Robust zero-energy modes in an electronic higher-order topological insulator: the dimerized Kagome lattice

Quantum simulators are an essential tool for understanding complex quantum materials. Platforms based on ultracold atoms in optical lattices and photonic devices led the field so far, but electronic quantum simulators are proving to be equally relevant. Simulating topological states of matter is one of the holy grails in the field. Here, we experimentally realize a higher-order electronic topological insulator (HOTI). Specifically, we create a dimerized Kagome lattice by manipulating carbon-monoxide (CO) molecules on a Cu(111) surface using a scanning tunneling microscope (STM). We engineer alternating weak and strong bonds to show that a topological state emerges at the corner of the non-trivial configuration, while it is absent in the trivial one. Contrarily to conventional topological insulators (TIs), the topological state has two dimensions less than the bulk, denoting a HOTI. The corner mode is protected by a generalized chiral symmetry, which leads to a particular robustness against perturbations. Our versatile approach to quantum simulation with artificial lattices holds promises of revealing unexpected quantum phases of matter

Band offsets at zincblende-wurtzite GaAs nanowire sidewall surfaces

The band structure and the Fermi level pinning at clean and well-ordered sidewall surfaces of zincblende (ZB)-wurtzite (WZ) GaAs nanowires are investigated by scanning tunneling spectroscopy and density functional theory calculations. The WZ-ZB phase transition in GaAs nanowires introduces p-i junctions at the sidewall surfaces. This is caused by the presence of numerous steps, which induce a Fermi level pinning at different energies on the non-polar WZ and ZB sidewall facets.This study was financially supported by the EQUIPEX program Excelsior, the European Community’s Seventh Framework Program (Grant No. PITN-GA-2012- 316751, “Nanoembrace” Project) and the Impuls- und Vernetzungsfonds of the Helmholtz-Gemeinschaft Deutscher Forschungszentren under Grant No. HIRG-0014. T. Xu acknowledges the support from the National Natural Science Foundation of China (Grant No. 61204014)

Bottom-up strategies for the assembling of magnetic systems using nanoclusters

International audienceIn the frame of the 20th Anniversary of the Journal of Nanoparticle Research (JNR), our aim is to start from the historical context twenty, years ago and to give some recent results and perspectives concerning nanomagnets prepared from clusters preformed in the gas phase using the Low Energy Cluster Beam Deposition (LECBD) technique. In this paper, we focus our attention on the typical case of Co clusters embedded in various matrices to study interface magnetic anisotropy and magnetic interactions as a function of volume concentrations, and on still current and perspectives through two examples of binary metallic 3d-5d TM (namely CoPt and FeAu) clusters assemblies to illustrate size-related and nanoalloy phenomena on magnetic properties in well-defined mass-selected clusters. The structural and magnetic properties of these cluster assemblies were investigated using various experimental techniques that include High Resolution Transmission Electron Microscopy (HRTEM), Superconducting Quantum Interference Device (SQUID) magnetometry, as well as synchrotron techniques such as Extended X-Ray Absorption Fine Structure (EXAFS) and X-Ray Magnetic Circular Dichroism (XMCD). Depending on the chemical nature of both NPs and matrix, we observe different magnetic responses compared to their bulk counterparts. In particular, we show how finite size effects (size reduction) enhance their magnetic moment and how specific relaxation in nanoalloys can impact their magnetic anisotropy

Compact localized boundary states in a quasi-1D electronic diamond-necklace chain

Zero-energy modes localized at the ends of one-dimensional (1D) wires hold great potential as qubits for fault-tolerant quantum computing. However, all the candidates known to date exhibit a wave function that decays exponentially into the bulk and hybridizes with other nearby zero-modes, thus hampering their use for braiding operations. Here, we show that a quasi-1D diamond-necklace chain exhibits a completely unforeseen type of robust boundary state, namely compact localized zero-energy modes that do not decay into the bulk. We theoretically engineer a lattice geometry to access this mode, and experimentally realize it in an electronic quantum simulator setup. Our work provides a general route for the realization of robust and compact localized zero-energy modes that could potentially be braided without the drawbacks of hybridization

Phase transformation of PbSe/CdSe nanocrystals from core-shell to Janus structure studied by photoemission spectroscopy

Photoelectron spectroscopic measurements have been performed, with synchrotron radiation on PbSe/CdSe heteronanocrystals that initially consist of core-shell structures. The study of the chemical states of the main elements in the nanocrystals shows a reproducible and progressive change in the valence-band and core-level spectra under photon irradiation, whatever the core and shell sizes are. Such chemical modifications are explained in light of transmission electron microscopy observations and reveal a phase transformation of the nanocrystals: The core-shell nanocrystals undergo a morphological change toward a Janus structure with the formation of semidetached PbSe and CdSe clusters. Photoelectron spectroscopy gives new insight into the reorganization of the ligands anchored at the surface of the nanocrystals and the modification of the electronic structure of these heteronanocrystals

Nonlinear Time Series Analysis of Nodulation Factor Induced Calcium Oscillations: Evidence for Deterministic Chaos?

Legume plants form beneficial symbiotic interactions with nitrogen fixing bacteria (called rhizobia), with the rhizobia being accommodated in unique structures on the roots of the host plant. The legume/rhizobial symbiosis is responsible for a significant proportion of the global biologically available nitrogen. The initiation of this symbiosis is governed by a characteristic calcium oscillation within the plant root hair cells and this signal is activated by the rhizobia. Recent analyses on calcium time series data have suggested that stochastic effects have a large role to play in defining the nature of the oscillations. The use of multiple nonlinear time series techniques, however, suggests an alternative interpretation, namely deterministic chaos. We provide an extensive, nonlinear time series analysis on the nature of this calcium oscillation response. We build up evidence through a series of techniques that test for determinism, quantify linear and nonlinear components, and measure the local divergence of the system. Chaos is common in nature and it seems plausible that properties of chaotic dynamics might be exploited by biological systems to control processes within the cell. Systems possessing chaotic control mechanisms are more robust in the sense that the enhanced flexibility allows more rapid response to environmental changes with less energetic costs. The desired behaviour could be most efficiently targeted in this manner, supporting some intriguing speculations about nonlinear mechanisms in biological signaling

Sheet resistance of multi-layer stacking of silicene

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Bile acids induce calcium signals in mouse pancreatic acinar cells: implications for bile-induced pancreatic pathology

The effect of the natural bile acid, taurolithocholic acid 3-sulfate (TLC-S), on calcium signalling in pancreatic acinar cells has been investigated. TLC-S induced global calcium oscillations and extended calcium transients as well as calcium signals localised to the secretory granule (apical) region of acinar cells. These calcium signals could still be triggered by TLC-S in a calcium-free external solution. TLC-S-induced calcium signals were not inhibited by atropine, but were abolished by caffeine or by depletion of calcium stores, due to prolonged application of ACh. Global calcium signals, produced by TLC-S application, displayed vectorial apical-to-basal polarity. The signals originated in the apical part and were then propagated to the basal region. Other natural bile acids, taurocholate (TC) and taurodeoxycholate (TDC), were also able to produce local and global calcium oscillations (but at higher concentrations than TLC-S). Bile, which can enter pancreas by reflux, has been implicated in the pathology of acute pancreatitis. The calcium releasing properties of bile acids suggest that calcium toxicity could be an important contributing factor in the bile acid-induced cellular damage

Electrical characterization of semiconductor nanowires by scanning tunneling microscopy

In order to understand the structural and electronic properties of semiconductor nanowires, scanning tunneling microscopy is an appealing technique that can supplement transmission electron microscopies and conventional electrical characterization techniques. It is able to probe the surface of semiconductor materials at the atomic scale and can be successfully applied to study the nanofaceting morphology, the atomic structure and the surface composition of oxide-free nanowire sidewalls. Based on the advantages provided by the unique geometry of semiconductor nanowires for a low-cost and efficient integration into nanoscale devices, additional characterization schemes performed with multiple probe scanning tunneling microscopy are also presented to get a deeper understanding of their transport properties