arXiv subjects

Ludovic Largeau

Publications and source records attributed to Ludovic Largeau.

15 recordsLinked to original sources

arXiv2026

Spin-wave softening across the uniform-to-stripe domain transition in iron garnet film

Spin-wave spectra across transitions between uniform and textured phases can offer deep insight into both symmetry-breaking physics and self-assembled magnonic bands. However, experiments require a material platform that combines low damping, well-defined textures, and spectroscopic access. Here, we study a Bi-doped iron-garnet film with perpendicular magnetic anisotropy (PMA), which undergoes a uniform-to-stripe-domain transition as a function of in-plane magnetic field. Real-space imaging by magnetic force microscopy reveals field-reorientable stripe domains aligned with the in-plane field, while reciprocal-space measurements using thermal microfocused Brillouin light scattering ($\mu$-BLS) reveal the softening of a low-frequency spin-wave branch near the transition and the appearance of additional modes in the stripe-domain state. Calculated dispersion relations identify finite-$k$ softening in the Damon-Eshbach geometry ($k \perp M$), with the corresponding wavelength matching the stripe periodicity at the transition. In addition, a $\mu$-BLS spectral model reproduces the measured mode frequencies and relative intensities at selected fixed fields. Micromagnetic simulations capture the field-driven formation of the stripe state and reproduce the experimental thermal $\mu$-BLS spectra. Our findings establish BiYIG with PMA as a model low-damping platform for studying spin-wave freezing, stripe-domain modes, and reconfigurable magnonic band structures.

cond-mat.mes-hall↗

arXiv2026

Unlocking extreme doping and strain in epitaxial monocrystalline silicon

Hyperdoping, overcoming the solubility limit of dopants in a crystalline semiconductor, is a fertile method for the enhancement of the electrical, structural and optical devices' performances and for the exploration of exotic phases such as superconductivity. We demonstrate an unprecedented control on the dopant concentration and lattice deformation via nanosecond laser doping in epitaxial boron doped silicon, achieving record carrier concentrations (8 at.%) and lattice deformations (3 %). Probing the microscopical hyperdoping limitations, we show that the relevant mechanisms are caught by a simple combinatorial model, which quantitatively explains both the experimental carrier concentration and lattice deformation evolution. First principle calculations complete and support such simple model. Indeed, at the high doping levels now attainable, the maximum carrier concentration is inherently limited by the probability of two or three substitutional dopants occupying neighboring lattice sites, forming partially inactive complexes that we detail. This description is valid in the case of perfect layers with no crystallographic defects and a fully substitutional dopant occupation, highlighting the quality of the epitaxial layers realized.

cond-mat.mtrl-sci↗

arXiv2025

Strong and Engineerable Optical Anisotropy in Easily Integrable Epitaxial SrO(SrTiO 3 ) N Ruddlesden--Popper Thin Layers

Optical anisotropy is a key property for numerous photonic devices. However, bulk anisotropic materials suitable for such applications remain relatively scarse and are often challenging to synthesize as thin films. Additionally, the optical losses as well as the complex structuration of anisotropic metamaterials hinder their integrability in photonic devices. Based on ellipsometry measurements coupled with reflectance, it is demonstrated here that Ruddlesden-Popper (RP) SrO(SrTiO 3 ) N phases (STO-RP N ), epitaxial thin films composed of a SrTiO 3 lattice periodically interrupted by one SrO atomic plane every N unit cells, exhibit pronounced dichroism and birefringence over a broad spectral range. Notably, this anisotropy is tunable by adjusting the RP order N. In contrast to most other anisotropic materials reported in the literature, STO-RP N thin layers can be fabricated using industry-standard growth processes. As it can be epitaxially grown on Si and GaAs using SrTiO 3 templates, the work paves the way for their compact integration on these photonic platforms.

cond-mat.mtrl-sci↗

arXiv2025

Growth of Highly Conductive PtSe2 Films Controlled by Raman Metrics for High-Frequency Photodetectors and Optoelectronic Mixers at 1.55 {\mu}m

Two-dimensional PtSe2 exhibits outstanding intrinsic properties such as high carrier mobility, tunable bandgap, broadband absorption and air stability, making it ideal for (opto)electronic applications. In particular, thick PtSe2 is semimetallic and well suited for ultrafast optoelectronics in the infrared domain. However, achieving PtSe2 films of high crystalline quality with controlled properties on low-cost and insulating substrates remains challenging. Here, highly crystalline semimetallic PtSe2 films are grown by molecular beam epitaxy on sapphire substrates. It is shown how an optimized post-growth annealing remarkably improves the out-of-plane crystallinity and leads to record sheet conductances, up to 1.6 mS. In-depth structural analyses reveal the strong influence of the domain arrangement within the films on their electrical properties. Films that are mainly composed of vertically single crystalline domains exhibit high sheet conductance (1.1 - 1.6 mS), whereas films that contain superimposed twisted domains present low sheet conductance (0.5 - 0.6 mS). Moreover, it is demonstrated that the A1g Raman peak width, in addition to the commonly used Eg peak width, are both effective metrics for evaluating the quality of PtSe2: films with narrower Eg and A1g peaks exhibit higher in-plane and out-of-plane crystalline quality, respectively, as well as higher sheet conductance. Finally, coplanar waveguides integrating a semimetallic PtSe2 channel are fabricated on a 2-inch sapphire substrate to demonstrate optoelectronic devices operating at the 1.55 {\mu}m telecom wavelength. This includes photodetectors with a record 60 GHz bandwidth and the first PtSe2-based optoelectronic mixer with a bandwidth above 30 GHz.

cond-mat.mtrl-sci↗

arXiv2023

Ferroelectricity in tetragonal ZrO$_2$ thin films

We report on the crystal structure and ferroelectric properties of epitaxial ZrO$_2$ films ranging from 7 to 42 nm thickness grown on La$_{0.67}$Sr$_{0.33}$MnO$_3$-buffered (110)-oriented SrTiO$_3$ substrate. By employing X-ray diffraction, we confirm a tetragonal phase at all investigated thicknesses, with slight in-plane strain due to the substrate in the thinnest films. Further confirmation of the tetragonal phase was obtained through Infrared absorption spectroscopy with synchrotron light, performed on ZrO$_2$ membrane transferred onto a high resistive Silicon substrate. Up to a thickness of 31 nm, the ZrO$_2$ epitaxial films exhibit ferroelectric behavior, at variance with the antiferroelectric behavior reported previously for the tetragonal phase in polycrystalline films. However, the ferroelectricity is found here to diminish with increasing film thickness, with a polarization of about 13 $\mu$C.cm$^{-2}$ and down to 1 $\mu$C.cm$^{-2}$ for 7 nm and 31 nm-thick ZrO$_2$ films, respectively. This highlights the role of thickness reduction, substrate strain, and surface effects in promoting polarization in the tetragonal ZrO$_2$ thin films. These findings provide new insights into the ferroelectric properties and structure of ZrO$_2$ thin films, and open up new directions to investigate the origin of ferroelectricity in ZrO$_2$ and to optimize this material for future applications.

cond-mat.mtrl-sci↗

arXiv2021

Stabilization of the epitaxial rhombohedral ferroelectric phase in ZrO2 by surface energy

Doped HfO2 and HfO2-ZrO2 compounds are gaining significant interest thanks to their ferroelectric properties in ultrathin films. Here, we show that ZrO2 could be a playground for doping and strain engineering to increase the thickness in epitaxial thin films. Based on surface energy considerations supported by ab initio calculations, we show that a pure ZrO2 epitaxial thin film exhibits a ferroelectric rhombohedral phase (r-phase, with R3m space group) more stable than for the Hf0.5Zr0.5O2 (HZO) and pure HfO2 cases. In particular, for a thickness up to 37 nm we experimentally evidence a single (111)-oriented r-phase in ZrO2 films deposited on La2/3Sr1/3MnO3-buffered DyScO3(110) substrate, while a tetragonal phase is observed alongside the rhombohedral one on SrTiO3(001). The formation of this r-phase is discussed and compared between HfO2, ZrO2 and HZO, highlighting the role of surface energy.

cond-mat.mtrl-sci↗

arXiv2021

Relaxation mechanism of GaP grown on 001 Sisubstrates: influence of defects on the growth of AlGaPlayers on GaP/Si templates

The mechanical stability of commercial GaP/Si templates during thermal an-nealing and subsequent MOCVD growth of GaP and AlGaP is investigated.Although the GaP layer of the template originally presents an excellent surfacemorphology, annealing at high enough temperatures to remove the native oxideprior to growth leads to plastic relaxation, accompanied by a variety of defects,including a dense grid of micro-twins. These micro-twins detrimentally affectGaP and AlGaP layers grown subsequently on the template.

cond-mat.mtrl-sci↗

arXiv2020

Stress measurements in silicon photonics by integrated Raman spectroscopy

Complex 3D integration of photonic and electronic integrated circuits is of particular interest to carry the photonics roadmap and to address challenges but involves mechanical stress, often detrimental for the behavior of optical components. Existing experiments failed to carefully analyze the stress in such integrated optical devices due to the requirement in terms of feature sizes, few hundreds of nanometers, and 3D-stacked integration. We present for the first time the characterization of the stress tensor of a silicon waveguide using Integrated Raman Spectroscopy (IRS). This experimental technique is directly sensitive to the effective stress, which involves changes in optical properties of the guided mode, at the working wavelength and polarization state of the photonic component. The experimental stress tensor is in good agreement with simulations.

physics.app-ph↗

arXiv2014

Surface acoustic wave driven ferromagnetic resonance in (Ga,Mn)(As,P) epilayers

Interdigitated transducers were used to generate and detect surface acoustic waves on a thin layer of (Ga,Mn)(As,P). The out-of-plane uniaxial magnetic anisotropy of this dilute magnetic semiconductor is very sensitive to the strain of the layer, making it an ideal test material for the dynamic control of magnetization via magneto-striction. The time-domain measurement of the amplitude and phase of the transmitted SAW during magnetic field sweeps indicated a clear resonant behavior at a field close to the one calculated to give a precession frequency equal to the SAW frequency. A resonance was observed from 5K to 85K, just below the Curie temperature of the layer. A full analytical treatment of the coupled magnetization/acoustic dynamics showed that the magneto-strictive coupling modifies the elastic constants of the material and accordingly the wave-vector solution to the elastic wave equation. The shape and position of the resonance were well reproduced by the calculations, in particular the fact that velocity (phase) variations resonated at lower fields than the acoustic attenuation variations.

cond-mat.mtrl-sci↗

arXiv2012

Electric-field control of the magnetic anisotropy in an ultrathin (Ga,Mn)As/(Ga,Mn)(As,P) bilayer

We report on the electric control of the magnetic anisotropy in an ultrathin ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with competing in-plane and out-of-plane anisotropies. The carrier distribution and therefore the strength of the effective anisotropy is controlled by the gate voltage of a field effect device. Anomalous Hall Effect measurements confirm that a depletion of carriers in the upper (Ga,Mn)As layer results in the decrease of the in-plane anisotropy. The uniaxial anisotropy field is found to decrease by a factor ~ 4 over the explored gate-voltage range, so that the transition to an out-of-plane easy-axis configuration is almost reached.

cond-mat.mtrl-sci↗

arXiv2008

Strain-Control of the magnetic anisotropy in (Ga,Mn)(As,P) ferromagnetic semiconductor layers

A small fraction of phosphorus (up to 10 %) was incorporated in ferromagnetic (Ga,Mn)As epilayers grown on a GaAs substrate. P incorporation allows reducing the epitaxial strain or even change its sign, resulting in strong modifications of the magnetic anisotropy. In particular a reorientation of the easy axis toward the growth direction is observed for high P concentration. It offers an interesting alternative to the metamorphic approach, in particular for magnetization reversal experiments where epitaxial defects stongly affect the domain wall propagation.

cond-mat.mtrl-sci↗

arXiv2007

Evolution of the magnetic anisotropy with carrier density in hydrogenated (Ga,Mn)As

The magnetic properties of (Ga,Mn)As thin films depend on both the Mn doping level and the carrier concentration. Using a post growth hydrogenation process we show that it is possible to decrease the hole density from 1.1021 cm-3 to <1017 cm-3 while maintaining the manganese concentration constant. For such a series of films we have investigated the variation of the magnetization, the easy and hard axes of magnetization, the critical temperatures, the coercive fields and the magnetocrystalline anisotropy constants as a function of temperature using magnetometry, ferromagnetic resonance and magneto-transport measurements. In particular, we evidenced that magnetic easy axes flipped from out-of-plane [001] to in-plane [100] axis, followed by the <110> axes, with increasing hole density and temperature. Our study concluded on a general agreement with mean-field theory predictions of the expected easy axis reversals, and of the weight of uniaxial and cubic anisotropies in this material.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic properties and domain structure of (Ga,Mn)As films with perpendicular anisotropy

The ferromagnetism of a thin GaMnAs layer with a perpendicular easy anisotropy axis is investigated by means of several techniques, that yield a consistent set of data on the magnetic properties and the domain structure of this diluted ferromagnetic semiconductor. The magnetic layer was grown under tensile strain on a relaxed GaInAs buffer layer using a procedure that limits the density of threading dislocations. Magnetometry, magneto-transport and polar magneto-optical Kerr effect (PMOKE) measurements reveal the high quality of this layer, in particular through its high Curie temperature (130 K) and well-defined magnetic anisotropy. We show that magnetization reversal is initiated from a limited number of nucleation centers and develops by easy domain wall propagation. Furthermore, MOKE microscopy allowed us to characterize in detail the magnetic domain structure. In particular we show that domain shape and wall motion are very sensitive to some defects, which prevents a periodic arrangement of the domains. We ascribed these defects to threading dislocations emerging in the magnetic layer, inherent to the growth mode on a relaxed buffer.

cond-mat.mtrl-sci↗

arXiv2006

Tuning the ferromagnetic properties of hydrogenated GaMnAs

Hydrogenation and posthydrogenation annealings have been used as a very efficient tool to tune the hole density over a wide range, at fixed magnetic moment concentration, in thin GaMnAs layers. Reduction of the hole density resulted in strong modifications of their ferromagnetic properties. In particular, we observed in magnetotransport experiments the decrease of the Curie temperature, along with modifications of the magnetic anisotropy, a behavior consistent with the mean-field theory.

cond-mat.mtrl-sci↗

arXiv2004

Buried dislocation networks designed to organize the growth of III-V semiconductor nanostructures

We first report a detailed transmission electron microscopy study of dislocation networks (DNs) formed at shallowly buried interfaces obtained by bonding two GaAs crystals between which we establish in a controlled manner a twist and a tilt around a k110l direction. For large enough twists, the DN consists of a twodimensional network of screw dislocations accommodating mainly the twist and of a one-dimensional network of mixed dislocations accommodating mainly the tilt. We show that in addition the mixed dislocations accommodate part of the twist and we observe and explain slight unexpected disorientations of the screw dislocations with respect to the k110l directions. By performing a quantitative analysis of the whole DN, we propose a coherent interpretation of these observations which also provides data inaccessible by direct experiments. When the twist is small enough, one screw subnetwork vanishes. The surface strain field induced by such DNs has been used to pilot the lateral ordering of GaAs and InGaAs nanostructures during metal-organic vapor phase epitaxy. We prove that the dimensions and orientations of the nanostructures are correlated with those of the cells of the underlying DN and explain how the interface dislocation structure governs the formation of the nanostructures.

cond-mat.mtrl-sci↗