Dr.jens.brede

The formation of extended electron states in one-dimensional nanostructures is of key importance for the function of molecular electronics devices. Here we study the effects of strong electron-phonon interaction on the formation of extended electronic states in intentionally created Cl vacancy pairs and chains in a NaCl bilayer on Cu(111). The interaction between the vacancies was tailored by fabricating vacancy pairs and chains of different orientation and separation with atomic precision using vertical manipulation. Small separation of divacancies led to the formation of symmetric and antisymmetric vacancy states and localized interface-states. By scanning tunneling spectroscopy (STS) we measured their energy splitting and broadening as a function of the inter-vacancy separation. Unexpectedly, the energy splitting between the vacancy states is enlarged by level repulsion resulting from phonon dressing of the electronic states, as evidenced by theory. Already for a few coupled vacancies we observe an emerging band structure of the defect band.

Motivated by a recent experiment in which zero-bias peaks have been observed in scanning tunneling microscopy (STM) experiments performed on chains of magnetic atoms on a superconductor, we show, by generalizing earlier work, that a multichannel ferromagnetic wire deposited on a spin-orbit coupled superconducting substrate can realize a non-trivial chiral topological superconducting state with Majorana bound states localized at the wire ends. The non-trivial topological state occurs for generic parameters requiring no fine tuning, at least for very large exchange spin splitting in the wire. We theoretically obtain the signatures which appear in the presence of an arbitrary number of Majorana modes in multi-wire systems incorporating the role of finite temperature, finite potential barrier at the STM tip, and finite wire length. These signatures are presented in terms of spatial profiles of STM differential conductance which clearly reveal zero energy Majorana end modes and the prediction of a multiple Majorana based fractional Josephson effect. A substantial part of this work is devoted to a detailed critical comparison between our theory and the recent STM experiment claiming the observation of Majorana fermions. The conclusion of this detailed comparison is that although the experimental observations are not manifestly inconsistent with our theoretical findings, the very small topological superconducting gap and the very high temperature of the experiment make it impossible to decisively verify the existence of a localized Majorana zero mode, as the spectral weight of the Majorana mode is necessarily spread over a very broad energy regime exceeding the size of the gap. Thus, although the experimental findings are indeed consistent with a highly broadened and weakened Majorana zero bias peak, much lower experimental temperatures are necessary for any definitive conclusion.

Article

Closely-spaced anisotropically-engineered single-domain nanomagnets may be exploited to encode and transmit binary information. Here, Gu et al . use time-resolved X-ray microscopy to image signal propagation at the intrinsic nanomagnetic switching limit in permalloy nanomagnet chains.

Nature Communications doi: 10.1038/ncomms7466

Authors: Zheng Gu, Mark E. Nowakowski, David B. Carlton, Ralph Storz, Mi-Young Im, Jeongmin Hong, Weilun Chao, Brian Lambson, Patrick Bennett, Mohmmad T. Alam, Matthew A. Marcus, Andrew Doran, Anthony Young, Andreas Scholl, Peter Fischer, Jeffrey Bokor

Emergent phenomena and functions arising from topological electron-spin textures in real space or momentum space are attracting growing interest for new concept of states of matter as well as for possible applications to spintronics. One such example is a magnetic skyrmion, a topologically stable nanoscale spin vortex structure characterized by a topological index. Real-space regular arrays of skyrmions are described by combination of multi-directional spin helixes. Nanoscale configurations and characteristics of the two-dimensional skyrmion hexagonal-lattice have been revealed extensively by real-space observations. Other three-dimensional forms of skyrmion lattices, such as a cubic-lattice of skyrmions, are also anticipated to exist, yet their direct observations remain elusive. Here we report real-space observations of spin configurations of the skyrmion cubic-lattice in MnGe with a very short period (~3 nm) and hence endowed with the largest skyrmion number density. The skyrmion lattices parallel to the {100} atomic lattices are directly observed using Lorentz transmission electron microscopes (Lorentz TEMs). It enables the first simultaneous observation of magnetic skyrmions and underlying atomic-lattice fringes. These results indicate the emergence of skyrmion-antiskyrmion lattice in MnGe, which is a source of emergent electromagnetic responses and will open a possibility of controlling few-nanometer scale skyrmion lattices through atomic lattice modulations.

We report the first occurrence of a natural quasicrystal with decagonal symmetry. The quasicrystal, with composition Al71Ni24Fe5, was discovered in the Khatyrka meteorite, a recently described CV3 carbonaceous chondrite. Icosahedrite, Al63Cu24Fe13, the first natural quasicrystal to be identified, was found in the same meteorite. The new quasicrystal was found associated with steinhardtite (Al38Ni32Fe30), Fe-poor steinhardtite (Al50Ni40Fe10), Al-bearing trevorite (NiFe2O4) and Al-bearing taenite (FeNi). Laboratory studies of decagonal Al71Ni24Fe5 have shown that it is stable over a narrow range of temperatures, 1120 K to 1200 K at standard pressure, providing support for our earlier conclusion that the Khatyrka meteorite reached heterogeneous high temperatures [1100 < T(K) ≤ 1500] and then rapidly cooled after being heated during an impact-induced shock that occurred in outer space 4.5 Gya. The occurrences of metallic Al alloyed with Cu, Ni, and Fe raises new questions regarding conditions that can be achieved in the early solar nebula.

Scientific Reports 5 doi: 10.1038/srep09111

Abstract

Author(s): J. Sforzini, L. Nemec, T. Denig, B. Stadtmüller, T.-L. Lee, C. Kumpf, S. Soubatch, U. Starke, P. Rinke, V. Blum, F. C. Bocquet, and F. S. Tautz

X-ray measurements of its vertical absorption height indicate that hydrogen-intercalated graphene on silicon carbide is effectively free-standing.

[Phys. Rev. Lett. 114, 106804] Published Tue Mar 10, 2015

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 spin structure arranged on a 3x3 lattice. Despite an almost vanishing total magnetization we predict the emergence of a large anomalous Hall effect, to which there is a significant topological contribution purely due to the real space spin texture at the surface.

Abstract

Article

Electron transport at the atom-level scale cannot be described by spatially averaged electric fields as it is in macroscopic systems. Here, the authors experimentally demonstrate the spatial extent of an atomically local scattering process that gives rise to resistivity in nanoscale devices.

Nature Communications doi: 10.1038/ncomms7399

Authors: Philip Willke, Thomas Druga, Rainer G. Ulbrich, M. Alexander Schneider, Martin Wenderoth

Article

There is currently interest in two-dimensional graphene-like materials incorporating heteroatoms. Here, the authors synthesize a solution-processable, holey two-dimensional network with C 2 N stoichiometry containing evenly distributed holes and nitrogen atoms, and use it to fabricate a field effect transistor.

Nature Communications doi: 10.1038/ncomms7486

Authors: Javeed Mahmood, Eun Kwang Lee, Minbok Jung, Dongbin Shin, In-Yup Jeon, Sun-Min Jung, Hyun-Jung Choi, Jeong-Min Seo, Seo-Yoon Bae, So-Dam Sohn, Noejung Park, Joon Hak Oh, Hyung-Joon Shin, Jong-Beom Baek

Article

Direct growth of graphene on h-BN is desired to improve device performance. Here, the authors demonstrate the direct growth of large-area and continuous graphene/h-BN vertical heterostructures via a co-segregation approach.

Nature Communications doi: 10.1038/ncomms7519

Authors: Chaohua Zhang, Shuli Zhao, Chuanhong Jin, Ai Leen Koh, Yu Zhou, Weigao Xu, Qiucheng Li, Qihua Xiong, Hailin Peng, Zhongfan Liu

Topological magnetic states, such as chiral skyrmions, are of great scientific interest and show huge potential for novel spintronics applications, provided their topological charges can be fully controlled. So far skyrmionic textures have been observed in noncentrosymmetric crystalline materials with low symmetry and at low temperatures. We propose theoretically and demonstrate experimentally the design of spin textures with topological charge densities that can be tailored at ambient temperatures. Tuning the interlayer coupling in vertically stacked nanopatterned magnetic heterostructures, such as a model system of a Co/Pd multilayer coupled to Permalloy, the in-plane non-collinear spin texture of one layer can be imprinted into the out-of-plane magnetised material. We observe distinct spin textures, e.g. vortices, magnetic swirls with tunable opening angle, donut states and skyrmion core configurations. We show that applying a small magnetic field, a reliable switching between topologically distinct textures can be achieved at remanence.

Scientific Reports 5 doi: 10.1038/srep08787

Nature Nanotechnology 10, 259 (2015). doi:10.1038/nnano.2014.326

Authors: Ben Warner, Fadi El Hallak, Henning Prüser, John Sharp, Mats Persson, Andrew J. Fisher & Cyrus F. Hirjibehedin

Phenomena that are highly sensitive to magnetic fields can be exploited in sensors and non-volatile memories. The scaling of such phenomena down to the single-molecule level may enable novel spintronic devices. Here, we report magnetoresistance in a single-molecule junction arising from negative differential resistance that shifts in a magnetic field at a rate two orders of magnitude larger than Zeeman shifts. This sensitivity to the magnetic field produces two voltage-tunable forms of magnetoresistance, which can be selected via the applied bias. The negative differential resistance is caused by transient charging of an iron phthalocyanine (FePc) molecule on a single layer of copper nitride (Cu2N) on a Cu(001) surface, and occurs at voltages corresponding to the alignment of sharp resonances in the filled and empty molecular states with the Cu(001) Fermi energy. An asymmetric voltage-divider effect enhances the apparent voltage shift of the negative differential resistance with magnetic field, which inherently is on the scale of the Zeeman energy. These results illustrate the impact that asymmetric coupling to metallic electrodes can have on transport through molecules, and highlight how this coupling can be used to develop molecular spintronic applications.

Article

Endohedral fullerenes are known to stabilize reactive radicals; however, the external magnetic manipulation of these species’ remains challenging. Here, the authors link a nitroxide radical to a paramagnetic fullerene system and are able to alter the spin behaviour of the fullerene via spin–spin interactions.

Nature Communications doi: 10.1038/ncomms7468

Authors: Bo Wu, Taishan Wang, Yongqiang Feng, Zhuxia Zhang, Li Jiang, Chunru Wang