Dr.jens.brede

Author(s): E. Simon, K. Palotás, L. Rózsa, L. Udvardi, and L. Szunyogh

We perform an extensive study of the spin-configurations in a PdFe bilayer on Ir(111) in terms of ab initio and spin-model calculations. We use the spin-cluster expansion technique to obtain spin model parameters, and solve the Landau-Lifshitz-Gilbert equations at zero temperature. In particular, we...

[Phys. Rev. B 90, 094410] Published Tue Sep 16, 2014

Article

Chemical derivatives of graphene are typically disordered or corrugated, impairing attempts to utilize them in monolayer devices. Here, the authors show that chair-C 2 F graphene is a stable material displaying long-range order, with functionalization on only one face in a given domain.

Nature Communications doi: 10.1038/ncomms5902

Authors: Reza J. Kashtiban, M Adam Dyson, Rahul R. Nair, Recep Zan, Swee L. Wong, Quentin Ramasse, Andre K. Geim, Ursel Bangert, Jeremy Sloan

Abstract

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2 \times 2)$ or a $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n-doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially as found by X-ray magnetic circular dichroism. The $(2 \times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.

Nature advance online publication 14 September 2014. doi:10.1038/nature13763

Authors: Mazhar N. Ali, Jun Xiong, Steven Flynn, Jing Tao, Quinn D. Gibson, Leslie M. Schoop, Tian Liang, Neel Haldolaarachchige, Max Hirschberger, N. P. Ong & R. J. Cava

Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors, in magnetic memory, and in hard drives at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.

Nature Materials. doi:10.1038/nmat4069

Authors: Ryosuke Senga, Hannu-Pekka Komsa, Zheng Liu, Kaori Hirose-Takai, Arkady V. Krasheninnikov & Kazu Suenaga

The spin textures of ultra-thin magnetic layers exhibit surprising variety. The loss of inversion symmetry at the interface of the magnetic layer and substrate gives rise to the so-called Dzyaloshinskii–Moriya interaction which favors non-collinear spin arrangements with unique rotational sense. Here we review the application of spin-polarized scanning tunneling microscopy to such systems, which has led to the discovery of interface-induced chiral domain walls and spin spirals. Recently, different interface-driven skyrmion lattices have been found, and the writing as well as the deleting of individual skyrmions based on local spin-polarized current injection has been demonstrated. These interface-induced non-collinear magnetic states offer new exciting possibilities to study fundamental magnetic interactions and to tailor material properties for spintronic applications.

The application of low temperature spin-polarized scanning tunneling microscopy and spectroscopy in magnetic fields for the quantitative characterization of spin polarization, magnetization reversal and magnetic anisotropy of individual nano structures is reviewed. We find that structural relaxation, spin polarization and magnetic anisotropy vary on the nm scale near the border of a bilayer Co island on Cu(1 1 1). This relaxation is lifted by perimetric decoration with Fe. We discuss the role of spatial variations of the spin-dependent electronic properties within and at the edge of a single nano structure for its magnetic properties.

Although the dispersion of magnons has been measured in many bulk materials, few studies deal with the changes in the dispersion when the material is in the form of a thin film, a system that is of interest for applications. Here we review inelastic tunneling spectroscopy studies of magnon dispersion in Mn/Cu 3 Au(1 0 0) and present new studies on Co and Ni thin films on Cu(1 0 0). The dispersion in Mn and Co films closely follows the dispersion of bulk samples with negligible dependence on thickness. The lifetime of magnons depends slightly on film thickness, and decreases considerably as the magnon energy increases. In Ni/Cu(1 0 0) films the thickness dependence of dispersion is much more pronounced. The measurements indicate a considerable mode softening for thinner films. Magnon lifetimes decrease dramatically near the edge of the Brillouin zone due to a close proximity of the Stoner continuum. The experimental study is supported by first-principles calculations.

Inelastic electron tunneling spectra (IETS) are evaluated for spin-1/2 Heisenberg chains showing different phases of their spin ordering. The spin ordering is controlled by the value of the two different Heisenberg couplings on the two sides of each of the chain's atoms (bond-alternating chains). The perfect anti-ferromagnetic phase, i.e. a unique exchange coupling, marks a topological quantum phase transition (TQPT) of the bond-alternating chain. Our calculations show that the TQPT is recognizable in the excited states of the chain and hence that IETS is in principle capable of discriminating the phases. We show that perfectly symmetric chains, such as closed rings mimicking infinite chains, yield the same spectra on both sides of the TQPT and IETS cannot reveal the nature of the spin phase. However, for finite size open chains, both sides of the TQPT are associated with different IETS spectra, especially on the edge atoms, thus outlining the transition.

Scanning tunneling microscopy (STM), with surface sensitivity, is an ideal tool to probe the intriguing properties of the surface state of topological insulators (TIs) and topological crystalline insulators (TCIs). We summarize the recent progress on those topological phases revealed by STM studies. STM observations have directly confirmed the existence of the topological surface states and clearly revealed their novel properties. We also discuss STM work on magnetic doped TIs, topological superconductors and crystalline symmetry-protected surface states in TCIs. The studies have greatly promoted our understanding of the exotic properties of the new topological phases, as well as put forward new challenges. STM will continue to play an important role in this rapidly growing field from the point view of both fundamental physics and applications.

Scanning tunneling microscopy and spectroscopy were performed to study transition metal adatoms (Fe, Co, Cu) and individual metal-dithiol complexes on insulating Cu 2 N islands. Adsorption of metal adatoms on Cu 2 N is surprisingly complex and in the case of Fe, we find two distinct adsorption states for each of two distinct adsorption sites. Connection of these metal adatoms to dithiol molecules was pursued to model a single molecule junction, with the aim of understanding further details about the nature of metal/molecule electrical contact. Pronounced changes in local density of states, magnetic anisotropy and Kondo interactions were observed for Co adatoms connected to dithiol molecules. These results illustrate some of the challenges and opportunities for STM studies of nanoscale magnetism in complex systems.

Recently, the family of high-resolution scanning probe imaging techniques using decorated tips has been complimented by a method based on inelastic electron tunneling spectroscopy (IETS). The new technique resolves the inner structure of organic molecules by mapping the vibrational energy of a single carbonmonoxide (CO) molecule positioned at the apex of a scanning tunnelling microscope (STM) tip. Here, we explain high-resolution IETS imaging by extending the model developed earlier for STM and atomic force microscopy (AFM) imaging with decorated tips. In particular, we show that the tip decorated with CO acts as a nanoscale sensor that changes the energy of the CO frustrated translation in response to the change of the local curvature of the surface potential. In addition, we show that high resolution AFM, STM and IETS-STM images can deliver information about intramolecular charge transfer for molecules deposited on a~surface. To demonstrate this, we extended our numerical model by taking into the account the electrostatic force acting between the decorated tip and surface Hartree potential.

Article

Topological excitations in insulating magnets have recently attracted great interest from a fundamental and applied perspective. Here, Pereiro et al. report on the conditions to generate, stabilize and control skyrmions, even at room temperature, in a kagome magnet.

Nature Communications doi: 10.1038/ncomms5815

Authors: Manuel Pereiro, Dmitry Yudin, Jonathan Chico, Corina Etz, Olle Eriksson, Anders Bergman

Interviews: The all-important job talk

Nature 513, 7516 (2014). doi:10.1038/nj7516-131a

Author: Amanda Mascarelli

How people present their research and aspirations during interviews for academic positions can make or break their career.

Nature Materials. doi:10.1038/nmat4061

Authors: Miguel M. Ugeda, Aaron J. Bradley, Su-Fei Shi, Felipe H. da Jornada, Yi Zhang, Diana Y. Qiu, Wei Ruan, Sung-Kwan Mo, Zahid Hussain, Zhi-Xun Shen, Feng Wang, Steven G. Louie & Michael F. Crommie

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring 2D semiconductor physics. Reduced screening in two dimensions results in markedly enhanced electron–electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects. Here we present a rigorous experimental observation of extraordinarily large exciton binding energy in a 2D semiconducting TMD. We determine the single-particle electronic bandgap of single-layer MoSe2 by means of scanning tunnelling spectroscopy (STS), as well as the two-particle exciton transition energy using photoluminescence (PL) spectroscopy. These yield an exciton binding energy of 0.55 eV for monolayer MoSe2 on graphene—orders of magnitude larger than what is seen in conventional 3D semiconductors and significantly higher than what we see for MoSe2 monolayers in more highly screening environments. This finding is corroborated by our ab initio GW and Bethe–Salpeter equation calculations which include electron correlation effects. The renormalized bandgap and large exciton binding observed here will have a profound impact on electronic and optoelectronic device technologies based on single-layer semiconducting TMDs.

Image-potential states of graphene on various substrates have been investigated by two-photon photoemission and scanning tunneling spectroscopy. They are used as a probe for the graphene-substrate interaction and resulting changes in the (local) work function. The latter is driven by the work function difference between graphene and the substrate. This results in a charge transfer which also contributes to core-level shifts in x-ray photoemission. In this review article, we give an overview over the theoretical models and the experimental data for image-potential states and work function of graphene on various substrates.

Article

When superconductivity emerges in a thin superconductor grown on a ferromagnet, it does so in an array of interacting superconducting and normally conducting channels. Maria Iavarone and colleagues use scanning tunnelling microscopy to image how these channels form and interact.

Nature Communications doi: 10.1038/ncomms5766

Authors: M. Iavarone, S. A. Moore, J. Fedor, S. T. Ciocys, G. Karapetrov, J. Pearson, V. Novosad, S. D. Bader

Author(s): S. Bertaina, C.-E. Dutoit, J. Van Tol, M. Dressel, B. Barbara, and A. Stepanov

We provide evidence for the coherence and Rabi oscillations of spin solitons pinned by the local breaking of translational symmetry in isotropic Heisenberg chains (simple antiferromagnetic Néel or spin Peierls). We show that these correlated spin systems that are made of hundreds of coupled spins be...

[Phys. Rev. B 90, 060404] Published Wed Aug 20, 2014

Author(s): James H. Atkinson, Ross Inglis, Enrique del Barco, and Euan K. Brechin

We report on a single-molecule magnet where the spatial arrangement of three manganese ions and their spin-orbit coupling tensor orientations result in threefold angular modulations of the magnetization tunneling rates and quantum interference patterns that mimic the form of a three-leaf clover. Alt...

[Phys. Rev. Lett. 113, 087201] Published Mon Aug 18, 2014

Nature Materials. doi:10.1038/nmat4060

Authors: Peter Nirmalraj, Damien Thompson, Agustín Molina-Ontoria, Marilyne Sousa, Nazario Martín, Bernd Gotsmann & Heike Riel

Author(s): J. Brüggemann, S. Weiss, P. Nalbach, and M. Thorwart

Spin currents could be used to carry away waste heat in nanoscale magnetic devices.

[Phys. Rev. Lett. 113, 076602] Published Thu Aug 14, 2014

Limits on fundamental limits to computation

Nature 512, 7513 (2014). doi:10.1038/nature13570

Authors: Igor L. Markov

An indispensable part of our personal and working lives, computing has also become essential to industries and governments. Steady improvements in computer hardware have been supported by periodic doubling of transistor densities in integrated circuits over the past fifty years. Such Moore scaling now requires ever-increasing efforts, stimulating research in alternative hardware and stirring controversy. To help evaluate emerging technologies and increase our understanding of integrated-circuit scaling, here I review fundamental limits to computation in the areas of manufacturing, energy, physical space, design and verification effort, and algorithms. To outline what is achievable in principle and in practice, I recapitulate how some limits were circumvented, and compare loose and tight limits. Engineering difficulties encountered by emerging technologies may indicate yet unknown limits.

Nature Physics. doi:10.1038/nphys3048

Authors: Ilya K. Drozdov, A. Alexandradinata, Sangjun Jeon, Stevan Nadj-Perge, Huiwen Ji, R. J. Cava, B. Andrei Bernevig & Ali Yazdani

Author(s): Joo-Von Kim, Felipe Garcia-Sanchez, João Sampaio, Constance Moreau-Luchaire, Vincent Cros, and Albert Fert

The dynamics of individual magnetic skyrmions confined in ultrathin film dots is studied theoretically. The systems considered are transition-metal ferromagnets possessing perpendicular magnetic anisotropy and particular attention is given to the dynamic response of the skyrmions to perpendicular ac...

[Phys. Rev. B 90, 064410] Published Mon Aug 11, 2014