Abstract
Active learning—the field of machine learning (ML) dedicated to optimal experiment design—has played a part in science as far back as the 18th century when Laplace used it to guide his ...discovery of celestial mechanics. In this work, we focus a closed-loop, active learning-driven autonomous system on another major challenge, the discovery of advanced materials against the exceedingly complex synthesis-processes-structure-property landscape. We demonstrate an autonomous materials discovery methodology for functional inorganic compounds which allow scientists to fail smarter, learn faster, and spend less resources in their studies, while simultaneously improving trust in scientific results and machine learning tools. This robot science enables science-over-the-network, reducing the economic impact of scientists being physically separated from their labs. The real-time closed-loop, autonomous system for materials exploration and optimization (CAMEO) is implemented at the synchrotron beamline to accelerate the interconnected tasks of phase mapping and property optimization, with each cycle taking seconds to minutes. We also demonstrate an embodiment of human-machine interaction, where human-in-the-loop is called to play a contributing role within each cycle. This work has resulted in the discovery of a novel epitaxial nanocomposite phase-change memory material.
Oxide ion conductors find important technical applications in electrochemical devices such as solid-oxide fuel cells (SOFCs), oxygen separation membranes and sensors. Na0.5Bi0.5TiO3 (NBT) is a ...well-known lead-free piezoelectric material; however, it is often reported to possess high leakage conductivity that is problematic for its piezo- and ferroelectric applications. Here we report this high leakage to be oxide ion conduction due to Bi-deficiency and oxygen vacancies induced during materials processing. Mg-doping on the Ti-site increases the ionic conductivity to ~0.01 S cm(-1) at 600 °C, improves the electrolyte stability in reducing atmospheres and lowers the sintering temperature. This study not only demonstrates how to adjust the nominal NBT composition for dielectric-based applications, but also, more importantly, gives NBT-based materials an unexpected role as a completely new family of oxide ion conductors with potential applications in intermediate-temperature SOFCs and opens up a new direction to design oxide ion conductors in perovskite oxides.
The structure and thermoelectric (TE) properties of La-doped, A-site-deficient SrTiO3 (Sr1–3x/2La x TiO3) ceramics sintered in air and N2/5% H2 have been investigated. Air-sintered ceramics with 0.10 ...≤ x < 0.30 appear cubic by X-ray diffraction (XRD) but exhibit superstructure consistent with a tetragonal cell (a 0 a 0 c –), according to electron diffraction (ED) studies. 0.30 ≤ x < 0.50 have additional short-range A-site vacancy (V A) ordering, and x ≥ 0.50 are orthorhombic with an a – a – c + tilt system and long-range V A ordering. 0.10 ≤ x ≤ 0.50 reduced in N2/5% H2 are oxygen-deficient and appear cubic in XRD patterns but exhibit superstructure compliant with an a 0 a 0 c – tilt system by ED. For x = 0.50, additional short-range V A order is observed, and x > 0.50 are orthorhombic with an a – a – c + tilt system and long-range V A ordering. x = 0.15 sintered in N2/5% H2 shows the largest dimensionless TE figure-of-merit ZT = 0.41 at 973 K reported for n-type SrTiO3-based ceramics, suggesting that the accommodation of La through formation of (V Sr) coupled with reduction in N2/5% H2 represents a new protocol for the development of oxide-based thermoelectrics.
Abstract
Ferromagnetism and superconductivity are two key ingredients for topological superconductors, which can serve as building blocks of fault-tolerant quantum computers. Adversely, ...ferromagnetism and superconductivity are typically also two hostile orderings competing to align spins in different configurations, and thus making the material design and experimental implementation extremely challenging. A single material platform with concurrent ferromagnetism and superconductivity is actively pursued. In this paper, we fabricate van der Waals Josephson junctions made with iron-based superconductor Fe(Te,Se), and report the global device-level transport signatures of interfacial ferromagnetism emerging with superconducting states for the first time. Magnetic hysteresis in the junction resistance is observed only below the superconducting critical temperature, suggesting an inherent correlation between ferromagnetic and superconducting order parameters. The 0-π phase mixing in the Fraunhofer patterns pinpoints the ferromagnetism on the junction interface. More importantly, a stochastic field-free superconducting diode effect was observed in Josephson junction devices, with a significant diode efficiency up to 10%, which unambiguously confirms the spontaneous time-reversal symmetry breaking. Our work demonstrates a new way to search for topological superconductivity in iron-based superconductors for future high T
c
fault-tolerant qubit implementations from a device perspective.
Data-centric applications are pushing the limits of energy-efficiency in today's computing systems, including those based on phase-change memory (PCM). This technology must achieve low-power and ...stable operation at nanoscale dimensions to succeed in high-density memory arrays. Here we use a novel combination of phase-change material superlattices and nanocomposites (based on Ge
Sb
Te
), to achieve record-low power density ≈ 5 MW/cm
and ≈ 0.7 V switching voltage (compatible with modern logic processors) in PCM devices with the smallest dimensions to date (≈ 40 nm) for a superlattice technology on a CMOS-compatible substrate. These devices also simultaneously exhibit low resistance drift with 8 resistance states, good endurance (≈ 2 × 10
cycles), and fast switching (≈ 40 ns). The efficient switching is enabled by strong heat confinement within the superlattice materials and the nanoscale device dimensions. The microstructural properties of the Ge
Sb
Te
nanocomposite and its high crystallization temperature ensure the fast-switching speed and stability in our superlattice PCM devices. These results re-establish PCM technology as one of the frontrunners for energy-efficient data storage and computing.
Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter ...wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS
, and WSe
with thermal resistance greater than 100 times thicker SiO
and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.
Epitaxial tetragonal 425 and 611 nm thick Pb(Zr0.45Ti0.55)O3 (PZT) films are deposited by pulsed laser deposition on SrRuO3‐coated (100) SrTiO3 24° tilt angle bicrystal substrates to create a single ...PZT grain boundary with a well‐defined orientation. On either side of the bicrystal boundary, the films show square hysteresis loops and have dielectric permittivities of 456 and 576, with loss tangents of 0.010 and 0.015, respectively. Using piezoresponse force microscopy (PFM), a decrease in the nonlinear piezoelectric response is observed in the vicinity (720–820 nm) of the grain boundary. This region represents the width over which the extrinsic contributions to the piezoelectric response (e.g., those associated with the domain density/configuration and/or the domain wall mobility) are influenced by the presence of the grain boundary. Transmission electron microscope (TEM) images collected near and far from the grain boundary indicate a strong preference for (101)/(1¯01) type domain walls at the grain boundary, whereas (011)/(01¯1) and (101)/(1¯01) are observed away from this region. It is proposed that the elastic strain field at the grain boundary interacts with the ferro‐electric/elastic domain structure, stabilizing (101)/(1¯01) rather than (011)/(01¯1) type domain walls, which inhibits domain wall motion under applied field and decreases non‐linearity.
A decrease in the nonlinear piezoelectric response is observed in the vicinity (720–820 nm) of the 24° tilt grain boundary. It is proposed that the elastic strain field at the grain boundary interacts with the ferro‐electric/elastic domain structure, stabilizing (101)/(1¯01) rather than (011)/(01¯1) type domain walls, which inhibits domain wall motion under applied field and decreases non‐linearity.
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