arXiv subjects

J. Coraux

Publications and source records attributed to J. Coraux.

At least 19 recordsLinked to original sources

arXiv2026

Magnetic-field-induced magnon portfolio in a van der Waals magnet

Magnonic excitations are investigated in chromium oxychloride (CrOCl), a van der Waal (vdW) antiferromagnet prone to a multitude of magnetic phase transitions, with absorption experiments in a broad continuous energy range. At low magnetic fields, the magnon spectra show a strong bi-axial anisotropy and inform on the relative weights of the effective exchange coupling and the system anisotropies. As the magnetic field increases, magnons characteristic of a canted phase are first observed, with peculiarities attributed to in-plane anisotropies and magnon-magnon coupling. Subsequently, a hysteretic magnon spectrum appears as the system transitions to a ferrimagnetic state, with two new magnon branches partly coexisting with the lower energy canted phase branch, indicating the formation of spatially separated magnetic phases. Further changes in the magnon spectrum in higher magnetic fields accompany transitions to the different canted magnetic phases previously reported. Our experiments show that competing exchange interactions and ground states broaden the options to generate different kinds of magnonic excitations in the same vdW material upon the variation of external parameters.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic field tuning of modulated magnetic orders in CrOCl at the two-dimensional limit

Chromium oxychloride is a van der Waals magnet with intrinsic competing exchange interactions, including a strong antiferromagnetic one, source of a very rich magnetic phase diagram, with ferrimagnetic, antiferromagnetic, and canted states, up to high magnetic fields. We investigate the sequence of these magnetic phases in thin layers of CrOCl using magneto-Raman scattering spectroscopy. We identify phases whose magnetic order is commensurate with the atomic lattice, and find signatures of strong magneto-striction, presumably of exchange origin. The coupling of the spin and atomic degrees of freedom in the crystal is observed down to the single-layer limit -- phonon modes significantly soften or stiffen, in a complex way due to the competition of interactions. The existence domains of the different phases change with the number of layers.

cond-mat.mes-hall↗

arXiv2025

Magnetic phases and zone-folded phonons in a frustrated van der Waals magnet

2D magnetic materials have attracted extensive research interest due to their potential application in nanospintronics, optospintronics, and in magnonics. Ferromagnetic as well as antiferromagnetic layered materials have been demonstrated and successfully inserted into van der Waals heterostructures. However, the effects of magnetic frustration in van der Waals materials and the possibilities offered by spin configurations characterized by nonlinear spin arrangements have not been fully considered yet. Herein, we establish the magnetic phase diagram of bulk CrOCl, a frustrated van der Waals magnet, using magnetization and magneto-optical spectroscopy techniques. In particular, we use the magnetic superstructures relative to the crystallographic unit cell and the associated rich zone-folded phonon series to describe the magnetic field induced phases. Theoretical calculations taking into account the competing nearest neighbors magnetic exchange interactions provide a unique insight into the lattice vibrations of this class of magnetic system. This study expands the scope of 2D magnetic materials and provides a methodology to characterize frustrated van der Waals magnets.

cond-mat.mes-hall↗

arXiv2025

Amp\`ere phase in frustrated magnets

We report a new class of algebraic spin liquids, in which the macroscopically degenerate ground state manifold is not Coulombic, like in spin ices, but Amp\`ere-like. The local constraint characterizing an Amp\`ere phase is not a Gauss law, but rather an Amp\`ere law, i.e., a condition on the curl of the magnetization vector field and not on its divergence. As a consequence, the excitations evolving in such a manifold are not magnetically charged scalar quasiparticles, the so-called magnetic monopoles in Coulomb phases, but instead vectorial magnetic loops (or fictional current lines). We demonstrate analytically that in a macroscopically degenerate manifold inheriting the properties of a cooperative paramagnet and subject to a local curl-free contraint, magnetic correlations decay in space with a power law whose exponent is the space dimension d: the Amp\`ere phase is a d-algebraic spin liquid. Using Monte Carlo simulations with appropriate cluster dynamics, we confirm this physics numerically in two- and three-dimensional examples, and illustrate how the Amp\`ere phase compares to its Coulomb counterpart.

cond-mat.str-el↗

arXiv2025

Magnetic imaging under high pressure with a spin-based quantum sensor integrated in a van der Waals heterostructure

Pressure is a powerful thermodynamic parameter for tuning the magnetic properties of van der Waals magnets owing to their weak interlayer bonding. However, local magnetometry measurements under high pressure still remain elusive for this important class of emerging materials. Here we introduce a method enabling in situ magnetic imaging of van der Waals magnets under high pressure with sub-micron spatial resolution. Our approach relies on a quantum sensing platform based on boron-vacancy (V$_\text{B}^-$) centers in hexagonal boron nitride (hBN), which can be placed in atomic contact of any type of two-dimensional (2D) material within a van der Waals heterostructure. We first show that the V$_\text{B}^-$ center can be used as a magnetic field sensor up to pressures of a few GPa, a pressure range for which the properties of a wide variety of van der Waals magnets are efficiently altered. We then use V$_\text{B}^-$ centers in a thin hBN layer to perform magnetic imaging of a van der Waals magnet under pressure. As a proof of concept, we study the pressure-dependent magnetization in micrometer-sized flakes of $1T$-CrTe$_2$, whose evolution is explained by a shift of the Curie temperature. Besides providing a new path for studying pressure-induced phase transitions in van der Waals magnets, this work also opens up interesting perspectives for exploring the physics of 2D superconductors under pressure via local measurements of the Meissner effect.

cond-mat.mtrl-sci↗

arXiv2023

Effective interaction quenching in artificial kagom\'e spin chains

Achieving thermal equilibrium in two-dimensional lattices of interacting nanomagnets has been a key issue on the route to study exotic phases in artificial frustrated magnets. We revisit this issue in artificial one-dimensional kagom\'e spin chains. Imaging arrested micro-states generated by a field demagnetization protocol and analyzing their pairwise spin correlations in real space, we unveil a non-equilibrated physics. Remarkably, this physics can be reformulated into an at-equilibrium one by rewriting the associated spin Hamiltonian in such a way that one of the coupling constants is quenched. We ascribe this effective behavior to a kinetic hinderance during the demagnetization protocol, which induces the formation of local flux closure spin configurations that sometimes compete with the magnetostatic interaction.

cond-mat.dis-nn↗

arXiv2023

Corrugation in a molecular C60 monolayer as a frustrated magnet

Under certain experimental conditions, the deposition of C60 molecules onto an atomically flat copper surface gives rise to the formation of corrugated islands. This corrugation, which reflects a molecular displacement perpendicular to the surface plane, presents an astonishing pattern: it is well described by a frustrated Ising spin Hamiltonian whose thermodynamics is compatible with a spin liquid about to transit towards an ordered zigzag state. Here we study the statistical properties of such a molecular corrugation using tools generally employed in frustrated magnetism. More specifically, the real and reciprocal space analysis of pairwise molecule correlations allows us to demonstrate that the C60/Cu system, in which magnetism is totally absent, has all the characteristics of a triangular Ising antiferromagnet. Our results indicate that the organization of two-dimensional matter, at the molecular length scale, sometimes turns out to be particularly close to that encountered in highly frustrated magnets.

cond-mat.mes-hall↗

arXiv2022

Magnetic imaging with spin defects in hexagonal boron nitride

Optically-active spin defects hosted in hexagonal boron nitride (hBN) are promising candidates for the development of a two-dimensional (2D) quantum sensing unit. Here, we demonstrate quantitative magnetic imaging with hBN flakes doped with negatively-charged boron-vacancy (V$_{\rm B}^-$) centers through neutron irradiation. As a proof-of-concept, we image the magnetic field produced by CrTe$_2$, a van der Waals ferromagnet with a Curie temperature slightly above $300$ K. Compared to other quantum sensors embedded in 3D materials, the advantages of the hBN-based magnetic sensor described in this work are its ease of use, high flexibility and, more importantly, its ability to be placed in close proximity to a target sample. Such a sensing unit will likely find numerous applications in 2D materials research by offering a simple way to probe the physics of van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Dispersing and semi-flat bands in the wide band gap two-dimensional semiconductor bilayer silicon oxide

Epitaxial bilayer silicon oxide is a transferable two-dimensional material predicted to be a wide band gap semiconductor, with potential applications for deep UV optoelectronics, or as a building block of van der Waals heterostructures. The prerequisite to any sort of such applications is the knowledge of the electronic band structure, which we unveil using angle-resolved photoemission spectroscopy and rationalise with the help of density functional theory calculations. We discover dispersing bands related to electronic delocalisation within the top and bottom planes of the material, with two linear crossings reminiscent of those predicted in bilayer AA-stacked graphene, and semi-flat bands stemming from the chemical bridges between the two planes. This band structure is robust against exposure to air, and can be controled by exposure to oxygen. We provide an experimental lower-estimate of the band gap size of 5 eV and predict a full gap of 7.36 eV using density functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2021

Characterization of room-temperature in-plane magnetization in thin flakes of CrTe$_2$ with a single spin magnetometer

We demonstrate room-temperature ferromagnetism with in-plane magnetic anisotropy in thin flakes of the CrTe$_2$ van der Waals ferromagnet. Using quantitative magnetic imaging with a single spin magnetometer based on a nitrogen-vacancy defect in diamond, we infer a room-temperature in-plane magnetization in the range of $M\sim 27$ kA/m for flakes with thicknesses down to $20$ nm. In addition, our measurements indicate that the orientation of the magnetization is not determined solely by shape anisotropy in micron-sized CrTe$_2$ flakes, which suggest the existence of a non-negligible magnetocrystalline anisotropy. These results make CrTe$_2$ a unique system in the growing family of van der Waals ferromagnets, as it is the only material platform known to date which offers an intrinsic in-plane magnetization and a Curie temperature above $300$ K in thin flakes.

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↗

arXiv2019

Interlayer Charge Transfer and Defect Creation in Type I van der Waals Heterostructures

Van der Waals heterostructures give access to a wide variety of new phenomena that emerge thanks to the combination of properties brought in by the constituent layered materials. We show here that owing to an enhanced interaction cross section with electrons in a type I van der Waals heterostructure, made of single layer molybdenum disulphide and thin boron nitride films, electrons and holes created in boron nitride can be transferred to the dichalcogenide where they form electron-hole pairs yielding luminescence. This cathodoluminescence can be mapped with a spatial resolution far exceeding what can be achieved in a typical photoluminescence experiment, and is highly valuable to understand the optoelectronic properties at the nanometer scale. We find that in heterostructures prepared following the mainstream dry transfer technique, cathodoluminescence is locally extinguished, and we show that this extinction is associated with the formation of defects, that are detected in Raman spectroscopy and photoluminescence. We establish that to avoid defect formation induced by low-energy electron beams and to ensure efficient transfer of electrons and holes at the interface between the layers, flat and uniform interlayer interfaces are needed, that are free of trapped species, airborne ones or contaminants associated with sample preparation. We show that heterostructure fabrication using a pick-up technique leads to superior, intimate interlayer contacts associated with significantly more homogeneous cathodoluminescence.

cond-mat.mes-hall↗

arXiv2019

Electronic band structure of ultimately thin silicon oxide on Ru(0001)

Silicon oxide can be formed in a crystalline form, when prepared on a metallic substrate. It is a candidate support catalyst and possibly the ultimately-thin version of a dielectric host material for two-dimensional materials (2D) and heterostructures. We determine the atomic structure and chemical bonding of the ultimately thin version of the oxide, epitaxially grown on Ru(0001). In particular, we establish the existence of two sub-lattices defined by metal-oxygen-silicon bridges involving inequivalent substrate sites. We further discover four electronic bands below Fermi level, at high binding energies, two of them forming a Dirac cone at K point, and two others forming semi-flat bands. While the latter two correspond to hybridized states between the oxide and the metal, the former relate to the topmost silicon-oxygen plane, which is not directly coupled to the substrate. Our analysis is based on high resolution X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and density functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2017

Toward Moiré engineering in 2D materials via dislocation theory

We present a framework that explains the strong connection in 2D materials between mechanics and electronic structure, via dislocation theory. Within this framework, Moiré patterns created by layered 2D materials may be understood as dislocation arrays, and vice versa. The dislocations are of a unique type that we describe as van der Waals dislocations, for which we present a complete geometrical description, connected to both stretch and twist Moiré patterns. A simple computational scheme, which reduces the complexity of the electronic interaction between layers in order to make the problem computationally tractable, is introduced to simulate these dislocation arrays, allowing us to predict and explain all of the observed Moiré patterns in 2D material systems within a unique framework. We extend this analysis as well to defects in Moiré patterns, which have been reported recently, and which are the result of defects of the same symmetry in the constituent 2D material layers. Finally, we show that linear defects in the Moiré space can be viewed as unidimensional topological states, and can be engineered using our framework

cond-mat.mtrl-sci↗

arXiv2016

Nanofaceting as a stamp for periodic graphene charge carrier modulations

The exceptional electronic properties of monoatomic thin graphene sheets triggered numerous original transport concepts, pushing quantum physics into the realm of device technology for electronics, optoelectronics and thermoelectrics. At the conceptual pivot point is the particular twodimensional massless Dirac fermion character of graphene charge carriers and its volitional modification by intrinsic or extrinsic means. Here, interfaces between different electronic and structural graphene modifications promise exciting physics and functionality, in particular when fabricated with atomic precision. In this study we show that quasiperiodic modulations of doping levels can be imprinted down to the nanoscale in monolayer graphene sheets. Vicinal copper surfaces allow to alternate graphene carrier densities by several 10^13 carriers per cm^2 along a specific copper high-symmetry direction. The process is triggered by a self-assembled copper faceting process during high-temperature graphene chemical vapor deposition, which defines interfaces between different graphene doping levels at the atomic level.

cond-mat.mtrl-sci↗

arXiv2016

Growth of Epitaxial Graphene: Theory and Experiment

A detailed review of the literature for the last 5-10 years on epitaxial growth of graphene is presented. Both experimental and theoretical aspects related to growth on transition metals and on silicon carbide are thoroughly reviewed. Thermodynamic and kinetic aspects of growth on all these materials, where possible, are discussed. To make this text useful for a wider audience, a range of important experimental techniques that have been used over the last decade to grow (e.g. CVD, TPG and segregation) and characterize (STM, LEEM, etc.) graphene are reviewed, and a critical survey of the most important theoretical techniques is given. Finally, we critically discuss various unsolved problems related to growth and its mechanism which we believe require proper attention in future research.

cond-mat.mtrl-sci↗

arXiv2015

Unconventional magnetisation texture in graphene / cobalt hybrids

Magnetic domain structure and spin-dependent reflectivity measurements on cobalt thin films intercalated at the graphene / Ir(111) interface are investigated using spin-polarised low-energy electron microscopy. We find that graphene-covered cobalt films have surprising magnetic properties. Vectorial imaging of magnetic domains reveals an unusually gradual thickness-dependent spin reorientation transition, in which magnetisation rotates from out-of-the-film plane to the in-plane direction by less than 10$^\circ$ per cobalt monolayer. During this transition, cobalt films have a meandering spin texture, characterised by a complex, three-dimensional, wavy magnetisation pattern. In addition, spectroscopy measurements indicate that the electronic band structure of the unoccupied states is essentially spin-independent already a few electron-Volts above the vacuum level. These properties strikingly differ from those of pristine cobalt films and could open new prospects in surface magnetism.

cond-mat.mes-hall↗

arXiv2014

Ripples and Charge Puddles in Graphene on a Metallic Substrate

Graphene on a dielectric substrate exhibits spatial doping inhomogeneities, forming electron-hole puddles. Understanding and controlling the latter is of crucial importance for unraveling many of graphene's fundamental properties at the Dirac point. Here we show the coexistence and correlation of charge puddles and topographic ripples in graphene decoupled from the metallic substrate it was grown on. The analysis of interferences of Dirac fermion-like electrons yields a linear dispersion relation, indicating that graphene on a metal can recover its intrinsic electronic properties.

cond-mat.mes-hall↗