Visualizing Giant Ferroelectric Gating Effects in Large-Scale WSe2/BiFeO3 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 BiFeO₃, a room-temperature multiferroic oxide, and WSe₂, a semiconducting two-dimensional material with potential for photovoltaics and photonics. WSe₂ is grown by molecular beam epitaxy and transferred on a centimeter-scale onto BiFeO₃ films. Using angle-resolved photoemission spectroscopy, we visualize the electronic structure of 1 to 3 monolayers of WSe₂ and evidence a giant energy shift as large as 0.75 eV induced by the ferroelectric polarization direction in the underlying BiFeO₃. Such a strong shift opens new perspectives in the efficient manipulation of TMD properties by proximity effects.

Unidirectional Rashba spin splitting in single layer WS2(1-x)Se2xalloy

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides have attracted considerable attention due to their tunable band gap, intriguing spin-valley physics, piezoelectric effects and potential device applications. Here we study the electronic properties of a single layer WS_1.4Se_0.6alloys. The electronic structure of this alloy, explored using angle resolved photoemission spectroscopy, shows a clear valence band structure anisotropy characterized by two paraboloids shifted in one direction of thek-space by a constant in-plane vector. This band splitting is a signature of a unidirectional Rashba spin splitting with a related giant Rashba parameter of 2.8 ± 0.7 eV Å. The combination of angle resolved photoemission spectroscopy with piezo force microscopy highlights the link between this giant unidirectional Rashba spin splitting and an in-plane polarization present in the alloy. These peculiar anisotropic properties of the WS_1.4Se_0.6alloy can be related to local atomic orders induced during the growth process due the different size and electronegativity between S and Se atoms. This distorted crystal structure combined to the observed macroscopic tensile strain, as evidenced by photoluminescence, displays electric dipoles with a strong in-plane component, as shown by piezoelectric microscopy. The interplay between semiconducting properties, in-plane spontaneous polarization and giant out-of-plane Rashba spin-splitting in this 2D material has potential for a wide range of applications in next-generation electronics, piezotronics and spintronics devices.

Quantitative investigation of polarization-dependent photocurrent in ferroelectric thin films

Ferroelectric thin films are investigated for their potential in photovoltaic (PV) applications, owing to their high open-circuit voltage and switchable photovoltaic effect. The direction of the ferroelectric polarization can control the sign of the photocurrent through the ferroelectric layer, theoretically allowing for 100% switchability of the photocurrent with the polarization, which is particularly interesting for photo-ferroelectric memories. However, the quantitative relationship between photocurrent and polarization remains little studied. In this work, a careful investigation of the polarization-dependent photocurrent of epitaxial Pb(Zr, Ti)O₃thin films has been carried out, and has provided a quantitative determination of the unswitchable part of ferroelectric polarization. These results represent a systematic approach to study and optimize the switchability of photocurrent, and more broadly to get important insights on the ferroelectric behavior in all types of ferroelectric layers in which pinned polarization is difficult to investigate.

Understanding nanoscale structural distortions in Pb(Zr0.2Ti0.8)O3 by utilizing X-ray nanodiffraction and clustering algorithm analysis

Hard X-ray nanodiffraction provides a unique nondestructive technique to quantify local strain and structural inhomogeneities at nanometer length scales. However, sample mosaicity and phase separation can result in a complex diffraction pattern that can make it challenging to quantify nanoscale structural distortions. In this work, a k-means clustering algorithm was utilized to identify local maxima of intensity by partitioning diffraction data in a three-dimensional feature space of detector coordinates and intensity. This technique has been applied to X-ray nanodiffraction measurements of a patterned ferroelectric PbZr_0.2Ti_0.8O₃ sample. The analysis reveals the presence of two phases in the sample with different lattice parameters. A highly heterogeneous distribution of lattice parameters with a variation of 0.02 Å was also observed within one ferroelectric domain. This approach provides a nanoscale survey of subtle structural distortions as well as phase separation in ferroelectric domains in a patterned sample.

Direct Evidence of Lithium Ion Migration in Resistive Switching of Lithium Cobalt Oxide Nanobatteries

Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two-terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the Li_x CoO₂ layer. These observations are very well correlated with the observed insulator-to-metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling - much further than the present cycling life of usual lithium-ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics.

Universal Fabrication of 2D Electron Systems in Functional Oxides

2D electron systems (2DESs) in functional oxides are promising for applications, but their fabrication and use, essentially limited to SrTiO3 -based heterostructures, are hampered by the need for growing complex oxide overlayers thicker than 2 nm using evolved techniques. It is demonstrated that thermal deposition of a monolayer of an elementary reducing agent suffices to create 2DESs in numerous oxides.

Memristive and neuromorphic behavior in a Li(x)CoO2 nanobattery

The phenomenon of resistive switching (RS), which was initially linked to non-volatile resistive memory applications, has recently also been associated with the concept of memristors, whose adjustable multilevel resistance characteristics open up unforeseen perspectives in cognitive computing. Herein, we demonstrate that the resistance states of Li(x)CoO2 thin film-based metal-insulator-metal (MIM) solid-state cells can be tuned by sequential programming voltage pulses, and that these resistance states are dramatically dependent on the pulses input rate, hence emulating biological synapse plasticity. In addition, we identify the underlying electrochemical processes of RS in our MIM cells, which also reveal a nanobattery-like behavior, leading to the generation of electrical signals that bring an unprecedented new dimension to the connection between memristors and neuromorphic systems. Therefore, these LixCoO2-based MIM devices allow for a combination of possibilities, offering new perspectives of usage in nanoelectronics and bio-inspired neuromorphic circuits.

Wavelength dependence of Pockels effect in strained silicon waveguides

We investigate the influence of the wavelength, within the 1.3μm-1.63μm range, on the second-order optical nonlinearity in silicon waveguides strained by a silicon nitride (Si₃N ₄) overlayer. The effective second-order optical susceptibility χxxy(2)¯ evolutions have been determined for 3 different waveguide widths 385 nm, 435 nm and 465 nm and it showed higher values for longer wavelengths and narrower waveguides. For wWG = 385 nm and λ = 1630 nm, we demonstrated χxxy(2)¯ as high as 336 ± 30 pm/V. An explanation based on the strain distribution within the waveguide and its overlap with optical mode is then given to justify the obtained results.

Two-dimensional electron gas with six-fold symmetry at the (111) surface of KTaO3

Two-dimensional electron gases (2DEGs) at transition-metal oxide (TMO) interfaces, and boundary states in topological insulators, are being intensively investigated. The former system harbors superconductivity, large magneto-resistance, and ferromagnetism. In the latter, honeycomb-lattice geometry plus bulk spin-orbit interactions lead to topologically protected spin-polarized bands. 2DEGs in TMOs with a honeycomb-like structure could yield new states of matter, but they had not been experimentally realized, yet. We successfully created a 2DEG at the (111) surface of KTaO3, a strong insulator with large spin-orbit coupling. Its confined states form a network of weakly-dispersing electronic gutters with 6-fold symmetry, a topology novel to all known oxide-based 2DEGs. If those pertain to just one Ta-(111) bilayer, model calculations predict that it can be a topological metal. Our findings demonstrate that completely new electronic states, with symmetries not realized in the bulk, can be tailored in oxide surfaces, promising for TMO-based devices.

Dual antiferromagnetic coupling at La0.67Sr0.33MnO3/SrRuO3 interfaces

We have studied the magnetic hysteresis cycle of La0.67Sr0.33MnO3/SrRuO3 antiferromagnetically coupled bilayers, by magnetometry and polarized neutron reflectometry. A positive exchange bias as well as an unusual asymmetry are observed on the magnetic reversal process of the La0.67Sr0.33MnO3 layer. Through an extended Stoner-Wohlfarth model comprising the magnetic anisotropy of both layers, we give experimental evidence that this asymmetry originates from two different but well-defined antiferromagnetic coupling strengths at the interface between the two magnetic oxides. The possible origin of this dual coupling is discussed in view of our experimental results.

Atomic-level control of the domain wall velocity in ultrathin magnets by tuning of exchange interactions

We demonstrate that the propagation velocity of field driven magnetic domain walls in ultrathin Au/Co/Au films with perpendicular anisotropy on vicinal substrates is anisotropic and strongly depends on the step density of the substrate. The velocity of walls oriented perpendicular to the steps drastically increases with increasing local step density while being unchanged or only weakly decreased for the walls oriented parallel to the steps. We develop an analytical model revealing the step-modified exchange interactions as the main driving force for this anisotropic behavior. The enhancement of the domain wall velocity at low magnetic fields far below the Walker instability threshold makes this phenomenon interesting for magnetic nanodevices.

Deformation of a "rigid" molecule in self-assembled nanostructures

A simple spirobifluorene molecule with pseudotetrahedral structure was investigated for its supposed conformational resilience upon adsorption. Through deposition at room temperature of this molecule on a Cu(111) surface and subsequent observation at 5 K with an ultrahigh vacuum scanning tunneling microscope, this "rigidity" upon physisorption is confirmed. However, an unexpected chemisorbed state was also found with the molecules arranged in trimers. The unique coexistence of physisorbed and chemisorbed states on the same substrate is thus demonstrated at the early stage of self-assembly.

Color view of atomic highs and lows in tunneling induced light emission

Based on a detailed experimental study of light emission stimulated with a scanning tunneling microscope, we put forward a consistent picture for the atomic-scale contrasts observed to date on noble metal surfaces. Divergent contrasts near various atomic steps and conflicting interpretations of light emission from a model atomic grating, (2 x 1) reconstructed Au(110), are accounted for. The light intensity modulation results from different spatial distributions of the local density of final states in the elastic and inelastic tunneling channels.

From meandering to faceting, is step flow growth ever stable?

Based on helium atom beam diffraction and scanning tunneling microscopy data, the coexistence of a meandering and a bunching instability during homoepitaxial step flow growth is established in a class of nonreconstructed, metallic vicinal surfaces, Cu (1,1,n), n=5,9,17. Specifically, the meandering instability is shown to act as a precursor to the bunching instability, indicating that a one-dimensional treatment of bunching in step flow growth is not sufficient. Our findings might be generic to step flow growth in kinetically restricted systems.

Surface-state Stark shift in a scanning tunneling microscope

We report a quantitative low-temperature scanning tunneling spectroscopy (STS) study on the Ag(111) surface state over an unprecedented range of currents (50 pA to 6 microA) through which we can tune the electric field in the tunnel junction of the microscope. We show that in STS a sizable Stark effect causes a shift of the surface-state binding energy E0. Data taken are reproduced by a one-dimensional potential model calculation, and are found to yield a Stark-free energy E0 in agreement with recent state-of-the-art photoemission spectroscopy measurements.