Dr.jens.brede

Clusters built from individual iron atoms adsorbed on surfaces (adatoms) were investigated by atomic force microscopy (AFM) with subatomic resolution. Single copper and iron adatoms appeared as toroidal structures and multiatom clusters as connected structures, showing each individual atom as a torus. For single adatoms, the toroidal shape of the AFM image depends on the bonding symmetry of the adatom to the underlying structure [twofold for copper on copper(110) and threefold for iron on copper(111)]. Density functional theory calculations support the experimental data. The findings correct our previous work, in which multiple minima in the AFM signal were interpreted as a reflection of the orientation of a single front atom, and suggest that dual and triple minima in the force signal are caused by dimer and trimer tips, respectively. Authors: Matthias Emmrich, Ferdinand Huber, Florian Pielmeier, Joachim Welker, Thomas Hofmann, Maximilian Schneiderbauer, Daniel Meuer, Svitlana Polesya, Sergiy Mankovsky, Diemo Ködderitzsch, Hubert Ebert, Franz J. Giessibl

Article

Hybrid inorganic-organic structures can overcome the limits of inorganic semiconductor light emitting devices but the energy level offset is an obstacle. Here, Schlesinger et al. lower the ZnO work function with an organometallic donor monolayer and enhance the radiative emission of the hybrid structure.

Nature Communications doi: 10.1038/ncomms7754

Authors: R. Schlesinger, F. Bianchi, S. Blumstengel, C. Christodoulou, R. Ovsyannikov, B. Kobin, K. Moudgil, S. Barlow, S. Hecht, S.R. Marder, F. Henneberger, N. Koch

Author(s): Michael Ruby, Benjamin W. Heinrich, Jose I. Pascual, and Katharina J. Franke

The type I superconductor lead (Pb) has been theoretically predicted to be a two-band superconductor. We use scanning tunneling spectroscopy (STS) to resolve two superconducting gaps with an energy difference of 150  μeV. Tunneling into Pb(111), Pb(110), and Pb(100) crystals reveals a strong depende...

[Phys. Rev. Lett. 114, 157001] Published Tue Apr 14, 2015

Article

Targeted synthesis of vertically stacked graphene (G) and hexagonal boron nitride (h-BN) heterostructures remains a challenge. Here, the authors achieve the selective, CVD growth of h-BN-G and G/h-BN through a temperature-triggered switching reaction.

Nature Communications doi: 10.1038/ncomms7835

Authors: Teng Gao, Xiuju Song, Huiwen Du, Yufeng Nie, Yubin Chen, Qingqing Ji, Jingyu Sun, Yanlian Yang, Yanfeng Zhang, Zhongfan Liu

Author(s): Hadj M. Benia, Carola Straßer, Klaus Kern, and Christian R. Ast

The much debated topology of Bi1-xSbx has been resolved in this very careful and thorough experiment. The authors optimized the Bi1-xSbx growth method and minimized surface imperfections. They performed direct measurements of the surface band structure and followed its evolution within a large window of Sb concentration. They did not detect a third surface band seen in previous experiments. Thus, they conclude that the experimental surface band structure of B1-xSbx in the topologically insulating regime now agrees well with the theoretical predictions.

[Phys. Rev. B 91, 161406(R)] Published Mon Apr 13, 2015

Author(s): T. Iffländer, S. Rolf-Pissarczyk, L. Winking, R. G. Ulbrich, A. Al-Zubi, S. Blügel, and M. Wenderoth

The electronic charge distribution at the iron/gallium interface is imaged directly with high spatial and energetic resolution. The metal-semiconductor electronic barrier properties are found to be dependent on both the charge distribution and chemical bonds between the metal and semiconductor atoms.

[Phys. Rev. Lett. 114, 146804] Published Thu Apr 09, 2015

The atomic-scale spin structure of individual isolated skyrmions in an ultrathin film is investigated in real space by spin-polarized scanning tunneling microscopy. Their axial symmetry as well as their unique rotational sense is revealed by using both out-of-plane and in-plane sensitive tips. The size and shape of skyrmions change as a function of magnetic field. An analytical expression for the description of skyrmions is proposed and applied to connect the experimental data to the original theoretical model describing chiral skyrmions. Thereby, the relevant material parameters responsible for skyrmion formation can be obtained.

In order to exploit the intriguing optical properties of graphene it is essential to gain a better understanding of the light-matter interaction in the material on ultrashort timescales. Exciting the Dirac fermions with intense ultrafast laser pulses triggers a series of processes involving interactions between electrons, phonons and impurities. Here we study these interactions in epitaxial graphene supported on silicon carbide (semiconducting) and iridium (metallic) substrates using ultrafast time- and angle-resolved photoemission spectroscopy (TR-ARPES) based on high harmonic generation. For the semiconducting substrate we reveal a complex hot carrier dynamics that manifests itself in an elevated electronic temperature and an increase in linewidth of the ? band. By analyzing these effects we are able to disentangle electron relaxation channels in graphene. On the metal substrate this hot carrier dynamics is found to be severely perturbed by the presence of the metal, and...

Abstract

On the occasion of its 25th anniversary, we revise Chen's derivative rule for electron tunneling [C.J. Chen, Phys. Rev. B 42, 8841 (1990)] for the purpose of computationally efficient simulations of scanning tunneling microscopy (STM) based on first principles electronic structure data. The revised model allows the weighting of tunneling matrix elements of different tip orbital characters by an arbitrary energy independent choice or based on energy dependent weighting coefficients obtained by an expansion of the tip single electron wavefunctions/density of states projected onto the tip apex atom. Tip-orbital interference in the STM junction is included in the model by construction and can be analyzed quantitatively. As a further advantage, arbitrary tip geometrical orientations are included in the revised model by rotating the coordinate system of the tip apex using Euler angles and redefining the weighting coefficients of the tunneling matrix elements. We demonstrate the reliability of the model by applying it to two functionalized surfaces of recent interest where quantum interference effects play an important role in the STM imaging process: N-doped graphene and a magnetic Mn2H complex on the Ag(111) surface. We find that the proposed tunneling model is 25 times faster than the Bardeen method concerning computational time, while maintaining good agreement. Our results show that the electronic structure of the tip has a considerable effect on STM images, and the Tersoff-Hamann model does not always provide sufficient results in view of quantum interference effects. For both studied surfaces we highlight the importance of interference between s and pz tip orbitals that can cause a significant contrast change in the STM images.

Author(s): Jérôme Lagoute, Frédéric Joucken, Vincent Repain, Yann Tison, Cyril Chacon, Amandine Bellec, Yann Girard, Robert Sporken, Edward H. Conrad, François Ducastelle, Mattias Palsgaard, Nick Papior Andersen, Mads Brandbyge, and Sylvie Rousset

Scanning tunneling microscopy experiments have been performed to measure the local electron injection in nitrogen-doped graphene on SiC(0001¯) and were successfully compared to ab initio calculations. In graphene, a gaplike feature is measured around the Fermi level due to a phonon-mediated tunnelin...

[Phys. Rev. B 91, 125442] Published Tue Mar 31, 2015

We investigate the impact of strained nanobubbles on the conductance characteristics of graphene nanoribbons using a combined molecular dynamics - tight-binding simulation scheme. We describe in detail how the conductance, density of states, and current density of zigzag or armchair graphene nanoribbons are modified by the presence of a nanobubble. In particular, we establish that low-energy electrons can be confined in the vicinity or within the nanobubbles by the delicate interplay between the pseudomagnetic field pattern created by the shape of the bubble, mode mixing, and substrate interaction. The coupling between confined evanescent states and propagating modes can be enhanced under different clamping conditions, which translates into Fano resonances in the conductance traces.

Spatially varying strained graphene can acquire interesting electronic properties because of the strain-induced valley-dependent gauge (pseudomagnetic) fields1,2. Here we report the realization of strained graphene regions located close to the step edges of Cu(111), obtained by using thermal strain engineering3,4. We study these strained structures with sub-nanometre-resolved scanning tunnelling microscopy and spectroscopy and identify their spatially modulated Dirac points, demonstrating the effect of overlap of Cu and graphene wave functions on the charge transfer between them5. By applying a magnetic field of 8 Tesla, electron confinement, as revealed by regularly spaced sharp resonances6,7, is observed in the strained graphene. In some regions of the strained graphene, repetitive pairs of resonance peaks appear in the tunnelling spectra. This provides direct and compelling evidence for lifting of valley degeneracy due to the coexistence of both the magnetic field and the pseudomagnetic field.

Article

Electron–phonon coupling influences the thermal and electronic properties of many solid materials. Zeljkovic et al . now combine Landau level spectroscopy and scanning tunnelling microscopy to extract quantitative information on electron–phonon coupling in the insulator lead selenide.

Nature Communications doi: 10.1038/ncomms7559

Authors: Ilija Zeljkovic, Kane L. Scipioni, Daniel Walkup, Yoshinori Okada, Wenwen Zhou, R Sankar, Guoqing Chang, Yung Jui Wang, Hsin Lin, Arun Bansil, Fangcheng Chou, Ziqiang Wang, Vidya Madhavan

Interactions between atomic and molecular objects are to a large extent defined by the nanoscale electrostatic potentials which these objects produce. We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with sub-nanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of a (qPlus tuning fork) atomic force microscope operated at 5 K, we quantitatively measure the quadrupole field of a single molecule and the dipole field of a single metal adatom, both adsorbed on a clean metal surface. Because of its high sensitivity, the technique can record electrostatic potentials at large distances from their sources, which above all will help to image complex samples with increased surface roughness.

Magnetic resonance in an ensemble of laser-cooled trapped Rb atoms is excited using a micro- cantilever with a magnetic tip. The cantilever is mounted on a multi-layer chip designed to capture, cool, and magnetically transport cold atoms. The coupling is observed by measuring the loss from a magnetic trap as the oscillating cantilever induces Zeeman state transitions in the atoms. Interfacing cold atoms with mechanical devices could enable probing and manipulating atomic spins with nanometer spatial resolution and single-spin sensitivity, leading to new capabilities in quantum computation, quantum simulation, or precision sensing.

Author(s): Q. Dubout, F. Donati, C. Wäckerlin, F. Calleja, M. Etzkorn, A. Lehnert, L. Claude, P. Gambardella, and H. Brune

Cobalt atoms exposed to hydrogen gas have higher spins, an effect that could be used to build magnetic nanostructures and lattices.

[Phys. Rev. Lett. 114, 106807] Published Wed Mar 11, 2015

Square ice in graphene nanocapillaries

Nature 519, 7544 (2015). doi:10.1038/nature14295

Authors: G. Algara-Siller, O. Lehtinen, F. C. Wang, R. R. Nair, U. Kaiser, H. A. Wu, A. K. Geim & I. V. Grigorieva

Bulk water exists in many forms, including liquid, vapour and numerous crystalline and amorphous phases of ice, with hexagonal ice being responsible for the fascinating variety of snowflakes. Much less noticeable but equally ubiquitous is water adsorbed at interfaces and confined in microscopic pores. Such low-dimensional water determines aspects of various phenomena in materials science, geology, biology, tribology and nanotechnology. Theory suggests many possible phases for adsorbed and confined water, but it has proved challenging to assess its crystal structure experimentally. Here we report high-resolution electron microscopy imaging of water locked between two graphene sheets, an archetypal example of hydrophobic confinement. The observations show that the nanoconfined water at room temperature forms ‘square ice’—a phase having symmetry qualitatively different from the conventional tetrahedral geometry of hydrogen bonding between water molecules. Square ice has a high packing density with a lattice constant of 2.83 Å and can assemble in bilayer and trilayer crystallites. Molecular dynamics simulations indicate that square ice should be present inside hydrophobic nanochannels independently of their exact atomic nature.

Author(s): Rico Friedrich, Vasile Caciuc, Nikolai S. Kiselev, Nicolae Atodiresei, and Stefan Blügel

We theoretically explore through systematic multiscale ab initio and Monte Carlo calculations how the surface magnetism of a ferromagnetic surface can be fine-tuned by nonmagnetic organic molecules containing a single π bond. We demonstrate that a magnetic hardening or softening can be induced depen...

[Phys. Rev. B 91, 115432] Published Wed Mar 25, 2015

Nature Chemistry 7, 317 (2015). doi:10.1038/nchem.2182

Authors: Dmitry V. Kondratuk, Luís M. A. Perdigão, Ayad M. S. Esmail, James N. O'Shea, Peter H. Beton & Harry L. Anderson

Biopolymers adopt functional tertiary structures through folding and multiplex formation. Synthetic molecules with protein-like dimensions — monodisperse cyclic porphyrin polymers with diameters of 13–21 nm — have now been shown to exhibit biomimetic self-organization by forming nested structures on a gold surface. These assemblies are formed both under vacuum and during deposition from solution.

Article

Magnetic domain walls can exhibit a variety of different spin textures. Chen et al . show that it is possible to switch these textures between left handed, right handed, cycloidal, helical and mixed domain wall structures by controlling uniaxial strain in iron/nickel bilayer thin films on tungsten.

Nature Communications doi: 10.1038/ncomms7598

Authors: Gong Chen, Alpha T. N’Diaye, Sang Pyo Kang, Hee Young Kwon, Changyeon Won, Yizheng Wu, Z. Q. Qiu, Andreas K. Schmid