Showing papers by "Simon Brown published in 2020"

Realization of Symmetry-Enforced Two-Dimensional Dirac Fermions in Nonsymmorphic α-Bismuthene.

Abstract: Two-dimensional (2D) Dirac-like electron gases have attracted tremendous research interest ever since the discovery of free-standing graphene. The linear energy dispersion and nontrivial Berry phase play a pivotal role in the electronic, optical, mechanical, and chemical properties of 2D Dirac materials. The known 2D Dirac materials are gapless only within certain approximations, for example, in the absence of spin-orbit coupling (SOC). Here, we report a route to establishing robust Dirac cones in 2D materials with nonsymmorphic crystal lattice. The nonsymmorphic symmetry enforces Dirac-like band dispersions around certain high-symmetry momenta in the presence of SOC. Through μ-ARPES measurements, we observe Dirac-like band dispersions in α-bismuthene. The nonsymmorphic lattice symmetry is confirmed by μ-low-energy electron diffraction and scanning tunneling microscopy. Our first-principles simulations and theoretical topological analysis demonstrate the correspondence between nonsymmorphic symmetry and Dirac states. This mechanism can be straightforwardly generalized to other nonsymmorphic materials. The results enlighten the search of symmetry-enforced Dirac fermions in the vast uncharted world of nonsymmorphic 2D materials.

Atomic Scale Dynamics Drive Brain-like Avalanches in Percolating Nanostructured Networks

Abstract: Self-assembled networks of nanoparticles and nanowires have recently emerged as promising systems for brain-like computation. Here, we focus on percolating networks of nanoparticles which exhibit brain-like dynamics. We use a combination of experiments and simulations to show that the brain-like network dynamics emerge from atomic-scale switching dynamics inside tunnel gaps that are distributed throughout the network. The atomic-scale dynamics emulate leaky integrate and fire (LIF) mechanisms in biological neurons, leading to the generation of critical avalanches of signals. These avalanches are quantitatively the same as those observed in cortical tissue and are signatures of the correlations that are required for computation. We show that the avalanches are associated with dynamical restructuring of the networks which self-tune to balanced states consistent with self-organized criticality. Our simulations allow visualization of the network states and detailed mechanisms of signal propagation.

Long-range temporal correlations in scale-free neuromorphic networks.

Abstract: Biological neuronal networks are the computing engines of the mammalian brain. These networks exhibit structural characteristics such as hierarchical architectures, small-world attributes, and scal...

Facile dissociation of molecular nitrogen using lanthanide surfaces: towards ambient temperature ammonia synthesis

Abstract: A combined experimental and computational study is reported on a hitherto unrecognised single lanthanide catalyst for the breaking of molecular nitrogen and formation of ammonia at ambient temperature and low pressure. We combine in situ electrical conductance and electron diffraction measurements to track the conversion from the lanthanide metals to the insulating lanthanide nitrides. The efficiency of the conversion is then interpreted using $\mathrm{DFT}+U$ calculations, suggesting a molecular nitrogen dissociation pathway separate from that well established for transition metals. Finally, we show that exposure of the lanthanide surfaces to both molecular nitrogen and hydrogen results in the formation of ammonia.

Removable capping layer for air-sensitive GdN.

Abstract: The strongly correlated rare earth nitrides display unusual coupled magnetic, electronic and superconducting properties, with predicted topological states. However, their air-sensitiveness has prevented in-depth investigations of their properties. In this paper, we show that a 100 nm thick epitaxial samarium layer provides adequate passivation of 100 nm thick thin films of gadolinium nitride (GdN), the prototypical rare earth nitride, enabling ex-situ magnetic and structural characterizations. Using reflection high-energy electron diffraction, atomic force microscopy and energy dispersive x-ray spectroscopy, we investigate the thermal desorption of the samarium layer under vacuum. We finally demonstrate successful removal of the samarium capping layer in a separate vacuum chamber after exposure to air using a combination of argon ion sputtering and thermal desorption at 400 °C, recovering the GdN surface.