Zhen Huang

Proton implantation induced modulation of quantum transport in LAO/STO heterostructures.

The SrTiO3-based conducting heterointerfaces, when laterally confined by nanopatterning or defined using a conductive atomic force microscope, exhibit superior electronic properties compared to their macroscopic two-dimensional counterparts. This enhancement is attributed to the modulation of nanoscale transport properties by the polarity of SrTiO3 domain walls. However, the impact of polar domain walls on macroscopic electronic behavior remains poorly explored. In this study, we observe two distinct resistance anomalies at approximately 80 and 40 K, characteristic temperatures of SrTiO3 domain wall polarity, by measuring the macroscopic electrical transport properties of the (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 conducting heterointerface. As the carrier density decreases with decreasing gating voltage, these anomalies become more pronounced, with a metal-insulator-metal transition occurring near 40 K at a carrier density threshold of approximately 1 × 1012 cm−2. Our findings provide valuable insights into the role of polar domain walls in shaping the electronic characteristics of SrTiO3-based heterostructures and highlight the potential for manipulating macroscopic electronic behavior through domain wall engineering.

The interfacial electronic properties of complex oxides are governed by a delicate balance between charge transfer, lattice distortions, and electronic correlations, posing a key challenge for controlled tunability in materials research. Here, we demonstrate that proton implantation serves as a precise tool for modulating interfacial transport in SrTiO3-based heterostructures. By introducing protons into the SrTiO3 substrate beneath an amorphous (La,Sr)(Al,Ta)O3 capping layer, we uncover a competition between disorder and charge doping induced by implantation. At low implantation fluences below 1x1015 protons/cm2 (1E15), charge doping dominates, leading to an increase in carrier density and mobility, analogous to electrostatic gating effect. This enables the emergence of quantum transport oscillations at low temperature. Conversely, at higher fluences (above 1E15), disorder scattering prevails, suppressing carrier mobility and inducing an insulating state. The nonmonotonic evolution of transport with implantation fluence underscores the critical interplay between electronic correlations and disorder, offering a new paradigm for the controlled engineering of interfacial quantum states in SrTiO3-based oxide heterostructures.

Relaxor ferroelectrics are promising for high-performance energy storage applications, particularly in miniaturized electronic devices and power systems. Herein, we investigate the influence of CeO2 doping on the structural and polarization behavior of (1 − x)BaTiO3−xCeO2 [(1 − x)BT−xC, 0.0 ≤ x ≤ 0.5] films. Our results show that the incorporation of CeO2 into BaTiO3 matrix induces chemical and structural heterogeneity, which effectively suppresses hysteresis, achieving a large energy storage density Ue of ∼37.6 J/cm3 and a high efficiency η of ∼80% for 0.7BT-0.3C films grown on Nb-SrTiO3 (001) substrates. The orientation control further refined the energy storage properties, with 0.7BT-0.3C films grown on (110)- and (111)-oriented Nb-SrTiO3 substrates exhibiting improved Ue of 44.8 and 46.8 J/cm3. The frequency and thermal stability analyses revealed that the nanostructured BaTiO3-based films maintained stable energy storage performance across a wide frequency range from 20 Hz to 10 kHz, and temperature range from 25 to 160 °C. These findings suggest the potential of nanostructure engineering and orientation control in improving the energy storage performance of BaTiO3-based films for advanced applications.

The control of interfacial oxygen vacancy provides additional functional tunability to correlated oxide heterostructures. Here, we studied the role of amorphous capping layers, such as LaAlO3 and LaMnO3, in controlling magnetic properties of (La,Sr)MnO3//SrTiO3 heterostructures. When (La,Sr)MnO3 layers are thin (∼4 nm), the amorphous LaAlO3 capping layer significantly suppresses the ferromagnetic interaction, denoted by a decreased Curie temperature TC (ΔTC over −300 K) and saturated moment MS (ΔMS at −2.9 μB/Mn) compared to the uncapped counterparts, irrespective of epitaxial orientations. This ferromagnetic suppression is attributed to the creation of interfacial oxygen vacancies, as evidenced by the recovery of ferromagnetism upon ex situ annealing (1 h at 625 °C in air). Moreover, when replacing LaAlO3 with LaMnO3, the reduction of oxygen affinity in the capping layer results in less pronounced magnetic modulations with ΔTC at −100 K and ΔMS at −1.1 μB/Mn. Also, the increase in (La,Sr)MnO3 layer thickness dilutes the capping-layer effect, leading to negligible changes on TC and MS in the LaAlO3-capped 40-nm-thick (La,Sr)MnO3//SrTiO3 samples. Our results not only shed some light on understanding the capping-layer effect in manganite-based heterostructures but also offer a feasible strategy for functional modulations at other oxide interfaces.

One preferred lead-free ferroelectric, (K,Na)NbO₃, offers prominent features of environmentally benign and excellent piezoelectricity, but suffers from poor ferroelectricity. Mn-doping has improved its electrical properties, yet its site occupancy remained unclear. In this study, Mn-atomic-layered antiphase boundaries were created in (K,Na)NbO₃-based films, revealing Mn occupying the A-site position. These boundaries stabilized ferroelectricity in these (K,Na)NbO₃-based films with a large twice remnant polarization (~72.5 μC/cm²) across a wide frequency range (20 Hz−10 kHz). High-resolution imaging shows densely arranged antiphase boundaries are grown along three crystal axes. These boundaries are Mn-atomic-enriched at a nanoscale width of a single unit cell, equilibrating the interfacial charges and clamp the interfacial strain, resulting in the highly squared hysteresis loops and high Curie temperature of ~400 °C in the films. Our results may provide a paradigm for designing high-performance lead-free ferroelectric films, unleashing their application potential for expelling lead-containing counterparts.

One preferred lead-free ferroelectric, (K,Na)NbO₃ (KNN), is gifted with the prominent features of environmentally benign and excellent piezoelectricity in both its ceramic and film forms, but it suffers from poor ferroelectricity. Albeit intensive researches have demonstrated the efficiency of Mn-doping in improving the electrical properties of KNN, the occupancy of Mn (A- or B-site) remains elusive. Here, we have formed Mn-atomic-layered antiphase boundaries in KNN-based films, and observed that Mn occupied the A-site position. The Mn-inlaid antiphase boundary has been shown to induce stable ferroelectricity with robust polarization (2P_r ~ 72.5 µC/cm²) in KNN-based films. This stability is retained across a wide range of test frequencies, from as low as 20 Hz, and up to 10 kHz, which is of great significance in KNN system. The atomically resolved STEM images reveal that densely arranged antiphase boundaries are formed and grown along three crystal axes by half unit cell lattice mismatch throughout the entire thickness of KNN-based films. These boundaries are Mn-atomic-enriched at a nanoscale width of a single unit cell, which could equilibrate the interfacial charges and clamp the vertical interfacial strain, resulting in the highly squared hysteresis loops and high Curie temperature of ~ 400 ℃ in our films. Meanwhile, these films also exhibit a high dielectric constant of 1200 and optical transmittance of ~ 75%. Our results may provide a new paradigm for designing new type of high-performance KNN-based lead-free ferroelectric films, unleashing their application potential for expelling lead-containing counterparts.