Dr.jens.brede

The spin dynamics of all ferromagnetic materials are governed by two types of collective excitations: spin waves and domain walls. The fundamental processes underlying these collective modes, such as exchange interactions and magnetic anisotropy, all originate at the atomic scale; yet, conventional probing techniques, based on neutron and photon scattering, provide high resolution in reciprocal space, and thereby poor spatial resolution. Here we present direct imaging of spin waves in individual chains of ferromagnetically coupled $S=2$ Fe atoms, assembled one by one on a Cu$_2$N surface using a scanning tunnelling microscope. We are able to map the spin dynamics of these designer nanomagnets with atomic resolution, in two complementary ways. First, atom to atom variations of the amplitude of the quantized spin wave excitations, predicted by theory, are probed using inelastic electron tunnelling spectroscopy. Second, we observe slow stochastic switching between two opposite magnetisation states, whose rate varies strongly depending on the location of the tip along the chain. Our observations, combined with model calculations, reveal that switches of the chain are initiated by a spin wave excited state which has its antinodes at the edges of the chain, followed by a domain wall shifting through the chain from one end to the other. This approach opens the way towards atomic scale imaging of other types of spin excitations, such as spinons and fractional end-states, in engineered spin chains.

We demonstrate that the ultrafast fast dynamics of the d–f exchange interaction, between conduction band electrons and lattice spins in EuTe, can be accessed using an all-optical technique. Our results reveal, in full detail, the time evolution of the d–f exchange interaction induced by a femtosecond laser pulse. Specifically, by monitoring the time resolved dynamics of the reflectivity changes and Kerr rotation of a weak light pulse reflected from the surface of the sample, it is shown that an intense femtosecond light pulse with photon energies higher than that of the bandgap, triggers spin waves in EuTe. The laser-induced spin waves modulate the d–f exchange interaction, and cause the bandgap to oscillate with an amplitude reaching 1 meV, at frequencies up to tens of GHz. The ability to control and monitor the dynamics of the exchange energy with our all-optical technique opens up new opportunities for the manipulation of magnetism at ultrafast time-scales.

Scientific Reports 4 doi: 10.1038/srep04368

Abstract

Nature Chemistry 6, 267 (2014). doi:10.1038/nchem.1906

Author: Michelle Francl

Michelle Francl suggests that self-plagiarism is a misleading term and that repeating yourself in publications isn't always a bad thing.

Nature Physics. doi:10.1038/nphys2928

Author: Bert Koopmans

Spin pumping and spin-to-charge conversion in hybrid metal–organic devices reveal the physical mechanisms at work in semiconducting polymers.

Nature Physics. doi:10.1038/nphys2901

Authors: Shun Watanabe, Kazuya Ando, Keehoon Kang, Sebastian Mooser, Yana Vaynzof, Hidekazu Kurebayashi, Eiji Saitoh & Henning Sirringhaus

Author(s): R. Piquerel, O. Gaier, E. Bonet, C. Thirion, and W. Wernsdorfer

Microwave-assisted switching of the magnetization is an efficient way to reduce the magnetic field required to reverse the magnetization of nanostructures. Here, the phase sensitivity of microwave-assisted switching of an individual cobalt nanoparticle is studied using a pump-probe technique. The pu...

[Phys. Rev. Lett. 112, 117203] Published Wed Mar 19, 2014

Technology transfer: Industry-funded academic inventions boost innovation

Nature 507, 7492 (2014). doi:10.1038/507297a

Authors: Brian D. Wright, Kyriakos Drivas, Zhen Lei & Stephen A. Merrill

Brian D. Wright and colleagues present data challenging the assumption that corporate-funded academic research is less accessible and useful to others.

Author(s): Hongliang Xin, Aleksandra Vojvodic, Johannes Voss, Jens K. Nørskov, and Frank Abild-Pedersen

The d-band shape of a metal site, governed by the local geometry and composition of materials, plays an important role in determining trends of the surface reactivity of transition-metal alloys. We discuss this phenomenon using the chemisorption of various adsorbates such as C, N, O, and their hydro...

[Phys. Rev. B 89, 115114] Published Thu Mar 13, 2014

Author(s): Andrea Droghetti, Sabine Steil, Nicolas Großmann, Norman Haag, Hongtao Zhang, Maureen Willis, William P. Gillin, Alan J. Drew, Martin Aeschlimann, Stefano Sanvito, and Mirko Cinchetti

It was recently established that spin injection from a ferromagnetic metal into an organic semiconductor depends largely on the formation of hybrid interface states. Here we investigate whether the magnetic properties of the interface between cobalt and tris(8-hydroxyquinolinato)-Al(III) (Alq3), the...

[Phys. Rev. B 89, 094412] Published Wed Mar 12, 2014

We investigate the rotational properties of molecular hydrogen and its isotopes physisorbed on the surfaces of graphene and hexagonal boron nitride ($h$-BN), grown on Ni(111), Ru(0001), and Rh(111), using rotational excitation spectroscopy (RES) with the scanning tunneling microscope. The rotational thresholds are in good agreement with $\Delta J=2$ transitions of freely spinning para-H$_2$ and ortho-D$_2$ molecules. The line shape variations in RES for H$_2$ among the different surfaces can be traced back and naturally explained by a resonance mediated tunneling mechanism. RES data for H$_2$/$h$-BN/Rh(111) suggests a local intrinsic gating on this surface due to lateral variations in the surface potential. An RES inspection of H$_2$, HD, and D$_2$ mixtures finally points to a multi molecule excitation, since either of the three $J=0\rightarrow2$ rotational transitions are simultaneously present, irrespective of where the spectra were recorded in the mixed monolayer.

Author(s): Chen Fang, Matthew J. Gilbert, and B. Andrei Bernevig

We study a new type of three-dimensional topological superconductor that exhibits Majorana zero modes (MZM) protected by a magnetic group symmetry, a combined antiunitary symmetry composed of a mirror reflection and time reversal. This new symmetry enhances the noninteracting topological classificat...

[Phys. Rev. Lett. 112, 106401] Published Mon Mar 10, 2014

Author(s): Stefan Schumacher, Daniel F. Förster, Feiming Hu, Thomas Frauenheim, Tim O. Wehling, and Thomas Michely

Through reactive molecular-beam epitaxy or Eu postoxidation and postannealing, large EuO(111) bilayer islands of high quality and exceptional stability are grown on Ir(111). We use scanning tunneling microscopy, low-energy electron diffraction, magneto-optical Kerr effect measurements, and density f...

[Phys. Rev. B 89, 115410] Published Tue Mar 11, 2014

Author(s): Mikkel Settnes, Stephen R. Power, Dirch H. Petersen, and Antti-Pekka Jauho

Experimental advances allow for the inclusion of multiple probes to measure the transport properties of a sample surface. We develop a theory of dual-probe scanning tunneling microscopy using a Green’s function formalism, and apply it to graphene. Sampling the local conduction properties at finite l...

[Phys. Rev. Lett. 112, 096801] Published Tue Mar 04, 2014

We have upgraded a low-temperature scanning tunnelling microscope (STM) with a radio-frequency (RF) modulation system to extend STM spectroscopy to the range of low energy excitations (

Abstract

We extend the orbital-dependent electron tunneling model implemented within the three-dimensional (3D) Wentzel-Kramers-Brillouin (WKB) atom-superposition approach to simulate spin-polarized scanning tunneling microscopy (SP-STM) above magnetic surfaces. The tunneling model is based on the electronic structure data of the magnetic tip and surface obtained from first principles. Applying our method, we analyze the orbital contributions to the tunneling current, and study the nature of atomic contrast reversals occurring on constant-current SP-STM images above the Fe(110) surface. We find an interplay of orbital-dependent tunneling and spin-polarization effects responsible for the contrast inversion, and we discuss its dependence on the bias voltage, on the tip-sample distance, and on the tip orbital composition.

Recent developments have led to an explosion of activity on skyrmions in three-dimensional (3D) chiral magnets. Experiments have directly probed these topological spin textures, revealed their nontrivial properties, and led to suggestions for novel applications. However, in 3D the skyrmion crystal phase is observed only in a narrow region of the temperature-field phase diagram. We show here, using a general analysis based on symmetry, that skyrmions are much more readily stabilized in two-dimensional (2D) systems with Rashba spin-orbit coupling. This enhanced stability arises from the competition between field and easy-plane magnetic anisotropy and results in a nontrivial structure in the topological charge density in the core of the skyrmions. We further show that, in a variety of microscopic models for magnetic exchange, the required easy-plane anisotropy naturally arises from the same spin-orbit coupling that is responsible for the chiral Dzyaloshinskii-Moriya interactions. Our results are of particular interest for 2D materials like thin films, surfaces, and oxide interfaces, where broken surface-inversion symmetry and Rashba spin-orbit coupling naturally lead to chiral exchange and easy-plane compass anisotropy. Our theory gives a clear direction for experimental studies of 2D magnetic materials to stabilize skyrmions over a large range of magnetic fields down to T=0.

We address the question to what extent the success of scientific articles is due to social influence. Analyzing a data set of over 100000 publications from the field of Computer Science, we study how centrality in the coauthorship network differs between authors who have highly cited papers and those who do not. We further show that a machine learning classifier, based only on coauthorship network centrality measures at time of publication, is able to predict with high precision whether an article will be highly cited five years after publication. By this we provide quantitative insight into the social dimension of scientific publishing - challenging the perception of citations as an objective, socially unbiased measure of scientific success.

The magnetic ground-state properties of the periodic Anderson model with a regular depletion of the correlated sites are analyzed within different theoretical approaches. We consider the model on the one-dimensional chain and on the two-dimensional square lattice with hopping between nearest neighbors. At half-filling and with correlated impurities present at every second site, the depleted Anderson lattice is the most simple system where the indirect magnetic coupling mediated by the conduction electrons is ferromagnetic. We discuss the underlying electronic structure and the possible mechanisms that result in ferromagnetic long-range order. To this end, different numerical and analytical concepts are applied to the depleted Anderson and also to the related depleted Kondo lattice and are contrasted with each other. This includes numerical approaches, i.e. Hartree-Fock theory, density-matrix renormalization and dynamical mean-field theory, as well as analytical concepts, namely a variant of the Lieb-Mattis theorem and the concept of flat-band ferromagnetism, and finally perturbative approaches, i.e. the effective RKKY exchange in the limit of weak and the "inverse indirect magnetic exchange" in the limit of strong coupling between the conduction band and the impurities.

Author(s): Rasmus Westerström, Jan Dreiser, Cinthia Piamonteze, Matthias Muntwiler, Stephen Weyeneth, Karl Krämer, Shi-Xia Liu, Silvio Decurtins, Alexey Popov, Shangfeng Yang, Lothar Dunsch, and Thomas Greber

Paramagnetic atoms inside nanometer sized fullerenes realize robust, and chemically protected, spin systems. Changing the stoichiometry of the endohedral clusters results in a variety of magnetic ground states, as it is demonstrated for DynSc3−nN@C80 (n=1,2,3). All three exhibit distinct hysteresis ...

[Phys. Rev. B 89, 060406] Published Thu Feb 27, 2014

Junctions comprised of ferromagnets and nonmagnetic materials are one of the key building blocks in spintronics. With the recent breakthroughs of spin injection in ferromagnet/graphene junctions it is possible to consider spin-based applications that are not limited to magnetoresistive effects. However, for critical studies of such structures it is crucial to establish accurate predictive methods that would yield atomically resolved information on interfacial properties. By focusing on Co(0001)/graphene junctions and their electronic structure, we illustrate the inequivalence of different spin polarizations. We show atomically resolved spin polarization maps as a useful approach to assess the relevance of Co(0001)/graphene for different spintronics applications.