arXiv subjects

Alexandre Gloter

Publications and source records attributed to Alexandre Gloter.

At least 19 recordsLinked to original sources

arXiv2026

Orbital and Spin Edelstein Effects in KTaO$_3$(110) Two-Dimensional Electron Gases

The orbital Edelstein effect converts an electric field into a non-equilibrium orbital polarization, opening new opportunities for orbitronics. Although signatures of the orbital Edelstein effect have been reported, its microscopic mechanisms and quantitative validation remain underexplored. Here, by directly linking the atomic structure of KTaO$_3$(110) two-dimensional electron gases to both their calculated and measured electronic band dispersions, we predict and provide experimental evidence for an orbital Edelstein effect that largely counterbalances its spin counterpart. Scanning transmission electron microscopy and electron energy-loss spectroscopy resolve the interfacial atomic configuration, which is used as input for density-functional calculations. Angle-resolved photoemission spectroscopy then confirms the resulting band structure, which is fitted by a tight-binding model enabling computation of the spin and orbital Edelstein responses. Harmonic magnetotransport indicates that a $\sim$20 \% contribution from the orbital Edelstein response is necessary to describe the magnitude and anisotropy of the effect. Our results establish KTaO$_3$(110) as a model platform for orbitronics and demonstrate a pathway to generate and harness orbital polarization in quantum oxide systems while also offering new insights into pairing mechanisms in their superconducting state.

cond-mat.mtrl-sci↗

arXiv2026

Decoding Superconductivity in La$_3$Ni$_2$O$_{7-\delta}$ Thin Films via Ozone-Driven Structure and Oxidation Tuning

The discovery of superconductivity in bulk Ruddlesden-Popper La$_3$Ni$_2$O$_7$(LNO327) under high hydrostatic pressure has redefined the recent experimental consensus that nickelate superconductivity is restricted to systems with a $3d^9$ electronic configuration and square-planar coordination. However, the structural and electronic prerequisites for stabilizing superconductivity, whether under pressure or at ambient conditions in the case of thin films, remain poorly understood, largely due to the metastable nature of the LNO327 phase. Here, we present a detailed structural study of epitaxial La$_3$Ni$_2$O$_{7-\delta}$ thin films by using scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS). Grown via pulsed laser deposition onto SrLaAlO$_4$ substrates, those films exhibit distinct superconducting properties as a function of the different post-annealing conditions used. By correlating the rich landscape of stacking polymorphs with transport behavior, this work establishes a framework for understanding the metastable superconducting phase in bilayer nickelate thin films. Our findings underscore the critical role of homogeneity in oxygen stoichiometry, epitaxial strain and structural motif in stabilizing superconductivity, offering a clear pathway for designing ambient-pressure superconducting nickelates.

cond-mat.supr-con↗

arXiv2026

A metallic CrS$_2$ phase bridging the gap between two- and three-dimensional dichalcogenides

We report on the high-pressure synthesis of a CrS$_2$ phase in the form of single-crystalline nanorods. A structural refinement of Precession Electron Diffraction Tomography data confirms the nominal CrS$_2$ composition and unveils a ladder-type structure formed by portions of 1T-type CrS$_2$ layers characteristic of two-dimensional (2D) dichalcogenides connected by chains of edge-sharing CrS$_6$ octahedra characteristic of 3D dichalcogenides with marcasite structure. Ab initio density functional theory calculations of the relaxed structure confirm the stability of this structure and indicate a strong overlap of the 3d states of Cr with the 3p states of S, thus suggesting strong covalent Cr-S bonds and metallic behavior. Electrical resistivity, $\varrho$, measurements on single nanorods confirm this behavior and yield $\varrho \sim 2-20$ m$\Omega$ cm at 4 K. The proposed ladder-like structure of CrS$_2$ forms open channels along the chain direction, which may be suitable for ionic conduction.

cond-mat.mtrl-sci↗

arXiv2025

Tailoring the breathing-mode distortions in nickelate-ferroelectric heterostructures

In transition metal oxides electron-electron interaction and lattice degree of freedom are basic ingredients of emergent phenomena, such as metal-to-insulator transition (MIT) and superconductivity. Perovskite rare-earth nickelates are largely studied for their temperature-driven MIT which is accompanied by a breathing mode distortion, and associated to a bond-disproportionation of the expanded (3d8L0) and compressed (3d8L2) NiO6 octahedra. Steric effects control the onset temperature of the MIT, the latter being concomitant or not with a complex antiferromagnetic spin arrangement depending upon the choice of the rare earth ion (TMIT>TNeel). Interface engineering of oxygen octahedra tilting, as imposed by the symmetry and orientation of the substrate, has resulted in an efficient pathway to modify both TMIT and TNeel, hence, suggesting a key role of the electron-phonon coupling for both transport and magnetic properties in nickelate thin films. Here, via a combination of resonant elastic X-ray scattering and transport experiments, we show a control over both TMIT and TNeel in heteroepitaxial PZT(d)/NNO(7 nm)//STO heterostructures, which are characterized by different strains and polarization states of the PZT layer grown at different thicknesses d. We found the expected NNO bulk behaviour, for a fully relaxed PZT layer showing a monodomain polarization state. On the other side, an almost 30 K difference, is found for a fully strained PZT characterized by a multidomain texture of the polarization state. We discuss our results in terms of an altered breathing distortion pattern of the underlying nickelate layer as supported by X-ray absorption spectroscopy measurements. We infer that locally different polar distortions controlled by a combination of polarization direction and strength of the strain state play the main role in the observed TMIT and TNeel variations.

cond-mat.mtrl-sci↗

arXiv2024

Achieving superconductivity in infinite-layer nickelate thin films by aluminum sputtering deposition

The recent discovery of superconductivity in infinite-layer (IL, ABO$_2$) nickelates has opened a new avenue to deepen the understanding of high-temperature superconductivity. However, progress in this field is slowed by significant challenges in material synthesis and the scarcity of research groups capable of producing high quality superconducting samples. IL nickelates are obtained from a reduction of the perovskite ABO$_3$ phase, typically achieved by annealing using CaH$_2$ as a reducing agent. Here, we present a new method to synthesize superconducting infinite-layer nickelate Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ thin films using an aluminum overlayer deposited by sputtering as a reducing agent. We systematically optimized the aluminum deposition parameters and obtained superconducting samples reduced either in situ or ex situ (after air exposure of the precursor ABO$_3$ films). A comparison of their crystalline quality and transport properties shows that in situ Al reduction enhances the quality of the superconducting Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ thin films, achieving a maximum superconducting transition temperature $T_{c}^{onset}$ of 17 K, in agreement with the optimum value reported for this compound. This simple synthesis route, much more accessible than existing methods, offers better control and reproducibility over the topotactic transformation, opening new opportunities to gain insights into the physics of superconductivity in nickelates.

cond-mat.supr-con↗

arXiv2024

Polarization boost and ferroelectricity down to one unit cell in layered Carpy-Galy La$_{2}$Ti$_{2}$O$_{7}$ thin films

Layered perovskite-based compounds offer a range of unconventional properties enabled by their naturally anisotropic structure. While most renowned for the superconductivity observed in the Ruddlesden-Popper phases, many of these layered compounds are also ferroelectric and exhibit a sizeable in-plane polarization. Among these, the Carpy-Galy phases (A${_n}$B${_n}$O$_{3n+2}$), characterized by 110-oriented perovskite planes interleaved with additional oxygen layers, have been debated as platforms for hosting not only a robust polarization but also multiferroicity and polar metallicity. However, the challenges associated with the synthesis of ultrathin Carpy-Galy films and understanding the impact of strain on their properties limit their integration into devices. Addressing this issue, our study focuses on La$_2$Ti$_2$O$_7$, an $n$=4 (A$_2$B$_2$O$_7$) representative of the Carpy-Galy family, exploring its growth and concurrent phase stability on various substrates under different strain conditions. Remarkably, we demonstrate that a 3% tensile strain from DyScO$_3$ (100) substrates promotes a controlled layer-by-layer growth mode, while SrTiO$_3$ (110) and LaAlO$_3$-Sr$_2$TaAlO$_6$ (110), that exert negligible and compressive strains respectively, require post-deposition annealing to achieve similar results. Using scanning probe microscopy, X-ray diffraction, scanning transmission electron microscopy, and polarization switching experiments, we confirm that these films possess exceptional ferroelectric properties, including a polarization of 18 $\mu$C/cm$^2$ - more than three times higher than previously reported - as well as persistence of ferroelectricity down to a single-unit-cell thickness. This study not only advances our understanding of Carpy-Galy phases in thin films but also lays a foundation for their application in advanced ferroelectric device architectures.

cond-mat.mtrl-sci↗

arXiv2024

Superconductivity in PrNiO2 infinite-layer nickelates

Several reports about infinite-layer nickelate thin films suggest that the superconducting critical temperature versus chemical doping phase diagram has a dome-like shape, similar to cuprates. Here, we demonstrate a highly reproducible superconducting state in undoped PrNiO$_2$ thin films grown onto SrTiO$_3$. Scanning transmission electron microscopy measurements demonstrate coherent and defect-free infinite-layer phase, a high structural quality with no unintentional chemical doping and a total absence of interstitial oxygen. X-ray absorption measurements show very sharp features at the Ni L$_{3,2}$-edges with a large linear dichroism, indicating the preferential hole-occupation of Ni$^{1+}$-3d$_{x^2-y^2}$ orbitals in a square planar geometry. Resonant inelastic X-ray scattering measurements reveal sharp magnon excitations of 200\,meV energy at magnetic Brillouin zone boundary, highly resonant at the Ni$^{1+}$ absorption peak. The results indicate that, when properly stabilized, infinite-layer nickelate thin films are superconducting without chemical doping.

cond-mat.supr-con↗

arXiv2024

Unraveling p-type and n-type interfaces in Superconducting Infinite-Layer Nickelate thin films

After decades of research, superconductivity was finally found in nickel-based analogs of superconducting cuprates, with infinite-layer (IL) structure. These results are so far restricted to thin films in the case of IL-nickelates. Therefore, the nature of the interface with the substrate, and how it couples with the thin film properties is still an open question. Here, using scanning transmission electron microscopy (STEM)- electron energy loss spectroscopy (EELS) and four-dimensional (4D)-STEM, a novel chemically sharp p-type interface is observed in a series of superconducting IL-praseodymium nickelate samples, and a comparative study is carried out with the previously reported n-type interface obtained in other samples. Both interfaces have strong differences, with the p-type interface being highly polar. In combination with ab-initio calculations, we find that the influence of the interface on the electronic structure is local, and does not extend beyond 2-3 unit cells into the thin film. This decouples the direct influence of the interface in driving the superconductivity, and indicates that the IL-nickelate thin films do not have a universal interface model. Insights into the spatial hole-distribution in SC samples, provided by monochromated EELS and total reflection-hard x-ray photoemission spectroscopy, suggest that this particular distribution might be directly influencing superconductivity.

cond-mat.supr-con↗

arXiv2024

Towards reliable synthesis of superconducting infinite layer nickelate thin films by topochemical reduction

Infinite layer nickelates provide a new route beyond copper oxides to address outstanding questions in the field of unconventional superconductivity. However, their synthesis poses considerable challenges, largely hindering experimental research on this new class of oxide superconductors. That synthesis is achieved in a two-step process that yields the most thermodynamically stable perovskite phase first, then the infinite-layer phase by topotactic reduction, the quality of the starting phase playing a crucial role. Here, we report on reliable synthesis of superconducting infinite-layer nickelate films after successive topochemical reductions of a parent perovskite phase with nearly optimal stoichiometry. Careful analysis of the transport properties of the incompletely reduced films reveals an improvement of the strange metal behaviour of their normal state resistivity over subsequent topochemical reductions, offering insight into the reduction process.

cond-mat.supr-con↗

arXiv2023

Emergent electronic landscapes in a novel valence-ordered nickelate with tri-component nickel coordination

The metal-hydride-based topochemical reduction process has produced novel thermodynamically unstable phases across various transition metal oxide series with unusual crystal structures and non-trivial ground states. Here, by such an oxygen (de-) intercalation method we synthesis a novel samarium nickelate with ordered nickel valences associated with tri-component coordination configurations. This structure, with a formula of Sm$_{9}$Ni$_{9}$O$_{22}$ as revealed by four-dimensional scanning transmission electron microscopy, emerges from the intricate planes of {303}$_{\text{pc}}$ ordered apical oxygen vacancies. X-ray spectroscopy measurements and ab-initio calculations show the coexistence of square-planar, pyramidal and octahedral Ni sites with mono-, bi- and tri-valences. It leads to an intense orbital polarization, charge-ordering, and a ground state with a strong electron localization marked by the disappearance of ligand-hole configuration at low-temperature. This new nickelate compound provides another example of previously inaccessible materials enabled by topotactic transformations and presents a unique platform where mixed Ni valence can give rise to exotic phenomena.

cond-mat.mtrl-sci↗

arXiv2023

Nanoscale domain engineering in SrRuO$_3$ thin films

We investigate nanoscale domain engineering via epitaxial coupling in a set of SrRuO$_3$/PbTiO$_3$/SrRuO$_3$ heterostructures epitaxially grown on (110)$_o$-oriented DyScO$_3$ substrates. The SrRuO$_3$ layer thickness is kept at 55 unit cells, whereas the PbTiO$_3$ layer is grown to thicknesses of 23, 45 and 90 unit cells. Through a combination of atomic force microscopy, x-ray diffraction and high resolution scanning transmission electron microscopy studies, we find that above a certain critical thickness of the ferroelectric layer, the large structural distortions associated with the ferroelastic domains propagate through the top SrRuO$_3$ layer, locally modifying the orientation of the orthorhombic SrRuO$_3$ and creating a modulated structure that extends beyond the ferroelectric layer boundaries.

cond-mat.mtrl-sci↗

arXiv2023

Charge distribution across capped and uncapped infinite-layer neodymium nickelate thin films

Charge ordering (CO) phenomena have been widely debated in strongly-correlated electron systems mainly regarding their role in high-temperature superconductivity. Here, we elucidate the structural and charge distribution in NdNiO$_{2}$ thin films prepared with and without capping layers, and characterized by the absence and presence of CO. Our microstructural and spectroscopic analysis was done by scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) and hard x-ray photoemission spectroscopy (HAXPES). Capped samples show Ni$^{1+}$, with an out-of-plane (o-o-p) lattice parameter of around 3.30 angstroms indicating good stabilization of the infinite-layer structure. Bulk-sensitive HAXPES on Ni-2p shows weak satellite feature indicating large charge-transfer energy. The uncapped samples evidence an increase of the o-o-p parameter up to 3.65 angstroms on the thin-film top, and spectroscopies show signatures of higher valence in this region (towards Ni$^{2+}$). Here, 4D-STEM demonstrates (3,0,3) oriented stripes which emerge from partially occupied apical oxygen. Those stripes form quasi-2D coherent domains viewed as rods in the reciprocal space with $Δ\text{q}_{z} \approx 0.24$ r.l.u. extension located at Q = ($\pm \frac{1}{3},0,\pm \frac{1}{3}$) r.l.u. and Q = ($\pm \frac{2}{3},0,\pm \frac{2}{3}$) r.l.u. The stripes associated with oxygen re-intercalation concomitant with hole doping suggests a possible link to the previously reported CO in infinite-layer nickelate thin films.

cond-mat.mtrl-sci↗

arXiv2023

Mapping the complex evolution of ferroelastic/ferroelectric domain patterns in epitaxially strained PbTiO3 heterostructures

We study the complex ferroelastic/ferroelectric domain structure in the prototypical ferroelectric PbTiO3 epitaxially strained on (110)o-oriented DyScO3 substrates, using a combination of atomic force microscopy, laboratory and synchrotron x-ray diffraction and high resolution scanning transmission electron microscopy. We observe that the anisotropic strain imposed by the orthorhombic substrate creates a large asymmetry in the domain configuration, with domain walls macroscopically aligned along one of the two in-plane directions. We show that the periodicity as a function of film thickness deviates from the Kittel law. As the ferroelectric film thickness increases, we find that the domain configuration evolves from flux-closure to a/c-phase, with a larger scale arrangement of domains into superdomains.

cond-mat.mtrl-sci↗

arXiv2022

Synthesis of infinite-layer nickelates and influence of the capping-layer on magnetotransport

The recent discovery of a zero-resistance state in nickel-based compounds has generated a re-excitement about the long-standing problem in condensed matter of high-critical-temperature superconductivity, in light of the analogies between infinite-layer nickelates and cuprates. However, despite some formal valence and crystal symmetry analogies, the electronic properties of infinite-layer nickelates are remarkably original accounting, among other properties, of a unique Nd5d-Ni3d hybridization. This designates infinite-layer nickelates as a new class of oxide superconductors which should be considered on their own. Here we report about Nd1-xSrxNiO2 (x = 0, 0.05 and 0.2) thin films synthesized with and without a SrTiO3 capping-layer, showing very smooth and step-terraced surface morphologies. Angle-dependent anisotropic magnetoresistance measurements performed with a magnetic field rotating in-plane or out-of-plane with respect to the sample surface, rendered important information about the magnetic properties of undoped SrTiO3-capped and uncapped samples. The results point at a key role of the capping-layer in controlling the magnitude and the anisotropy of the anisotropic magnetoresistance properties. We discuss this control in terms of a combined effect between the Nd-Ni hybridization and an intra-atomic exchange coupling between the Nd-4f and Nd-5d states, the latter essentially contributing to the (magneto)transport. Further studies foresee the influence of the capping layer on infinite-layer nickelates with no magnetic rare-earth.

cond-mat.mtrl-sci↗

arXiv2021

From low-field Sondheimer oscillations to high-field very large and linear magnetoresistance in a SrTiO$_3$-based two-dimensional electron gas

Quantum materials harbor a cornucopia of exotic transport phenomena challenging our understanding of condensed matter. Among these, a giant, non-saturating linearmagnetoresistance (MR) has been reported in various systems, from Weyl semi-metals to topological insulators. Its origin is often ascribed to unusual band structure effects but it may also be caused by extrinsic sample disorder. Here, we report a very large linear MR in a SrTiO$_3$ two-dimensional electron gas and, by combining transport measurements with electron spectro-microscopy, show that it is caused by nanoscale inhomogeneities that are self-organized during sample growth. Our data also reveal semi-classical Sondheimer oscillations arising from interferences between helicoidal electron trajectories, from which we determine the 2DEG thickness. Our results bring insight into the origin of linear MR in quantum materials, expand the range of functionalities of oxide 2DEGs and suggest exciting routes to explore the interaction of linear MR with features like Rashba spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2020

Can Copper Nanostructures Sustain High-Quality Plasmons?

Silver is considered to be the king among plasmonic materials because it features low inelastic absorption in the visible and infrared (vis-IR) spectral regions compared to other metals. In contrast, copper is commonly regarded as being too lossy for plasmonic applications. Here, we experimentally demonstrate vis-IR plasmons in long copper nanowires (NWs) with quality factors that exceed a value of 60, as determined by spatially resolved, high-resolution electron energy-loss spectroscopy (EELS) measurements. We explain this counterintuitive result by the fact that plasmons in these metal wires have most of their electromagnetic energy outside the metal, and thus, they are less sensitive to inelastic losses in the material. We present an extensive set of data acquired on long silver and copper NWs of varying diameters supporting this conclusion and further allowing us to understand the relative roles played by radiative and nonradiative losses in plasmons that span a wide range of energies down to $<20\,$meV. At such small plasmon energies, thermal population of these modes becomes significant enough to enable the observation of electron energy gains associated with plasmon absorption events. Our results support the use of copper as an attractive cheap and abundant material platform for high quality plasmons in elongated nanostructures.

cond-mat.mtrl-sci↗

arXiv2020

Bismuth iron garnet: ab initio study of electronic properties

Bismuth iron garnet (BIG), i.e. Bi3Fe5O12, is a strong ferrimagnet that also possess outstanding magneto-optical properties such as the largest known Faraday rotation. These properties are related with the distribution of magnetic moments on octahedral and tetrahedral sites, the presence of spin gaps in the density of state and a strong spin-orbit coupling. In this work, first-principles ab initio calculations are performed to study the structural, electronic and magnetic properties of BIG using Density Functional Theory with Hubbard+U (DFT+U) correction including spin-orbit coupling and HSE06 hybrid functional. We found that the presence of spin gaps in the electronic structure results from the interplay between exchange and correlation effects and the crystal field strengths for tetrahedral and octahedral iron sublattices. The DFT+U treatment tends to close the spin-gaps for larger U due to over-localization effects, notably in the octahedral site. On the other hand, the hybrid functional confirms the occurrences of three spin gaps in the iron states of the conduction band as expected from optical measurements. A strong exchange splitting at the top of the valence bands associated with a lone-pair type mixture of O p and Bi s,p states is also obtained. Similar exchange splitting was not previously observed for other iron based garnets, such as for yttrium iron garnet. It follows that hole doping, as obtained by Ca substitution at Bi sites, results in a full spin polarized density at the Fermi energy. This work helps to shed more light on the theoretical comprehension of the properties of BIG and opens the route towards the use of advanced Many Body calculations to predict the magneto-optical coupling effects in BIG in a direct comparison with the experimental measurements.

cond-mat.str-el↗

arXiv2019

Tailored nanoscale plasmon-enhanced vibrational electron spectroscopy

Vibrational optical spectroscopies can be enhanced by surface plasmons to reach molecular-sized limits of detection and characterization. The level of enhancement strongly depends on microscopic details of the sample that are generally missed by macroscopic techniques. Here we investigate phonons in h-BN by coupling them to silver-nanowire plasmons, whose energy is tuned by modifying the nanowire length. Specifically, we use electron beam milling to accurately and iteratively change the nanowire length, followed by electron energy-loss spectroscopy to reveal the plasmon-enhanced vibrational features of h-BN. This allows us to investigate otherwise hidden bulk phonons and observe strong plasmon-phonon coupling. The new milling-and-spectroscopy technique holds great potential for resolving vibrational features in material nanostructures.

cond-mat.mes-hall↗