Biaxial Epitaxial Strain Research Articles

Functional substrates – a novel approach to tailor transport properties and flux-line pinning in YBa2Cu3O7 – x thin films

Abstract A concept is introduced, using substrates for high-temperature superconductor (HTS) thin-film deposition beyond their usual application as chemical inert, lattice-matched support for the films. The substrates are applied as functional elements in order to controllably modify the growth mode of YBa2Cu3O7– x thin films in the case of vicinal cut SrTiO3 single crystals and, thus, generate regular arrays of pinning centers. Furthermore, the use of biaxial epitaxial strain, caused by lattice misfit of film and substrate, is briefly reviewed for HTS thin films and the application of this concept to tailor the properties of perovskite thin films in general is shown.

Engineering polar vortex from topologically trivial domain architecture

Topologically nontrivial polar structures are not only attractive for high-density data storage, but also for ultralow power microelectronics thanks to their exotic negative capacitance. The vast majority of polar structures emerging naturally in ferroelectrics, however, are topologically trivial, and there are enormous interests in artificially engineered polar structures possessing nontrivial topology. Here we demonstrate reconstruction of topologically trivial strip-like domain architecture into arrays of polar vortex in (PbTiO3)10/(SrTiO3)10 superlattice, accomplished by fabricating a cross-sectional lamella from the superlattice film. Using a combination of techniques for polarization mapping, atomic imaging, and three-dimensional structure visualization supported by phase field simulations, we reveal that the reconstruction relieves biaxial epitaxial strain in thin film into a uniaxial one in lamella, changing the subtle electrostatic and elastostatic energetics and providing the driving force for the polar vortex formation. The work establishes a realistic strategy for engineering polar topologies in otherwise ordinary ferroelectric superlattices.

Optical properties of LaNiO3 films tuned from compressive to tensile strain

Materials with strong electronic correlations host remarkable -- and technologically relevant -- phenomena such as magnetism, superconductivity and metal-insulator transitions. Harnessing and controlling these effects is a major challenge, on which key advances are being made through lattice and strain engineering in thin films and heterostructures, leveraging the complex interplay between electronic and structural degrees of freedom. Here we show that the electronic structure of LaNiO3 can be tuned by means of lattice engineering. We use different substrates to induce compressive and tensile biaxial epitaxial strain in LaNiO3 thin films. Our measurements reveal systematic changes of the optical spectrum as a function of strain and, notably, an increase of the low-frequency free carrier weight as tensile strain is applied. Using density functional theory (DFT) calculations, we show that this apparently counter-intuitive effect is due to a change of orientation of the oxygen octahedra.The calculations also reveal drastic changes of the electronic structure under strain, associated with a Fermi surface Lifshitz transition. We provide an online applet to explore these effects. The experimental value of integrated spectral weight below 2 eV is significantly (up to a factor of 3) smaller than the DFT results, indicating a transfer of spectral weight from the infrared to energies above 2 eV. The suppression of the free carrier weight and the transfer of spectral weight to high energies together indicate a correlation-induced band narrowing and free carrier mass enhancement due to electronic correlations. Our findings provide a promising avenue for the tuning and control of quantum materials employing lattice engineering.

A Partial Anion Disorder in SrVO2H Induced by Biaxial Tensile Strain

SrVO2H, obtained by a topochemical reaction of SrVO3 perovskite using CaH2, is an anion-ordered phase with hydride anions exclusively at the apical site. In this study, we conducted a CaH2 reduction of SrVO3 thin films epitaxially grown on KTaO3 (KTO) substrates. When reacted at 530 °C for 12 h, we observed an intermediate phase characterized by a smaller tetragonality of c/a = 0.96 (vs. c/a = 0.93 for SrVO2H), while a longer reaction of 24 h resulted in the known phase of SrVO2H. This fact suggests that the intermediate phase is a metastable state stabilized by applying tensile strain from the KTO substrate (1.4%). In addition, secondary ion mass spectrometry (SIMS) revealed that the intermediate phase has a hydrogen content close to that of SrVO2H, suggesting a partially disordered anion arrangement. Such kinetic trapping of an intermediate state by biaxial epitaxial strain not only helps to acquire a new state of matter but also advances our understanding of topochemical reaction processes in extended solids.

Mott insulator to metal transition driven by oxygen incorporation in epitaxial LaTiO3 films

Lanthanum titanate, LaTiO3, is an antiferromagnetic Mott insulator with a Ti 3d1 electronic configuration and exhibits an intriguing metallic state in its epitaxial film grown on the SrTiO3 substrate. Here, we explore the driving force of the Mott insulator to metal transition in LaTiO3 epitaxial films by a systematic study of the film growth conditions and biaxial strain using reactive molecular beam epitaxy. Within the achievable range (up to −2.4%) of the biaxial compressive strain in our study, we found that the oxygen incorporation plays a more crucial role than the biaxial epitaxial strain in the Mott insulator to metal transition in LaTiO3 films.

Manipulating superconductivity in ruthenates through Fermi surface engineering

The key challenge in superconductivity research is to go beyond the historical mode of discovery-driven research. We put forth a new strategy, which is to combine theoretical developments in the weak-coupling renormalization group approach with the experimental developments in lattice strain driven Fermi surface-engineering. For concreteness we theoretically investigate how superconducting tendencies will be affected by strain engineering of ruthenates' Fermi surface. We first demonstrate that our approach qualitatively reproduces recent experiments under uniaxial strain. We then note that order few $\%$ strain readily accessible to epitaxial thin films, can bring the Fermi surface close to van Hove singularity. Using the experimental observation of the change in the Fermi surface under biaxial epitaxial strain and ab-initio calculations, we predict ${\rm T}_{\rm c}$ for triplet pairing to be maximized by getting close to the van Hove singularities without tuning on to the singularity.

(Invited) Ab Initio Studies of Strain Effects on Perovskite Oxygen Vacancy Formation and Migration Energetics

Oxygen active materials are capable of rapidly transporting oxygen and exchanging it with the environment and have a wide range of applications, including solid oxide fuel cells, gas separation membranes, oxygen sensors, chemical looping devices, and memristors. Recent work has shown that epitaxial strain can play a significant role in altering the transport of oxygen through oxygen active materials, particularly in fluorite structured compounds. However, transition metal perovskites are an important class of oxygen active materials that have received only limited attention. In this work we use ab initio methods to model the coupling of biaxial epitaxial strain with oxygen vacancy formation and migration energetics in a range of transition metal perovskites. We focus on materials of the form LaBO3, where B = [Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga]. We predict the formation volume and strain response of oxygen vacancy formation energy, demonstrating that non-linear coupling of strain and vacancy formation can yield responses qualitatively different than those expected from simple elasticity arguments. In particular, as first observed in Ref. [1], we show that in some cases the vacancy can weaken the bonding and lower the elastic constants, which allows the formation of a vacancy to reduce the strain energy. This effect leads to stabilization of the vacancy under both compressive and tensile strain, even though the vacancy has positive formation volume. We also predict the migration volume and the strain response of oxygen migration energetics, here finding that values are quite consistent with a simple elastic strain model.[2] We find that tensile (compressive) biaxial strain reduces (enhances) the oxygen vacancy migration barrier approximately linearly across the systems studied (Figure 1). The average decrease in migration energy is 66 meV per percent strain for a single selected hop, with a low of 36 and a high of 89 meV decrease in migration barrier per percent strain across all the systems studied. In general we find that the oxygen transport kinetics can be altered by orders of magnitude in perovskites through strain effects on both vacancy content and migration energetics. These effects could play a significant role on materials performance at interfaces and in thin-film devices. [1] W. Donner, C. L. Chen, M. Liu, A. J. Jacobson, Y. L. Lee, M. Gadre, and D. Morgan, Epitaxial Strain-Induced Chemical Ordering in La0.5Sr0.5CoO3-delta Films on SrTiO3, Chemistry of Materials 23, p. 984-988 (2011). [2] T. Mayeshiba and D. Morgan, Strain Effects on Oxygen Migration in Perovskites, Phys. Chem. Chem. Phys. 17, p. 2715-2721 (2015 ). Figure 1

Geometric ferroelectricity in fluoroperovskites

The authors analyze the unusual origin of the ferroelectric instability in ABF3 fluoroperovskites using first-principles calculations, and predict that a member of this family, NaMnF3, will become multiferroic under biaxial epitaxial strain.

Tuning the magnetic anisotropy in LSMO manganite films through non-magnetic nanoparticles

We report the observation of anomalous magnetic anisotropy driven by nonmagnetic ZrO2 nanoparticles in epitaxial La2/3Sr1/3MnO (LSMO) films grown on LaAlO3 (LAO) substrates. The compressive epitaxial strain imposed by the lattice mismatch of substrate and film is tuned by the density of ZrO2 nanoparticles embedded in the film matrix and affects the magnetic anisotropy as well as the magnetotransport properties.Epitaxial 54nm thick LSMO thin films with different concentrations of ZrO2 nanoparticles demonstrate anisotropic hysteresis loops concomitant with anisotropic magnetotresistance behavior. The biaxial epitaxial strain, induced by the substrate/film lattice parameter mismatch is partially relaxed by increasing the density of precipitates and they serve as a tuning parameter for the strain state. We interpret our results by a strain-induced interplay of impurity scattering, weak localization and magnetic domain structure.

Tailoring a two-dimensional electron gas at the LaAlO 3 /SrTiO 3 (001) interface by epitaxial strain

Recently a metallic state was discovered at the interface between insulating oxides, most notably LaAlO 3 and SrTiO 3 . Properties of this two-dimensional electron gas (2DEG) have attracted significant interest due to its potential applications in nanoelectronics. Control over this carrier density and mobility of the 2DEG is essential for applications of these unique systems, and may be achieved by epitaxial strain. However, despite the rich nature of strain effects on oxide materials properties, such as ferroelectricity, magnetism, and superconductivity, the relationship between the strain and electrical properties of the 2DEG at the LaAlO 3 /SrTiO 3 heterointerface remains largely unexplored. Here, we use different lattice constant single-crystal substrates to produce LaAlO 3 /SrTiO 3 interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile-strained SrTiO 3 destroys the conducting 2DEG, while compressively strained SrTiO 3 retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO 3 /SrTiO 3 interface. We have also found that the critical LaAlO 3 overlayer thickness for 2DEG formation increases with SrTiO 3 compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO 3 layer is responsible for this behavior. The polarization is directed away from the interface and hence creates a negative polarization charge opposing that of the polar LaAlO 3 layer. This behavior both increases the critical thickness of the LaAlO 3 layer, and reduces carrier concentration above the critical thickness, in agreement with our experimental results. Our findings suggest that epitaxial strain can be used to tailor 2DEGs properties of the LaAlO 3 /SrTiO 3 heterointerface.

Controlled Modifications of HTSC Thin-Film Properties by Tailoring Substrate Surfaces

This chapter introduces the use of substrates for high-temperature superconductor (HTSC) thin-film deposition beyond their usual purpose as chemically inert, lattice-matched support for the films. Substrates are used as functional elements in order to controllably modify the growth mode of YBa 2 Cu 3 O 7 thin films in the case of vicinal-cut SrTiO 3 single crystals or to locally modify the surface morphology and film/substrate lattice mismatch in the case of ion implanted SrTiO 3 . Furthermore, the use of biaxial epitaxial strain is briefly reviewed for HTSC thin films, and the application of this concept to tailor the properties of perovskite thin films in general is shown.