Dr.jens.brede

There are two types of intrinsic surface states in solids. The first type is formed on the surface of topological insulators. Recently, transport of massless Dirac fermions in the band of “topological” states has been demonstrated. States of the second type were predicted by Tamm and Shockley long ago. They do not have a topological background and are therefore strongly dependent on the properties of the surface. We study the problem of the conductivity of Tamm-Shockley edge states through direct transport experiments. Aharonov-Bohm magneto-oscillations of resistance are found on graphene samples that contain a single nanohole. The effect is explained by the conductivity of the massless Dirac fermions in the edge states cycling around the nanohole. The results demonstrate the deep connection between topological and non-topological edge states in 2D systems of massless Dirac fermions.

Scientific Reports 4 doi: 10.1038/srep07578

Nature Materials. doi:10.1038/nmat4169

Authors: Achim Woessner, Mark B. Lundeberg, Yuanda Gao, Alessandro Principi, Pablo Alonso-González, Matteo Carrega, Kenji Watanabe, Takashi Taniguchi, Giovanni Vignale, Marco Polini, James Hone, Rainer Hillenbrand & Frank H. L. Koppens

We study the properties of edge states in in-plane heterostructures made of adjacent zigzag graphene and BN ribbons. While in pure zigzag graphene nanoribbons, gapless edge states are nearly flat and cannot contribute significantly to the conduction, at BN/Graphene interfaces the properties of these states are significantly modified. They are still strongly localized at the zigzag edges of graphene but they exhibit a high group velocity up to 4.3x10^5 m/s at the B/C interface and even 7.4x10^5 m/s at the N-C interface. For a given wave vector the velocities of N/C and B/C hybrid interface states have opposite signs. Additionally, in the case of asymmetric structure BN/Graphene/BN, a bandgap of about 207 meV is open for sub-ribbon widths of 5 nm. These specific properties suggest new ways to engineer and control the transport properties of graphene nanostructures.

Magnetization curves of two rectangular metal-organic coordination networks formed by the organic ligand TCNQ (7,7,8,8-tetracyanoquinodimethane) and two different (Mn and Ni) 3d transition metal atoms [M(3d)] show marked differences that are explained using first principles density functional theory and model calculations. We find that the existence of a weakly dispersive hybrid band with M(3d) and TCNQ character crossing the Fermi level is determinant for the appearance of ferromagnetic coupling between metal centers, as it is the case of the metallic system Ni-TCNQ but not of the insulating system Mn-TCNQ. The spin magnetic moment localized at the Ni atoms induces a significant spin polarization in the organic molecule; the corresponding spin density being delocalized along the whole system. The exchange interaction between localized spins at Ni centers and the itinerant spin density is ferromagnetic. Based on two different model Hamiltonians, we estimate the strength of exchange couplings between magnetic atoms for both Ni- and Mn-TCNQ networks that results in weak ferromagnetic and very weak antiferromagnetic correlations for Ni- and Mn-TCNQ networks, respectively.

Author(s): I. I. Klimovskikh, S. S. Tsirkin, A. G. Rybkin, A. A. Rybkina, M. V. Filianina, E. V. Zhizhin, E. V. Chulkov, and A. M. Shikin

The electronic and spin structure of a graphene monolayer synthesized on Pt(111) has been investigated experimentally by angle- and spin-resolved photoemission with different polarizations of incident synchrotron radiation and using density functional theory calculations. It is shown that despite th...

[Phys. Rev. B 90, 235431] Published Mon Dec 22, 2014

Inspired by the recent experimental observation of topological superconductivity in ferromagnetic chains, we consider a dilute 2D lattice of magnetic atoms deposited on top of a superconducting surface with a Rashba spin-orbit coupling. We show that the studied system supports a generalization of $p_x+ip_y$ superconductivity and that its topological phase diagram contains Chern numbers higher than $\xi/a$ $(\gg1)$, where $\xi$ is the superconducting coherence length and $a$ is the distance between the magnetic atoms. The signatures of nontrivial topology can be observed by STM spectroscopy in finite-size islands.

Abstract

Functionalized olefin cross-coupling to construct carbon–carbon bonds

Nature 516, 7531 (2014). doi:10.1038/nature14006

Authors: Julian C. Lo, Jinghan Gui, Yuki Yabe, Chung-Mao Pan & Phil S. Baran

Carbon–carbon (C–C) bonds form the backbone of many important molecules, including polymers, dyes and pharmaceutical agents. The development of new methods to create these essential connections in a rapid and practical fashion has been the focus of numerous organic chemists. This endeavour relies heavily on

Refereeing: What football can teach science

Nature 516, 7531 (2014). doi:10.1038/516329e

Author: Arturo Sala

One solution to the challenges posed by voluntary peer review (M.ArnsNature515, 46710.1038/515467a (2014) and see Nature515, 480–482;10.1038/515480a2014) might be to create a professional, independent body of reviewers

Targeted at fueling future transportation and sustaining smart grids, lithium-ion batteries (LIBs) are undergoing intensive investigation for improved durability and energy density. Atomic layer deposition (ALD), enabling uniform and conformal nanofilms, has recently made possible many new advances for superior LIBs. The progress was summarized by Liu and Sun in their latest review [1], offering many insightful views, covering the design of nanostructured battery components (i.e., electrodes and solid electrolytes), and nanoscale modification of electrode/electrolyte interfaces. This work well informs peers of interesting research conducted and it will also further help boost the applications of ALD in next-generation LIBs and other advanced battery technologies.

Nature Physics. doi:10.1038/nphys3173

Authors: Fabian Calleja, Héctor Ochoa, Manuela Garnica, Sara Barja, Juan Jesús Navarro, Andrés Black, Mikhail M. Otrokov, Evgueni V. Chulkov, Andrés Arnau, Amadeo L. Vázquez de Parga, Francisco Guinea & Rodolfo Miranda

The electronic band structure of a material can acquire interesting topological properties in the presence of a magnetic field or as a result of the spin–orbit coupling. We study graphene on Ir, with Pb monolayer islands intercalated between the graphene sheet and the Ir surface. Although the graphene layer is structurally unaffected by the presence of the Pb islands, its electronic properties change markedly, with regularly spaced resonances appearing. We interpret these resonances as the effect of a strong and spatially modulated spin–orbit coupling, induced in graphene by the Pb monolayer. As well as confined electronic states, the electronic spectrum has a series of gaps with non-trivial topological properties, resembling a realization of the quantum spin Hall effect proposed by Bernevig and Zhang.

Controlling domain wall (DW) generation and dynamics behaviour in ferromagnetic nanowire is critical to the engineering of domain wall-based non-volatile logic and magnetic memory devices. Previous research showed that DW generation suffered from a random or stochastic nature and that makes the realization of DW based device a challenging task. Conventionally, stabilizing a Néel DW requires a long pulsed current and the assistance of an external magnetic field. Here, we demonstrate a method to deterministically produce single DW without having to compromise the pulse duration. No external field is required to stabilize the DW. This is achieved by controlling the stray field magnetostatic interaction between a current-carrying strip line generated DW and the edge of the nanowire. The natural edge-field assisted domain wall generation process was found to be twice as fast as the conventional methods and requires less current density. Such deterministic DW generation method could potentially bring DW device technology, a step closer to on-chip application.

Scientific Reports 4 doi: 10.1038/srep07459

Article

Plasmons in metallic nanostructures provide light enhancement that amplifies their nonlinear optical response. This study shows that graphene nanoislands also give rise to an amplified nonlinear polarizability that can be tuned electrically to surpass those of other nonlinear media by orders of magnitude.

Nature Communications doi: 10.1038/ncomms6725

Authors: Joel D. Cox, F. Javier García de Abajo

Author(s): Bent Weber, Hoon Ryu, Y.-H. Matthias Tan, Gerhard Klimeck, and Michelle Y. Simmons

The recent observation of ultralow resistivity in highly doped, atomic-scale silicon wires has sparked interest in what limits conduction in these quasi-1D systems. Here we present electron transport measurements of gated Si∶P wires of widths 4.6 and 1.5 nm. At 4.6 nm we find an electron mobility, μ_...

[Phys. Rev. Lett. 113, 246802] Published Wed Dec 10, 2014

Author(s): D. Rakhmilevitch, R. Korytár, A. Bagrets, F. Evers, and O. Tal

The satellite peaks accompaning the zero-bias conductance peak in single molecule junctions of copper-phtalocyanide are experimentally shown to orignate from electron-phonon interactions.

[Phys. Rev. Lett. 113, 236603] Published Fri Dec 05, 2014

Atomically precise armchair graphene nanoribbons of width $N=7$ (7-AGNRs) are investigated by scanning tunneling spectroscopy (STS) on Au(111). The analysis of energy-dependent standing wave patterns of finite length ribbons allows, by Fourier transformation, the direct extraction of the dispersion relation of frontier electronic states. Aided by density functional theory calculations, we assign the states to the valence band, the conduction band and the next empty band of 7-AGNRs, determine effective masses of $0.42\pm 0.08\,m_e$, $0.40\pm 0.18\,m_e$ and $0.20\pm 0.03\,m_e$, respectively, and a band gap of $2.37\pm 0.06$ eV.

Defects play a key role in determining the properties of most materials and, because they tend to be highly localized, characterizing them at the single-defect level is particularly important. Scanning tunneling microscopy (STM) has a history of imaging the electronic structure of individual point defects in conductors, semiconductors, and ultrathin films, but single-defect electronic characterization at the nanometer-scale remains an elusive goal for intrinsic bulk insulators. Here we report the characterization and manipulation of individual native defects in an intrinsic bulk hexagonal boron nitride (BN) insulator via STM. Normally, this would be impossible due to the lack of a conducting drain path for electrical current. We overcome this problem by employing a graphene/BN heterostructure, which exploits graphene's atomically thin nature to allow visualization of defect phenomena in the underlying bulk BN. We observe three different defect structures that we attribute to defects within the bulk insulating boron nitride. Using scanning tunneling spectroscopy (STS), we obtain charge and energy-level information for these BN defect structures. In addition to characterizing such defects, we find that it is also possible to manipulate them through voltage pulses applied to our STM tip.