional (2D) materials is subjected to intense investigation because of its potential application in 2D spintronics and valley-related magnetic phenomena. We report here a bottom-up strategy using molecular beam epitaxy to grow and dope large-area (cm$^2$) few-layer MoSe$_2$ with Mn as a magnetic dopant. High-quality Mn-doped MoSe$_2$ layers are obtained for Mn content of less than 5 % (atomic). When increasing the Mn content above 5 % we observe a clear transition from layer-by-layer to cluster growth. Magnetic measurements involving a transfer process of the cm$^2$-large doped layers on 100-micron-thick silicon substrate, show plausible proof of high-temperature ferromagnetism of 1 % and 10 % Mn-doped MoSe$_2$. Although we could not point to a correlation between magnetic and electrical properties, we demonstrate that the transfer process described in this report permits to achieve conventional electrical and magnetic measurements on the doped layers transferred on any substrate. Therefore, this study provides a promising route to characterize stable ferromagnetic 2D layers, which is broadening the current start-of-the-art of 2D materials-based applications.

cond-mat.mtrl-sci↗

arXiv2019

Observation of Large Unidirectional Rashba Magnetoresistance in Ge(111)

Relating magnetotransport properties to specific spin textures at surfaces or interfaces is an intense field of research nowadays. Here, we investigate the variation of the electrical resistance of Ge(111) grown epitaxially on semi-insulating Si(111) under the application of an external magnetic field. We find a magnetoresistance term which is linear in current density j and magnetic field B, hence odd in j and B, corresponding to a unidirectional magnetoresistance. At 15 K, for I = 10 $μ$A (or j = 0.33 A/m) and B = 1 T, it represents 0.5 % of the zero field resistance, a much higher value compared to previous reports on unidirectional magnetoresistance. We ascribe the origin of this magnetoresistance to the interplay between the externally applied magnetic field and the current-induced pseudo-magnetic field in the spin-splitted subsurface states of Ge(111). This unidirectional magnetoresistance is independent of the current direction with respect to the Ge crystal axes. It progressively vanishes, either using a negative gate voltage due to carrier activation into the bulk (without spin-splitted bands), or by increasing the temperature due to the Rashba energy splitting of the subsurface states lower than $\sim$58 k$_B$. The highly developed technologies on semiconductor platforms would allow the rapid optimization of devices based on this phenomenon.

cond-mat.mtrl-sci↗

arXiv2019

Van der Waals solid phase epitaxy to grow large-area manganese-doped MoSe$_2$ few-layers on SiO$_2$/Si

Large-area growth of continuous transition metal dichalcogenides (TMDCs) layers is a prerequisite to transfer their exceptional electronic and optical properties into practical devices. It still represents an open issue nowadays. Electric and magnetic doping of TMDC layers to develop basic devices such as p-n junctions or diluted magnetic semiconductors for spintronic applications are also an important field of investigation. Here, we have developed two different techniques to grow MoSe$_2$ mono- and multi-layers on SiO$_2$/Si substrates over large areas. First, we co-deposited Mo and Se atoms on SiO$_2$/Si by molecular beam epitaxy in the van der Waals regime to obtain continuous MoSe$_2$ monolayers over 1 cm$^2$. To grow MoSe$_2$ multilayers, we then used the van der Waals solid phase epitaxy which consists in depositing an amorphous Se/Mo bilayer on top of a co-deposited MoSe$_2$ monolayer which serves as a van der Waals growth template. By annealing, we obtained continuous MoSe$_2$ multilayers over 1 cm$^2$. Moreover, by inserting a thin layer of Mn in the stack, we could demonstrate the incorporation of up to 10 \% of Mn in MoSe$_2$ bilayers.

cond-mat.mtrl-sci↗

arXiv2019

Spin dependent transport characterization in metallic lateral spin valves using 1D and 3D modeling

We present the analysis of the spin signals obtained in NiFe based metallic lateral spin valves. We exploit the spin dependent diffusive equations in both the conventional 1D analytic modeling as well as in 3D Finite Element Method simulations. Both approaches are used for extracting the spin diffusion length $l_{sf}^{N}$ and the effective spin polarization $P_{eff}$ in Py/Al, Py/Cu and Py/Au based lateral nano-structures at both $300\,K$ and $77\,K$. Both the analytic modeling and 3D Finite Element Method simulations give consistent results. Combination of both models provides a powerful tool for reliable spin transport characterization in all metallic spin valves and gives an insight into the spin/charge current and spin accumulations 3D distributions in these devices. We provide the necessary ingredients to develop the 3D finite element modeling of diffusive spin transport.

cond-mat.mtrl-sci↗