Dr.jens.brede

Recent experiments have provided evidence that one-dimensional (1D) topological superconductivity can be realized experimentally by placing transition metal atoms that form a ferromagnetic chain on a superconducting substrate. We address some properties of this type of systems by using a Slater-Koster tight-binding model. We predict that topological superconductivity is nearly universal when ferromagnetic transition metal chains form straight lines on superconducting substrates and that it is possible for more complex chain structures. The proximity induced superconducting gap is $\sim \Delta E_{so} / J$ where $\Delta$ is the $s$-wave pair-potential on the chain, $E_{so}$ is the spin-orbit splitting energy induced in the normal chain state bands by hybridization with the superconducting substrate, and $J$ is the exchange-splitting of the ferromagnetic chain $d$-bands. Because of the topological character of the 1D superconducting state, Majorana end modes appear within the gaps of finite length chains. We find, in agreement with experiment, that when the chain and substrate orbitals are strongly hybridized, Majorana end modes are substantially reduced in amplitude when separated from the chain end by less than the coherence length defined by the $p$-wave superconducting gap. We conclude that Pb is a particularly favorable substrate material for ferromagnetic chain topological superconductivity because it provides both strong $s-$wave pairing and strong Rashba spin-orbit coupling, but that there is an opportunity to optimize properties by varying the atomic composition and structure of the chain. Finally, we note that in the absence of disorder a new chain magnetic symmetry, one that is also present in the crystalline topological insulators, can stabilize multiple Majorana modes at the end of a single chain.

Article

Modelling magnetic data for lanthanide clusters is challenging due to spin–orbit coupling and crystal field effects. Here, the authors use multi-frequency electron paramagnetic resonance spectroscopy to measure directly the interaction between two dysprosium(III) ions in a dimeric system.

Nature Communications doi: 10.1038/ncomms6243

Authors: Eufemio Moreno Pineda, Nicholas F. Chilton, Raphael Marx, María Dörfel, Daniel O. Sells, Petr Neugebauer, Shang-Da Jiang, David Collison, Joris van Slageren, Eric J.L. McInnes, Richard E.P. Winpenny

Author(s): Alberto Hinojosa, Rafael M. Fernandes, and Andrey V. Chubukov

We argue that superconductivity in the coexistence region with spin-density-wave (SDW) order in weakly doped Fe pnictides erdiffers qualitatively from the ordinary s+- state outside the coexistence region as it develops an additional gap component which is a mixture of intrapocket singlet (s++) and ...

[Phys. Rev. Lett. 113, 167001] Published Mon Oct 13, 2014

Recent experimental conductance measurements performed on paramagnetic molecular adsorbates on a superconducting surface, using superconducting scanning tunneling microscopy techniques, are theoretically investigated. For low temperatures, we demonstrate that tunneling current assisted excitations of the local magnetic moment cannot occur for voltage biases smaller than the superconducting gap of the scanning tunneling microscope. The magnetic moment is only excited for voltages corresponding to the sum of the superconducting gap and the spin excitation energies. In excellent agreement with experiment, we show that pumping into higher excitations give additional current signatures by accumulation of density in the lower ones. Using external magnetic fields, we Zeeman split possible degeneracy and thereby resolve all excitations comprised in the magnetic moment.

Majorana fermions are predicted to localize at the edge of a topological superconductor, a state of matter that can form when a ferromagnetic system is placed in proximity to a conventional superconductor with strong spin-orbit interaction. With the goal of realizing a one-dimensional topological superconductor, we have fabricated ferromagnetic iron (Fe) atomic chains on the surface of superconducting lead (Pb). Using high-resolution spectroscopic imaging techniques, we show that the onset of superconductivity, which gaps the electronic density of states in the bulk of the Fe chains, is accompanied by the appearance of zero energy end states. This spatially resolved signature provides strong evidence, corroborated by other observations, for the formation of a topological phase and edge-bound Majorana fermions in our atomic chains.

Author(s): Xiaopeng Duan, Yuriy G. Semenov, and Ki Wook Kim

Exotic properties of topological insulators (TIs) and graphene have captivated many condensed matter physicists, but are practical outcomes actually within reach? The authors propose an efficient, beyond-CMOS spin logic platform exploiting the strong exchange coupling between a ferromagnet and the Dirac fermion states of a TI and of graphene. A detailed theoretical analysis illustrates the desired ultralow-power performance under realistic conditions, leading to predictions for practical devices to be controlled by signals as small as atto- (10−18) Joules!

[Phys. Rev. Applied 2, 044003] Published Thu Oct 09, 2014

Author(s): Chih-Pin Lu, Guohong Li, K. Watanabe, T. Taniguchi, and Eva Y. Andrei

Molybdenum disulfide may provide an ideal substrate for supporting graphene.

[Phys. Rev. Lett. 113, 156804] Published Thu Oct 09, 2014

Intermolecular features in atomic force microscopy (AFM) images of organic molecules have been ascribed to intermolecular bonds. A recent theoretical study [P. Hapala et al., Phys. Rev. B 90, 085421 (2014)] showed that these features can also be explained by the flexibility of molecule terminated tips. We probe this effect by carrying out AFM experiments on a model system that contains regions where intermolecular bonds should and should not exist between close-by molecules. Intermolecular features are observed in both regions, demonstrating that intermolecular contrast cannot be directly interpreted as intermolecular bonds.

Author(s): A. Eich, M. Michiardi, G. Bihlmayer, X.-G. Zhu, J.-L. Mi, Bo B. Iversen, R. Wiesendanger, Ph. Hofmann, A. A. Khajetoorians, and J. Wiebe

The band structure and intra- and interband scattering processes of the electrons at the surface of a bismuth bilayer on Bi2Se3 have been experimentally investigated by low-temperature Fourier-transform scanning tunneling spectroscopy. The observed complex quasiparticle interference patterns are com...

[Phys. Rev. B 90, 155414] Published Wed Oct 08, 2014

Experimental realization of universal geometric quantum gates with solid-state spins

Nature 514, 7520 (2014). doi:10.1038/nature13729

Authors: C. Zu, W.-B. Wang, L. He, W.-G. Zhang, C.-Y. Dai, F. Wang & L.-M. Duan

Experimental realization of a universal set of quantum logic gates is the central requirement for the implementation of a quantum computer. In an ‘all-geometric’ approach to quantum computation, the quantum gates are implemented using Berry phases and their non-Abelian extensions, holonomies, from geometric transformation of quantum states in the Hilbert space. Apart from its fundamental interest and rich mathematical structure, the geometric approach has some built-in noise-resilience features. On the experimental side, geometric phases and holonomies have been observed in thermal ensembles of liquid molecules using nuclear magnetic resonance; however, such systems are known to be non-scalable for the purposes of quantum computing. There are proposals to implement geometric quantum computation in scalable experimental platforms such as trapped ions, superconducting quantum bits and quantum dots, and a recent experiment has realized geometric single-bit gates in a superconducting system. Here we report the experimental realization of a universal set of geometric quantum gates using the solid-state spins of diamond nitrogen–vacancy centres. These diamond defects provide a scalable experimental platform with the potential for room-temperature quantum computing, which has attracted strong interest in recent years. Our experiment shows that all-geometric and potentially robust quantum computation can be realized with solid-state spin quantum bits, making use of recent advances in the coherent control of this system.

Author(s): S. Just, S. Zimmermann, V. Kataev, B. Büchner, M. Pratzer, and M. Morgenstern

Monolayer graphene grown by chemical vapor deposition and transferred to SiO2 is used to introduce vacancies by Ar+ ion bombardment at a kinetic energy of 50 eV. The density of defects visible in scanning tunneling microscopy is considerably lower than the ion fluence, implying that most of the defe...

[Phys. Rev. B 90, 125449] Published Mon Sep 29, 2014

Nature Physics. doi:10.1038/nphys3084

Authors: Ying-Shuang Fu, M. Kawamura, K. Igarashi, H. Takagi, T. Hanaguri & T. Sasagawa

Massless Dirac electrons in condensed matter are, unlike conventional electrons, described by two-component wavefunctions associated with the spin degrees of freedom in the surface state of topological insulators. Hence, the ability to observe the two-component wavefunction is useful for exploring novel spin phenomena. Here we show that the two-component nature is manifest in Landau levels, the degeneracy of which is lifted by a Coulomb potential. Using spectroscopic-imaging scanning tunnelling microscopy, we visualize energy and spatial structures of Landau levels in Bi2Se3, a prototypical topological insulator. The observed Landau-level splitting and internal structures of Landau orbits are distinct from those in a conventional electron system and are well reproduced by a two-component model Dirac Hamiltonian. Our model further predicts energy-dependent spin-magnetization textures in a potential variation and provides a way for manipulating spins in the topological surface state.

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 the switching probability of a single quantum spin for two characteristic symmetries encountered in recent experiments and suggest a third one that exhibits robust protection against surface induced spin flips. Further we illuminate how the switching of an adatom spin exhibits characteristic behavior with respect to low energy excitations from which the symmetry of the system can be inferred.

How to draw perfect figures

Nature 513, 7519 (2014). doi:10.1038/513463e

Jumbled charts and misleading graphs — illustrations in a paper can go wrong in many ways. Now, a treatise that attempts to rescue science from bad figures has been getting rave reviews on social media (N. P. Rougier et al. PLoS Comput. Biol.10,

Abstract

We report a combined experimental and theoretical study of the Kondo effect in a series of binuclear metal-organic complexes of the form [(Me(hfacac)_2)_2(bpym)]^0, with Me = Nickel (II), Manganese(II), Zinc (II); hfacac = hexafluoroacetylacetonate, and bpym = bipyrimidine, adsorbed on Cu(100) surface. While Kondo-features did not appear in the scanning tunneling spectroscopy spectra of non-magnetic Zn_2, a zero bias resonance was resolved in magnetic Mn_2 and Ni_2 complexes. The case of Ni_2 is particularly interesting as the experiments indicate two adsorption geometries with very different properties. For Ni_2-complexes we have employed density functional theory to further elucidate the situation. Our simulations show that one geometry with relatively large Kondo temperatures T_K ~ 10K can be attributed to distorted Ni_2 complexes, which are chemically bound to the surface via the bipyrimidine unit. The second geometry, we assign to molecular fragmentation: we suggest that the original binuclear molecule decomposes into two pieces, including Ni(hexafluoroacetylacetonate)_2, when brought into contact with the Cu-substrate. For both geometries our calculations support a picture of the (S=1)-type Kondo effect emerging due to open 3d shells of the individual Ni^{2+} ions.

Nature Physics. doi:10.1038/nphys3085

Authors: Matthias Germann, Xin Tong & Stefan Willitsch

Spectroscopic transitions in atoms and molecules that are not allowed within the electric-dipole approximation, but occur because of higher-order terms in the interaction between matter and radiation, are termed dipole-forbidden. These transitions are extremely weak and therefore exhibit very small natural linewidths. Dipole-forbidden optical transitions in atoms form the basis of next-generation atomic clocks and of high-fidelity qubits used in quantum information processors and quantum simulators. In molecules, however, such transitions are much less characterized, reflecting the considerable challenges to address them. Here, we report direct observation of dipole-forbidden, electric-quadrupole-allowed infrared (IR) transitions in a molecular ion. Their detection was enabled by the very long interrogation times of several minutes afforded by the sympathetic cooling of individual quantum-state-selected molecular ions into the nearly perturbation-free environment of a Coulomb crystal. The present work paves the way for new mid-IR frequency standards and precision spectroscopic measurements on single molecules in the IR domain.

Condensed-matter physics: Catching relativistic electrons

Nature 513, 7518 (2014). doi:10.1038/513319a

Authors: Zhihuai Zhu & Jennifer E. Hoffman

Low-energy electrons have been found to mimic relativistic high-energy particles in cadmium arsenide. This defines the first stable '3D Dirac semimetal', which holds promise for fundamental-physics exploration and practical applications.

Magnetic resonance spectroscopy enables detection and manipulation of subtle spin interactions in organic semiconductors. [Also see Perspective by Bobbert] Authors: H. Malissa, M. Kavand, D. P. Waters, K. J. van Schooten, P. L. Burn, Z. V. Vardeny, B. Saam, J. M. Lupton, C. Boehme

Probing the interplay between the electronic and nuclear spins in organic semiconductors [Also see Report by Malissa et al.] Author: Peter A. Bobbert

On 3 July 1880, “a weekly journal of scientific progress” published its first research articles under the banner Science. From that original print medium, the inaugural set of papers has been reprinted, photocopied, posted online, converted to PDF, and even etched in glass blocks to serve as special gifts. Being part of the vanguard is indeed a privilege, and opportunities to be one of the first are rare. For that reason, I proudly announce that Science Advances, a new open-access journal for all of the sciences, officially joins the Science family of journals (Science, Science Translational Medicine, and Science Signaling) and is now accepting submissions (go to www.scienceadvances.org). In February 2015, the inaugural papers will be released, heralding the start of something big in scope, reach, and influence. Author: Marcia McNutt