Dr.jens.brede

Nature Physics. doi:10.1038/nphys3264

Authors: Christian Pauly, Bertold Rasche, Klaus Koepernik, Marcus Liebmann, Marco Pratzer, Manuel Richter, Jens Kellner, Markus Eschbach, Bernhard Kaufmann, Lukasz Plucinski, Claus M. Schneider, Michael Ruck, Jeroen van den Brink & Markus Morgenstern

Helical locking of spin and momentum and prohibited backscattering are the key properties of topologically protected states. They are expected to enable novel types of information processing by providing pure spin currents, or fault tolerant quantum computation by using the Majorana fermions at interfaces of topological states with superconductors. So far, the required helical conduction channels used to realize Majorana fermions are generated through the application of an axial magnetic field to conventional semiconductor nanowires. Avoiding the magnetic field enhances the possibilities for circuit design significantly. Here, we show that subnanometre-wide electron channels with natural helicity are present at surface step edges of the weak topological insulator Bi14Rh3I9 (ref. ). Scanning tunneling spectroscopy reveals the electron channels to be continuous in both energy and space within a large bandgap of 200 meV, evidencing its non-trivial topology. The absence of these channels in the closely related, but topologically trivial compound Bi13Pt3I7 corroborates the channels’ topological nature. The backscatter-free electron channels are a direct consequence of Bi14Rh3I9’s structure: a stack of two-dimensional topologically insulating, graphene-like planes separated by trivial insulators. We demonstrate that the surface of Bi14Rh3I9 can be engraved using an atomic force microscope, allowing networks of protected channels to be patterned with nanometre precision.

We review different electron transport methods to probe the magnetic properties, such as the magnetic anisotropy, of an individual Fe 4 SMM. The different approaches comprise first and higher order transport through the molecule. Gate spectroscopy, focusing on the charge degeneracy-point, is presented as a robust technique to quantify the longitudinal magnetic anisotropy of the SMM in different redox states. We provide statistics showing the robustness and reproducibility of the different methods. In addition, conductance measurements typically show high-energy excited states well beyond the ground spin multiplet of SMM. Some of these excitations have their origin in excited spin multiplets, others in vibrational modes of the molecule. The interplay between vibrations, charge and spin may yield a new approach for spin control.

We present a detailed study of an electric-field induced phase transition of a single layer of Fe on a Ni(111) substrate. Scanning tunneling microscopy at 4 K substrate temperature is used to provide the necessary electric field and to follow the transition from face-centered cubic to hexagonal closepacked stacking with atomic resolution.

The opening of a gap in single-layer graphene is often ascribed to the breaking of the equivalence between the two carbon sublattices. We show by angle-resolved photoemission spectroscopy that Ir- and Na-modified graphene grown on the Ir(111) surface presents a very large unconventional gap that can be described in terms of a phenomenological "massless" Dirac model. We discuss the consequences and differences of this model in comparison of the standard massive gap model, and we investigate the conditions under which such anomalous gap can arise from a spontaneous symmetry breaking.

Article

Precise surface modification of titanium dioxide is useful for numerous applications. Here, the authors report that high ion dose bombardment transforms the surface of titanium dioxide (110) into single-crystalline titanium oxide (001) thin film, unlike previous lower energy ion bombardment studies.

Nature Communications doi: 10.1038/ncomms7147

Authors: B.M. Pabón, J.I. Beltrán, G. Sánchez-Santolino, I. Palacio, J. López-Sánchez, J. Rubio-Zuazo, J.M. Rojo, P. Ferrer, A. Mascaraque, M.C. Muñoz, M. Varela, G.R. Castro, O. Rodríguez de la Fuente

Nature Physics. doi:10.1038/nphys3240

Authors: Dimitri Roditchev, Christophe Brun, Lise Serrier-Garcia, Juan Carlos Cuevas, Vagner Henrique Loiola Bessa, Milorad Vlado Milošević, François Debontridder, Vasily Stolyarov & Tristan Cren

Superconducting correlations may propagate between two superconductors separated by a tiny insulating or metallic barrier, allowing a dissipationless electric current to flow. In the presence of a magnetic field, the maximum supercurrent oscillates and each oscillation corresponding to the entry of one Josephson vortex into the barrier. Josephson vortices are conceptual blocks of advanced quantum devices such as coherent terahertz generators or qubits for quantum computing, in which on-demand generation and control is crucial. Here, we map superconducting correlations inside proximity Josephson junctions using scanning tunnelling microscopy. Unexpectedly, we find that such Josephson vortices have real cores, in which the proximity gap is locally suppressed and the normal state recovered. By following the Josephson vortex formation and evolution we demonstrate that they originate from quantum interference of Andreev quasiparticles, and that the phase portraits of the two superconducting quantum condensates at edges of the junction decide their generation, shape, spatial extent and arrangement. Our observation opens a pathway towards the generation and control of Josephson vortices by applying supercurrents through the superconducting leads of the junctions, that is, by purely electrical means without any need for a magnetic field, which is a crucial step towards high-density on-chip integration of superconducting quantum devices.

Article

It is challenging to determine thermodynamic quantities for single molecules. Here, the authors access single-molecule thermodynamic information via a microscopic and computational study of a confined molecule, for which the resulting patterns represent a real-space equilibrium probability distribution.

Nature Communications doi: 10.1038/ncomms7210

Authors: Carlos-Andres Palma, Jonas Björk, Florian Klappenberger, Emmanuel Arras, Dirk Kühne, Sven Stafström, Johannes V. Barth

Author(s): Rasmus Westerström, Anne-Christine Uldry, Roland Stania, Jan Dreiser, Cinthia Piamonteze, Matthias Muntwiler, Fumihiko Matsui, Stefano Rusponi, Harald Brune, Shangfeng Yang, Alexey Popov, Bernd Büchner, Bernard Delley, and Thomas Greber

The interaction between the endohedral unit in the single-molecule magnet Dy₂ScN@C₈₀ and a rhodium (111) substrate leads to alignment of the Dy 4f orbitals. The resulting orientation of the Dy₂ScN plane parallel to the surface is inferred from comparison of the angular anisotropy of x-ray absorption...

[Phys. Rev. Lett. 114, 087201] Published Mon Feb 23, 2015

A coupling-limited approach for the Ullmann reaction-like on-surface synthesis of a two-dimensional covalent organic network starting from a halogenated metallo-porphyrin is demonstrated. Copper-octabromo-tetraphenylporphyrin molecules can diffuse and self-assemble when adsorbed on the inert Au(111) surface. Splitting-off of bromine atoms bonded at the macrocyclic core of the porphyrin starts at room temperature after the deposition and is monitored by X-ray photoelectron spectroscopy for different annealing steps. Direct coupling between the reactive carbon sites of the molecules is, however, hindered by the molecular shape. This leads initially to an ordered non-covalently interconnected supramolecular structure. Further heating to 300{\deg}C and an additional hydrogen dissociation step is required to link the molecular macrocycles via a phenyl group and form large ordered polymeric networks. This approach leads to a close-packed covalently bonded network of overall good quality. The structures are characterized using scanning tunneling microscopy. Different kinds of lattice defects and, furthermore, the impact of polymerization on the HOMO-LUMO gap are discussed. Density functional theory calculations corroborate the interpretations and give further insight into the adsorption of the debrominated molecule on the surface and the geometry and coupling reaction of the polymeric structure.

Author(s): Kirsten von Bergmann, Markus Ternes, Sebastian Loth, Christopher P. Lutz, and Andreas J. Heinrich

Spin-resolved scanning tunneling microscopy is employed to quantitatively determine the spin polarization of the magnetic field-split Kondo state. Tunneling conductance spectra of a Kondo-screened magnetic atom are evaluated within a simple model taking into account inelastic tunneling due to spin e...

[Phys. Rev. Lett. 114, 076601] Published Fri Feb 20, 2015

Nature journals offer double-blind review

Nature 518, 7539 (2015). doi:10.1038/518274b

Starting in March, Nature and the monthly Nature research journals will offer an alternative to conventional peer review. Authors will be able to request that their names and affiliations are withheld from reviewers of their papers — a form of peer review known as

Article

Interfaces between two materials often show interesting properties. Here, the authors demonstrate that diamond and cubic boron nitride, the hardest materials known, can be grown on top of each other through a novel misfit accommodation mechanism, forming a two-dimensional electron gas at the interface.

Nature Communications doi: 10.1038/ncomms7327

Authors: Chunlin Chen, Zhongchang Wang, Takeharu Kato, Naoya Shibata, Takashi Taniguchi, Yuichi Ikuhara

Article

Artificial photonic graphene, a honeycomb array of evanescently coupled waveguides, has proven to be a useful tool for investigating graphene physics in various optical settings. Here, Song et al. demonstrate pseudospin-mediated vortex generation and topological charge flipping in otherwise uniform optical beams.

Nature Communications doi: 10.1038/ncomms7272

Authors: Daohong Song, Vassilis Paltoglou, Sheng Liu, Yi Zhu, Daniel Gallardo, Liqin Tang, Jingjun Xu, Mark Ablowitz, Nikolaos K. Efremidis, Zhigang Chen

Quantum systems are inherently dissipation-less, making them excellent candidates even for classical information processing. We propose to use an array of large-spin quantum magnets for realizing a device which has two modes of operation: memory and data-bus. While the weakly interacting low-energy levels are used as memory to store classical information (bits), the high-energy levels strongly interact with neighboring magnets and mediate the spatial movement of information through quantum dynamics. Despite the fact that memory and data-bus require different features, which are usually prerogative of different physical systems -- well isolation for the memory cells, and strong interactions for the transmission -- our proposal avoids the notorious complexity of hybrid structures. The proposed mechanism can be realized with different setups. We specifically show that molecular magnets, as the most promising technology, can implement hundreds of operations within their coherence time, while adatoms on surfaces probed by a scanning tunneling microscope is a future possibility.

The diffusion of Pb atoms under a graphene layer deposited on a Si(5 5 3)-Au surface is studied by the first-principles density functional theory. The presence of graphene locks the moving Pb atoms inside tube-like closed objects, formed by grooves near steps of the Si(5 5 3)-Au surface and limited from the top by the graphene layer. As a result, the diffusion processes are well-separated from the environment. The methods of experimental verification of the undercover diffusion are also proposed.

Nature Materials. doi:10.1038/nmat4215

Authors: Ilija Zeljkovic, Yoshinori Okada, Maksym Serbyn, R. Sankar, Daniel Walkup, Wenwen Zhou, Junwei Liu, Guoqing Chang, Yung Jui Wang, M. Zahid Hasan, Fangcheng Chou, Hsin Lin, Arun Bansil, Liang Fu & Vidya Madhavan

Abstract

Electric double layers (EDLs) of ionic liquids have been used in superconducting field-effect transistors as nanogap capacitors. Because of the freezing of the ionic motion below ~200 kelvin, modulations of the carrier density have been limited to the high-temperature regime. Here we observe carrier-doping–induced superconductivity in an organic Mott insulator with a photoinduced EDL based on a photochromic spiropyran monolayer. Because the spiropyran can isomerize reversibly between nonionic and zwitterionic isomers through photochemical processes, two distinct built-in electric fields can modulate the carrier density even at cryogenic conditions. Authors: Masayuki Suda, Reizo Kato, Hiroshi M. Yamamoto

Article

Surface strain affects the performance of catalysts. Here, the authors present computational evidence that mechanical strain of late transition metals can modify binding energies at stepped surfaces through a mechanical energy contribution yielding chemical trends unique from the established d-band model.

Nature Communications doi: 10.1038/ncomms7261

Authors: M. F. Francis, W. A. Curtin

Author(s): Zhenzhao Jia, Baoming Yan, Jingjing Niu, Qi Han, Rui Zhu, Dapeng Yu, and Xiaosong Wu

Enhancement of the spin-orbit coupling in graphene may lead to various topological phenomena and also find applications in spintronics. Adatom adsorption has been proposed as an effective way to achieve the goal. In particular, great hope has been held for indium in strengthening the spin-orbit coup…

[Phys. Rev. B 91, 085411] Published Thu Feb 12, 2015