Dispersing and semi-flat bands in the wide band gap two-dimensional semiconductor bilayer silicon oxide
G. Kremer, J. C. Alvarez-Quiceno, T. Pierron, C. González, M. Sicot, B. Kierren, L. Moreau, J. E. Rault, P. Le Fèvre, F. Bertran, Y. J. Dappe, J. Coraux, P. Poc… 展开作者
G. Kremer, J. C. Alvarez-Quiceno, T. Pierron, C. González, M. Sicot, B. Kierren, L. Moreau, J. E. Rault, P. Le Fèvre, F. Bertran, Y. J. Dappe, J. Coraux, P. Pochet, Y. Fagot-Revurat
Epitaxial bilayer silicon oxide is a transferable two-dimensional material predicted to be a wide band gap semiconductor, with potential applications for deep UV optoelectronics, or as a building block of van der Waals heterostructures. The prerequisite to any sort of such applications is the knowledge of the electronic band structure, which we unveil using angle-resolved photoemission spectroscopy and rationalise with the help of density functional theory calculations. We discover dispersing bands related to electronic delocalisation within the top and bottom planes of the material, with two linear crossings reminiscent of those predicted in bilayer AA-stacked graphene, and semi-flat bands stemming from the chemical bridges between the two planes. This band structure is robust against exposure to air, and can be controled by exposure to oxygen. We provide an experimental lower-estimate of the band gap size of 5 eV and predict a full gap of 7.36 eV using density functional theory calculations.
We address the production of indium nanoparticles (In NPs) from In thin films thermally evaporated on both c-Si substrates and sputtered indium tin oxide (ITO) as well as from sputtered ITO thin films, exposed to a hydrogen (H2) plasma. On the one hand, we show that evaporated In thin films grow in Volmer-Weber (VW) mode; H2 plasma reduces their surface oxide and substrate annealing reshapes them from flat islands into spheres, without any remarkable surface migration or coalescence. On the other hand, we studied the In NPs formation on the ITO thin films and on In/ITO bilayer structures, by v…
Thin films of perovskite oxides offer the possibility of combining emerging concepts of strongly correlated electron phenomena and spin current in magnetic devices. However, spin transport and magnetization dynamics in these complex oxide materials are not well understood. Here, we experimentally quantify spin transport parameters and magnetization damping in epitaxial perovskite ferromagnet/paramagnet bilayers of La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ (LSMO/SRO) by broadband ferromagnetic resonance spectroscopy. From the SRO thickness dependence of Gilbert damping, we estimate a short spin dif…
Correlated electron systems on a honeycomb lattice have emerged as a fertile playground to explore exotic electronic phenomena. Theoretical and experimental work has appeared to realize novel behavior, including quantum Hall effects and valleytronics, mainly focusing on van der Waals compounds, such as graphene, chalcogenides, and halides. In this article, we review our theoretical study on perovskite transition-metal oxides (TMOs) as an alternative system to realize such exotic phenomena. We demonstrate that novel quantum Hall effects and related phenomena associated with the honeycomb struct…
We reported here a high-performance In2O3/InZnO bilayer metal-oxide (BMO) thin-film transistor (TFT) using ultra-thin solution-processed ZrOx dielectric. A thin layer of In2O3 offers a higher carrier concentration, thereby maximizing the charge accumulation and yielding high carrier mobility. A thick layer of InZnO controls the charge conductance resulting in low off-state current and suitable threshold voltage. As a consequence, the BMO TFT showed higher filed-effect mobility (37.9 cm2/V s) than single-layer InZnO TFT (7.6 cm2/V s). More importantly, an on/off current ratio of 109, a subthres…
This paper reports experiments investigating the reaction of H$_{2}$ with uranium metal-oxide bilayers. The bilayers consist of $\leq$ 100 nm of epitaxial $α$-U (grown on a Nb buffer deposited on sapphire) with a UO$_{2}$ overlayer of thicknesses of between 20 and 80 nm. The oxides were made either by depositing via reactive magnetron sputtering, or allowing the uranium metal to oxidise in air at room temperature. The bilayers were exposed to hydrogen, with sample temperatures between 80 and 200 C, and monitored via in-situ x-ray diffraction and complimentary experiments conducted using Scanni…
A commercially available membrane filter paper composed of mixed cellulose esters bearing typically an interconnected pore structure was transformed into a stimuli-responsive bilayer actuator by depositing a thin film of poly(ionic liquid)-modified graphene oxide sheets (GO-PIL) onto the filter paper. In acetone vapor, the as-synthesized bilayer actuator bent readily into multiple loops at a fast speed with the GO-PIL top film inwards. Upon pulling back into air the actuator recovered their original shape. The asymmetric swelling of the top GO-PIL film and the bottom porous filter paper toward…
The 5d transition-metal oxides have been an intriguing platform to demonstrate efficient charge to spin current conversion due to a unique electronic structure dominated by strong spin-orbit coupling. Here, we report on stacking-order effect of spin-orbit torque (SOT), spin-Hall magnetoresistance, and magnetic anisotropy in bilayer Ni$_{81}$Fe$_{19}$-5d iridium oxide, IrO$_2$. While all the IrO$_2$ and Pt control samples exhibit large dampinglike-SOT generation stemming from the efficient charge to spin current conversion, the magnitude of the SOT is larger in the IrO$_2$ (Pt)-bottom sample th…
The number of atomic layers confined in a two-dimensional structure is crucial for the electronic and magnetic properties. Single-layer and bilayer $J_{\rm eff} = 1/2$ square lattices are well-known examples where the presence of the extra layer turns the XY-anisotropy to the $c$-axis anisotropy. We report on experimental realization of a hybrid SrIrO$_3$/SrTiO$_3$ superlattice that integrates monolayer and bilayer square lattices in one layered structure. By synchrotron x-ray diffraction, resonant x-ray magnetic scattering, magnetization, and resistivity measurements, we found that the hybrid…
Honeycomb layered oxides with monovalent or divalent, monolayered cationic lattices generally exhibit myriad crystalline features encompassing rich electrochemistry, geometries and disorders, which particularly places them as attractive material candidates for next-generation energy storage applications. Herein, we report global honeycomb layered oxide compositions, ${\rm Ag_2}M_2{\rm TeO_6}$ ($M = \rm Ni, Mg, \textit{etc}.$) exhibiting $\rm Ag$ atom bilayers with sub-valent states within Ag-rich crystalline domains of ${\rm Ag_6}M_2{\rm TeO_6}$ and $\rm Ag$-deficient domains of ${\rm Ag}_{2 -…
High-quality epitaxial ferrites, such as low-damping MgAl-ferrite (MAFO), are promising nanoscale building blocks for all-oxide heterostructures driven by pure spin current. However, the impact of oxide interfaces on spin dynamics in such heterostructures remains an open question. Here, we investigate the spin dynamics and chemical and magnetic depth profiles of 15-nm-thick MAFO coherently interfaced with an isostructural $\approx$1-8-nm-thick overlayer of paramagnetic CoCr$_2$O$_4$ (CCO) as an all-oxide model system. Compared to MAFO without an overlayer, effective Gilbert damping in MAFO/CCO…
Extraordinary electronic phases can form in artificial oxide heterostructures, which will provide a fertile ground for new physics and also give rise to novel device functions. Based on a systematic first-principles density functional theory study of the magnetic and electronic properties of the (111) superlattices (ABO3)2/(AB'O3)10 of 4d and 5d transition metal perovskite (B = Ru, Rh, Ag, Re, Os, Ir, Au; AB'O3 = LaAlO3, SrTiO3), we demonstrate that due to quantum confinement, bilayers (LaBO3)2 (B = Ru, Re, Os) and (SrBO3)2 (B = Rh, Os, Ir) are ferromagnetic with ordering temperatures up to ro…
The 5d transition-metal oxides have a unique electronic structure dominated by strong spin-orbit coupling and hence they can be an intriguing platform to explore spin-current physics. Here, we report on room-temperature generation of spin-orbit torque (SOT) from a conductive 5d iridium oxide, IrO2. By measuring second-harmonic Hall resistance of Ni81Fe19/IrO2 bilayers, we find both dampinglike and fieldlike SOTs. The former is larger than the latter, enabling easier control of magnetization. We also observe that the dampinglike SOT efficiency has a significant dependence on IrO2 thickness, whi…
In the early days of high temperature superconductivity it was already recognized that magnetic properties of these materials are intimately related to the superconducting ones . When doped, the long-range ordered antiferromagnetic background of pristine copper-oxide insulators melts away and makes room for a spin liquid and superconductivity. By resonant inelastic x-ray scattering (RIXS) in the soft regime we probe the hitherto inaccessible dynamical multiple-spin correlations of the magnetic background in a series of parent compounds and in high Tc materials [NCCO (Nd2-xCexCuO4) and LSCO (La…
Two-dimensional (2D) transition-metal oxide perovskites greatly expand the field of available 2D multifunctional material systems. Here, based on density functional theory calculations, we predicted the presence of ferromagnetism orders accompanying with an insulator-metal phase transition in bilayer $KNbO_{3}$ and $KTaO_{3}$ by applying strain engineering and/or external electric field. Our results will contribute to the applications of few-layer transition metal oxide perovskites in the emerging spintronics and straintronics.
Chemical vapor deposition (CVD) has been established as a versatile route for the large-scale synthesis of transition metal dichalcogenides, such as tungsten disulfide (WS2). Yet, the role of the precursor composition on the efficiency of the CVD process remains largely unknown and yet to be explored. Here, we employ Pulsed Laser Deposition (PLD) in a two-stage process to tune the oxygen content in tungsten oxide (WO3-x) precursors and demonstrate that the presence of oxygen vacancies in the precursor films leads to a more facile conversion from WO3-x to WS2. Using a joint study based on ab in…
Manipulation of oxygen vacancies (V_O) in single oxide layers by varying the electric field can result in significant modulation of the ground state. However, in many oxide multilayers with strong application potentials, e.g. ferroelectric tunnel junctions and solid-oxide fuel cells, understanding V_O behaviour in various layers under an applied electric field remains a challenge, owing to complex V_O transport between different layers. By sweeping the external voltage, a reversible manipulation of V_O and a corresponding fixed magnetic phase transition sequence in cobaltite/manganite (SrCoO3-…
By combining angle-resolved photoemission spectroscopy and scanning tunneling microscopy we reveal the structural and electronic properties of multilayer graphene on Ru(0001). We prove that large ethylene exposure allows to synthesize two distinct phases of bilayer graphene with different properties. The first phase has Bernal AB stacking with respect to the first graphene layer, displays weak vertical interaction and electron doping. The long-range ordered moiré pattern modulates the crystal potential and induces replicas of the Dirac cone and minigaps. The second phase has AA stacking sequen…
One of the key technologies in spintronics is to tame spin-orbit coupling (SOC) that links spin and motion of electrons, giving rise to intriguing magneto-transport properties in itinerant magnets. Prominent examples of such SOC-based phenomena are anomalous and topological Hall effects. However, controlling them by electric field has remained unachieved since electric field tends to be screened in itinerant magnets. Here we demonstrate that both anomalous and topological Hall effects can be modulated by electric field in oxide heterostructures consisting of ferromagnetic SrRuO$_{3}$ and nonma…
Two-dimensional (2D) ferroelectricity has attracted extensive attention since its discovery in the monolayers of van der Waals materials. Here we show that 2D ferroelectricity induced by octahedral rotation distortion is widely present in the perovskite bilayer system through first-principles calculations. The perovskite tolerance factor plays a crucial role in the lattice dynamics and ground-state structure of the perovskite monolayers and bilayers, thus providing an important indicator for screening this hybrid improper ferroelectricity. Generally, the ferroelectric switching via an orthorho…
We use temperature-dependent resistivity in small-angle twisted double bilayer graphene to measure bandwidths and gaps of the bands. This electron-hole asymmetric system has one set of non-dispersing bands that splits into two flat bands with the electric field - distinct from the twisted bilayer system. With electric field, the gap between two emergent flat bands increases monotonically and bandwidth is tuned from 1 meV to 15 meV. These two flat bands with gap result in a series of thermally induced insulator to metal transitions - we use a model, at charge neutrality, to measure the bandwidt…
Quasicrystalline oxide interfaces by twisting SrTiO3 membranes
University of Minnesota（明尼苏达大学）；California Institute of Technology（加州理工学院）；Massachusetts Institute of Technology（麻省理工学院）；Hankuk University of Foreign Studies（韩国外国语大学）；Technical University of Denmark（丹麦技术大学）；University of Antwerpen（安特卫普大学）；Temple University（天普大学）；University of Illinois, Urbana-Champaign（伊利诺伊大学厄巴纳-香槟分校）；Helmholtz-Zentrum Berlin für Materialien und Energie GmbH（柏林亥姆霍兹材料与能源中心）；Kyung Hee University（庆熙大学）
Topological insulators are characterized by a nontrivial band topology driven by the spin-orbit coupling. To fully explore the fundamental science and application of topological insulators, material realization is indispensable. Here we predict, based on tight-binding modeling and first-principles calculations, that bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are potential candidates for two-dimensional topological insulators. The topological band structure of these materials can be fine-tuned by changing dopant ions, substrates, and external…
Magnetoelectric (ME) interactions in layered structures of magnetostrictive and piezoelectric phases are mediated by mechanical deformation. Here we discuss the frequency dependence of ME coupling in bilayers and trilayers of Permendur, a ferromagnetic alloy, and lead zirconate titanate. Data on ME voltage coefficient versus frequency profiles reveal a giant ME coupling at electromechanical resonance. The maximum voltage coefficient of 90 V/cm Oe is three orders of magnitude higher than low-frequency values. The ME interactions for transverse fields is an order of magnitude stronger than for l…
The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3*10^3 is demonstrated at 20 K. The dens…
We study the effects of insulating oxides in their crystalline forms on the energy band structure of monolayer and bilayer graphene using a \textit{first principles} density functional theory based electronic structure method and a local density approximation. We consider the dielectric oxides, SiO$_{2}$ ($α$-quartz) and Al$_{2}$O$_{3}$ (alumina or $α$-sapphire) each with two surface terminations. Our study suggests that atomic relaxations and resulting equilibrium separations play a critical role in perturbing the linear band structure of graphene in contrast to the less critical role played …
Two-dimensional (2D) magnets have broad application prospects in the spintronics, but how to effectively control them with a small electric field is still an issue. Here we propose that 2D magnets can be efficiently controlled in a multiferroic heterostructure composed of 2D magnetic material and perovskite oxide ferroelectric (POF) whose dielectric polarization is easily flipped under a small electric field. We illustrate the feasibility of such strategy in the bilayer CrI3/BiFeO3(001) heterostructure by using the first-principles calculations. Different from the traditional POF multiferroic …
Emergent quantum phases driven by electronic interactions can manifest in materials with narrowly dispersing, i.e. "flat", energy bands. Recently, flat bands have been realized in a variety of graphene-based heterostructures using the tuning parameters of twist angle, layer stacking and pressure, and resulting in correlated insulator and superconducting states. Here we report the experimental observation of similar correlated phenomena in twisted bilayer tungsten diselenide (tWSe2), a semiconducting transition metal dichalcogenide (TMD). Unlike twisted bilayer graphene where the flat band appe…
Using first-principle calculations, we demonstrate several approaches to manipulate Dzyaloshinskii-Moriya Interaction (DMI) in ultrathin magnetic films. First, we find that DMI is significantly enhanced when the ferromagnetic (FM) layer is sandwiched between nonmagnetic (NM) layers inducing additive DMI in NM/FM/NM structures. For instance, as Pt and Ir below Co induce DMI of opposite chirality, inserting Co between Pt (below) and Ir (above) in Ir/Co/Pt trilayers enhances the DMI of Co/Pt bilayers by 15\%. Furthermore, in case of Pb/Co/Pt trilayers (Ir/Fe/Co/Pt multilayers), DMI can be enhance…
We report a novel fabrication method of graphene Al2O3 gate insulators based on sputtering. Electrical performance of dual-gated mono- and bilayer exfoliated graphene devices is presented. Sputtered Al2O3 layers possess comparable quality to oxides obtained by atomic layer deposition (ALD) with respect to a high relative dielectric constant of about 8, as well as low-hysteresis performance and high breakdown voltage. We observe a moderate carrier mobility of about 1000 cm2/Vs in graphene and 350 cm2/Vs in its bilayer due to increased resonant scattering on atomic scale defects. Most likely thi…
Advances in honeycomb layered oxides: Part II -- Theoretical advances in the characterisation of honeycomb layered oxides with optimised lattices of cations
The k-projection method provides an approach to separate the contributions from different constituents in heterostructure systems and can act as an aid to connect the results of experiments and calculations. We show that the technique can be used to "unfold" the calculated electronic bands of interfaces and supercells, and provide local band structure by integrating the projected states over specified regions of space, a step that can be implemented efficiently using fast Fourier transforms. We apply the method to investigate the effects of interfaces in heterostructures consisting of a graphe…
Transistors are the fundamental building block of modern electronic devices. So far, all transistors are based on various types of semiconductor junctions. The most common bipolar-junction transistors and metal-oxide-semiconductor field-effect transistors contain p-n junctions to control the current, depending on applied biases across the junctions. Thin-film transistors need metal-semiconductor junctions for injecting and extracting electrons from their channels. Here, by coating a heavily-doped thin indium-tin-oxide (ITO) film through a shadow mask onto a biopolymer chitosan/ITO/glass substr…
In this work, we present an experimental study of spin mediated enhanced negative magnetoresistance in Ni80Fe20 (50 nm)/p-Si (350 nm) bilayer. The resistance measurement shows a reduction of ~2.5% for the bilayer specimen as compared to 1.3% for Ni80Fe20 (50 nm) on oxide specimen for an out-of-plane applied magnetic field of 3T. In the Ni80Fe20-only film, the negative magnetoresistance behavior is attributed to anisotropic magnetoresistance. We propose that spin polarization due to spin-Hall effect is the underlying cause of the enhanced negative magnetoresistance observed in the bilayer. Sili…
We present a first-principles study of the atomic, electronic, and magnetic properties of two-dimensional (2D), single and bilayer ZnO in honeycomb structure and its armchair and zigzag nanoribbons. In order to reveal the dimensionality effects, our study includes also bulk ZnO in wurtzite, zincblende, and hexagonal structures. The stability of 2D ZnO, its nanoribbons and flakes are analyzed by phonon frequency, as well as by finite temperature ab initio molecular-dynamics calculations. 2D ZnO in honeycomb structure and its armchair nanoribbons are nonmagnetic semiconductors but acquire net ma…
We present results from a Monte Carlo investigation of a simple bilayer model with geometrically frustrated interactions similar to those found in the mixed layer pnictide oxides $(Sr_{2}Mn_{3}Pn_{2}O_{2}, Pn=As,Sb).$ Our model is composed of two inequivalent square lattices with nearest neighbor intra- and interlayer interactions. We find a ground state composed of two independent Néel ordered layers when the interlayer exchange is an order of magnitude weaker than the intralayer exchange, as suggested by experiment. We observe this result independent of the number of layers in our model. We …
Memristive switching serves as the basis for a new generation of electronic devices. Memristors are two-terminal devices in which the current is turned on and off by redistributing point defects, e.g., vacancies, which is difficult to control. Memristors based on alternative mechanisms have been explored, but achieving both the high On/Off ratio and the low switching energy desirable for use in electronics remains a challenge. Here we report memristive switching in a La0.7Ca0.3MnO3/PrBa2Cu3O7 bilayer with an On/Off ratio greater than 10^3 and demonstrate that the phenomenon originates from a n…
In search of an improved strategy to form low resistance contacts to MoS2 and related semiconducting transition metal dichalcogenides, we use ab initio density functional electronic structure calculations in order to determine the equilibrium geometry and electronic structure of MoO3/MoS2 and MoO2/MoS2 bilayers. Our results indicate that, besides a rigid band shift associated with charge transfer, the presence of molybdenum oxide modifies the electronic structure of MoS2 very little. We find that the charge transfer in the bilayer provides a sufficient degree of hole doping to MoS2, resulting …
Based on first-principles calculations we performed a systematic study of the energetic stability, structural characterization, and electronic properties of the fully oxidized $A_{2}B$, electrenes, with the following combinations, (i) $A$=Ca, Sr, and Ba for $B$=N; (ii) $A$=Sr and Ba for $B$=P; and Y$_{2}$C, and Ba$_{2}$As. We have considered one side oxidation of single layer electrenes $(O/A_{2}B)$, and two side oxidation of bilayer electrenes $(O/(A_{2}B)_{2}/O)$. We show that the hexagonal lattice of the pristine host is no longer the ground state structure in the oxidized systems. Our tota…
Dirac points in two-dimensional electronic structures are a source for topological electronic states due to the $\pm π$ Berry phase that they sustain. Here we show that two rutile multilayers (namely (WO$_2$)$_2$/(ZrO$_2$)$_n$ and (PtO$_2$)$_2$/(ZrO$_2$)$_n$, where an active bilayer is sandwiched by a thick enough (n=6 is sufficient) band insulating substrate, show semi-metallic Dirac dispersions with a total of four Dirac cones along the $Γ-M$ direction. These become gapped upon the introduction of spin-orbit coupling, giving rise to an insulating ground state comprising four edge states. We …
Topological phases have attracted much interest in recent years. While there are a number of three-dimensional materials exhibiting topological properties, there are relatively few two-dimensional examples aside from the well-known quantum Hall systems. Here we make materials-specific predictions for topological phases using density functional theory combined with Hartree-Fock theory that includes the full orbital structure of the relevant iridum $d$-orbitals and the strong but finite spin-orbit coupling strength. We find Y$_2$Ir$_2$O$_7$ bilayer and trilayer films grown along the [111] direct…
We have investigated the effects of number of layers on the structural, vibrational, electronic and optical properties of $α$-PbO using first principles calculations. Our theoretical calculations have shown that four Raman active modes of $α$-PbO tend to red-shift from bulk to monolayer due to decreasing of force constants and increasing of bond lengths. It has been shown that while bulk and multilayer $α$-PbO have an indirect band gap, monolayer form has a direct band gap value of 2.59 eV. Although lead atoms have 5d states, spin-orbit coupling does not significantly affect the band structure…
