Dr.jens.brede

Author(s): P. Sessi, M. M. Otrokov, T. Bathon, M. G. Vergniory, S. S. Tsirkin, K. A. Kokh, O. E. Tereshchenko, E. V. Chulkov, and M. Bode

We performed a scanning tunneling spectroscopy investigation of the electronic properties of the topological insulator Bi₂Te₃ in the energy range between −200 and +700 meV with respect to the Fermi level. For unoccupied states, tunneling into topological surface states dominates. Analysis of Fourier...

[Phys. Rev. B 88, 161407] Published Fri Oct 25, 2013

Author(s): Fang Yang, Y. R. Song, H. Li, K. F. Zhang, X. Yao, Canhua Liu, Dong Qian, C. L. Gao, and Jin-Feng Jia

The surface magnetic property plays a key role in determining magnetic related quantum phenomena of magnetic topological insulators. Using spin-polarized scanning tunneling microscopy, we investigate the surface magnetism and anisotropy of a Cr doped topological insulator: Cr_0.05Sb_1.95Te₃. It is fou...

[Phys. Rev. Lett. 111, 176802] Published Fri Oct 25, 2013

The apex atom of a W scanning probe tip reveals a non-spherical charge distribution as probed by a CO molecule bonded to a Cu(111) surface [Welker et al. Science, 336, 444 (2012)]. Three high-symmetry images were observed and related to three low-index crystallographic directions of the W bcc crystal. Open questions remained, however, including the verification that the tip was indeed W-terminated, and whether this method can be easily applied to distinguish other atomic species. In this work, we investigate bulk Cu and Fe tips. In both cases we can associate our data with the fcc (Cu) and bcc (Fe) crystal structures. A model is presented, based on the partial filling of d orbitals, to relate the AFM images to the angular orientation of the tip structure.

The effect of the Dzyaloshinsky-Moriya interaction on the magnetic structure of an ordered FePt monolayer deposited on Pt (111) surface has been investigated. In the ground state, the pronounced anisotropic geometry of the FePt layer with alternating Fe and Pt chains gives rise to a helimagnetic structure with a strong difference in the helicity period along the chains and perpendicular to them. In the presence of an external magnetic field, the region of stable Skyrmion magnetic structures in the $B-T$ phase diagram has been demonstrated via Monte Carlo simulations using the parameters obtained within first-principles electronic structure calculations. The present study demonstrates clearly that the ratio of the exchange coupling parameters J/D for a deposited magnetic film - being of central importance for the formation of Skyrmions - can be manipulated by growing an overlayer of 2-dimensional (2D) compounds with the atoms carrying spontaneous magnetic moments separated by the atoms of non-magnetic elements.

The Scanning Tunneling Microscope is a powerful tool for studying the electronic properties at the atomic level, however it's relatively small scanning range and the fact that it can only operate on conducting samples prevents its application to study heterogeneous samples consisting on conducting and insulating regions. Here we present a long-range scanning tunneling microscope capable of detecting conducting micro and nanostructures on insulating substrates using a technique based on the capacitance between the tip and the sample and performing STM studies.

Author(s): D. Jacob, M. Soriano, and J. J. Palacios

Using a state-of-the art combination of density functional theory and impurity solver techniques, we present a complete and parameter-free picture of the Kondo effect in the high-spin (S=3/2) coordination complex known as manganese phthalocyanine adsorbed on the Pb(111) surface. We calculate the cor...

[Phys. Rev. B 88, 134417] Published Fri Oct 18, 2013

Author(s): Christian F. Hermanns, Matthias Bernien, Alex Krüger, Christian Schmidt, Sören T. Waßerroth, Gelavizh Ahmadi, Benjamin W. Heinrich, Martin Schneider, Piet W. Brouwer, Katharina J. Franke, Eugen Weschke, and Wolfgang Kuch

Using magnetic endohedral fullerenes for molecular spintronics requires control over their encapsulated magnetic moments. We show by field-dependent x-ray magnetic circular dichroism measurements of Gd₃N@C₈₀ endohedral fullerenes adsorbed on a Cu surface that the magnetic moments of the encapsulated...

[Phys. Rev. Lett. 111, 167203] Published Tue Oct 15, 2013

Author(s): F. Delgado, C. D. Batista, and J. Fernández-Rossier

Spin chains are among the simplest physical systems in which electron-electron interactions induce novel states of matter. Here we propose to combine atomic scale engineering and spectroscopic capabilities of state of the art scanning tunnel microscopy to probe the fractionalized edge states of indi...

[Phys. Rev. Lett. 111, 167201] Published Mon Oct 14, 2013

We use first-principles calculations based on density functional theory to investigate the magnetic exchange interaction of Fe clusters on Rh(111) and Ru(0001). We consider dimers, trimers, tetramers, and pentamers of different shape in fcc and hcp stacking as well as infinite atomic and biatomic chains. From the dimer calculations we extract the exchange interaction as a function of adatom distance by mapping total energies to a Heisenberg model. The nearest-neighbor (NN) exchange constant is about one order of magnitude smaller than reported for other substrates due to the strong hybridization between the Fe atoms and the partly filled $4d$-band of the surface. We also find a transition from a ferromagnetic NN exchange interaction for Fe dimers on Rh(111) to an antiferromagnetic one on Ru(0001). The distance-dependent exchange coupling displays a RKKY-like oscillatory behavior which is nearly inverted for Fe dimers on the Rh(111) surface compared to those on Ru(0001). Unexpectedly, for Fe clusters beyond dimers, a complex trend of the magnetic ground state is observed which alternates between ferro- and antiferromagnetic configurations depending on cluster size and shape. In view of the exchange constants obtained for dimers, it is surprising that on both surfaces small compact clusters are ferromagnetic while open structures such as linear trimers or tetramers become antiferromagnetic. We demonstrate that both vertical and lateral structural relaxations of the clusters are crucial in order to understand this unexpected trend of magnetic order and connected to the competition of direct ferromagnetic exchange among Fe atoms in the cluster and the hybridization with the substrate.

We study skyrmions in magnetic thin films with structural inversion asymmetry perpendicular to the film plane. We determine the magnetization texture of a single skyrmion and its dependence on the strength of the Dzyaloshinskii-Moriya interaction relative to the magnetostatic energy. Furthermore, we construct a phenomenological model that describes the interaction between the motion of skyrmions and electric currents to lowest order in spin-orbit coupling. We estimate the experimental verifiable velocities for current-driven motion of skyrmion textures based on available results obtained from domain walls dynamics.

Abstract

Abstract

Author(s): A. A. Khajetoorians, T. Schlenk, B. Schweflinghaus, M. dos Santos Dias, M. Steinbrecher, M. Bouhassoune, S. Lounis, J. Wiebe, and R. Wiesendanger

We demonstrate using inelastic scanning tunneling spectroscopy and simulations based on density functional theory that the amplitude and sign of the magnetic anisotropy energy for a single Fe atom adsorbed onto the Pt(111) surface can be manipulated by modifying the adatom binding site. Since the ma...

[Phys. Rev. Lett. 111, 157204] Published Tue Oct 08, 2013

Author(s): S. Marocchi, P. Ferriani, N. M. Caffrey, F. Manghi, S. Heinze, and V. Bellini

Using first-principles calculations we demonstrate sizable exchange coupling between a magnetic molecule and a magnetic substrate via a graphene layer. As a model system we consider cobaltocene (CoCp₂) adsorbed on graphene deposited on Ni(111). We find that the magnetic coupling is antiferromagnetic...

[Phys. Rev. B 88, 144407] Published Tue Oct 08, 2013

Author(s): Fabien Jean, Tao Zhou, Nils Blanc, Roberto Felici, Johann Coraux, and Gilles Renaud

The effects of the temperature on the structure of a single layer of graphene on Ir(111) have been investigated in situ in the growth chamber by grazing incidence x-ray diffraction between 10 and 1300 K. In addition, the effect of two growth temperatures has been studied. The graphene lattice parame...

[Phys. Rev. B 88, 165406] Published Mon Oct 07, 2013

Article

Understanding the anisotropic electronic structure of lanthanide complexes is useful for a range of advanced applications. Here, the authors present a simple electrostatic method, capable of predicting the magnetic anisotropy of low-symmetry mononuclear and polynuclear dysprosium(III) complexes.

Nature Communications doi: 10.1038/ncomms3551

Authors: Nicholas F. Chilton, David Collison, Eric J. L. McInnes, Richard E. P. Winpenny, Alessandro Soncini

Nature Physics. doi:10.1038/nphys2766

Authors: Maciej Misiorny, Michael Hell & Maarten R. Wegewijs

Author(s): Nuala M. Caffrey, Paolo Ferriani, Simone Marocchi, and Stefan Heinze

Using first-principles calculations, we show that the magnetic properties of a two-dimensional antiferromagnetic transition-metal surface are modified on the atomic scale by the adsorption of small organic molecules. We consider benzene (C₆H₆), cyclooctatetraene (C₈H₈), and a small transition-metal–...

[Phys. Rev. B 88, 155403] Published Fri Oct 04, 2013

Magnetic moments strongly coupled to the spins of conduction electrons in a nanostructure can confine the conduction-electron motion due to scattering at almost localized Kondo singlets. We study the resulting local-moment formation in the conduction-electron system and the magnetic exchange coupling mediated by the Kondo singlets. Its distance dependence is oscillatory and induces robust ferro- or antiferromagnetic order in multi-impurity systems.

Zhang et al. reply

Nature 501, 7468 (2013). doi:10.1038/nature12590

Authors: X. Z. Zhang, C. H. Wan, X. L. Gao, J. M. Wang & X. Y. Tan

replying to J. Luo et al.Nature501, http://dx.doi.org/10.1038/nature12589 (2013).We agree with Luo et al. that the magnetoresistance effects that we reported were dependent on the method used to measure them. The reason that there is a difference in the results depending on whether method 1 or method 2 is used (adopting the measurement notation of ref. 1) is that there are two voltage-stabilizing diodes in the Keithley 2400 instrument we used. We were unaware that when this instrument was used both as current source and voltmeter, one diode connected the input port of the current source to the input port of the voltmeter, whereas the other diode connected the output port of the current source to the output port of the voltmeter. The diodes caused a crossover of the Hall coefficient from negative to positive when the instrument was used to conduct a Hall measurement in this configuration, leading us to propose an invalid mechanism for the abnormal magnetoresistance. Therefore the mechanism we proposed—minority injection and an induced p-n boundary—does not provide a correct explanation for the observed geometry-enhanced magnetoresistance. Although such a mechanism does not operate in our samples, we note that a p–n boundary could still enhance magnetoresistance in certain circumstances according to our and others’ theoretical calculations and experiments.

Low-voltage magnetoresistance in silicon

Nature 501, 7468 (2013). doi:10.1038/nature12589

Authors: Jun Luo, Peisen Li, Sen Zhang, Hongyu Sun, Hongping Yang & Yonggang Zhao

Arising from C. H. Wan, X. Z. Zhang, X. L. Gao, J. M. Wang & X. Y. Tan Nature477, 304–307 (2011).Magnetoresistance exhibited by non-magnetic semiconductors has attracted much attention. In particular, Wan et al. reported room-temperature magnetoresistance in silicon to reach 10% at 0.07 T and 150,000% at 7 T—“an intrinsically spatial effect”. Their supply voltage was approximately 10 V (ref. 12), which is low and approaches the industrial requirement. However, we have found their large magnetoresistance values to be experimental artefacts caused by their method of measurement. The true room-temperature magnetoresistance of the devices described in ref. 12 is low with a magnetic field of up to 7 T and a supply voltage of around 10 V and hence these devices cannot offer large magnetoresistance with low supply voltage to industry. There is a Reply to this Brief Communication Arising by Zhang, X. Z., Wan, C. H., Gao, X. L., Wang, J. M. & Tan, X. Y. Nature501,http://dx.doi.org/10.1038/nature12590 (2013).

Carbon nanotube computer

Nature 501, 7468 (2013). doi:10.1038/nature12502

Authors: Max M. Shulaker, Gage Hills, Nishant Patil, Hai Wei, Hong-Yu Chen, H.-S. Philip Wong & Subhasish Mitra

The miniaturization of electronic devices has been the principal driving force behind the semiconductor industry, and has brought about major improvements in computational power and energy efficiency. Although advances with silicon-based electronics continue to be made, alternative technologies are being explored. Digital circuits based on transistors fabricated from carbon nanotubes (CNTs) have the potential to outperform silicon by improving the energy–delay product, a metric of energy efficiency, by more than an order of magnitude. Hence, CNTs are an exciting complement to existing semiconductor technologies. Owing to substantial fundamental imperfections inherent in CNTs, however, only very basic circuit blocks have been demonstrated. Here we show how these imperfections can be overcome, and demonstrate the first computer built entirely using CNT-based transistors. The CNT computer runs an operating system that is capable of multitasking: as a demonstration, we perform counting and integer-sorting simultaneously. In addition, we implement 20 different instructions from the commercial MIPS instruction set to demonstrate the generality of our CNT computer. This experimental demonstration is the most complex carbon-based electronic system yet realized. It is a considerable advance because CNTs are prominent among a variety of emerging technologies that are being considered for the next generation of highly energy-efficient electronic systems.

Author(s): J. P. Cleuziou, N. V. N’Guyen, S. Florens, and W. Wernsdorfer

We report on cotunneling spectroscopy magnetoconductance measurements of multihole ultraclean carbon nanotube quantum dots in the SU(4) Kondo regime with strong spin-orbit coupling. Successive shells show a gradual weakening of the Kondo effect with respect to the spin-orbital splittings, leading to...

[Phys. Rev. Lett. 111, 136803] Published Tue Sep 24, 2013