Dr.jens.brede

Author(s): Shiyong Wang, Liang Z. Tan, Weihua Wang, Steven G. Louie, and Nian Lin

We demonstrate that Dirac fermions can be created and manipulated in a two-dimensional electron gas (2DEG). Using a cryogenic scanning tunneling microscope, we arranged coronene molecules one by one on a Cu(111) surface to construct artificial graphene nanoribbons with perfect zigzag (ZGNRs) or arm-...

[Phys. Rev. Lett. 113, 196803] Published Fri Nov 07, 2014

Nature Nanotechnology 9, 869 (2014). doi:10.1038/nnano.2014.273

We are making author anonymity an option in our peer review.

Author(s): E. I. Baibekov, M. R. Gafurov, D. G. Zverev, I. N. Kurkin, B. Z. Malkin, and B. Barbara

We study coherent dynamics in a system of dipolar coupled spin qubits diluted in a solid and subjected to a driving microwave field. In the case of rare earth ions, an anisotropic crystal background results in anisotropic g tensor and thus modifies the dipolar coupling. We develop a microscopic theo...

[Phys. Rev. B 90, 174402] Published Tue Nov 04, 2014

Author(s): C. Hübner, B. Baxevanis, A. A. Khajetoorians, and D. Pfannkuche

Highly symmetric magnetic environments have been suggested to stabilize the magnetic information stored in magnetic adatoms on a surface. Utilized as memory devices such systems are subjected to electron tunneling and external magnetic fields. We analyze theoretically how such perturbations affect t...

[Phys. Rev. B 90, 155134] Published Fri Oct 31, 2014

Article

The spin texture of a topological insulator is defined by spin-momentum locked Dirac fermions in its non-trivial surface states. Here, Sessi et al. show how the spin texture of Bi 2 Te 3 may be modified by extremely dilute magnetic adatoms, with magnetic order mediated via the RKKY interaction.

Nature Communications doi: 10.1038/ncomms6349

Authors: Paolo Sessi, Felix Reis, Thomas Bathon, Konstantin A. Kokh, Oleg E. Tereshchenko, Matthias Bode

Author(s): Leo Gross, Bruno Schuler, Fabian Mohn, Nikolaj Moll, Niko Pavliček, Wolfram Steurer, Ivan Scivetti, Konstantinos Kotsis, Mats Persson, and Gerhard Meyer

Noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) have become important tools for nanotechnology; however, their contrast mechanisms on the atomic scale are not entirely understood. Here we used chlorine vacancies in NaCl bilayers on Cu(111) as a model system to in...

[Phys. Rev. B 90, 155455] Published Thu Oct 30, 2014

Room-temperature magnetic order on zigzag edges of narrow graphene nanoribbons

Nature 514, 7524 (2014). doi:10.1038/nature13831

Authors: Gábor Zsolt Magda, Xiaozhan Jin, Imre Hagymási, Péter Vancsó, Zoltán Osváth, Péter Nemes-Incze, Chanyong Hwang, László P. Biró & Levente Tapasztó

The possibility that non-magnetic materials such as carbon could exhibit a novel type of s–p electron magnetism has attracted much attention over the years, not least because such magnetic order is predicted to be stable at high temperatures. It has been demonstrated that atomic-scale structural defects of graphene can host unpaired spins, but it remains unclear under what conditions long-range magnetic order can emerge from such defect-bound magnetic moments. Here we propose that, in contrast to random defect distributions, atomic-scale engineering of graphene edges with specific crystallographic orientation—comprising edge atoms from only one sub-lattice of the bipartite graphene lattice—can give rise to a robust magnetic order. We use a nanofabrication technique based on scanning tunnelling microscopy to define graphene nanoribbons with nanometre precision and well-defined crystallographic edge orientations. Although so-called ‘armchair’ ribbons display quantum confinement gaps, ribbons with the ‘zigzag’ edge structure that are narrower than 7 nanometres exhibit an electronic bandgap of about 0.2–0.3 electronvolts, which can be identified as a signature of interaction-induced spin ordering along their edges. Moreover, upon increasing the ribbon width, a semiconductor-to-metal transition is revealed, indicating the switching of the magnetic coupling between opposite ribbon edges from the antiferromagnetic to the ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hopes of graphene-based spintronic devices operating under ambient conditions.

Semiconductor nanowires with strong spin-orbit coupling and proximity-induced s-wave superconductivity in an external magnetic field have been the most promising settings for approaches towards experimental evidence of topological Majorana zero-modes. We investigate the effect of tilting the magnetic field relative to the spin-orbit coupling direction in a simple continuum model and provide an analytical derivation of the critical angle, at which the topological states disappear. We also obtain the differential conductance characteristic of a junction with a normal wire for different tilting angles and propose the qualitative change of the dependence of the zero-energy differential conductance on the tunnel barrier strength at the critical angle as a new criterion for establishing the topological nature of the observed signal.

Detection of the extremely weak signals in spectroscopy over an extremely wide frequency region is central to diverse sciences, including materials science, biology, astronomy and chemistry. Here we show a new type of atomic-scale spectroscopy, heterodyne scanning tunnelling spectroscopy (HSTS), which is based on the innovative application of the nonlinear heterodyne-mixing detection at the metal-insulator-metal (MIM) heterojunction of STM tip–vacuum–sample. The principle of HSTS is identical to that of the Atacama Large Millimeter Array (ALMA) space telescope in terms of using heterojunction for detecting extremely weak signals by converting from terahertz region to lower frequency regions. The MIM detector of ALMA, which is composed of niobium–titanium–nitride (NbTiN) tip-insulator-NbTiN, is very similar in shape and size to that of HSTS. We successfully detect a heterodyne beat signal f3 (= |f2 − f1|) and intermodulation distortion via tunnelling current by superimposing two different AC signals, f1 and f2, onto the DC tunnelling current at a highly oriented pyrolytic graphite (HOPG) surface. We then obtain spectra of the localized electronic states of HOPG by using f3. HSTS can be performed with a high resolution and over a wide energy range, including the terahertz range.

Scientific Reports 4 doi: 10.1038/srep06711

Abstract

Abstract

Article

Inorganic two-dimensional atomic crystals exhibit a variety of unusual but practically useful properties. Here, the authors produce an organic counterpart, atomically smooth monolayers of a molecular crystal, and use this organic analogue of graphene in high-performance organic field-effect transistors.

Nature Communications doi: 10.1038/ncomms6162

Authors: Daowei He, Yuhan Zhang, Qisheng Wu, Rui Xu, Haiyan Nan, Junfang Liu, Jianjun Yao, Zilu Wang, Shijun Yuan, Yun Li, Yi Shi, Jinlan Wang, Zhenhua Ni, Lin He, Feng Miao, Fengqi Song, Hangxun Xu, K. Watanabe, T. Taniguchi, Jian-Bin Xu, Xinran Wang

Author(s): A. Cavallin, F. D. Natterer, S. Ouazi, G. Moulas, A. Lehnert, S. Rusponi, and H. Brune

We report on the magnetization reversal mechanism of Co islands on Pt(111) as a function of their size and shape. We measure the zero-field susceptibility χ(T) and low-temperature magnetization curves M(H) with in situ magneto-optical Kerr effect. Together with the island morphology deduced from sca...

[Phys. Rev. B 90, 144427] Published Wed Oct 22, 2014

Article

Spin textures, such as skyrmions, could be useful in future low-power-consumption memory devices, but they are usually only seen in materials with a strong spin-orbit interaction. Phark et al. now, however, observe such non-collinear magnetic order in nanometre-scale bilayer iron islands.

Nature Communications doi: 10.1038/ncomms6183

Authors: S. -H. Phark, J. A. Fischer, M. Corbetta, D. Sander, K. Nakamura, J. Kirschner

Author(s): F. Donati, L. Gragnaniello, A. Cavallin, F. D. Natterer, Q. Dubout, M. Pivetta, F. Patthey, J. Dreiser, C. Piamonteze, S. Rusponi, and H. Brune

We determine the magnetic properties of individual Co atoms adsorbed on graphene (G) with x-ray absorption spectroscopy and magnetic circular dichroism. The magnetic ground state of Co adatoms strongly depends on the choice of the metal substrate on which graphene is grown. Cobalt atoms on G/Ru(0001...

[Phys. Rev. Lett. 113, 177201] Published Mon Oct 20, 2014

We describe a broadly-applicable theory of spin relaxation in materials with incoherent charge transport; examples include amorphous inorganic semiconductors, organic semiconductors, quantum dot arrays, and systems displaying trap-controlled transport or transport within an impurity band. The theory can incorporate many different relaxation mechanisms, so long as electron-electron correlations can be neglected. We focus primarily on spin relaxation caused by spin-orbit effects, which manifest through inhomogeneities in the $g$-factor and non-spin-conserving carrier hops, scattering, trapping, or detrapping. Analytic and numerical results from the theory are compared in various regimes with Monte Carlo simulations. Our results should assist in evaluating the suitability of various disordered materials for spintronic devices.

Author(s): Mie Andersen, Liv Hornekær, and Bjørk Hammer

The coverage-dependent intercalation of oxygen, CO, hydrogen, alkali metals, and halogens between graphene and Ir(111) is investigated using density functional theory with van der Waals corrections. By comparing adsorption on clean Ir to intercalation we show that the presence of the graphene layer ...

[Phys. Rev. B 90, 155428] Published Thu Oct 16, 2014