arXiv subjects

Agnès Barthélémy

Publications and source records attributed to Agnès Barthélémy.

16 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

Reprogrammable magnonic logic in a multiferroic heterostructure via magnetoelectric coupling

The realization of fully reconfigurable, voltage-controlled, and programmable on-chip magnonic devices is essential to fully harness the potential of spin waves for signal processing, logic and neuromorphic computing. Yet, existing demonstrations of electrical tuning of magnonic responses are either volatile, current-driven and thus energy-inefficient, or rely on local strain modification limiting their scalability for wafer-scale integration. Here, we address this challenge using a BiFeO3/La0.67Sr0.33MnO3 multiferroic thin film heterostructure. We show that ferroelectric domain engineering in BiFeO3 enables deterministic tuning of the magnon dispersion of La0.67Sr0.33MnO3, producing frequency shifts up to $\sim 150 MHz$ and allowing reconfigurable waveguiding. Micro-focused Brillouin light scattering directly images these effects, revealing electrically defined magnonic waveguides and spatially programmable dispersion. Compared to conventional approaches, this method provides non-volatile and reversible control. Furthermore, using an inverse-design simulation code, we demonstrate the capability of our platform to perform advanced magnonic functions such as frequency demultiplexing. Our results open a new avenue for using magnetoelectric heterostructures for magnonic logic, with further applicability to reservoir and neuromorphic computing and AI driven magnonic devices.

cond-mat.mes-hall↗

arXiv2025

Spontaneous nonreciprocal transport in a gate-tunable ferromagnetic Rashba 2-dimensional electron gas

The broken inversion symmetry at interfaces of complex oxides gives rise to emergent phenomena, including ferromagnetism and Rashba spin-orbit coupling (SOC), which profoundly influence the electronic structure by entangling spin and momentum. While the interplay between Rashba SOC and ferromagnetism is theoretically intriguing, its experimental manifestations remain largely unexplored. Here, we demonstrate that ferromagnetic 2DEGs at SrTiO$_3$-based interfaces exhibit spontaneous nonreciprocal transport - a distinctive hallmark of Rashba ferromagnets - even in the absence of an external magnetic field. This nonreciprocal response, along with clear signatures of ferromagnetism such as anisotropic magnetoresistance and the anomalous Hall effect (AHE), is strongly tunable by gate voltage. Remarkably, the AHE not only varies in amplitude but even reverses sign, reflecting a subtle interplay between Fermi level position and Berry curvature distribution. These results establish SrTiO$_3$ 2DEGs as a model platform for studying Rashba ferromagnetism and demonstrate active control over transport phenomena in time- and inversion-symmetry-broken systems, paving the way for gate-tunable spintronic devices.

cond-mat.str-el↗

arXiv2023

Electronic bandstructure of superconducting KTaO3 (111) interfaces

Two-dimensional electron gases(2DEGs)based on KTaO3 are emerging as a promising platform for spin-orbitronics due to their high Rashba spin-orbit coupling (SOC) and gate-voltage tunability. The recent discovery of a superconducting state in KTaO3 2DEGs now expands their potential towards topological superconductivity. Although the band structure of KTaO3 surfaces of various crystallographic orientations has already been mapped using angle-resolved photoemission spectroscopy(ARPES), this is not the case for superconducting KTaO3 2DEGs. Here, we reveal the electronic structure of superconducting 2DEGs based on KTaO3 (111) single crystals through ARPES measurements. We fit the data with a tight-binding model and compute the associated spin textures to bring insight into the SOC-driven physics of this fascinating system.

cond-mat.mtrl-sci↗

arXiv2023

Boosting the Edelstein effect of two-dimensional electron gases by ferromagnetic exchange

Strontium titanate (SrTiO$_3$) two-dimensional electron gases (2DEGs) have broken spatial inversion symmetry and possess a finite Rashba spin-orbit coupling. This enables the interconversion of charge and spin currents through the direct and inverse Edelstein effects, with record efficiencies at low temperature, but more modest effects at room temperature. Here, we show that making these 2DEGs ferromagnetic enhances the conversion efficiency by nearly one order of magnitude. Starting from the experimental band structure of non-magnetic SrTiO$_3$ 2DEGs, we mimic magnetic exchange coupling by introducing an out-of-plane Zeeman term in a tight-binding model. We then calculate the band structure and spin textures for increasing internal magnetic fields and compute the Edelstein effect using a semiclassical Boltzmann approach. We find that the conversion efficiency first increases strongly with increasing magnetic field, then shows a maximum and finally decreases. This field dependence is caused by the competition of the exchange coupling with the effective Rashba interaction. While enhancing the splitting of band pairs amplifies the Edelstein effect, weakening the in-plane Rashba-type spin texture reduces it.

cond-mat.mes-hall↗

arXiv2023

Gate-voltage switching of non-reciprocal transport in oxide-based Rashba interfaces

The linear magnetoelectric effect (ME) is the phenomenon by which an electric field produces a magnetization. Its observation requires both time-reversal and space-inversion symmetries to be broken, as in multiferroics. While the ME effect has only been studied in insulating materials, it can actually exist in non-centrosymmetric conductors such as two-dimensional electron gases (2DEGs) with Rashba spin-orbit coupling. It is then coined the Edelstein effect (EE), by which a bias voltage -- generating a charge current -- produces a transverse spin density, i.e. a magnetization. Interestingly, 2D systems are sensitive to voltage gating, which provides an extra handle to control the EE. Here, we show that the sign of the EE in a SrTiO$_3$ 2DEG can be controlled by a gate voltage. We propose various logic devices harnessing the dual control of the spin density by current and gate voltages and discuss the potential of our findings for gate-tunable non-reciprocal electronics.

cond-mat.mes-hall↗

arXiv2021

Spin-charge interconversion in KTaO$_3$ two-dimensional electron gases

Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO$_3$ (STO), albeit in separate studies focusing on one or the other. The demonstration of these effects remains mostly elusive in other oxide interface systems despite their blossoming in the last decade. Here, we report the observation of both the DEE and IEE in a new interfacial two-dimensional electron gas (2DEG) based on the perovskite oxide KTaO$_3$. We generate 2DEGs by the simple deposition of Al metal onto KTaO$_3$ single crystals, characterize them by angle-resolved photoemission spectroscopy and magnetotransport, and demonstrate the DEE through unidirectional magnetoresistance and the IEE by spin-pumping experiments. We compare the spin-charge interconversion efficiency with that of STO-based interfaces, relate it to the 2DEG electronic structure, and give perspectives for the implementation of KTaO$_3$ 2DEGs into spin-orbitronic devices.

cond-mat.mes-hall↗

arXiv2021

Metal/SrTiO$_3$ two-dimensional electron gases for spin-to-charge conversion

SrTiO$_3$-based two-dimensional electron gases (2DEGs) can be formed through the deposition of epitaxial oxides like LaAlO$_3$ or of reactive metals such as Al. Such 2DEGs possess a finite Rashba spin-orbit coupling that has recently been harnessed to interconvert charge and spin currents through the direct and inverse Edelstein and spin Hall effects. Here we compare the formation and properties of 2DEGs generated in SrTiO$_3$ by the growth of Al, Ta and Y ultrathin films by magnetron sputtering. By combining in situ and ex situ X-ray photoelectron spectroscopy (XPS) we gain insight into the reduction of the SrTiO$_3$ and the appearance of Ti$^{3+}$ states associated with 2DEG formation, its reoxidation by exposure to the air, and the transformation of the metal into its binary oxides. We extract the carrier densities through magnetotransport and compare them with the XPS data. Finally, working with samples covered by an extra layer of NiFe, we perform spin-pumping ferromagnetic resonance experiments and investigate spin-charge conversion as a function of gate voltage. We identify trends in the data across the different sample systems and discuss them as a function of the carrier density and the transparency of the metal oxide tunnel barrier.

cond-mat.mes-hall↗

arXiv2020

A switchable two-dimensional electron gas based on ferroelectric Ca:SrTiO$_3$

Two-dimensional electron gases (2DEGs) can form at the surface of oxides and semiconductors or in carefully designed quantum wells and interfaces. Depending on the shape of the confining potential, 2DEGs may experience a finite electric field, which gives rise to relativistic effects such as the Rashba spin-orbit coupling. Although the amplitude of this electric field can be modulated by an external gate voltage, which in turn tunes the 2DEG carrier density, sheet resistance and other related properties, this modulation is volatile. Here, we report the design of a ''ferroelectric'' 2DEG whose transport properties can be electrostatically switched in a non-volatile way. We generate a 2DEG by depositing a thin Al layer onto a SrTiO$_3$ single crystal in which 1 percent of Sr is substituted by Ca to make it ferroelectric. Signatures of the ferroelectric phase transition at 25 K are visible in the Raman response and in the temperature dependences of the carrier density and sheet resistance that shows a hysteretic dependence on electric field as a consequence of ferroelectricity. We suggest that this behavior may be extended to other oxide 2DEGs, leading to novel types of ferromagnet-free spintronic architectures.

cond-mat.mtrl-sci↗

arXiv2019

Giant topological Hall effect in correlated oxide thin films

Strong electronic correlations can produce remarkable phenomena such as metal-insulator transitions and greatly enhance superconductivity, thermoelectricity, or optical non-linearity. In correlated systems, spatially varying charge textures also amplify magnetoelectric effects or electroresistance in mesostructures. However, how spatially varying spin textures may influence electron transport in the presence of correlations remains unclear. Here we demonstrate a very large topological Hall effect (THE) in thin films of a lightly electron-doped charge-transfer insulator, (Ca, Ce)MnO3. Magnetic force microscopy reveals the presence of magnetic bubbles, whose density vs. magnetic field peaks near the THE maximum, as is expected to occur in skyrmion systems. The THE critically depends on carrier concentration and diverges at low doping, near the metal-insulator transition. We discuss the strong amplification of the THE by correlation effects and give perspectives for its non-volatile control by electric fields.

cond-mat.mtrl-sci↗

arXiv2017

Reproducibility and off-stoichiometry issues in nickelate thin films grown by pulsed laser deposition

Rare-earth nickelates are strongly correlated oxides displaying a metal-to-insulator transition at a temperature tunable by the rare-earth ionic radius. In PrNiO$_3$ and NdNiO$_3$, the transition is very sharp and shows an hysteretic behavior akin to a first-order transition. Both the temperature at which the transition occurs and the associated resistivity change are extremely sensitive to doping and therefore to off-stoichiometry issues that may arise during thin film growth. Here we report that strong deviations in the transport properties of NdNiO$_3$ films can arise in films grown consecutively under nominally identical conditions by pulsed laser deposition; some samples show a well-developed transition with a resistivity change of up to five orders of magnitude while others are metallic down to low temperatures. Through a detailed analysis of \textit{in-situ} X-ray photoelectron spectroscopy data, we relate this behavior to large levels of cationic off-stoichoimetry that also translate in changes in the Ni valence and bandwidth. Finally, we demonstrate that this lack of reproducibility can be remarkably alleviated by using single-phase NdNiO$_3$ targets.

cond-mat.mtrl-sci↗

arXiv2016

Reconciling the ionic and covalent pictures in rare-earth nickelates

The properties of AMO3 perovskite oxides, where M is a 3d transition metal, depend strongly on the level of covalency between the metal d and oxygen p orbitals. With their complex spin orders and metal-insulator transition, rare-earth nickelates verge between dominantly ionic and covalent characters. Accordingly, the nature of their ground state is highly debated. Here, we reconcile the ionic and covalent visions of the insulating state of nickelates. Through first-principles calculations, we show that it is reminiscent of the ionic charge disproportionation picture (with strictly low-spin 4+ and high-spin 2+ Ni sites) while exhibiting strong covalence effects with oxygen electrons shifted toward the depleted Ni cations, mimicking a configuration with identical Ni sites. Our results further hint at strategies to control electronic and magnetic phases of transition metal oxide perovskites.

cond-mat.mtrl-sci↗

arXiv2014

Interplay between ferroic orders at the FeRh/BaTiO$_3$ interfaces

It has been recently demonstrated that the magnetic state of FeRh can be controlled by electric fields in FeRh/BaTiO$_{\text{3}}$ heterostructures [R.O. Cherifi et al. Nature Mater. 13, 345 (2014)]. Voltage-controlled changes in the ferroelastic domain structure of BaTiO$_3$ appeared to drive this effect, with charge accumulation and depletion due to ferroelectricity playing a more elusive role. To make this electric-field control of magnetic order non-volatile, the contribution of ferroelectric field-effect must be further enhanced, which requires understanding the details of the interface between FeRh and BaTiO$_3$. Here we report on the atomic structure and electron screening at this interface through density functional theory simulations. We relate different screening capabilities for the antiferromagnetic and ferromagnetic states of FeRh to different density of states at the Fermi level of corresponding bulk structures. We predict that the stability of the ferroelectric state in adjacent very thin BaTiO$_3$ films will be affected by magnetic order in FeRh. This control of ferroelectricity by magnetism can be viewed as the reciprocal effect of the voltage-controlled magnetic order previously found for this system.

cond-mat.mtrl-sci↗

arXiv2012

A ferroelectric memristor

Memristors are continuously tunable resistors that emulate synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, but the mechanism remains controversial. Purely electronic memristors have recently emerged based on well-established physical phenomena with albeit modest resistance changes. Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers yield memristive behaviour with resistance variations exceeding two orders of magnitude and a 10 ns operation speed. Using models of ferroelectric-domain nucleation and growth we explain the quasi-continuous resistance variations and derive a simple analytical expression for the memristive effect. Our results suggest new opportunities for ferroelectrics as the hardware basis of future neuromorphic computational architectures.

cond-mat.mtrl-sci↗

arXiv2011

Nanoscale ferroelectric manipulation of magnetic flux quanta

Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O7-δ), we demonstrate the modulation of the superconducting condensate at the nanoscale via ferroelectric field effects. Through this mechanism, a nanoscale pattern of normal regions that mimics the ferroelectric domain structure can be created in the superconductor. This yields an energy landscape for magnetic flux quanta and, in turn, couples the local ferroelectric polarization to the local magnetic induction. We show that this form of magnetoelectric coupling, together with the possibility to reversibly design the ferroelectric domain structure, allows the electrostatic manipulation of magnetic flux quanta.

cond-mat.supr-con↗

arXiv2010

Imaging ferroelectric domains in multiferroics using a low-energy electron microscope in the mirror operation mode

We report on low-energy electron microscopy imaging of ferroelectric domains with submicron resolution. Periodic strips of 'up' and 'down'-polarized ferroelectric domains in bismuth ferrite -a room temperature multiferroic- serve as a model system to compare low-energy electron microscopy with the established piezoresponse force microscopy. The results confirm the possibility of full-field imaging of ferroelectric domains with short acquisition times by exploiting the sensitivity of ultraslow electrons to small variations of the electric potential near surfaces in the "mirror" operation mode.

physics.ins-det↗