Artificial Superlattices Research Articles

Strain‐Mediated Inverse Photoresistivity in SrRuO3/La0.7Sr0.3MnO3 Superlattices

In the pursuit of novel functionalities by utilizing the lattice degree of freedom in complex oxide heterostructure, the control mechanism through direct strain manipulation across the interfaces is still under development, especially with various stimuli, such as electric field, magnetic field, light, etc. In this study, the superlattices consisting of colossal‐magnetoresistive manganites La0.7Sr0.3MnO3 (LSMO) and photostrictive SrRuO3 (SRO) have been designed to investigate the light‐dependent controllability of lattice order in the corresponding functionalities and rich interface physics. Two substrates, SrTiO3 (STO) and LaAlO3 (LAO), have been employed to provide the different strain environments to the superlattice system, in which the LSMO sublayers exhibit different orbital occupations. Subsequently, by introducing light, we can modulate the strain state and orbital preference of LSMO sublayers through light‐induced expansion of SRO sublayers, leading to surprisingly opposite changes in photoresistivity. The observed photoresistivity decreases in the superlattice grown on STO substrate while increases in the superlattice grown on LAO substrate under light illumination. This work has presented a model system that demonstrates the manipulation of orbital–lattice coupling and the resultant functionalities in artificial oxide superlattices via light stimulus.

Odd-parity superconductivity by competing spin-orbit coupling and orbital effect in artificial heterostructures

We show that odd-parity superconductivity occurs in multilayer Rashba systems without requiring spin-triplet Cooper pairs. A pairing interaction in the spin-singlet channel stabilizes the odd-parity pair-density-wave (PDW) state in the magnetic field parallel to the two-dimensional conducting plane. It is shown that the layer-dependent Rashba spin-orbit coupling and the orbital effect play essential roles for the PDW state in binary and tricolor heterostructures. We demonstrate that the odd-parity PDW state is a symmetry-protected topological superconducting state characterized by the one-dimensional winding number in the symmetry class BDI. The superconductivity in the artificial heavy-fermion superlattice ${\mathrm{CeCoIn}}_{5}/{\mathrm{YbCoIn}}_{5}$ and bilayer interface ${\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}$ is discussed.

Ultrahigh tunnel magnetoresistance using an artificial superlattice barrier with copper and aluminum oxide

This paper proposes an ultrahigh tunnel magnetoresistance that is achieved by a magnetic tunnel junction with an artificial superlattice barrier that is composed of alternate layers of copper and aluminium oxide. By designing proper thickness filling factor of the superlattice barrier, ultrahigh magnetoresistance can be achieved. The tunnel magnetoresistance increases as the number of cells in the superlattice barrier increases. This ultrahigh magnetoresistance effect is attributed to the crystalline property of superlattices, similarly to the high magnetoresistance effect achieved by traditional crystalline MgO. There are more adjustable parameters, such as the lattice constant and the barrier height, in the artificial superlattice barrier than in a traditional crystalline-MgO barrier. This ultrahigh magnetoresistance effect may be used to design spintronic devices.

Organization of Artificial Superlattices Utilizing Nanosheets as a Building Block and Exploration of Their Advanced Functions

This review covers some of the latest developments in the organization of artificial superlattice assemblies utilizing colloidal oxide or hydroxide nanosheets bearing a negative or positive charge, respectively. Various solution-based procedures, e.g., flocculation, electrostatic sequential adsorption, and Langmuir-Blodgett deposition, have been introduced for the self-assembly of 2D nanosheets. Superlattice composites or films integrated with different nanosheets may yield concerted or synergistic modulation, e.g., soft coupling or new electronic states at interfaces. This behavior offers an unprecedented opportunity for the exploration of high-performance devices, as well as advanced or novel functions that cannot be achieved with a single-component material.

Direct observation of electron propagation and dielectric screening on the atomic length scale.

The propagation and transport of electrons in crystals is a fundamental process pertaining to the functioning of most electronic devices. Microscopic theories describe this phenomenon as being based on the motion of Bloch wave packets. These wave packets are superpositions of individual Bloch states with the group velocity determined by the dispersion of the electronic band structure near the central wavevector in momentum space. This concept has been verified experimentally in artificial superlattices by the observation of Bloch oscillations--periodic oscillations of electrons in real and momentum space. Here we present a direct observation of electron wave packet motion in a real-space and real-time experiment, on length and time scales shorter than the Bloch oscillation amplitude and period. We show that attosecond metrology (1 as = 10(-18) seconds) now enables quantitative insight into weakly disturbed electron wave packet propagation on the atomic length scale without being hampered by scattering effects, which inevitably occur over macroscopic propagation length scales. We use sub-femtosecond (less than 10(-15) seconds) extreme-ultraviolet light pulses to launch photoelectron wave packets inside a tungsten crystal that is covered by magnesium films of varied, well-defined thicknesses of a few ångströms. Probing the moment of arrival of the wave packets at the surface with attosecond precision reveals free-electron-like, ballistic propagation behaviour inside the magnesium adlayer--constituting the semi-classical limit of Bloch wave packet motion. Real-time access to electron transport through atomic layers and interfaces promises unprecedented insight into phenomena that may enable the scaling of electronic and photonic circuits to atomic dimensions. In addition, this experiment allows us to determine the penetration depth of electrical fields at optical frequencies at solid interfaces on the atomic scale.

Geometric tuning of charge and spin correlations in manganite superlattices

We report a modulation of the in-plane magnetotransport in artificial manganite superlattice [(NdMnO3)n/(SrMnO3)n/(LaMnO3)n]m by varying the layer thickness n while keeping the total thickness of the structure constant. Charge transport in these heterostructures is confined to the interfaces and occurs via variable range hopping. Upon increasing n, the interfacial separation rises, leading to a suppression of the electrostatic screening between carriers of neighboring interfaces and the opening of a Coulomb gap at the Fermi level (EF). The high-field magnetoresistance (MR) is universally negative due to progressive spin alignment. However, at a critical thickness of n = 5 unit cells (u.c.), an exchange field coupling between ferromagnetically ordered interfaces results in positive MR at low magnetic field (H). Our results demonstrate the ability to geometrically tune the electrical transport between regimes dominated by either charge or spin correlations.

Exploring multiferroic materials based on artificial superlattice LaFeO3-YMnO3 and natural superlattice n-LaFeO3-Bi4Ti3O12 thin films

Combining ferroelectric with antiferromagentic materials in nanometer scale is an effective method for exploring multiferroic materials. We preflent two kinds of systems to show the possibility of multiferroic properties in such nanometer composites. One is the artificial superlattice LaFeO3-YMnO3, and the other is the natural layered Aurivillius material Bi4Ti3O12 doped with different layers of LaFeO3, BiFeO3. Both materials were synthesized by pulsed laser deposition method on SrTiO3 substrates. Microstructural charterizations with XRD, TEM, and EELS in scanning transmission electron microscopy mode substantiate that the samples have atomically sharp interfaces between neighboring layers; this is important for producing possible magneto-electric coupling in multiferroic materials. Magnetic characterization proves that these materials have ferrimagnetic properties, in spite of their anti-ferromagnetic nature before coupling. Magnetic characterization also proves that there is 0.55-0.9 B remanant magnetization generated at LaFeO3-YMnO3 interface. And the 0.5 and 1.5LaFeO3-Bi4Ti3O12 samples show ferrimagnetism which can remain even up to room temperature. Ferroelectric tests prove that there is a large leakage current in LaFeO3-YMnO3 superlattice and BiFeO3-inserted Bi4Ti3O12, but 0.5LaFeO3-Bi4Ti3O12 shows ferroelectric hysteresis loops. It can be therefore concluded that 0.5LaFeO3-Bi4Ti3O12 is a multiferroic material. If more perovskite layers (3-layer SrTiO3 or 2.5-layer LaFeO3) are inserted, the Aurivillius structure of Bi4Ti3O12 may appear structural instability that can be observed in our HRTEM measureflent. Our first principles calculations show that the degeneracy of formation enthalpies is the reason why the intergrowth in these materials forms and their structures are not stable. Our work may provide some examples for exploring new multiferroics by means of nano-meter composite.

Charge transfer across transition-metal oxide interfaces: Emergent conductance and electronic structure

We perform density functional theory plus dynamical mean field theory calculations to investi- gate internal charge transfer in an artificial superlattice composed of alternating layers of vanadate and manganite perovskite and Ruddlesden-Popper structure materials. We show that the elec- tronegativity difference between vanadium and manganese causes moderate charge transfer from VO2 to MnO2 layers in both perovskite and Ruddlesden-Popper based superlattices, leading to hole doping of the VO2 layer and electron doping of the MnO2 layer. Comparison of the perovskite and Ruddlesden-Popper based heterostructures provides insights into the role of the apical oxy- gen. Our first principles simulations demonstrate that the combination of internal charge transfer and quantum confinement provided by heterostructuring is a powerful approach to engineering electronic structure and tailoring correlation effects in transition metal oxides.

Structural and electrical properties of Bi0.5Na0.5 TiO3 based superlattices grown by pulsed laser deposition

Artificial superlattices of ferroelectric Bi0.5Na0.5TiO3 and BaTiO3 have been successfully grown on (001) insulator or conductive SrTiO3 substrates by pulsed laser deposition. In these epitaxial layered structures, the BaTiO3 layers were shown to contribute to an improvement of the two dimensional growth of the Bi0.5Na0.5TiO3 layers. The influence of the superlattice period Λ, between 5 and 20 nm, was investigated. We observe an increase in the in-plane tensile strain as Λ is reduced from 20 nm to 10 nm, accompanied by a decrease in the density of dislocations. A concomitant enhancement of the dielectric permittivity was measured, demonstrating the strain tunability of these superlattices. A significant reduction of the dielectric losses is also obtained with decreasing Λ. Furthermore, a minimum value of the coercive field of less than 70 kV/cm, close to that of Bi0.5Na0.5TiO3 bulk ceramics, was reached by decreasing the period. We demonstrate that the dielectric and ferroelectric properties can be explained as the result of the in-plane tensile strain which contributes to improve the structural properties in the superlattices.

X-Ray, Dielectric, and Ferroelectric Properties of La0.67Sr0.33MnO3/ Pb(Zr0.53Ti0.47)0.60(Fe0.5Ta0.5)0.40O3 Artificial Multilayers and Superlattices structures

Superlattice (SLs) is an artificial periodic structure consisting of alternate layers of two different material. Multiferroic SLs exhibit superior properties like low loss, high dielectric constant, very good ferroelectric and magnetic properties which make them attractive candidates for thin film devices applications. Multilayers (MLs) and SLs of the colossal magnetoresistive La0.67Sr0.33MnO3 (LSMO) and low loss multiferroic Pb(Zr0.53Ti0.47)0.60(Fe0.5Ta0.5)0.40O3 (PZTFT) materials have been grown by pulsed laser deposition on (100) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT) substrate with LSMO or LaNiO3 as bottom electrode. Depositions were performed at 600 oC in 150 mTorr oxygen pressure with an energy density of ~3 J/cm2, and a laser repetition rate of 5 Hz. X-ray diffraction revealed well oriented (00l) perovskite structure in all structures, in case of MLs two peaks correspond for LSMO and PZTFT were observed, while in SLs the so-called satellite peaks typical of modulated structures were present. The computed modulation period (Λ) by Bragg’s condition revealed that the SLs structure was observed below of Λ ~ 20 nm. The atomic Force Microscopy studies showed a systematic decrease in grain size with increase in number of layers in PZTFT/LSMO structures. All thin film structures were found to be densely packed, smooth and free of cracks. Piezo force microscopy showed that most of the ferroelectric domains are switching under ± 9 V external bias field in both MLs and SLs films. PZTFT/LSMO bilayer strcuture showed a dielectric constant of ~835 at 1kHz, while the ferroelectric loop showed saturation and remanent polarization of ~ 34 μC/cm2 and 6 μC/cm2 respectively, with a coercive field of 60 kV/cm. The saturation magnetization of LSMO/PZTFT structure grown on LSMO/LSAT was found to be ~45 emu/cm3. A systematic study of the effect of modulation period on dielectric, ferroelectric and magetic properties of the MLs and SLs structures will be discussed.

Magnetic edge anisotropy in graphenelike honeycomb crystals.

The independent predictions of edge ferromagnetism and the quantum spin Hall phase in graphene have inspired the quest of other two-dimensional honeycomb systems, such as silicene, germanene, stanene, iridates, and organometallic lattices, as well as artificial superlattices, all of them with electronic properties analogous to those of graphene, but a larger spin-orbit coupling. Here, we study the interplay of ferromagnetic order and spin-orbit interactions at the zigzag edges of these graphenelike systems. We find an in-plane magnetic anisotropy that opens a gap in the otherwise conducting edge channels that should result in large changes of electronic properties upon rotation of the magnetization.

Molecular-scale heteroassembly of redoxable hydroxide nanosheets and conductive graphene into superlattice composites for high-performance supercapacitors.

Artificial superlattice nanocomposites are successfully prepared by electrostatic heteroassembly of redoxable Co-Al or Co-Ni layered double hydroxide (LDH) nanosheets with graphene. The superlattice electrodes exhibit a high capacity up to ca. 650 F/g, which is approximately 6 times that of pure graphene. The composites are found to be capable of superfast charging and discharging, up to ca. 100 Hz, comparable with the high-power performance of graphene electrodes.

Tc up to 50 K in superlattices of insulating oxides

Here we report a structural and electronic investigation of the recently discovered CaCuO2/SrTiO3 superconducting artificial superlattices based on two insulating oxides. We show that Tc = 50 K is reached in the superlattices grown on NdGaO3 substrate under optimized growth conditions. If the superlattices are grown on LaAlO3 substrate the Tc is drastically reduced due to the much higher lattice mismatch between CaCuO2 and LaAlO3, which has lattice parameters substantially smaller than NdGaO3. The consequent in-plane lattice contraction also makes the incorporation of extra oxygen atoms more difficult, leading to a decrease of hole doping and Tc. Finally, x-ray absorption spectroscopy study as a function of the thickness of the CaCuO2 block in CaCuO2/SrTiO3 superconducting superlattices has revealed the important role played by the interfaces as the location of hole doping and superconductivity in these systems.

Artificial Solids by Design: Assembly and Electron Microscopy Study of Nanosheet-Derived Heterostructures

Two-dimensional materials do not only attract interest owing to their anisotropic properties and quantum confinement effects but also lend themselves as well-defined building blocks for the rational design of 3D materials with custom-made structures and, hence, properties. Here, we present the bottom-up fabrication of an artificial superlattice derived from positively charged layered double hydroxide (LDH) and negatively charged perovskite layers sequentially assembled by electrostatic layer-by-layer deposition. In contrast to previously employed bulk methods averaging out the elemental distribution within such stacks, we use a combination of HRTEM, STEM, and EEL spectroscopy to elucidate the structure and composition of the multilayer stack with a high spatial resolution on the subnanometer scale. Atomic column resolved STEM coupled with EELS line scans confirms the periodic arrangement of individual nanosheets by evaluation of the Ca-L2,3 and Mn-L2,3 edges. Furthermore, HRTEM confirms the formation of u...

Enhanced electrical and ferroelectric properties in a multiferroic (BiFeO3/Bi0.5Na0.5TiO3)3/LaNiO3 superlattices structure

Artificial multiferroic superlattices (SL), consisting of BiFeO3 (16 nm)/Bi0.5Na0.5TiO3 (5 nm) (BFO/BNT SL), were grown on (001) SrTiO3 single crystal by pulsed laser deposition method. The cross-sectional, surface morphology, and crystallographic structure of BFO/BNT SL and BFO single layer were investigated. It was found that the electrical, ferroelectric, and magnetic properties of BFO/BNT SL exhibit a remarkably enhancement compared with BFO single layer. The influence of BNT buffering layer, lattice strain, and interfaces interplay of the SL structure are supposed to benefit their ferroelectric and ferromagnetic properties. Our works suggested the BFO/BNT SL with an improved multiferroic characteristics have a promising application for the future informational storage devices.

Compositional engineering of BaTiO3/(Ba,Sr)TiO3 ferroelectric superlattices

Epitaxial strain is one of the major factors influencing physical properties of artificial superlattice (SL) structures. One way to control the local stress in epitaxial films is altering the lattice parameters by doping. Superlattices of BT/Ba(1−x)SrxTiO3 (BT/BST) with x = (0, 0.3, 0.4, 0.5, 0.6, 0.7, 1) with a modulation period of about 80–90 Å were grown on La0.67Sr0.33MnO3 coated (100) MgO substrates by pulsed laser deposition technique. The modulated structure of the thin films was confirmed by x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The increase in Sr concentration in BST layers of the SLs results in an increase in in-plane (a) and out-plane (c) compressive/tensile misfit strains in the BT/BST layers, respectively. The highest value of the dielectric constant was obtained for BT/Ba0.3Sr0.7TiO3 (BT/BST3070) and BT/Ba0.7Sr0.3TiO3 (BT/BST7030) SLs. Slim, asymmetric, but well-saturated ferroelectric hysteresis loops were observed in all SLs. Additionally, BT/BST SLs exhibited exceptionally high electric field stress sustainability over a wide range of frequency (10 kHz) and temperature (80–350 K). Temperature-dependent dielectric and ferroelectric properties show a ferroelectric relaxor behavior when the Sr content is increased in the BST layer of the SLs. Analysis of polarization versus temperature data using Landau-Devonshire theory suggests a second-order ferroelectric phase transition in these SLs. This structure can therefore be attractive in the design of a new kind of dielectric device capable in both high power and high energy density applications.

Electric control of the magnetization in BiFeO3/LaFeO3superlattices

First-principles techniques are used to investigate the behavior of BiFeO$_{3}$/LaFeO$_{3}$ perovskite oxide superlattices epitaxially grown on a (001)-SrTiO$_3$ substrate. The calculations show that 1/1 superlattices exhibit a $Pmc2_1$ ground state combining a trilinear coupling of one polar and two oxygen rotational lattice modes, and weak ferromagnetism. The microscopic mechanism allowing one to manipulate the magnetization with an electric field in such systems is presented and its dependence on strain and chemical substitution is discussed. BiFeO$_{3}$/LaFeO$_{3}$ artificial superlattices appear to be good candidates to achieve electric switching of magnetization at room temperature.

Superconducting anisotropy in (CaCuO2)n/(SrTiO3)m superlattices

The superconducting properties of (CaCuO2)n/(SrTiO3)m artificial superlattices have been investigated via transport measurements under the application of external magnetic fields. The coherence lengths in the plane of the substrate and in the direction perpendicular to it (ξab and ξc, respectively) have been measured while varying m, the thickness of the SrTiO3 block. The results show that with increasing m, i.e. with increasing structural anisotropy, the superconducting anisotropy γ = ξab/ξc decreases. This apparent anomalous relation between the structural and the superconducting anisotropies suggests that γ is more affected by local doping at the interface rather than by the separation between the superconducting blocks. This interpretation of the experimental results has been confirmed by both the irreversibility lines and the magnetic field dependence of the activation energy for fluxon motion.

Complex-Stripe Phases Induced by Staggered Rashba Spin–Orbit Coupling

We study superconducting phases in a quasi-two-dimensional multilayer system without local inversion symmetry. Broken local inversion symmetry induces layer-dependent Rashba-type spin–orbit couplings. We find that a complex-stripe phase, which is the intermediate phase between the Fulde–Ferrell (FF) phase and Larkin–Ovchinnikov (LO) phase, is realized in the magnetic field applied parallel to the layers. A crossover from the FF phase to the LO phase appears by tuning temperature and magnetic field. We show the local density of states that characterizes the complex-stripe phase. As a possible realization of the complex-stripe phase, we discuss the artificial superlattices of CeCoIn5.

Naturally Oxidized FeCo as a Magnetic Coupling Layer for Electrically Isolated Read/Write Paths in mLogic

Recently, a nonvolatile, low power circuit scheme based on current-induced domain wall motion and perpendicular magnetic tunnel junctions known as mLogic has been proposed that requires electrically isolated, magnetically coupled read and write paths. Here, we suggest naturally oxidized FeCo for the magnetic coupling layer. Relevant properties of the FeCo-oxide were evaluated by a preliminary investigation of [FeCo/FeCo-Oxide] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> artificial superlattices. It is found that FeCo thin films form an insulating 11 Å passivating oxide layer with a magnetization of 500 emu/cc. Experimental measurements show that FeCo-oxide can couple two perpendicular FeCoB layers with a coupling strength greater than 0.35 ergs/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .