Dr.jens.brede

We calculate the spatially resolved tunneling conductance of topological superconductors (TSCs) based on ferromagnetic chains, measured by means of spin-polarised scanning tunneling microscopy (SPSTM). Our analysis reveals novel signatures of MFs arising from the interplay of their strongly anisotropic spin-polarisation and the magnetisation content of the tip. We focus on the deep Yu-Shiba-Rusinov (YSR) limit where only YSR bound states localised in the vicinity of the adatoms govern the low-energy as also the topological properties of the system. Under these conditions, we investigate the occurence of zero/finite bias peaks (ZBPs/FBPs) for a single or two coupled TSC chains forming a Josephson junction. Each TSC can host up to two Majorana fermions (MFs) per edge if chiral symmetry is preserved. Here we retrieve the conductance for all the accessible configurations of the MF number of each chain. Our results illustrate innovative spin-polarisation-sensitive experimental routes for arresting the MFs by either restoring or splitting the ZBP in a predictable fashion via: i) weakly breaking chiral symmetry, e.g. by the SPSTM tip itself or by an external Zeeman field and ii) tuning the superconducting phase difference of the TSCs, which is encoded in the 4$\pi$-Josephson coupling of neighbouring MFs.

Author(s): M. Hoffmann, J. Weischenberg, B. Dupé, F. Freimuth, P. Ferriani, Y. Mokrousov, and S. Heinze

We predict the occurrence of a novel type of atomic-scale spin lattice in an Fe monolayer on the Ir(001) surface. Based on density functional theory calculations we parametrize a spin Hamiltonian and solve it numerically using Monte Carlo simulations. We find the stabilization of a three-dimensional…

[Phys. Rev. B 92, 020401(R)] Published Mon Jul 06, 2015

Graphene–metal interface as one of the interesting graphene-based objects attracts much attention from both application and fundamental science points of view. This paper gives a timely review of the recent experimental works on the growth and the electronic properties of the graphene–metal interfaces. This work makes a link between huge amount of experimental and theoretical data allowing one to understand the influence of the metallic substrate on the electronic properties of a graphene overlayer and how its properties can be modified in a controllable way. The further directions of studies and applications of the graphene–metal interfaces are discussed.

Author(s): Z. F. Wang and Feng Liu

Graphene, the first discovered 2D Dirac material, has had a profound impact on science and technology. In the last decade, we have witnessed huge advances in graphene related fundamental and applied research. Here, based on first-principles calculations, we propose a new 2D Dirac band on the Si(111)…

[Phys. Rev. Lett. 115, 026803] Published Tue Jul 07, 2015

Article

Molecular wires with high conductivity and conformable flexibility are desirable in nanoelectronics. Here, Nacci et al. grow flexible molecular wires, composed of alternating donor and acceptor units, on a metal surface, and show that high conductance can be achieved without electronic delocalization.

Nature Communications doi: 10.1038/ncomms8397

Authors: Christophe Nacci, Francisco Ample, David Bleger, Stefan Hecht, Christian Joachim, Leonhard Grill

Author(s): Dennis Huang, Can-Li Song, Tatiana A. Webb, Shiang Fang, Cui-Zu Chang, Jagadeesh S. Moodera, Efthimios Kaxiras, and Jennifer E. Hoffman

We use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO3(001). We map the momentum-space band structure by combining quasiparticle interference imaging with decay length spectroscopy. In addition to quan…

[Phys. Rev. Lett. 115, 017002] Published Thu Jul 02, 2015

Author(s): Sujoy Karan, David Jacob, Michael Karolak, Christian Hamann, Yongfeng Wang, Alexander Weismann, Alexander I. Lichtenstein, and Richard Berndt

Small changes in the configuration of a molecule can significantly influence its resistive behavior in a circuit.

[Phys. Rev. Lett. 115, 016802] Published Thu Jul 02, 2015

Nature Nanotechnology 10, 603 (2015). doi:10.1038/nnano.2015.78

Authors: T. Scrace, Y. Tsai, B. Barman, L. Schweidenback, A. Petrou, G. Kioseoglou, I. Ozfidan, M. Korkusinski & P. Hawrylak

Materials often exhibit fundamentally new phenomena in reduced dimensions that potentially lead to novel applications. This is true for single-layer, two-dimensional semiconductor crystals of transition-metal dichalcogenides, MX2 (M = Mo, W and X = S, Se). They exhibit direct bandgaps with energies in the visible region at the two non-equivalent valleys in the Brillouin zone. This makes them suitable for optoelectronic applications that range from light-emitting diodes to light harvesting and light sensors, and to valleytronics. Here, we report the results of a magnetoluminescence study of WS2 single-layer crystals in which the strong spin–orbit interaction additionally locks the valley and spin degrees of freedom. The recombination of the negatively charged exciton in the presence of a two-dimensional electron gas (2DEG) is found to be circularly polarized at zero magnetic field despite being excited with unpolarized light, which indicates that the existence of a valley polarized 2DEG is caused by valley and spin locking and strong electron–electron interactions.

Nature Nanotechnology 10, 589 (2015). doi:10.1038/nnano.2015.113

Authors: K. Shibata, J. Iwasaki, N. Kanazawa, S. Aizawa, T. Tanigaki, M. Shirai, T. Nakajima, M. Kubota, M. Kawasaki, H. S. Park, D. Shindo, N. Nagaosa & Y. Tokura

Mechanical control of magnetism is an important and promising approach in spintronics. To date, strain control has mostly been demonstrated in ferromagnetic structures by exploiting a change in magnetocrystalline anisotropy. It would be desirable to achieve large strain effects on magnetic nanostructures. Here, using in situ Lorentz transmission electron microscopy, we demonstrate that anisotropic strain as small as 0.3% in a chiral magnet of FeGe induces very large deformations in magnetic skyrmions, as well as distortions of the skyrmion crystal lattice on the order of 20%. Skyrmions are stabilized by the Dzyaloshinskii–Moriya interaction, originating from a chiral crystal structure. Our results show that the change in the modulation of the strength of this interaction is amplified by two orders of magnitude with respect to changes in the crystal lattice due to an applied strain. Our findings may provide a mechanism to achieve strain control of topological magnetic structures based on the Dzyaloshinskii–Moriya interaction.

As an emerging class of new materials, two-dimensional (2D) non-graphene materials, including layered and non-layered, and their heterostructures are currently attracting increasing interest due to their promising applications in electronics, optoelectronics and clean energy. In contrast to traditional semiconductors, such as Si, Ge and III–V group materials, 2D materials show significant merits of ultrathin thickness, very high surface-to-volume ratio, and high compatibility with flexible devices. Owing to these unique properties, while scaling down to ultrathin thickness, devices based on these materials as well as artificially synthetic heterostructures exhibit novel and surprising functions and performances. In this review, we aim to provide a summary on the state-of-the-art research activities on 2D non-graphene materials. The scope of the review will cover the preparation of layered and non-layered 2D materials, construction of 2D vertical van der Waals and lateral ultrathi...

An electro-mechanical setup for the measurement of AC-forces in a low-temperature tunnelling microscope has been developed, which enables extremely high force resolution. The crosstalk of vibrations onto the tunnelling current is used to measure the deflection of a force-sensing cantilever beam. We demonstrate its capability to measure the noise of the force at a tunnelling contact using polycrystalline Iridium. Depending on temperature, spring constant and current, a resolution in the range of $\rm {fN}/\sqrt{\rm Hz}$ is possible. We observe peak levels of the force-noise at the energy of the expected phonon maximal density of states, which suggests that inelastic transport processes contribute to force fluctuations.

Article

Ligand design contributes to dictating the magnetic properties of lanthanide-based single-molecule magnets. Here, the authors report a series of phosphorus-ligated dysprosium complexes, and show that the dynamic magnetic properties change as the ligand is varied from phosphine to phosphide to phosphinidene.

Nature Communications doi: 10.1038/ncomms8492

Authors: Thomas Pugh, Floriana Tuna, Liviu Ungur, David Collison, Eric J.L. McInnes, Liviu F. Chibotaru, Richard A. Layfield

Author(s): C.-Z. Xu, Y. Liu, R. Yukawa, L.-X. Zhang, I. Matsuda, T. Miller, and T.-C. Chiang

The complex behavior of photoemission in topological insulators is a result of quantum interference involving three photoemission channels according to abinitio calculations.

[Phys. Rev. Lett. 115, 016801] Published Tue Jun 30, 2015

Nature Physics. doi:10.1038/nphys3369

Authors: R. Cireasa, A. E. Boguslavskiy, B. Pons, M. C. H. Wong, D. Descamps, S. Petit, H. Ruf, N. Thiré, A. Ferré, J. Suarez, J. Higuet, B. E. Schmidt, A. F. Alharbi, F. Légaré, V. Blanchet, B. Fabre, S. Patchkovskii, O. Smirnova, Y. Mairesse & V. R. Bhardwaj

Chiral molecules that are non-superimposable mirror images of each other, known as enantiomers, have identical chemical and physical properties unless they interact with another chiral entity, such as chiral light. Chiroptical effects arising from such interactions are used to detect enantiomers in mixtures and to induce enantioselective synthesis and catalysis. Chiroptical effects often arise from the interplay between light-induced electric- and magnetic-dipole transitions in a molecule and evolve on ultrafast electronic timescales. Here we use high-harmonic generation from a randomly oriented gas of molecules subjected to an intense laser field, to probe chiral interactions on these sub-femtosecond timescales. We show that a slight disparity in the laser-driven electron dynamics in the two enantiomers is recorded and amplified by several orders of magnitude in the harmonic spectra. Our work shows that chiroptical detection can go beyond detecting chiral structure to resolving and controlling chiral dynamics on electronic timescales.

Article

Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Here, Stetsovych et al . demonstrate the potential of simultaneously combining atomic force microscopy and scanning tunnelling microscopy to identify the atomic species populating the (101) surface of anatase.

Nature Communications doi: 10.1038/ncomms8265

Authors: Oleksandr Stetsovych, Milica Todorović, Tomoko K. Shimizu, César Moreno, James William Ryan, Carmen Pérez León, Keisuke Sagisaka, Emilio Palomares, Vladimír Matolín, Daisuke Fujita, Ruben Perez, Oscar Custance

The high index (441) surface of bismuth has been studied using Scanning Tunnelling Microscopy (STM), Angle Resolved Photoemission Spectroscopy (APRES) and spin-resolved ARPES. The surface is strongly corrugated, exposing a regular array of (110)-like terraces. Two surface localised states are observed, both of which are linearly dispersing in one in-plane direction ($k_x$), and dispersionless in the orthogonal in-plane direction ($k_y$), and both of which have a Dirac-like crossing at $k_x$=0. Spin ARPES reveals a strong in-plane polarisation, consistent with Rashba-like spin-orbit coupling. One state has a strong out-of-plane spin component, which matches with the miscut angle, suggesting its {possible} origin as an edge-state. The electronic structure of Bi(441) has significant similarities with topological insulator surface states and is expected to support one dimensional Quantum Spin Hall-like coupled spin-charge transport properties with inhibited backscattering, without requiring a topological insulator bulk.

In strained graphene, lattice deformation can create pseudo-magnetic fields and result in zero-field Landau level-like quantization. In the presence of an external magnetic field, valley-polarized Landau levels are predicted to be observed because the pseudo-magnetic fields are of opposite directions in the K and K' valleys of graphene. Here, we present experimental spectroscopic measurements by scanning tunneling microscopy of strained graphene on Rh foil. We direct observed valley splitting of the Landau level induced by the coexistence of the pseudo-magnetic fields and external magnetic fields. The observed result paves the way to exploit novel electronic properties in graphene through the combination of the pseudo-magnetic fields and the external magnetic fields.

Abstract

Article

Modulation of the spin states of molecules adsorbed on metal surfaces facilitates potential spintronic applications. Here, the authors demonstrate the tuning of the electronic interaction between a gold surface and the ligand of a corrole complex, tuning its spin state through fused-ring-expansion.

Nature Communications doi: 10.1038/ncomms8547

Authors: Fan Wu, Jie Liu, Puneet Mishra, Tadahiro Komeda, John Mack, Yi Chang, Nagao Kobayashi, Zhen Shen

Author(s): Bruno Schuler, Mats Persson, Sami Paavilainen, Niko Pavliček, Leo Gross, Gerhard Meyer, and Jascha Repp

The formation of extended electron states in one-dimensional nanostructures is of key importance for the function of molecular electronics devices. In this paper, using a combination of atomic force microscopy, scanning tunneling spectroscopy, and tight-binding calculations the authors investigate the confinement of electronic states to controlled Cl vacancy pairs in a NaCl bilayer on Cu(111). They reveal that electron-phonon coupling has great influence on these quantities.

[Phys. Rev. B 91, 235443] Published Wed Jun 24, 2015

Article

A fundamental understanding of electron–phonon interactions in graphene is crucial to developing graphene-based electronic and photonic devices. Here, the authors control the properties of phonons in graphene by tuning the interaction strength between graphene and an underlying platinum substrate.

Nature Communications doi: 10.1038/ncomms8528

Authors: Hyo Won Kim, Wonhee Ko, JiYeon Ku, Insu Jeon, Donggyu Kim, Hyeokshin Kwon, Youngtek Oh, Seunghwa Ryu, Young Kuk, Sung Woo Hwang, Hwansoo Suh

The macroscopic magnetic moment of a superparamagnetic system has to overcome an energy barrier in order to switch its direction. This barrier is formed by magnetic anisotropies in the material and may be surmounted typically after 10^9 to 10^12 attempts per second by thermal fluctuations. In a first step, the associated switching rate may be described by a Neel-Brown-Arrhenius law, in which the energy barrier is assumed as constant or a given temperature. Yet, magnetic anisotropies in general depend on temperature themselves which is known to modify the Neel-Brown-Arrhenius law. We illustrate quantitatively the implications of a temperature-dependent anisotropy on the switching rate and in particular for the interpretation of the prefactor as an attempt frequency. In particular, we show that realistic numbers for the attempt frequency are obtained when the temperature dependence of the anisotropy is taken into account.

Author(s): Christian Karlewski, Michael Marthaler, Tobias Märkl, Timofey Balashov, Wulf Wulfhekel, and Gerd Schön

Due to underlying symmetries, the ground states of magnetic adatoms may be highly stable, which opens perspectives for application as single-atom memory. A specific example is a single holmium atom (with J=8) on a platinum (111) surface for which exceptionally long lifetimes were observed in recent …

[Phys. Rev. B 91, 245430] Published Tue Jun 23, 2015

Author(s): Michael Schecter, Mark S. Rudner, and Karsten Flensberg

We investigate magnetic order in a lattice of classical spins coupled to an isotropic gas of one-dimensional conduction electrons via local exchange interactions. The frequently discussed Ruderman-Kittel-Kasuya-Yosida effective exchange model for this system predicts that spiral order is always pref…

[Phys. Rev. Lett. 114, 247205] Published Thu Jun 18, 2015

To improve graphene-based multifunctional devices at nanoscale, a stepwise and controllable fabrication procedure must be elucidated. Here, a series of structural transition of bismuth (Bi) adatoms, adsorbed on monolayer epitaxial graphene (MEG), is explored at room temperature. Bi adatoms undergo a structural transition from one-dimensional (1D) linear structures to two-dimensional (2D) triangular islands and such 2D growth mode is affected by the corrugated substrate. Upon Bi deposition, a little charge transfer occurs and a characteristic peak can be observed in the tunneling spectrum, reflecting the distinctive electronic structure of the Bi adatoms. When annealed to ~500 K, 2D triangular Bi islands aggregate into Bi nanoclusters (NCs) of uniform size. A well-controlled fabrication method is thus demonstrated. The approaches adopted herein provide perspectives for fabricating and characterizing periodic networks on MEG and related systems, which are useful in realizing graphene-based electronic, energy, sensor and spintronic devices.

Scientific Reports 5 doi: 10.1038/srep11623

Nature Physics. doi:10.1038/nphys3371

Authors: J-X. Yin, Zheng Wu, J-H. Wang, Z-Y. Ye, Jing Gong, X-Y. Hou, Lei Shan, Ang Li, X-J. Liang, X-X. Wu, Jian Li, C-S. Ting, Z-Q. Wang, J-P. Hu, P-H. Hor, H. Ding & S. H. Pan

In superconductors, electrons are paired and condensed into the ground state. An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap. The characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state. A zero-energy in-gap state is particularly noteworthy, because it can be the consequence of non-trivial pairing symmetry or topology. Here we use scanning tunnelling microscopy/spectroscopy to demonstrate that an isotropic zero-energy bound state with a decay length of ∼10 Å emerges at each interstitial iron impurity in superconducting Fe(Te,Se). More noticeably, this zero-energy bound state is robust against a magnetic field up to 8 T, as well as perturbations by neighbouring impurities. Such a spectroscopic feature has no natural explanation in terms of impurity states in superconductors with s-wave symmetry, but bears all the characteristics of the Majorana bound state proposed for topological superconductors, indicating that the superconducting state and the scattering mechanism of the interstitial iron impurities in Fe(Te,Se) are highly unconventional.