SrTiO3 Single Crystals Research Articles

Superconducting (Ba,K)Fe2As2 epitaxial films on single and bicrystal SrTiO3 substrates

The realization of single crystal and bicrystal films of superconducting materials is of great interest because they allow the investigation of the intragrain performance as well as the understanding of potential limitations in the grain boundary transparency. For many years, the realization of a high-quality (Ba,K)Fe2As2 film has been challenging. Here, the realization of (Ba,K)Fe2As2 epitaxial thin films on single crystal SrTiO3(001) and [001]-tilt-type SrTiO3 bicrystal substrates with high superconducting properties is demonstrated. The epitaxial growth of (Ba,K)Fe2As2 was enabled by implementing an undoped BaFe2As2 buffer layer between the SrTiO3 substrate and (Ba,K)Fe2As2 film. The film exhibits a high Tc of 38.0 K and an extremely high Jc of 14.3 MA/cm2 at 4.2 K. Artificial grain boundaries of (Ba,K)Fe2As2 were also successfully achieved on bicrystals with misorientation angles up to 36.8° by the same preparation methods. The artificial grain boundaries exhibited an identical Tc of 38.0 K and an excellent transfer of the grain orientation from the bicrystal substrates with high crystallinity comparable to that of the high-quality Ba(Fe,Co)2As2 films. This enables the investigation of the intrinsic (Ba,K)Fe2As2 grain boundary nature, which will clarify its potential for superconducting applications, like Josephson junctions, wires, and magnets.

Measuring inverse flexoelectric effect at the macro scale and flexoelectric actuator

Abstract The flexoelectric effect is a two-way mechanical-electrical coupling. The dielectric is polarized when subjected to bending moments, and inversely, the electric field can also induce strain gradients within the dielectric. Although equally important, research on the inverse flexoelectric effect has lagged far behind that on the direct effect, and investigations of the inverse effect on a macroscopic scale are noticeably lacking. This dilemma impedes the design of flexoelectric actuators. To go out of the dilemma, in this work, we design an experimental method to achieve inverse flexoelectricity and propose a method to measure the inverse flexoelectric effect with a lower voltage at the macroscopic scale. The result shows that the flexoelectric coefficient of SrTiO3 (STO) single crystal from the inverse flexoelectric experiment has the same order of magnitude as that of the direct flexoelectric experiments. Furthermore, this method can be utilized to design an STO flexoelectric actuator on a macroscopic scale. The displacement resolution of flexoelectric actuators is as low as 0.42 pm V−1, which is three orders of magnitude lower than that of piezoelectric actuators. This type of flexoelectric actuator is important for precise driving and positioning.

Comprehensive study of epitaxial (Nd,Eu,Gd)Ba2Cu3O7−δ films grown on textured templates

We report the successful epitaxial growth and comprehensive study of structural and superconducting properties of (Nd,Eu,Gd)Ba2Cu3O7−δ films with a thickness of up to 1.2 μm prepared by on-axis pulsed laser deposition on SrTiO3 (STO) single crystals as well as on ion-beam-assisted deposition (IBAD) and rolling-assisted biaxially textured substrate (RABiTS)-based metal templates. X-ray diffraction demonstrates the epitaxial quality of the grown films on both single crystalline substrates and metal-based templates. In addition, no significant changes in surface morphology were found with increasing thickness for films on all types of substrates. On all substrates, a T c value of about 89 K was observed, however, significant inhomogeneities indicating small superconducting volume were found for all films on STO. In general, the best transport characteristics over a wide range of magnetic fields and temperatures were observed for thicker films on IBAD-MgO and RABiTS templates, respectively. The J c values at 77 K for the films on IBAD-MgO templates are comparable to YBCO films, but inferior in the low temperature region and high magnetic fields, whereas the films on RABiTS and especially on STO showed much lower J c over the entire temperature and magnetic field range. A maximum pinning force of about 7 GN·m−3 at 65 K was found for the 900 nm thick films on IBAD-MgO template. In addition, the pinning mechanism seems to depend on both temperature and type of substrate. In general, no correlation between local microstructure and transport characteristics was revealed for NEG films on both metal templates.

Nanometer-Scale Acoustic Wave Packets Generated by Stochastic Core-Level Photoionization Events

We demonstrate that the absorption of femtosecond hard x-ray pulses excites quasispherical, high-amplitude, and high-wave-vector coherent acoustic phonon wave packets using an all hard-x-ray pump-probe scattering experiment. The time- and momentum-resolved diffuse scattering signal is consistent with an ensemble of 3D strain wave packets induced by the rapid electron cascade dynamics following photoionization at uncorrelated excitation centers. We quantify key parameters of this process, including the localization size of the stress field and the photon energy conversion efficiency into elastic energy. The parameters are determined by the photoelectron and Auger electron cascade dynamics, as well as the electron-phonon interaction. In particular, we obtain the localization size of the observed strain wave packet to be 1.5 and 2.5 nm for bulk SrTiO3 and KTaO3 single crystals, respectively. The results provide crucial information on the mechanism of x-ray energy deposition into matter and shed light on the shortest collective length scales accessible to coherent acoustic phonon generation using x-ray excitation. Published by the American Physical Society 2024

Strain-oxygen vacancies coupling in topotactic (La,Sr)CoO3-δ thin films

Oxygen defect engineering is a widely used approach for tuning physical properties in oxides. Multivalent transition metal oxide La0.7Sr0.3CoO3-δ (LSCO) shows oxygen vacancy-driven metal-to-insulator transition (MIT) due to topotactic phase transition and its high oxygen vacancy tolerance. Here, we introduce strain as a new degree of freedom to study the strain-oxygen vacancy coupling effects and elucidate its impact on the electronic property in oxygen-deficient LSCO epitaxial thin films grown on SrTiO3 (100) single crystal. By combining the experimental results with density functional theory plus U (DFT+U) calculations, we reveal that 2.1 % in-plane tensile strain can stabilize the insulating state of LSCO with a surprisingly low concentration of oxygen vacancies, <0.5 %. This study reveals that the MIT in LSCO is governed by the combination of oxygen vacancies and strain, offering the potential for additional tuning knob of the material's electronic properties.

Tuning Van Der Waals Heterostructure of WS2/SrTiO3 Via in Situ Transmission Electron Microscopy

Monolayer two-dimensional (2D) materials have been widely researched and discussed in recent years, while multi-layers have received comparatively less attention. Nowadays, combining different materials to create heterostructures has become mainstream. In semiconductor industry, heterostructure can form devices such as lasers, light-emitting diodes, fast transistors and so on. Heterostructures can create well electronic properties by combining two different materials together. Therefore, heterostructures become a great tool to manipulate the electronic structures. In traditional heterostructures, the interfaces between two different materials are formed by covalent bonds, hence it requires similar lattice constants and structures. On the other hand, novel van der Waal heterostructures (vdW-heterostructure), formed by 2D materials, do not require the same strict similarity as traditional ones. VdW-heterostructures, which create 3D structures through van der Waals forces, provide a new perspective. In this study, we created a vdW-heterostructures by stacking tungsten disulfide (WS2) with single-crystal strontium titanate, SrTiO3 (STO), shown in Figure 1a,b. The crystalline atomic-layer WS2 and STO were synthesized by chemical vapor deposition (CVD) and pulsed laser deposition (PLD), respectively. They are analyzed by high-resolution transmission electron microscope (HRTEM) shown in Figure 1c-h. Once the vdW-heterostructures are formed, we conducted in situ heating by TEM. Through the advanced in situ technique, we observed the change of the twisted stacking angle by heating the vdW-heterostructure. Ultimately, the lattices will shift and rotate, minimizing the free energy, so that the vdW-heterostructure will be aligned and formed epitaxially. In addition, we utilized atomic resolution scanning transmission electron microscope (STEM) to identify the stacking images and the moiré pattern of the vdW-heterostructure. During the rotation, we can observed different moiré patterns by various twisted stacking angles. These different moiré patterns can produce different electrical properties, which can be applied to advanced semiconductor devices. Keywords: high-resolution TEM/STEM, in situ heating, 2D materials, van der Waal heterostructures, moiré pattern Figure 1. The vdW-heterostructure of STO/WS2. a) The diagram showing that the formation process of WS2/STO vdW-heterostructures which have different twisted stacking angles. b) The low-magnification SEM image showing that the single crystal STO thin film is transferred onto the TEM heating chips. The circles in the middle are the observation windows for transmission electrons. c-e) HRTEM images. f-g) Diffraction patterns of HRTEM. c,f) Single crystal STO thin film with d-spacing (1−10) = 0.284 nm. d,g) Monolayer single crystal WS2 with d-spacing (−110) = 0.274 nm. e,h) WS2(red)/STO(yellow) vdW-heterostructure with stacking angle 3.6°. Figure 1

Importance of growth method and substrate-induced crystalline quality in Al/Gd0.2Ca0.8MnO3/Au memristor devices

We report on the impact of the growth method and substrate-induced crystalline quality on the performance of planar Al/Gd0.2Ca0.8MnO3/Au memristor devices. Structural, magnetic, and resistive properties were thoroughly examined for fundamental characterizations, with a particular emphasis on their correlation with the memristive properties of fabricated devices. Our findings suggest that memristor structures grown on single crystal SrTiO3 substrates using pulsed laser deposition (PLD) consistently exhibit superior crystalline quality compared to those fabricated using chemical solution deposition and on silicon-based substrates. Despite variations in growth method and substrate, all memristor structures display typical resistive switching (RS) behaviour, distinguishing between high-resistance and low-resistance states. However, endurance and retention measurements demonstrate that memristor structures produced via PLD on single crystalline SrTiO3 demonstrate the most favourable RS properties. To elucidate the mechanisms underlying the differences in RS behavior across substrates and deposition methods, we extensively discuss these issues in the context of structural distortion and conduction mechanisms.

Pulsed laser deposition and structural analysis of Ba2Ti6O13 epitaxial thin film on the (210) surface of SrTiO3 single crystal

A single crystalline thin film of Ba2Ti6O13 was grown on the (210) surface of a SrTiO3 single-crystal substrate by pulsed laser deposition. Scanning transmission electron microscopy shows that the Ba2Ti6O13 thin film grew epitaxially with the a-plane, and the b-/c-axes of Ba2Ti6O13 parallel to the (210) plane and the [00 1¯ ]/[ 1¯ 20] directions of SrTiO3, respectively. The electrical conductance of the Ba2Ti6O13 film was approximately one order larger than that of the reduced SrTiO3−δ single crystal with oxygen deficiency of roughly δ = 0.05. Electronic structure calculations predict that Ba2Ti6O13 is an n-type degenerate semiconductor with spin-split states primarily of Ti 3d orbital.

Top-seeded solid-state single crystal growth of 0.75(Na0.5Bi0.5)TiO3-0.25SrTiO3 single crystals in oxygen and their characterization

Top-seeded solid-state single crystal growth of 0.75(Na0.5Bi0.5)TiO3-0.25SrTiO3 single crystals in oxygen and their characterization

Colossal dielectric behavior of Nb-doped SrTiO3 single crystal

Crystals are usually used to explore the origin of material properties due to the fact that they have no grain boundaries and no pores. Although doped SrTiO3 crystal have remarkable dielectric properties, the origin of this improvement is still not fully understood. Herein, Nb5+ has been selected as a donor ion to study the dielectric properties of SrTiO3 due to its radius close to Ti4+. It has been found that the dielectric permittivity of the Nb-doped crystal is greater than 94,500 and the dielectric loss is maintained below 0.002 in the frequency range from 20 Hz to 0.2 MHz. X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy indicate that the dielectric properties of doped crystals were improved due to the existence of oxygen vacancies and there was no effect of interface polarization. It was beneficial for the Nb-doped crystal to be annealed in a N2 atmosphere resulting in oxygen loss more oxygen vacancies, and a higher concentration of Ti3+. The electron can be bound by VO•• and Ti4+ and form the defect dipole [Ti4+∙e − VO•• − Ti4+∙e], which showed a higher permittivity and lower dielectric loss. In summary, the origin and optimization of dielectric properties of SrTiO3-based materials can be deeply understood when from doped SrTiO3 crystals.

Pore walls as high-way for efficient bulk charge transfer in porous SrTiO3 single crystals boosting photocatalytic overall water splitting

Suppressing carrier recombination in bulk and facilitating carrier transfer to surface via rational structure design is of great significance to improve solar-to-H2 conversion efficiency. We demonstrate a facile hydrothermal method to synthesize porous SrTiO3 single crystals (SrTiO3-P) with exposed (001) facets by introducing carbon spheres as templates. The obviously increased surface photovoltage and photocurrent response indicate that the interconnected pore walls act as enormous charge transfer “highways”, accelerating carrier transport from bulk to surface. Furthermore, the absence of grain boundaries and high crystallinity could also lower the carrier recombination rate. Thus, the SrTiO3-P photocatalyst loaded with Rh/Cr2O3 as cocatalyst exhibits 1.5 times higher overall water splitting activity than that of solid SrTiO3, with gas evolution rate of 19.99 μmol h−1 50 mg−1 for H2 and 11.37 μmol h−1 50 mg−1 for O2. Additionally, SrTiO3-P also shows superior stability without any decay during cycling testing. This work provides a new insight into designing efficient multicomponent photocatalysts with a single-crystal porous structure.

Enhanced energy storage performance in Bi4Ti3O12 thin films with oxygen depletion layers

The imprint effect in ferroelectric materials can significantly enhance the performance of energy storage devices. Bi4Ti3O12 (BTO) and oxygen-deficient Bi4Ti3O11.2 (DBTO) thin films were deposited on single-crystal Nb-doped SrTiO3 substrates using pulsed laser deposition. In stark contrast, multilayer DBTO/BTO thin films incorporating an oxygen-deficient layer were also fabricated, with the DBTO layers having thicknesses of 5 nm and 10 nm, respectively. Both BTO and DBTO thin films exhibited epitaxial growth and c-axis-oriented crystallinity. Regarding ferroelectric properties, DBTO/BTO capacitors with an oxygen-deficient layer demonstrated a pronounced imprint effect in their ferroelectric hysteresis loops, compared to the BTO and DBTO capacitors. This effect was particularly noticeable in the DBTO/BTO capacitor with a 10 nm oxygen-deficient layer, as opposed to the thin film with a 5 nm oxygen-deficient layer. We suggest that the imprint effect can be attributed to localized electric field variations and imbalances in charge transport at the interface between the oxygen-depleted layer and the remaining thin film. Consequently, the imprint effect observed in the DBTO/BTO capacitors contributed to an enhancement in energy storage density and efficiency, compared to BTO and DBTO capacitors. This study demonstrates the potential of multilayer DBTO/BTO thin films in advancing capacitive energy storage systems.

Polarization near dislocation cores in SrTiO3 single crystals: The role of flexoelectricity

Spontaneous polarization as large as ∼28 μC/cm2 was recently observed around the dislocation cores in non-polar SrTiO3 bulk crystals, and its origin was attributed to the flexoelectric effect, i.e., polarization induced by strain gradients. However, the roles of flexoelectricity, relative to other electromechanical contributions, and the nature of dislocations, i.e., edge vs screw dislocations in the induced polarization, are not well understood. In this work, we study the role of flexoelectricity in inducing polarization around three types of dislocation cores in SrTiO3: b=a(100) edge dislocation, b=a(110) edge dislocation, and b=a(010) screw dislocation, where b is the Burgers vector. For the edge dislocations, polarization can be induced by electrostriction alone, while flexoelectricity is essential for stabilizing the symmetric polarization pattern. The shear component of the flexoelectric tensor has a dominant effect on the magnitude and spatial distribution of the flexoelectric polarization. In contrast, no polarization is induced around the b=a(010) screw dislocation through either electrostriction or flexoelectricity. Our findings provide an in-depth understanding of the role of flexoelectricity in inducing polarization around dislocation cores and offer insights into the defect engineering of dielectric/ferroelectric materials.

Quantum interference in a high-transition-temperature superconductor based on nanoslits on SrTiO3 substrate

The Josephson junction, a key component of superconducting quantum circuits, is much less mature, especially for the high-transition (high-Tc) temperature cuprate superconductors. Herein, we report on the realization of high-Tc Josephson junction based on nanoslits on a SrTiO3 substrate. We demonstrate the ability to manipulate the current–voltage characteristics of YBa2Cu3O7−x bridges continuously from superconducting current-dominated to Josephson junction behavior by changing the irradiation parameters of the focused Ga+ ion beam on a single-crystal SrTiO3 substrate. The periodic critical current that depends on the magnetic flux coupled into the superconducting quantum interference devices was observed, which exactly reflects the effects of quantum tunneling and flux quantization. To some extent, this weak link within the framework of the semiconductor manufacturing process shows the potential to provide a cost-effective, highly efficient, and reliable pathway for scaling up quantum mechanical superconducting circuits, which is promising for the fabrication process.

Navigating the magnetic contribution of the commonly used single-crystal substrates SrTiO3 (100) and LaAlO3 (100) in weak magnetic thin films

In recent years, thin films of weak ferromagnetic materials have been in huge demand; however, probing their magnetic characteristics has been difficult due to contributions from underlying substrates. In the present study, we have analyzed the magnetic properties of the commonly used single-crystal SrTiO3 (100) and LaAlO3 (100) substrates and performed a time-dependent annealing protocol in vacuum and ambient oxygen pressure to mitigate the intrinsic weak ferromagnetic contributions from these substrates arising due to the presence of disorder or defects such as vacancies in the pristine substrates. It is shown that after proper air annealing, the substrate magnetic background becomes diamagnetic. When such air-annealed diamagnetic substrates are used for the deposition of low-thickness films carrying low magnetic moments such as SrRuO3 and SrMnO3 thin films, their magnetic transitions are explicitly observed. The proposed annealing protocols help to improve the signal from weak magnetic samples. This allows us to analyze the film’s magnetic properties without worrying about the contribution from the substrate.

Crystal Facet Engineering on SrTiO3 Enhances Photocatalytic Overall Water Splitting.

Single-crystal semiconductor-based photocatalysts exposing unique crystallographic facets show promising applications in energy and environmental technologies; however, crystal facet engineering through solid-state synthesis for photocatalytic overall water splitting is still challenging. Herein, we develop a novel crystal facet engineering strategy through solid-state recrystallization to synthesize uniform SrTiO3 single crystals exposing tailored {111} facets. The presynthesized low-crystalline SrTiO3 precursors enable the formation of well-defined single crystals through kinetically improved crystal structure transformation during solid-state recrystallization process. By employing subtle Al3+ ions as surface morphology modulators, the crystal surface orientation can be precisely tuned to a controlled percentage of {111} facets. The photocatalytic overall water splitting activity increases with the exposure percentage of {111} facets. Owing to the outstanding crystallinity and favorable anisotropic surface structure, the SrTiO3 single crystals with 36.6% of {111} facets lead to a 3-fold enhancement of photocatalytic hydrogen evolution rates up to 1.55 mmol·h-1 in a stoichiometric ratio of 2:1 than thermodynamically stable SrTiO3 enclosed with isotropic {100} facets.

Mechanistic insights into photo-current enhancement in photo-active SrTiO3 heterojunctions under UV illumination

Photo-active heterojunctions were prepared by growing thin La0.9Sr0.1CrO3-δ films on SrTiO3 (STO) single crystals substrates via pulsed laser deposition. The photo-electrochemical characteristics of these heterojunctions under UV illumination were investigated by DC measurements and electrochemical impedance spectroscopy (EIS). Under UV illumination, the photo-current produced by these junctions increases over time by almost an order of magnitude to 1 to 2 mA cm-2 at 350 °C. Two effects governing this increase can be distinguished: A first process, independent of the surrounding atmosphere, is related to a stoichiometry polarization of the STO single crystal upon current flow. A second process is highly sensitive to contaminants in the surrounding atmosphere and is strongly accelerated in pure, synthetic atmospheres. We suggest that it is related to photo-oxidation of the STO single crystal. EIS revealed that both processes result in a decrease of both the electronic transport resistance through the STO single crystal and, to a lesser degree, the space charge resistance of the photo-active junction. Operando EIS under illumination and with DC bias was used to record the cells' photo-power characteristics and identify individual loss processes. Owing to the illumination induced strong increase of the STO single crystal conductivity, the photo-currents of such enhanced cells are limited by the space charge resistance.

Resurrected and Tunable Conductivity and Ferromagnetism in the Secondary Growth La0.7Ca0.3MnO3 on Transferred SrTiO3 Membranes.

To avoid the epitaxy dilemma in various thin films, such as complex oxide, silicon, organic, metal/alloy, etc., their stacking at an atomic level and secondary growth are highly desired to maximize the functionality of a promising electronic device. The ceramic nature of complex oxides and the demand for accurate and long-range-ordered stoichiometry face severe challenges. Here, the transport and magnetic properties of the La0.7Ca0.3MnO3 (LCMO) secondary growth on single-crystal freestanding SrTiO3 (STO) membranes are demonstrated. It has been experimentally found that on an only 10 nm thick STO membrane, the LCMO can offer a bulk-like Curie temperature (TC) of 253 K and negative magnetoresistance of -64%, with a weak dependence on the thickness. The resurrected conductivity and ferromagnetism in LCMO confirm the advantages of secondary growth, which benefits from the excellent flexibility and transferability. Additionally, this study explores the integration strategy of complex oxides with other functional materials.

A method with ultra-high angular resolution for X-ray diffraction experiments.

In X-ray diffraction measurements, the angular resolution has a detection limit due to the receiving size of the detector. In many cases this detection limit is too large and must be breached to obtain the desired information. A novel method is proposed here by making the detector simultaneously measuring and moving. Using the deconvolution algorithm to remove the convolution effect, the pixel size limitation is finally broken. The algorithm used is not a common one, and suppresses signals at high frequencies, ensuring the reliability of the peak shape after restoration. The feasibility of this method is verified by successfully measuring the crystal truncation rod signal of SrTiO3 single crystal, and the resolution is nearly ten times higher than that of a single pixel. Moreover, this method greatly reduces the noise and improves the signal-to-noise ratio.

Preparation of p/n-type YBa2Cu3O7−δ/Nd1.85Ce0.15CuO4 superconducting heterostructures

Due to the great differences in electron pairing characteristics between the hole-type (p-type) high-temperature superconductor (HTS) and the electron-type (n-type) HTS, when the Cooper pairs enter from a p-type to an n-type HTS or an n-type to a p-type HTS, pairing adjustment or even the recombination of them will inevitably occur at the interface, and the superconducting current dominated by them will be bound to change. Therefore, it is possible to better understand the electron pairing mechanism of the HTS and develop new HTS junction devices by studying these changes. In this paper, p-/n-type YBa2Cu3O7−δ/Nd1.85Ce0.15CuO4 (YBCO/NCCO) heterostructures were prepared via pulsed laser deposition on (00l)-oriented single-crystal SrTiO3 substrates. X-ray diffraction measurements indicate that the NCCO films prepared on YBCO do not exhibit good c-axis epitaxial growth, while excellent c-axis epitaxial growth is obtained for YBCO/NCCO heterostructures with NCCO at the bottom. Due to the high-temperature oxygen atmosphere and the deoxygenation process, the superconducting electrical properties of the bilayer structure are seriously degraded, and it is not possible to obtain good superconducting electrical properties for both the upper and lower layers using the traditional preparation process based on an oxygen atmosphere. Subsequently, the effects of different growth atmospheres on the electrical properties of the YBCO superconductors were studied, and high-quality YBCO superconducting films could be grown in oxygen, nitrous oxide, and nitrogen atmospheres. However, the oxygen and nitrous oxide atmospheres degrade the superconducting electrical properties of the underlying NCCO layer, while the nitrogen atmosphere does not seem to affect it significantly. YBCO/NCCO superconducting bilayers with critical transition temperatures of 85 and 8 K for YBCO and NCCO, respectively, were finally prepared by growing NCCO in an oxygen atmosphere and YBCO in a nitrogen atmosphere using a low-temperature oxygenation process. The successful preparation of the p-/n-type HTS heterostructure will help further study on the HTS.