Oxide Superlattices Research Articles

First-Principles Study of Ferroelectric Oxide Epitaxial Thin Films and Superlattices: Role of the Mechanical and Electrical Boundary Conditions

In this review, we propose a summary of the most recent advances in the first-principles study of ferroelectric oxide epitaxial thin films and multilayers. We discuss in detail the key roles of mechanical and electrical boundary conditions, providing to the reader the basic background for a simple and intuitive understanding of the evolution of the ferroelectric properties in many nanostructures. Going further we also highlight promising new avenues and future challenges within this exciting field or researches.

Self-Assembly Synthesis and Electromagnetic Peculiarities of 2-D Transition Metal Oxide Superlattices with Molecular Assembly Template

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Revealing latent structural instabilities in perovskite ferroelectrics by layering and epitaxial strain: A first-principles study of Ruddlesden-Popper superlattices

Utilizing first-principles computational techniques, we have mapped out structural instabilities in the Ruddlesden-Popper homologous oxide superlattice families with a general chemical formula ${A}_{n\ensuremath{-}1}{A}_{2}^{\ensuremath{'}}{\text{Ti}}_{n}{\text{O}}_{3n+1}$, $A=\text{Sr}$, Ba, and Pb (perovskite-type slab) and ${A}^{\ensuremath{'}}=\text{Sr}$ (rocksalt-type insert) for $n=1--5$. Our calculations show that each superlattice family has a unique set of ``instability footprints,'' combining the ferroelectric, antiferroelectric, and antiferrodistortive types, and that the competition among the instabilities can be influenced by epitaxial strain and changing thickness of the perovskite-type slab, granting us wide flexibility to fine tune the properties of these materials for various device applications.

First-principles effective Hamiltonian for ferroelectric polarization inBaTiO3/SrTiO3 superlattices

We present an effective Hamiltonian for the description of ferroelectric polarizations in perovskite oxide superlattices. To understand the ferroelectric behavior of (BaTiO3)n/(SrTiO3)m superlattices, we constrained the local distortion modes along the c direction only and set up the effective Hamiltonian based on the local modes that capture the physics of long-wavelength acoustic modes (strain) and lowest energy transverse optical phonon modes (soft modes) as prescribed by the localized Wannier functions. All the parameters in this effective Hamiltonian were predetermined from the first-principles density-functional theory calculations of each BaTiO3 and SrTiO3 components. As an application of the model parameters, we calculated the polarizations of (BaTiO3)n/(SrTiO3)m with n+m=5, the results of which are in good agreement with those of the previous first-principles calculations of average polarizations as well as local polarizations. This effective Hamiltonian procedure can provide guidance for developing ferroelectric model of other kinds of oxide superlattices.

Atomically flat single-terminated SrTiO3 (111) surface

The authors have reproducibly obtained an atomically well-defined SrTiO3 (111) surface by a combined chemical etching and thermal annealing process. Although thermodynamic mixed termination is preferred as a means of suppressing the surface dipole, the kinetics-driven etching process, via selective etching of SrO34−, enables a single-terminated surface to be obtained. Subsequent O2 annealing of the etched surface produces a clear step-and-terrace structure. Atomically flat terraces and only one-unit-cell-high step edges are observed, signifying a single-terminated surface. This study might pave the way for constructing (111)-oriented perovskite oxide superlattices, which would be expected to demonstrate new and better physical phenomena and functionalities.

Towards Interface Studies by Cs-Corrected STEM

Interfaces in ceramics play an important role on the various properties. It has been known that the addition of small amount of dopants strongly improve the mechanical and functional properties in polycrystalline ceramics. Z-contrast images obtained by scanning transmission electron microscopy (STEM) is powerful technique to experimentally determine the location of the dopants segregated at grain boundaries and interfaces. As the image intensity in the Z-contrast is approximately proportional to the square of the atomic number, STEM technique is especially well suited for understanding the role of heavy impurities in grain boundaries and interfaces composed of much lighter ions. In this review paper, our recent results obtained for ceramic grain boundaries and interfaces by Cs-corrected STEM are introduced. Several examples are demonstrated for the gain boundaries of varistor and structural ceramics, dislocation in alumina, oxide superlattice, lithium battery and so on. These results indicate that the atomistic mechanism of the properties can be unraveled by the combination of STEM characterization and the first principles calculations. This approach will be a breakthrough for new materials science and engineering in the next generation.

Oxide Superlattices

Oxide Superlattices

Optical Magnetoelectric Effect of Patterned Oxide Superlattices with Ferromagnetic Interfaces

Nonreciprocal directional dichroism, termed the optical magnetoelectric (OME) effect, has been observed in patterned superlattice (SL) composed of perovskite oxides, LaMnO3, SrMnO3, and LaAlO3. Such a tricolor SL with ferromagnetic interfaces is expected to artificially break both space-inversion and time-reversal symmetries and hence to show the OME effect. The Bragg diffraction from the grating structure with a period of 4 microm fabricated on the SL was employed to sensitively detect the OME effect, yielding the relative change of the diffracted light intensity (~0.2%-0.5%) upon a reversal of either the in-plane magnetization or the propagation vector of the diffracted light.

Ferroelectricity in Artificial Bicolor Oxide Superlattices

We report on the growth and properties of high quality bicolor oxide superlattices, composed of two perovskites out of BaTiO{sub 3}, CaTiO{sub 3}, and SrTiO{sub 3}. The artificially grown superlattices are structurally unique and have a macroscopically homogeneous phase, which is not feasible to recreate in bulk form. By artificial structuring, it is found that the polarization of such superlattices can be highly increased as compared to pseudo-binary ceramics with the same overall composition. Such strong enhancement in superlattice is attributed to newly-developed ionic motions of A-site cations at the hetero-interfaces due to the interfacial coupling of electrostatic and elastic interactions, which cannot be found in single phase materials.

Optical magnetoelectric effect of patterned oxide superlattices with ferromagnetic interfaces

This is a copy of the slides presented at the meeting but not formally written up for the volume. Recent development of pulsed laser deposition technique makes it possible to build up different kinds of perovskite oxides and to create new electric and magnetic properties of heterointerfaces, which can neither be realized in bulk properties of the constitute oxides and nor be treated as the simple combination of them. The extraction of unique properties of interfaces thus crucially relies on the development of a new method to selectively detect their electronic properties accompanied with magnetism. Here we show a new approach with use of the optical magnetoelectric (ME) effect to address the unique properties of ferromagnetic oxide superlattices (SLs). The ME effect, that is, the control of the polarization P by a magnetic field H or inversely the control of the magnetization M by an electric field E, can be considered as a typical manifestation of cross-correlation phenomena in solids. Even at optical frequencies, such a cross-correlation response coming from the ME effect is known to show up in materials with a lack of both space-inversion and time-reversal symmetries. This is referred to as the optical ME effect. The optical ME response emerges as a change of reflection and transmission when the wavevector k of light is set to parallel or antiparallel to the toroidal moment T defined as P X M, which in turn enables us to control the intensity of light by changing the directions of E and/or H. In this work, we fabricated SLs composed of LaMnO3, SrMnO3, and LaAlO3 and exploited the use of their unique properties of heterointerfaces as a medium for optical ME effect. Such ’tricolor’ SLs are expected to artificially break both space-inversion and time-reversal symmetries, which are induced by asymmetric stacking sequences of three different oxides and by the charge-transfer-induced magnetism at LaMnO3/SrMnO3 interfaces, respectively. We patterned the grating structure with a period of 4 im on SLs and employed the Bragg diffraction geometry to sensitively detect the optical ME effect. The optical ME effect was clearly observed when the diffracted light was used as a probe. The optical ME response depending on PABC X Minterface is a direct consequence of the symmetry breaking at interfaces. Its magnitude per interface was thus estimated to be tilde operator = varies with = similar to 0.01% in H of 2 kOe, which are relatively large as compared to previously reported values in bulk materials showing the optical ME effect. Our data provide that the present method would be used as a tool for the study of oxide heterointerfaces.

New electronic structure in short-period complex oxide superlattices

This is a copy of the slides presented at the meeting but not formally written up for the volume. At heterojunctions between different oxide perovskite phases both lattice and electronic structure is modified by the junction. One interesting question that several groups have studied is just how far into the neighboring materials these perturbations extend. We have studied this for insulating phases as well as conducting phases. For insulating phases it appears that the lattice distortions are healed in a layer about one unit cell thick. By stacking different materials each of which is only a single unit cell thick we have obtained materials that exhibit new properties determined by the stacking architecture. For example, superlattices that lack inversion symmetry have a built-in polarization that is controlled by the direction of the strain asymmetry. For conducting phases, the electronic structure also seems to be modified mainly in a layer only a few unit cells thick. We have studied this in superlattices of SrTiO3 and LaMnO3 in which we vary the thickness of the layers. We use optical conductivity to probe the electronic structure in the near infrared to near ultraviolet spectral region. The conductivity is close to the average of the two constituents, but differs in certain spectral regions, especially for the films with the thinnest supercells.This work was supported by the Department of Energy Basic Energy Sciences program at the Fredrick Seitz Materials Research Laboratory at the University of Illinois, Urbana, IL.

Interfaces in semiconductor/metal radial superlattices

Semiconductor/metal radial superlattices are produced by the roll-up of inherently strained InGaAs∕Ti∕Au as well as InAlGaAs∕GaAs∕Cr films. Cross sections of the obtained structures are prepared and investigated in detail by diverse transmission electron microscopy as well as microanalysis techniques. Special attention is paid to the interfaces of the semiconductor/metal hybrid superlattice. The study reveals amorphous, noncrystalline layers for the semiconductor/metal as well as for the metal/semiconductor interface. The chemical analysis suggests that the observed interlayers are oxides giving rise to a semiconductor/oxide/metal/oxide superlattice rather than a pure semiconductor/metal superlattice.

Structural and magnetic properties of La 2/3Ca 1/3MnO 3/LaNiO 3 artificial superlattices prepared by rf magnetron sputtering

Magnetic oxide superlattices consisting of ferromagnetic oxide La 2/3Ca 1/3MnO 3 (LCMO) and paramagnetic oxide LaNiO 3 (LNO) have been successfully grown on SrTiO 3 (STO) substrate by rf magnetron sputtering. Measurements of X-ray reflectivity and high-resolution X-ray diffraction were employed to characterize the microstructure of these films. An azimuthal scan about the surface Bragg peak of the film shows the epitaxial relationship between the film and the substrate. The clearly discernible main feature and satellite features observed in the (002) crystal-truncation-rod indicate the high quality of the LCMO/LNO artificial superlattice structure formed on a SrTiO 3 substrate. Hysteresis measurements show magnetic properties to be highly correlated with interfacial strain between the substrate and the film: the smaller is the LNO spacer thickness, the larger is the in-plane lattice strain of the films present, and the larger is the ratio of remnant magnetization to saturation magnetization and the magnetic coercivity field.

Oxide superlattices for multiferroics: opportunities, issues, and challenges

Multiferroics form a rare class of materials, which simultaneously display various extraordinary properties that are usually absent in the classical case. Consequently, they are potential candidates for various novel devices which are unachievable by the conventional materials utilized for device applications. In this letter, we detail the alternative methodology to design an artificial multiferroic by the superlattice approach, which will circumvent the looming problem of their rarity. Details of the way of selecting the various superlattice components, controlling their properties, the role of interfaces, and the methodology of inducing the coupling between distinct orders are presented. Furthermore, we also discuss the various limitations, issues, and challenges in designing such heterostructures.

Enhanced Phonon Scattering in Oxide Superlattices to Improve Laser Induced Thermoelectric Voltages

AbstractOptimizing the figure of merit for thermoelectric applications, ZT = S2σ2T/κ is currently at the core of materials oriented research in thermoelectricity. Here, one promising approach is to reduce ther thermal conductivity without sacrificing the electrical conductivity. Constructing superlattices of structurally compatible materials is one way to accomplish this goal. We report an enhanced laser induced thermoelectric voltage (LITV) effect observed in (YBa2Cu3O7/La1-xPbxMnO3)n multilayer thin films for the first time. Two groups of multilayer thin films grown on vicinal cut LaAlO3 substrates were prepared by pulsed laser deposition technique. The first group were grown on different substrates vicinal cut at different angles, and were used for checking the mechanism of the induced voltages. The second group samples were made at different period number n and for studying the number dependence of the peak values of LITV. The substrate angle dependence proved that this is a thermoelectric effect [1]. It was found that the LITV signals were enhanced significantly for these multilayer thin films comparing with the single layer ones. It is natural that the conductivity is going to be anisotropic due to the layered structure, and the same holds for the Seebeck coefficients. The enlarged Seebeck anisotropy will lead to higher induced voltages. Another possible reason is the reduced thermal conductivity in the layered structure. The maximum enhancement of LITV signals takes place at period number of 7, which seems in agreement with the prediction of minimum thermal conductivity in superlattices by Simkin and Mahan.

Study on interfacial strain behavior of functional oxide heterostructures

Oxide films of BaTiO3 and MgO were deposited on SrTiO3 single crystal substrates by laser molecular beam epitaxy. Reflection high-energy electron diffraction was performed in situ to investigate the change of growth mode and the lattice relaxation. Due to the different mismatches between epitaxial oxide films and substrates, two kinds of strain relaxation behavior during the growth process can be observed. For the BaTiO3∕SrTiO3 system with the small mismatch of 2.18%, the coherent epitaxial growth can be maintained within the critical thickness. The experimental data were well in accord with the elastic strain theory in the small mismatch system. The coherent epitaxial growth is crucial to fabricate the functional oxide superlattice. However, strained island can form at the initial period to release strain energy in the case of MgO∕SrTiO3 system with the large mismatch of 7.8%. It provides a useful practical method to realize self-organized nanostructures by introducing the moderate strain at interface. Our systematic study on the interface strain of functional oxide heteroepitaxial growth could give an instructive method to realize different heterostructures, e.g., superlattice and nanostructures.

Charge compensation and mixed valency inLaAlO3∕SrTiO3heterointerfaces studied by the FLAPW method

The electronic structures and properties of heterointerfaces in the perovskite oxide superlattices ${\mathrm{LaAlO}}_{3}∕{\mathrm{SrTiO}}_{3}\phantom{\rule{0.3em}{0ex}}[001]$ are presented using the first-principles all-electron full-potential linearized augmented plane-wave (FLAPW) method. Superlattices with three types of interfaces, (i) electron-doped, (ii) hole-doped, and (iii) both electron- and hole-doped, are studied and compared. For the electron-doped interface, the mixed valency of Ti along with the Jahn-Teller effect are found to explain the metallicity, in agreement with experiment, whereas for the hole-doped interface metallicity is found, in contrast to experiment. Oxygen vacancies introduce an additional $n$-type carrier to compensate the holes present at the interface, which now becomes insulating and agrees well with experiment. For a system with both types of interfaces, the metallicity found for the unrelaxed structure is changed to insulating after relaxation is included. For all cases, the relaxation results in a complicated buckled geometry, which is a good indication of the adjustment due to polarity discontinuity. Our findings support recent experimental results, namely, (i) the mixed-valence character of Ti at the electron-doped interface, and (ii) the existence and importance of oxygen vacancies at the hole-doped interface.

Magnetotransport Properties in Epitaxial Exchange-Biased La$scriptstyle_2/3$Ca$scriptstyle_1/3$MnO$scriptstyle_3$/La$scriptstyle_1/3$Ca$scriptstyle_2/3$MnO$scriptstyle_3$Superlattices

Superlattices of ferromagnetic (F) and antiferromagnetic (AF) oxide materials have attracted increased attention given the exchange bias or exchange anisotropy phenomenon. Research on magnetic oxide superlattices has focused toward the technological goals of increasing magnetoresistance in lower applied magnetic fields and more useful temperature ranges for various applications. Superlattices of F-LCMO and AF-LCMO layers were grown on (001)-oriented SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrates. We studied the temperature dependence of magnetotransport properties for a series of [AF-LCMO (7.6 nm)/F-LCMO (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> )] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> superlattices as a function of the ferromagnetically doped layer thickness, t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> , while the antiferromagnetic (AF) layer thickness, t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> , was kept constant. Magnetotransport measurements at different temperatures were done on a series of [AF(7.6 nm)/F(t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> )] <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> superlattices. The ZFC resistance has a maximum at negative fields, for all multilayers, with descending fields (from 1 to -1 kOe). In all samples a shift along the field axis of the FC loop with respect to that of the ZFC loop is observed. The H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> values determined from these displacements of the FC loops of the magnetoresistance loops increase when temperature decreases and its magnitude also depends on the ferromagnetic layer thickness

Physical Properties of Strained Artificial Superlattices of ( La0.7Ba0.3 ) MnO3 ∕ LaNiO3

Oxide superlattices composed of ferromagnetic (LBMO) and paramagnetic (LNO) were grown on (001) substrates with dual-gun radio-frequency magnetron sputtering. The (00L) crystal-truncation-rod intensity profiles confirm the formation of a superlattice structure. The LBMO layer in the artificial superlattice structure is under biaxial compressive stress, whereas the LNO layer is under biaxial tensile stress. Experiments involving X-ray scattering, absorption, and atomic-force microscopy provide structural information about the designed superlattices. The observed Curie temperature of LBMO epi layers exhibits a strong correlation with lattice strain. All superlattices are coupled ferromagnetically at low temperature.

Electrical properties of SrTiO 3/(Sr 1− x,La x)TiO 3 superlattices grown by laser molecular beam epitaxy

We have successfully fabricated SrTiO 3/(Sr 1− x ,La x )TiO 3 (STO/SLTO) superlattices on step-terrace structured SrTiO 3, in which La defect is distributed in a regular manner of one dimension. The superlattices contain one unit cell thick SLTO layer with various stacking sequences. The charge carrier density of the superlattices was in the range of 1.7 × 10 19–2 × 10 16 cm − 3 . In addition, the mobility and resistivity of STO/SLTO superlattice were 11 cm 2/Vs and 0.18 Ω cm at the STO 1/SLTO 1 stacking sequence, respectively. The charge carrier density of STO/SLTO superlattices increased significantly with the change in a stacking sequence, while the mobility is relatively insensitive to the change in the stacking sequence. The resulting resistivity of the superlattics changed significantly with the change in the stacking sequence due to mainly the change of the charge carrier density. The oxide superlattice approach provides an useful way of nonlinear control of charge carrier density and electrical resistivity.