arXiv subjects

Laurent Limot

Publications and source records attributed to Laurent Limot.

11 recordsLinked to original sources

arXiv2025

Moiré pattern multiplicity driven by electronic effects in two-dimensional CrCl3/Au heterostructures

Moiré patterns are a central motif in van der Waals heterostructures arising from the superposition of two-dimensional (2D) incommensurate lattices. These patterns reveal a wealth of correlated effects, influencing electronic, magnetic, and structural phenomena. While diffraction techniques typically resolve multiple moiré wave-vectors corresponding to the incommensurate nature of the underlying lattices, Scanning Tunneling Microscopy (STM) often reveals only a dominant superperiod. In this work, we address this apparent discrepancy through an STM study of a twisted monolayer of CrCl3 on Au(111). We observe the coexistence of several moiré patterns at a fixed twist angle, whose relative intensity depends on the tunneling bias. Fourier analysis of STM data uncovers hidden higher-order moiré components not visible in STM topographic images, while spectroscopy maps reveal that the spectral weight of each pattern varies with electron energy. Our results establish that STM selectively probes on the same area distinct moiré modulations depending on electronic confinement, providing a unified framework that reconciles real space and reciprocal space observations of complex moiré superstructures.

cond-mat.other↗

arXiv2024

One dimensional chains of nickelocene fragments on Au(111)

We investigate the temperature-dependent deposition of nickelocene (NiCp$_2$) molecules on a single crystal Au(111) substrate, revealing distinct adsorption behaviors and structural formations. At low temperatures (4.2 K), individual NiCp$_2$ molecules adsorb on the herringbone elbows and step edges, forming ordered patterns as molecular coverage increases. However, at 77 K, the molecules dissociate, yielding two main fragments: NiCp fragments that are Ni atoms capped by cyclopentadienyl (Cp) rings, which preferentially adsorb at FCC hollow sites, and Cp radical fragments exhibiting strong substrate interactions. NiCp fragments self-assemble into one-dimensional (1-D) chains along the $\langle 1 1 \bar{2} \rangle$ directions, displaying higher protrusion in STM images. The strain and steric hindrance from the Cp protons induce chiral patterns within the chains, which are well-reproduced by our DFT simulations. In contrast, the Cp fragments maintain distances due to short-range repulsive forces and exhibit low diffusion barriers. Interestingly, the fragments are non-magnetic, as confirmed by both STM measurements and DFT calculations, in contrast to the magnetic signals from intact Nc molecules. In addition to linear chains, dimers of the Ni-Cp fragments form along the $\langle 1 \bar{1} 0\rangle$ directions, requiring gold adatoms for their creation. These results demonstrate the feasibility of constructing complex nanostructures based on metallocenes via on-surface synthesis, opening the possibility for realizing low-dimensional magnetic systems by selecting substrates that preserve the magnetic moment of the fragments.

cond-mat.mes-hall↗

arXiv2024

Structural Manipulation of Spin Excitations in a Molecular Junction

Single metallocene molecules act as sensitive spin detectors when decorating the probe of a scanning tunneling microscope (STM). However, the impact of the atomic-scale electrode details on the molecular spin state has remained elusive to date. Here, a nickelocene (Nc) STM junction is manipulated in an atom-wise manner showing clearly the dependence of the spin excitation spectrum on the anchoring of Nc to Cu(111), a Cu monomer and trimer. Moreover, while the spin state of the same Nc tip is a triplet with tunable spin excitation energies upon contacting the surface, it transitions to a Kondo-screened doublet on a Cu atom. Notably, the non-trivial magnetic exchange interaction of the molecular spin with the electron continuum of the substrate determines the spectral line shape of the spin excitations.

cond-mat.mes-hall↗

arXiv2024

Higher order topological defects in a moiré lattice

Topological defects are ubiquitous, they manifest in a wide variety of systems such as liquid crystals, magnets or superconductors. The recent quest for nonabelian anyons in condensed matter physics stimulates the interest for topological defects since they can be hosted in vortices in quantum magnets or topological superconductors. In addition to these vortex defects, in this study we propose to investigate edge dislocations in 2D magnets as new building blocks for topological physics since they can be described as vortices in the structural phase field. Here we demonstrate the existence of higher order topological dislocations within the higher order moiré pattern of the van der Waals 2D magnet CrCl3 deposited on Au(111). Surprizingly, these higher order dislocations arise from ordinary simple edge dislocations in the atomic lattice of CrCl3. We provide a theoretical framework explaining the higher order dislocations as vortex with a winding Chern number of 2. We expect that these original defects could stabilize some anyons either in a 2D quantum magnet or within a 2D superconductor coupled to it.

cond-mat.mes-hall↗

arXiv2024

Two molecular devices for superconducting spintronics

We create two molecular devices with superconducting junctions, using nickelocene molecules, single Fe atoms, and Pb electrodes at low temperature. We find contrasting behavior based on the coordination of the Fe atom: one device shows low-bias features in its differential conductance due to the superposition of multiple Andreev reflections (MAR) and Fe-induced in-gap states. The other reveals interference between MAR and in-gap states, showcasing the diversity achievable in atomically engineered devices with identical components.

cond-mat.supr-con↗

arXiv2020

Molecular templates of spin textures on superconducting surfaces

We create ordered islands of magnetically anisotropic nickelocene molecules on a Pb (111) substrate. By using inelastic electron tunneling spectra (IETS) and density functional theory, we characterize the magnetic response of these islands. This allows us to conclude that the islands present local and collective magnetic excitations. Furthermore, we show that nickelocene islands present complex non-collinear spin patterns on the superconducting Pb (111) surface, opening the possibility of using molecular arrays to engineer spin textures with important implications on topological superconductivity.

cond-mat.mes-hall↗

arXiv2019

Spin-current induced Kondo-resonance splitting of a single cobalt atom

We use a low-temperature scanning tunneling microscope to study the interplay between the Kondo effect of a single-atom contact and a spin current. To this end, a nickel tip is coated by a thick layer of copper and brought into contact with a single Co atom adsorbed on a Cu(100) surface. We show that upon contact the Kondo resonance of Co is spin split and attribute the splitting to the spin current produced by the nickel tip and flowing across the copper spacer. A quantitative line shape analysis indicates that the spin polarization of the junction amounts up to 18%, but decreases when a pristine nickel tip is directly contacted to the Co atom.

cond-mat.str-el↗

arXiv2017

Imaging isodensity contours of molecular states with STM

We present an improved way for imaging the local density of states with a scanning tunneling microscope, which consists in mapping the surface topography while keeping the differential conductance (d$I$/d$V$) constant. When archetypical C$_{60}$ molecules on Cu(111) are imaged with this method, these so-called iso-d$I$/d$V$ maps are in excellent agreement with theoretical simulations of the isodensity contours of the molecular orbitals. A direct visualization and unambiguous identification of superatomic C$_{60}$ orbitals and their hybridization is then possible.

cond-mat.mes-hall↗

arXiv2016

From tunneling to contact in a magnetic atom: the non-equilibrium Kondo effect

A low-temperature scanning tunneling microscope was employed to study the differential conductance in an atomic junction formed by an adsorbed Co atom on a Cu(100) surface and a copper-covered tip. A zero-bias anomaly (ZBA) reveals spin scattering off the Co atom, which is assigned to a Kondo effect. The ZBA exhibits a characteristic asymmetric lineshape when electrons tunnel between tip and sample, while upon the tip-Co contact it symmetrizes and broadens. Through density functional theory calculations and the non-equilibrium non-crossing approximation we show that the lineshape broadening is mainly a consequence of the additional coupling to the tip, while non-equilibrium effects only modify the large-bias tails of the ZBA.

cond-mat.mes-hall↗

arXiv2013

Non-equilibrium spin-current detection with a single Kondo impurity

We present a theoretical study based on the Anderson model of the transport properties of a Kondo impurity (atom or quantum dot) connected to ferromagnetic leads, which can sustain a non-equilibrium spin current. We analyze the case where the spin current is injected by an external source and when it is generated by the voltage bias. Due to the presence of ferromagnetic contacts, a static exchange field is produced that eventually destroys the Kondo correlations. We find that such a field can be compensated by an appropriated combination of the spin-dependent chemical potentials leading to the restoration of the Kondo resonance. In this respect, a Kondo impurity may be regarded as a very sensitive sensor for non-equilibrium spin phenomena.

cond-mat.mes-hall↗

arXiv2005

Correlations, spin dynamics, defects: the highly-frustrated Kagomé bilayer

The SrCr$\_{9p}$Ga$\_{12-9p}$O$\_{19}$ and Ba$\_{2}$Sn$\_{2}$ZnGa$\_{10-7p}$Cr$\_{7p}$O$\_{22}$ compounds are two highly-frustrated magnets possessing a quasi-two-dimensional Kagomé bilayer of spin 3/2 chromium ions with antiferromagnetic interactions. Their magnetic susceptibility was measured by local Nuclear Magnetic Resonance and non-local (SQUID) techniques, and their low-temperature spin dynamics by Muon Spin Resonance. Consistent with the theoretical picture drawn for geometrically frustrated systems, the Kagomé bilayer is shown here to exhibit: (i) short range spin-spin correlations down to a temperature much lower than the Curie-Weiss temperature, no conventional long-range transition occurring; (ii) a Curie contribution to the susceptibility from paramagnetic defects generated by spin vacancies; (iii) low-temperature spin fluctuations, at least down to 30 mK, which are a trademark of a dynamical ground state. These properties point to a spin-liquid ground state, possibly built on Resonating Valence Bonds with unconfined spinons as the magnetic excitations.

cond-mat.str-el↗