arXiv subjects

Manuel Bibes

Publications and source records attributed to Manuel Bibes.

At least 37 records · Page 2Linked to original sources

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↗

arXiv2023

All-electrical detection of the spin-charge conversion in nanodevices based on SrTiO3 two-dimensional electron gases

The Magnetoelectric Spin-Orbit (MESO) technology aims to bring logic into memory by combining a ferromagnet with a magnetoelectric (ME) element for information writing, and a spin-orbit (SO) element for information read-out through spin-charge conversion. Among candidate SO materials to achieve a large MESO output signal, oxide Rashba two-dimensional electron gases (2DEGs) have shown very large spin-charge conversion efficiencies, albeit mostly in spin-pumping experiments. Here, we report all-electrical spin-injection and spin-charge conversion experiments in nanoscale devices harnessing the inverse Edelstein effect of SrTiO3 2DEGs. We have designed, patterned and fabricated nanodevices in which a spin current injected from a cobalt layer into the 2DEG is converted into a charge current. We optimized the spin-charge conversion signal by applying back-gate voltages, and studied its temperature evolution. We further disentangled the inverse Edelstein contribution from spurious effects such as the planar Hall effect, the anomalous Hall effect or the anisotropic magnetoresistance. The combination of non-volatility and high energy efficiency of these devices could potentially lead to new technology paradigms for beyond-CMOS computing architectures.

cond-mat.mes-hall↗

arXiv2023

Spin-Hall Current and Nonlocal Transport in Ferromagnet-Free Multi-band models for SrTiO3-Based Nanodevices in the presence of impurities

We compute the spin-Hall conductance in a multiband model describing the two-dimensional electron gas formed at a LaAlO3/SrTiO3 interface in the presence of a finite concentration of impurities. Combining linear response theory with a systematic calculation of the impurity contributions to the self-energy, as well as to the vertex corrections of the relevant diagrams, we recover the full spin-Hall vs. sheet conductance dependence of LaAlO3/SrTiO3 as reported in Trier et al. [Nano Lett. 20, 395 (2020)], finding a very good agreement with the experimental data below and above the Lifshitz transition. In particular, we demonstrate that the multiband electronic structure leads to only a partial, instead of a complete, screening of the spin-Hall conductance, which decreases with increasing the carrier density. Our method can be generalized to other two-dimensional systems characterized by a broken inversion symmetry and multiband physics.

cond-mat.mes-hall↗

arXiv2023

Field-free switching of perpendicular magnetization in an ultrathin epitaxial magnetic insulator

For energy efficient and fast magnetic memories, switching of perpendicular magnetization by the spin-orbit torque (SOT) appears as a very promising solution, even more using magnetic insulators that suppress electrical shunting. This SOT switching generally requires the assistance of an in-plane magnetic field to break the symmetry. Here, we present experiments demonstrating the field-free SOT switching of perpendicularly magnetized layers of the thulium iron garnet (Tm_{3}Fe_{5}O_{12}) magnetic insulator. The polarity of the switching loops, clockwise (CW) or counter-clockwise (CCW), is determined by the direction of the initial current pulses, in contrast with field-assisted switchings in which this polarity is controlled by the direction of the field. After an independent determination of Dzyaloshinskii-Moriya interaction (DMI), we relate the field free switching to the interplay of SOT and DMI and the polarity of the loops to the imprint of a Néel domain wall induced by the first pulse, in agreement with Kerr imaging. Our observation and interpretation of field-free electrical switching of a magnetic insulator is an important milestone for future low power spintronic devices.

physics.app-ph↗

arXiv2022

Patterning of superconducting two-dimensional electron gases based on AlO$_x$/KTaO$_3$(111) interfaces

The versatility of properties displayed by two-dimensional electron gases (2DEGs) at oxide interfaces has fostered intense research in hope of achieving exotic electromagnetic effects in confined systems. Of particular interest is the recently discovered superconducting state appearing in (111)-oriented KTaO$_3$ interfaces, with a critical temperature $T_c \approx 2$ K, almost ten times higher than that of SrTiO$_3$-based 2DEGs. Just as in SrTiO$_3$-based 2DEGs, fabricating devices in this new system is a technical challenge due to the fragility of the 2DEG and the propensity of bulk KTaO$_3$ to become conducting outside the devices upon adventitious oxygen vacancy doping. Here, we present three different techniques for patterning Hall bars in AlO$_x$/KTaO$_3$~(111) heterostructures. The devices show superconducting transitions ranging from 1.3 K to 1.78 K, with limited degradation from the unpatterned thin film, and enable an efficient tuning of the carrier density by electric field effect. The array of techniques allows for the definition of channels with a large range of dimensions for the design of various kinds of devices to explore the properties of this system down to the nanoscale.

cond-mat.mtrl-sci↗

arXiv2022

Visualizing giant ferroelectric gating effects in large-scale WSe$_2$/BiFeO$_3$ heterostructures

Multilayers based on quantum materials (complex oxides, topological insulators, transition-metal dichalcogenides, etc) have enabled the design of devices that could revolutionize microelectronics and optoelectronics. However, heterostructures incorporating quantum materials from different families remain scarce, while they would immensely broaden the range of possible applications. Here we demonstrate the large-scale integration of compounds from two highly-multifunctional families: perovskite oxides and transition-metal dichalcogenides (TMDs). We couple BiFeO$_3$, a room-temperature multiferroic oxide, and WSe$_2$, a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe$_2$ is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO$_3$ films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe$_2$ and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO$_3$. Such a strong shift opens new perspectives in the efficient manipulation of TMDs properties by proximity effects.

cond-mat.mtrl-sci↗

arXiv2022

Direct visualization of Rashba-split bands and spin/orbital-charge interconversion at KTaO$_3$ interfaces

Rashba interfaces have emerged as promising platforms for spin-charge interconversion through the direct and inverse Edelstein effects. Notably, oxide-based two-dimensional electron gases (2DEGs) display a large and gate-tunable conversion efficiency, as determined by transport measurements. However, a direct visualization of the Rashba-split bands in oxide 2DEGs is lacking, which hampers an advanced understanding of their rich spin-orbit physics. Here, we investigate KTaO$_3$-2DEGs and evidence their Rashba-split bands using angle resolved photoemission spectroscopy. Fitting the bands with a tight-binding Hamiltonian, we extract the effective Rashba coefficient and bring insight into the complex multiorbital nature of the band structure. Our calculations reveal unconventional spin and orbital textures, showing compensation effects from quasi-degenerate band pairs which strongly depend on in-plane anisotropy. We compute the band-resolved spin and orbital Edelstein effects, and predict interconversion efficiencies exceeding those of other oxide 2DEGs. Finally, we suggest design rules for Rashba systems to optimize spin-charge interconversion performance.

cond-mat.mes-hall↗

arXiv2022

Ultra-High Carrier Mobilities in Ferroelectric Domain Wall Corbino Cones at Room Temperature

Recently, electrically conducting heterointerfaces between dissimilar band-insulators (such as lanthanum aluminate and strontium titanate) have attracted considerable research interest. Charge transport has been thoroughly explored and fundamental aspects of conduction firmly established. Perhaps surprisingly, similar insights into conceptually much simpler conducting homointerfaces, such as the domain walls that separate regions of different orientations of electrical polarisation within the same ferroelectric band-insulator, are not nearly so well-developed. Addressing this disparity, we herein report magnetoresistance in approximately conical 180o charged domain walls, which occur in partially switched ferroelectric thin film single crystal lithium niobate. This system is ideal for such measurements: firstly, the conductivity difference between domains and domain walls is extremely and unusually large (a factor of at least 1013) and hence currents driven through the thin film, between planar top and bottom electrodes, are overwhelmingly channelled along the walls; secondly, when electrical contact is made to the top and bottom of the domain walls and a magnetic field is applied along their cone axes (perpendicular to the thin film surface), then the test geometry mirrors that of a Corbino disc, which is a textbook arrangement for geometric magnetoresistance measurement. Our data imply carriers at the domain walls with extremely high room temperature Hall mobilities of up to ~ 3,700cm2V-1s-1. This is an unparalleled value for oxide interfaces (and for bulk oxides too) and is most comparable to mobilities in other systems typically seen at cryogenic, rather than at room, temperature.

cond-mat.mtrl-sci↗

arXiv2022

A multiferroic two-dimensional electron gas

Multiferroics are compounds in which at least two ferroic orders coexist - typically (anti)ferromagnetism and ferroelectricity. While magnetic order can arise in both insulating and conducting compounds, ferroelectricity is in principle not allowed in metals although a few two-dimensional (semi)metals were reported to behave as ferroelectrics. Yet, the combination with magnetic order to realize multiferroic metals remains elusive. Here, by combining x-ray spectroscopy and magnetotransport, we show the coexistence of ferroelectricity and magnetism in an oxide-based two-dimensional electron gas (2DEG). The data evidence a non-volatile switching of the polar displacements and of the anomalous Hall effect by the polarization direction, demonstrating a magnetoelectric coupling. Our findings provide new opportunities in quantum matter stemming from the interplay between ferroelectricity, ferromagnetism and Rashba spin-orbit coupling in a 2DEG.

cond-mat.mtrl-sci↗

arXiv2022

Non-volatile electric control of spin-orbit torques in an oxide two-dimensional electron gas

Spin-orbit torques (SOTs) have opened a novel way to manipulate the magnetization using in-plane current, with a great potential for the development of fast and low power information technologies. It has been recently shown that two-dimensional electron gases (2DEGs) appearing at oxide interfaces provide a highly efficient spin-to-charge current interconversion. The ability to manipulate 2DEGs using gate voltages could offer a degree of freedom lacking in the classical ferromagnetic/spin Hall effect bilayers for spin-orbitronics, in which the sign and amplitude of SOTs at a given current are fixed by the stack structure. Here, we report the non-volatile electric-field control of SOTs in an oxide-based Rashba-Edelstein 2DEG. We demonstrate that the 2DEG is controlled using a back-gate electric-field, providing two remanent and switchable states, with a large resistance contrast of 1064%. The SOTs can then be controlled electrically in a non-volatile way, both in amplitude and in sign. This achievement in a 2DEG-CoFeB/MgO heterostructures with large perpendicular magnetization further validates the compatibility of oxide 2DEGs for magnetic tunnel junction integration, paving the way to the advent of electrically reconfigurable SOT MRAMS circuits, SOT oscillators, skyrmion and domain-wall-based devices, and magnonic circuits.

cond-mat.mtrl-sci↗

arXiv2021

Structural instabilities of infinite-layer nickelates from first-principles simulations

Rare-earth nickelates RNiO$_2$ adopting an infinite-layer phase show superconductivity once La, Pr or Nd are substituted by a divalent cation. Either in the pristine or doped form, these materials are reported to adopt a high symmetry, perfectly symmetric, P4/mmm tetragonal cell. Nevertheless, bulk compounds are scarce, hindering a full understanding of the role of chemical pressure or strain on lattice distortions that in turn could alter magnetic and electronic properties of the 2D nickelates. Here, by performing a full analysis of the prototypical YNiO$_2$ compound with first-principles simulations, we identify that these materials are prone to exhibit O$_4$ group rotations whose type and amplitude are governed by the usual R-to-Ni cation size mismatch. We further show that these rotations can be easily tuned by external stimuli modifying lattice parameters such as pressure or strain. Finally, we reveal that H intercalation is favored for any infinite-layer nickelate member and pushes the propensity of the compounds to exhibit octahedra rotations.

cond-mat.mtrl-sci↗

arXiv2021

Nonreciprocal transport in a Rashba ferromagnet, delafossite PdCoO$_2$

Rashba interfaces yield efficient spin-charge interconversion and give rise to nonreciprocal transport phenomena. Here, we report magnetotransport experiments in few-nanometer-thick films of PdCoO$_2$, a delafossite oxide known to display a large Rashba splitting and surface ferromagnetism. By analyzing the angle dependence of the first- and second-harmonic longitudinal and transverse resistivities, we identify a Rashba-driven unidirectional magnetoresistance that competes with the anomalous Nernst effect below the Curie point. We estimate a Rashba coefficient of 0.75 {\pm} 0.3 eV Å and argue that our results qualify delafossites as a new family of oxides for nano-spintronics and spin-orbitronics, beyond perovskite materials.

cond-mat.mes-hall↗

arXiv2021

Non-collinear and strongly asymmetric polar moments at back-gated SrTiO3 interfaces

The highly mobile electrons at the interface of SrTiO3 with other oxide insulators, such as LaAlO3 or AlOx, are of great current interest. A vertical gate voltage allows controlling a metal/superconductor-to-insulator transition, as well as electrical modulation of the spin-orbit Rashba coupling for spin-charge conversion. These findings raise important questions about the origin of the confined electrons as well as the mechanisms that govern the interfacial electric field. Here we use infrared ellipsometry and confocal Raman spectroscopy to show that an anomalous polar moment is induced at the interface that is non-collinear, highly asymmetric and hysteretic with respect to the vertical gate electric field. Our data indicate that an important role is played by the electromigration of oxygen vacancies and their clustering at the antiferrodistortive domain boundaries of SrTiO3, which generates local electric and possibly also flexoelectric fields and subsequent polar moments with a large lateral component. Our results open new perspectives for the defect engineering of lateral devices with strongly enhanced and hysteretic local electric fields that can be manipulated with various other parameters, like strain, temperature, or photons.

cond-mat.mtrl-sci↗

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

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↗

arXiv2021

Voltage-Controlled Reconfigurable Magnonic Crystal at the Submicron Scale

Multiferroics offer an elegant means to implement voltage-control and on the fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic materials. These properties are particularly interesting for the field of magnonics, where spin waves are used to perform advanced logical or analogue functions. Recently, the emergence of nano-magnonics {\color{black} is expected to} eventually lead to the large-scale integration of magnonic devices. However, a compact voltage-controlled, on demand reconfigurable magnonic system has yet to be shown. Here, we introduce the combination of multiferroics with ferromagnets in a fully epitaxial heterostructure to achieve such voltage-controlled and reconfigurable magnonic systems. Imprinting a remnant electrical polarization in thin multiferroic $\mathrm{BiFeO_3}$ with a periodicity of $500\,\mathrm{nm}$ yields a modulation of the effective magnetic field in the micron-scale, ferromagnetic $\mathrm{La_{2/3}Sr_{1/3}MnO_3}$ magnonic waveguide. We evidence the magneto-electrical coupling by characterizing the spin wave propagation spectrum in this artificial, voltage induced, magnonic crystal and demonstrate the occurrence of a robust magnonic bandgap with $>20 \,\mathrm{dB}$ rejection.

cond-mat.mes-hall↗