The morphology and dimension of the conductive filament formed in a memristive device are strongly influenced by the thickness of its switching medium layer. Aggressive scaling of this active layer ...thickness is critical toward reducing the operating current, voltage, and energy consumption in filamentary‐type memristors. Previously, the thickness of this filament layer has been limited to above a few nanometers due to processing constraints, making it challenging to further suppress the on‐state current and the switching voltage. Here, the formation of conductive filaments in a material medium with sub‐nanometer thickness formed through the oxidation of atomically thin two‐dimensional boron nitride is studied. The resulting memristive device exhibits sub‐nanometer filamentary switching with sub‐pA operation current and femtojoule per bit energy consumption. Furthermore, by confining the filament to the atomic scale, current switching characteristics are observed that are distinct from that in thicker medium due to the profoundly different atomic kinetics. The filament morphology in such an aggressively scaled memristive device is also theoretically explored. These ultralow energy devices are promising for realizing femtojoule and sub‐femtojoule electronic computation, which can be attractive for applications in a wide range of electronics systems that desire ultralow power operation.
A nonvolatile memristive device with a sub‐nanometer thick switching layer, sub‐picoampere operating current, and femtojoule per bit energy consumption is demonstrated. The ultrathin medium layer is formed through the oxidation of atomically thin hexagonal boron nitride. Due to the atomic‐scale confinement of the filament length, current switching characteristics disparate from that in a thicker medium are observed resulting from the distinct ionic kinetics.
The inverse spinel ferrimagnetic NiCo2O4 presents a unique model system for studying the competing effects of crystalline fields, magnetic exchange, and various types of chemical and lattice disorder ...on the electronic and magnetic states. Here, magnetotransport anomalies in high‐quality epitaxial NiCo2O4 thin films resulting from the complex energy landscape are reported. A strong out‐of‐plane magnetic anisotropy, linear magnetoresistance, and robust anomalous Hall effect above 300 K are observed in 5–30 unit cell NiCo2O4 films. The anomalous Hall resistance exhibits a nonmonotonic temperature dependence that peaks around room temperature, and reverses its sign at low temperature in films thinner than 20 unit cells. The scaling relation between the anomalous Hall conductivity and longitudinal conductivity reveals the intricate interplay between the spin‐dependent impurity scattering, band intrinsic Berry phase effect, and electron correlation. This study provides important insights into the functional design of NiCo2O4 for developing spinel‐based spintronic applications.
Strong perpendicular magnetic anisotropy, intrinsic linear magnetoresistance, and robust anomalous Hall effect above 300 K are realized in ultrathin NiCo2O4 films. The anomalous Hall resistance peaks at around room temperature, and reverses its sign at low temperature in thinner films, reflecting the intricate interplay between the spin‐dependent impurity scattering, band intrinsic Berry curvature, and electron correlation effect.
Material defects remain as the main bottleneck to the progress of topological insulators (TIs). In particular, efforts to achieve thin TI samples with dominant surface transport have always led to ...increased defects and degraded mobilities, thus making it difficult to probe the quantum regime of the topological surface states. Here, by utilizing a novel buffer layer scheme composed of an In2Se3/(Bi0.5In0.5)2Se3 heterostructure, we introduce a quantum generation of Bi2Se3 films with an order of magnitude enhanced mobilities than before. This scheme has led to the first observation of the quantum Hall effect in Bi2Se3.
In the quest to understand high-temperature superconductivity in copper oxides, debate has been focused on the pseudogap-a partial energy gap that opens over portions of the Fermi surface in the ...'normal' state above the bulk critical temperature
. The pseudogap has been attributed to precursor superconductivity, to the existence of preformed pairs and to competing orders such as charge-density waves
. A direct determination of the charge of carriers as a function of temperature and bias could help resolve among these alternatives. Here we report measurements of the shot noise of tunnelling current in high-quality La
Sr
CuO
/La
CuO
/La
Sr
CuO
(LSCO/LCO/LSCO) heterostructures fabricated using atomic layer-by-layer molecular beam epitaxy at several doping levels. The data delineate three distinct regions in the bias voltage-temperature space. Well outside the superconducting gap region, the shot noise agrees quantitatively with independent tunnelling of individual charge carriers. Deep within the superconducting gap, shot noise is greatly enhanced, reminiscent of multiple Andreev reflections
. Above the critical temperature and extending to biases much larger than the superconducting gap, there is a broad region in which the noise substantially exceeds theoretical expectations for single-charge tunnelling, indicating pairing of charge carriers. These pairs are detectable deep into the pseudogap region of temperature and bias. The presence of these pairs constrains current models of the pseudogap and broken symmetry states, while phase fluctuations limit the domain of superconductivity.
Ferroelectric domain walls, topological entities separating domains of uniform polarization, are promising candidates as active elements for nanoscale memories. In such applications, controlled ...nucleation and stabilization of domain walls are critical. Here, using in situ transmission electron microscopy and phase‐field simulations, a controlled nucleation of vertically oriented 109° domain walls in (110)‐oriented BiFeO3 (BFO) thin films is reported. In the switching experiment, reversed domains that are nucleated preferentially at the nanoscale edges of the “crest and sag” pattern‐like electrode under external bias subsequently grow into a stable stripe configuration. In addition, when triangular pockets (with an in‐plane polarization component) are present, these domain walls are pinned to form stable flux‐closure domains. Phase field simulations show that i) field enhancement at the edges of the electrode causes site‐specific domain nucleation, and ii) the local electrostatics at the domain walls drives the formation of flux closure domains, thus stabilizing the striped pattern, irrespective of the initial configuration. The results demonstrate how flux closure pinning can be exploited in conjunction with electrode patterning and substrate orientation to achieve a desired topological defect configuration. These insights constitute critical advancements in exploiting domain walls in next generation ferroelectronic devices.
The results of in situ TEM investigations of domain switching under applied bias and observation of the fine structure of domain walls down to the atomic scale reveal the dynamic behavior in (110) oriented BiFeO3 thin films. Site‐specific domain nucleation and alternating vertical domain configuration can be achieved by electrode patterning, substrate orientation, and flux‐closure pinning.
Intrinsic 2D ferromagnetic semiconductors are an important class of materials for spin‐charge conversion applications. Cr2Ge2Te6 retains long‐range magnetic order in the bilayer at cryogenic ...temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X‐ray scattering, electronic, thermal transport and first‐principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr2Ge2Te6 are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr2Ge2Te6—based devices by electron–phonon‐ and spin–orbit coupling‐based tailoring of polaron properties.
a) Atomic resolution HADF STEM image showing stacking fault. b) Electrical resistivity fitted by adiabatic small polaron hopping model ρ(T) = ATexp(Eρ/kBT) with Eρ being the activation energy. c) S(T) versus 1000/T curve fitted by polaron model S(T) = (kB/e)(α + ES/kBT) with ES being the activation energy.
•Proposed a refinement procedure for artifact-free magnetic induction map based on Lorentz 4D-STEM through the refinement of peak shape and position in the Circular Hough Transform filtered central ...diffraction disk.•Performed dynamic simulations of 4D-STEM patterns built upon superstructures consisting of millions of atoms to account for edge geometries, showing how multiple scattering affects disk shape and intensity distribution in 4D-STEM.•Through quantitative analysis and comparing experiment with calculation, we demonstrate that non-spin-related artifacts that cause the disk shift can be minimized and high-quality spin signals retrieval is possible.
Recent advancement in scanning transmission electron microscopy (STEM) allows the use of 4D-STEM, a technique that captures an electron diffraction pattern at each scan point in STEM, to measure electrostatic and magnetic potential and field in materials. However, accurate measurement, separation of the magnetic and electric signals, and removal of artifacts remain challenging, especially in the presence of complex non-uniform diffraction contrast within the disks. Here, based on dynamic simulations of 4D-STEM patterns built upon superstructures consisting of millions of atoms to account for different sample thickness and edge geometries, we show how the shape and intensity distribution of the central disk are affected by multiple scattering. We propose a robust refinement procedure through iteration of the spin-sensitive peak position of the disk-center in the circular Hough transform filtered images from experimental Lorentz 4D-STEM dataset after minimizing the possible artifacts, such as those due to the change of thickness, dynamic scattering, and scanning process. We verify that caution must be taken as in practice the rigid-disk-shift model used to reconstruct induction maps can easily break down due to disk-protrusion when there exists a nonconstant phase gradient or thickness within the width of the probe. Through quantitative analysis and comparing experiment with calculation the effect of the non-spin-related intensity distribution inside the disk as well as that causes the disk shift due to the intensity-protrusion can be removed, and high-quality magnetic field mapping is possible.
Semiconductors, featuring tunable electrical transport, and magnets, featuring tunable spin configurations, form the basis of many information technologies. A long-standing challenge has been to ...realize materials that integrate and connect these two distinct properties. Two-dimensional (2D) materials offer a platform to realize this concept, but known 2D magnetic semiconductors are electrically insulating in their magnetic phase. Here we demonstrate tunable electron transport within the magnetic phase of the 2D semiconductor CrSBr and reveal strong coupling between its magnetic order and charge transport. This provides an opportunity to characterize the layer-dependent magnetic order of CrSBr down to the monolayer via magnetotransport. Exploiting the sensitivity of magnetoresistance to magnetic order, we uncover a second regime characterized by coupling between charge carriers and magnetic defects. The magnetoresistance within this regime can be dynamically and reversibly tuned by varying the carrier concentration using an electrostatic gate, providing a mechanism for controlling charge transport in 2D magnets.
Abstract
In Mott materials strong electron correlation yields a spectrum of complex electronic structures. Recent synthesis advancements open realistic opportunities for harnessing Mott physics to ...design transformative devices. However, a major bottleneck in realizing such devices remains the lack of control over the electron correlation strength. This stems from the complexity of the electronic structure, which often veils the basic mechanisms underlying the correlation strength. This study presents control of the correlation strength by tuning the degree of orbital overlap using picometer‐scale lattice engineering. This study illustrates how bandwidth control and concurrent symmetry breaking can govern the electronic structure of a correlated SrVO
3
model system. This study shows how tensile and compressive biaxial strain oppositely affect the SrVO
3
in‐plane and out‐of‐plane orbital occupancy, resulting in the partial alleviation of the orbital degeneracy. The spectral weight redistribution under strain is derived and explained, which illustrates how high tensile strain drives the system toward a Mott insulating state. Implementation of such concepts can push correlated electron phenomena closer toward new solid‐state devices and circuits. These findings therefore pave the way for understanding and controlling electron correlation in a broad range of functional materials, driving this powerful resource for novel electronics closer toward practical realization.
Two-dimensional (2D) van der Waals (vdW) materials show a range of profound physical properties that can be tailored through their incorporation in heterostructures and manipulated with external ...forces. The recent discovery of long-range ferromagnetic order down to atomic layers provides an additional degree of freedom in engineering 2D materials and their heterostructure devices for spintronics, valleytronics, and magnetic tunnel junction switches. Here, using direct imaging by cryo-Lorentz transmission electron microscopy we show that topologically nontrivial magnetic-spin states, skyrmionic bubbles, can be realized in exfoliated insulating 2D vdW Cr2Ge2Te6. Due to the competition between dipolar interactions and uniaxial magnetic anisotropy, hexagonally packed nanoscale bubble lattices emerge by field cooling with magnetic field applied along the out-of-plane direction. Despite a range of topological spin textures in stripe domains arising due to pair formation and annihilation of Bloch lines, bubble lattices with single chirality are prevalent. Our observation of topologically nontrivial homochiral skyrmionic bubbles in exfoliated vdW materials provides a new avenue for novel quantum states in atomically thin insulators for magneto-electronic and quantum devices.
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