Observations of unexpected grain boundary migration in SrTiO[subscript 3]
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Muralikrishnan, Vivekanand↗
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Strain-induced room-temperature ferroelectricity in SrTiO3 membranes
Not Available
36 MATERIALS SCIENCE↗
Quenched lattice fluctuations in optically driven SrTiO3
Abstract Crystal lattice fluctuations, which are known to influence phase transitions of quantum materials in equilibrium, are also expected to determine the dynamics of light-induced phase changes. However, they have only rarely been explored in these dynamical settings. Here we study the time evolution of lattice fluctuations in the quantum paraelectric SrTiO 3 , in which mid-infrared drives have been shown to induce a metastable ferroelectric state. Crucial in these physics is the competition between polar instabilities and antiferrodistortive rotations, which in equilibrium frustrate the formation of long-range ferroelectricity. We make use of high-intensity mid-infrared optical pulses to resonantly drive the Ti–O-stretching mode at 17 THz, and we measure the resulting change in lattice fluctuations using time-resolved X-ray diffuse scattering at a free-electron laser. After a prompt increase, we observe a long-lived quench in R-point antiferrodistortive lattice fluctuations. Their enhancement and reduction are theoretically explained by considering the fourth-order nonlinear phononic interactions to the driven optical phonon and third-order coupling to lattice strain, respectively. These observations provide a number of testable hypotheses for the physics of light-induced ferroelectricity.
36 MATERIALS SCIENCE↗
Epitaxial growth of conducting perovskite SrTiO3 on (0001) c-axis sapphire using a γ-Al2O3 intermediate layer
Epitaxial growth of complex oxides on large-area wafers, such as sapphire and silicon, represents a key step toward scalable oxide device production. Solid phase epitaxy allows the synthesis of γ-Al 2 O 3 on α-Al 2 O 3 and provides a template with a matched lattice constant and appropriate cubic symmetry for subsequent heteroepitaxial growth of perovskite complex oxides. Nb-doped SrTiO 3 thin films were deposited epitaxially on (111)-oriented γ-Al 2 O 3 intermediate layers on (0001) c-axis-oriented sapphire α-Al 2 O 3 crystals using pulsed laser deposition. The Nb:SrTiO 3 thin films with a thickness of 53 nm, grown at 700 °C on γ-Al 2 O 3 , reached fully relaxed lattice parameters and were epitaxially oriented with respect to the substrate. Nb:SrTiO 3 layers deposited using identical deposition conditions directly on α-Al 2 O 3 , without the γ-Al 2 O 3 intermediate layer, were polycrystalline. The sheet conductivity of Nb:SrTiO 3 grown on γ-Al 2 O 3 /α-Al 2 O 3 is more than ten times higher than that of Nb:SrTiO 3 grown directly on α-Al 2 O 3 without the γ-Al 2 O 3 layer. The results point to new directions for the integration of (111)-oriented pseudocubic perovskite complex oxides and the integration of epitaxial complex oxides over larger areas using α-Al 2 O 3 single-crystal substrates.
Atomic force microscopy↗
Local rare-earth dopant structure in a complex-oxide/semiconductor heterojunction: Molecular beam epitaxy grown Yb-doped SrTiO3 on Si(001)
We have investigated the structural and electronic properties of Yb-doped SrTiO3/Si(001) grown by molecular beam epitaxy. Other rare-earth donor dopants that result in n-type conductivity typically substitute for Sr at the A-sites in the perovskite lattice. In contrast, Yb has been found to substitute predominantly for Ti at the perovskite B-sites based on data from atomically resolved scanning transmission electron microscopy and spectroscopy, as well as extended x-ray absorption fine structure. Yb exhibits two distinct charge states as determined by x-ray absorption spectroscopy and associated modeling, +2.7 and +2.1. These aliovalent substitutional dopants are compensated by donor electrons from oxygen vacancies that form during epitaxial film growth. An atom beam flux mismatch was present during film depositions because it was assumed that Yb would occupy A-sites, leading to a flux matching formula given by Yb + Sr = Ti. However, the formation of YbTi rather than YbSr results in Sr vacancies and extraneous (i.e. non-lattice) Ti atoms in the films, or on the film surfaces. The presence of these defects, together with oxygen vacancies, leads to deep-level electron traps that were detected by resonant photoemission and much higher sheet resistance than that associated with, for instance, La-doped films.
Chambers, Scott A. (ORCID:000000025415043X)↗
Charge transport mechanisms in SrTiO3:Rh nanoparticle photocatalysts for Z-scheme water splitting.
Abstract not provided.
Zutter, Brian↗
Charge Transport in SrTiO3:Rh Nanoparticle Photocatalysts for Z-Scheme Water Splitting
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Zutter, Brian Travis↗
Oxygen Deficiency and Migration-Mediated Electric Polarization in Magnetic Fe,Co-Substituted SrTiO3−δ
We use density functional theory (DFT) calculations to show that oxygen vacancies (vO) and mobility induce noncentrosymmetric polar structures in SrTi1−x−yFexCoyO3−δ (STFC, x=y=0.125) with δ={0.125,0.25}, enhance the saturation magnetization, and give rise to large changes in the electric polarization |ΔP|. We present an intuitive set of rules to describe the properties of STFC, which are based on the interplay between (Co/Fe)-vO defects, magnetic cation coordination, and topological vacancy disorder. STFC structures consist of layered crystals with sheets of linearly organized O4,5,6-coordinated Fe–Co pairs, sandwiched with layers of O5-coordinated Ti. (Co/Fe)-vO defects are the source of crystal distortions, cation off-centering and bending of the oxygen octahedra which, considering the charge redistribution mediated by vO and the cations’ electronegativity and valence states, triggers an effective electric polarization. Oxygen migration for δ=0.125 leads to |ΔP|>∼10 µC/cm2 due to quantum-of-polarization differences between δ=0.125 structures. Increasing the oxygen deficiency to δ=0.25 yields |ΔP|, the O migration of which resolved polarization for δ=0.25 is >∼3 µC/cm2. Magnetism is dominated by the Fe,Co spin states for δ=0.125, and there is a contribution from Ti magnetic moments (∼1 μB) for δ=0.25. Magnetic and electric order parameters change for variations of δ or oxygen migration for a given oxygen deficiency. Our results capture characteristics observed in the end members of the series SrTi(Co,Fe)O3, and suggest the existence of a broader set of rules for oxygen-deficient multiferroic oxides.
36 MATERIALS SCIENCE↗
Revival and Technology Transfer of 90SrTiO3 Production for Heat Source Applications
Demand for radioisotope power systems (RPS) appears to be on the rise, and it is unlikely that this new demand can be met by plutonium-238. As a result, new heat source designs are under development. One such design is Z1. Z1 is a strontium-90 demonstration heat source developed by Zeno Power Systems in collaboration with the University of Dayton Research Institute and the Pacific Northwest National Laboratory. SrTiO3 was chosen as the fuel form for Z1. SrTiO3 was used to produce dozens of terrestrial RPS decades ago. While this indicates the technology is proven, the skill and experience necessary to effectively produce SrTiO3 has been lost. Recapturing SrTiO3 production technology therefore became necessary. Development started with non-radioactive surrogate experiments using natural strontium. This work showed that the old Oak Ridge flowsheets for SrTiO3 production are quite robust, but some critical pieces of the technology are left out of the old documentation. Specifically, particle sizes of the reagents are critical to producing a phase pure SrTiO3. If the reagent particles are not the right size, then a significant Sr3Ti2O7 phase impurity is observed. This phase impurity was proven to be the result of an incomplete reaction between the strontium and titanium precursors. This technology was transferred to Pacific Northwest National Laboratory where the technology was adapted for use in a hot cell. With the adaptations complete, this technology was used to produce the fuel for Z1 late last year. This represents the first new heat source design produced in the U.S. in over 40 years.
90Sr, strontium titanate, SrTi03, RTG, Radioisotop↗
Experimental determination of electron attenuation lengths in complex materials by means of epitaxial film growth: advantages and challenges
Accurate electron attenuation lengths are of critical importance in using electron spectroscopic methods to quantitatively characterize complex materials. Here we show that analysis of core-level and valence-band x-ray photoelectron spectra excited with monochromatic AlK? x-rays from the substrate and measured as a function of film thickness can be used to determine electron attenuation lengths in epitaxial SrTiO3 films on Ge(001). Closely lattice-matched epitaxial heterojunctions are ideal systems for determining attenuation lengths provided the films grow in a layer-by-layer fashion, leading to atomically flat surfaces, and the buried interfaces are atomically abrupt. In principle, either the rate of attenuation of substrate peak intensities or the rate of increase of film peak intensities can be used for this purpose. However, we find that structural nonuniformities in the films reduce the accuracy of electron attenuation lengths determined from photoelectrons that originate within the films. A more reliable source of information is found in photoelectrons from the substrate which traverse the film. By using the energy dependence of calculated electron attenuation lengths from the NIST database in combination with Ge 3d core and Ge-derived valence band intensities, we determine electron attenuation length as a function of kinetic energy for SrTiO3.
Chambers, Scott A.↗
Photoelectrolysis of water at high current density - Use of ultraviolet laser excitation
The behavior of TiO2 and SrTiO3 photoanodes in cells for the photoelectrolysis of H2O has been investigated for high-intensity 351-,364-nm excitation from an Ar ion laser. Intensities up to 380 W/sq cm have been used. For TiO2 a small amount of surface decomposition is found after irradiation at high intensity, whereas SrTiO3 undergoes no detectable changes. Current-voltage properties for both electrodes are essentially independent of light intensity up to the level of 380 W/sq cm, and there is little if any change in quantum efficiency for electron flow. Photocurrent densities have been shown to exceed 5 A/sq cm for O2 evolution. Data show that the energy storage rate associated with the SrTiO3 photoelectrolysis can exceed 30 W/sq cm; this represents the highest demonstrated rate of sustained optical-to-chemical energy conversion.
Bocarsly, A. B.↗
Characterization of ZrO2 buffer layers for sequentially evaporated Y-Ba-CuO on Si and Al2O3 substrates
Thin film high temperature superconductors have the potential to change the microwave technology for space communications systems. For such applications it is desirable that the films be formed on substrates such as Al2O3 which have good microwave properties. The use of ZrO2 buffer layers between Y-Ba-Cu-O and the substrate has been investigated. These superconducting films have been formed by multilayer sequential electron beam evaporation of Cu, BaF2 and Y with subsequent annealing. The three layer sequence of Y/BaF2/Cu is repeated four times for a total of twelve layers. Such a multilayer film, approximately 1 micron thick, deposited directly on SrTiO3 and annealed at 900 C for 45 min produces a film with a superconducting onset of 93 K and critical temperature of 85 K. Auger electron spectroscopy in conjunction with argon ion sputtering was used to obtain the distribution of each element as a function of depth for an unannealed film, the annealed film on SrTiO3 and annealed films on ZrO2 buffer layers. The individual layers were apparent. After annealing, the bulk of the film on SrTiO3 is observed to be fairly uniform while films on the substrates with buffer layers are less uniform. The Y-Ba-Cu-O/ZrO2 interface is broad with a long Ba tail into the ZrO2, suggesting interaction between the film and the buffer layer. The underlying ZrO2/Si interface is sharper. The detailed Auger results are presented and compared with samples annealed at different temperatures and durations.
Valco, George J.↗
Laser ablated high T(sub c) superconducting thin YBa2Cu3O(7-x) films on substrates suitable for microwave applications
The development of high temperature superconducting YBa2Cu3O(7-x) thin films on substrates suitable for microwave applications is of great interest for evaluating their applications for space radar, communication, and sensor systems. Thin films of YBa2Cu3O(7-x) were formed on SrTiO3, ZrO2, MgO, and LaAlO3 substrates by laser ablation. The wavelength used was 248 nm from a KrF excimer laser. During deposition the films were heated to 600 C in a flowing oxygen environment, and required no post annealing. The low substrate temperature during deposition with no post annealing gave films which were smooth, which had their c-axis aligned to the substrates, and which had grains ranging from 0.2 to 0.5 microns in size. The films being c-axis aligned gave excellent surface resistance at 35 GHz which was lower than that of copper at 77 K. At present, LaAlO3 substrates with a dielectric constant of 22, appears suitable as a substrate for microwave and electronic applications. The films were characterized by resistance-temperature measurements, scanning electron microscopy, and x ray diffraction. The highest critical transition temperatures (T sub c) are above 89 K for films on SrTiO3 and LaAlO3, above 88 K for ZrO2, and above 86 K for MgO. The critical current density (J sub c) of the films on SrTiO3 is above 2 x 10(exp 6) amperes/sq cm at 77 K. The T(sub c) and J(sub c) are reported as a function of laser power, composition of the substrate, and temperature of the substrate during deposition.
Warner, J. D.↗
Construction and performance of a high-temperature-superconductor composite bolometer
A high-Tc superconducting bolometer has been constructed using a YBa2Cu3O(x) thin-film meander line 20 microns wide and 76,000 microns long, deposited on a SrTiO3 substrate. Radiation is absorbed by a thin film of Bi with well-characterized absorption properties deposited on a Si substrate in contact with the SrTiO3. At 1.8 Hz the measured bolometer response to a 500-K blackbody is 5.2 V/W (820 V/W extrapolated to dc). The impact of apparent nonohmic behavior at the transition is discussed, as are ways of reducing the observed 1/f noise. The response time is 32 s and is dominated by the heat capacity of the SrTiO3 substrate.
Brasunas, J. C.↗
Microwave properties and characterization of co-evaporated BSCCO thin films
An extensive characterization of Bi-Sr-Ca-Cu-O (BSCCO) thin films deposited by co-evaporation on LaAlO3 and SrTiO3 substrates was performed. The films had a T(sub c) (R = O) of approximately 78 K, and were predominantly c-axis oriented, with critical current densities (J(sub c)) at 4.5 K of 1.6 x 10(exp 6) and 1.1 x 10(exp 6) A cm(sup -2), for the samples on SrTiO3 and LaAlO3, respectively. The microwave properties of the films were examined by three techniques. The complex conductivity sigma(sub *) = sigma(sub 1) - j(sigma(sub 2)) and the magnetic penetration depth (A) were measured by power transmission at 30.6 GHz; the surface resistance (R(sub s)) was measured using a cavity resonator at 58.9 GHz, and the transmission line losses were determined by measuring the quality factor (Q) of a linear microstrip resonator at 10.4 and 20.2 GHz. The complex conductivity for the film on LaAlO3 was determined to be (2.0-j10) x 10(exp 5) S/m at 77 K. It was observed that in the superconducting state sigma(sub 1) deviates from both the Bardeen-Cooper-Schrieffer (BCS) theory and the two-fluid model. Values of lambda were found to be approximately 2.0 and 1.1 microns at 77 K and 20 K respectively, and were obtained for the film on LaAlO3. The value of lambda at 20 K was approximately three times larger than that of BSCCO single crystals. R(sub s) values of 865 and 1391 mOmega were obtained for the films on SrTiO3 and LaAlO3, respectively, at 77 K and 58.9 GHz. Unloaded Q factors at 20 K of approximately 1100 and 800 at 10.4 and 20.2 GHz respectively, were measured for the BSCCO resonator. Unloaded Q values of 290 and 405 at 20 K were obtained at 10.4 GHz and 20.2 GHz respectively, for an all gold (Au) resonator.
Miranda, F. A.↗
High-Temperature Superconducting/Ferroelectric Structures for Tunable Microwave Components
At the NASA Lewis Research Center, ferroelectric films, such as SrTiO3 and Ba(x)Sr(1-x)TiO3, are being used in conjunction with YBa2Cu3O(7-delta) high-temperature superconducting (HTS) thin films to fabricate tunable microwave components, such as filters, varactors, and local oscillators. These structures capitalize on the variation of the dielectric constant of the ferroelectric film upon the application of a dc electric field as well as on the low microwave losses exhibited by the high-temperature superconducting films relative to their conventional conductor counterparts. (For example, the surface resistance for a YBa2Cu3O(7-delta) thin film at 10 GHz and 77 K is more than two orders of magnitude lower than that of copper at the same frequency and temperature.) SrTiO3 and Ba(x)Sr(1-x)TiO3 films are used because their crystal structure and lattice parameters are similar to those of YBa2Cu3O(7-delta), thus enabling the growth of highly textured YBa2Cu3O(7-delta) films with high critical current densities on the underlying ferroelectric film, or alternatively, of highly textured ferroelectric film on the underlying YBa2Cu3O(7-delta) film. Our efforts have been concentrated so far in determining the deposition parameters required for optimal ferroelectric thin-film growth (i.e., maximum tunability and lowest loss) and in investigating different varactor configurations to determine which geometry is the most advantageous in terms of tunability, losses, and required bias for a given communication application. For example, we have observed that for optimized SrTiO3 films in a parallel plate capacitor, tunabilities of up to 47 percent and dissipation losses (tan d) of 0.05 are attainable at 1 MHz , 80 K, and within the 0- to 5-V bias range. In contrast, for an interdigital configuration, tunabilities of up to 70 percent and tan d ranging from 0.015 to 0.001 (depending on bias) have been observed at 1 MHz and 77 K within the 0- to 100-V bias range. Efforts are underway to use these results in developing tunable receiver front-end preselect filters as well as in low-phase noise, tunable local oscillators for K-band applications. These components represent a hitherto unavailable technology to meet the stringent performance requirements of foreseeable satellite and wireless communication systems (e.g., bandwidth, in-band insertion losses, out-of-band rejection, and noise, amongst others) in a more advantageous fashion than with currently available technology (e.g., dielectric-filled cavity and waveguide filters, and dielectric resonator oscillators). Prototypes of high-temperature superconducting/ferroelectric tunable components such as a low-phase noise K-band local oscillator, a preselect C-band filter, and a low-loss K-band phase shifter are under development at NASA Lewis.
Miranda, Felix A.↗
Interfacial Strong Coupling and Negative Dispersion of Propagating Polaritons in Freestanding Oxide Membranes
Membranes of complex oxides like perovskite SrTiO3 extend the multi-functional promise of oxide electronics into the nanoscale regime of 2D materials. Here, it is demonstrated that freestanding oxide membranes supply a reconfigurable platform for nano-photonics based on propagating surface phonon polaritons. Infrared near-field imaging and spectroscopy enabled by a tunable ultrafast laser are applied to study pristine nano-thick SrTiO3 membranes prepared by hybrid molecular beam epitaxy. As predicted by coupled mode theory, it is found that strong coupling of interfacial polaritons realizes symmetric and antisymmetric hybridized modes with simultaneously tunable negative and positive group velocities. By resolving reflection of these propagating modes from membrane edges, defects, and substrate structures, their dispersion is quantified with position-resolved nano-spectroscopy. Remarkably, polariton negative dispersion is found to be both robust and tunable through choice of membrane dielectric environment and thickness, and proposes a novel design for in-plane Veselago lensing harnessing this control. This work lays the foundation for tunable transformation optics at the nanoscale using polaritons in a wide range of freestanding complex oxide membranes.
freestanding oxide membranes↗