Dr.jens.brede

The electronic and magnetic properties of Mn coordinated to 1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have been investigated by combining scanning tunneling microscopy and X-ray magnetic circular dichroism (XMCD) performed at low temperature (3 K). When formed on Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn sites with similar single-ion anisotropy energies, but different coupling constants. Low-temperature XMCD spectra show that the local environment of the Mn centers differs appreciably for the two substrates. Multiplet structure calculations were used to derive the corresponding ligand field parameters confirming an in-plane uniaxial anisotropy. The observed interatomic coupling is discussed in terms of superexchange as well as substrate-mediated magnetic interactions.

This paper gives a broad summary of the physical phenomena associated with the quench of a superconducting magnet.

The structure of the [001]-oriented single crystalline tungsten probes sharpened in ultra-high vacuum using electron beam heating and ion sputtering has been studied using scanning and transmission electron microscopy. The electron microscopy data prove reproducible fabrication of the single-apex tips with nanoscale pyramids grained by the {011} planes at the apexes. These sharp, [001]-oriented tungsten tips have been successfully utilized in high resolution scanning tunneling microscopy imaging of HOPG(0001), SiC(001) and graphene/SiC(001) surfaces. The electron microscopy characterization performed before and after the high resolution STM experiments provides direct correlation between the tip structure and picoscale spatial resolution achieved in the experiments.

Scientific Reports 4 doi: 10.1038/srep03742

Gold nanoparticle assemblies possess diverse application potential, ranging from industrial nanotechnology to medical biotechnology. Because the structures and properties of assemblies are directly affected by the stabilization mechanism between the organic molecules serving as protecting ligands and the gold nanoparticle surface, it is crucial to find and investigate new stabilization mechanisms. Here, we report that π-conjugated phthalocyanine rings can serve as stabilizing ligands for gold nanoparticles. Bis(phthalocyaninato)lutetium(III) (LuPc2) or bis(phthalocyaninato)terbium(III) (TbPc2), even though complex, do not have specific binding units and stabilize gold nanoparticles through van der Waals interaction between parallel adsorbed phthalocyanine ligands and the gold nanoparticle surface. AC magnetic measurements and the electron-transport properties of the assemblies give direct evidence that the phthalocyanines are isolated from each other. Each nanoparticle shows weak electronic coupling despite the short internanoparticle distance (~1 nm), suggesting Efros–Shklovskii-type variable-range hopping and collective single-electron tunnelling behaviours.

Scientific Reports 4 doi: 10.1038/srep03758

Article

The local coordination of the edge atoms of graphene may affect its electronic or magnetic properties. Here the authors experimentally demonstrate that non-functionalized graphene edges can exist in vacuum, contrary to some previous theoretical predictions

Nature Communications doi: 10.1038/ncomms4040

Authors: Kuang He, Gun-Do Lee, Alex W. Robertson, Euijoon Yoon, Jamie H. Warner

Military science: The evolving science of war

Nature 505, 7482 (2014). doi:10.1038/505156a

Author: Sharon Weinberger

Sharon Weinberger assesses two studies probing the roles of physics and psychology in conflicts past, present and future.

Author(s): Thorsten Deilmann, Peter Krüger, Michael Rohlfing, and Daniel Wegner

We investigate the adsorption of a single tetracyanoethylene (TCNE) molecule on the silver (001) surface. Adsorption structures, electronic properties, and scanning tunneling microscopy (STM) images are calculated within density-functional theory. Adsorption occurs most favorably in on-top configura...

[Phys. Rev. B 89, 045405] Published Thu Jan 09, 2014

Article

Ferromagnets are an integral part of spintronics because of their spin selectivity, but in combination with superconductors selectivity between different Cooper pairs is required. Here, the authors find evidence for this selectivity in a ferromagnet–superconductor–ferromagnet spin valve.

Nature Communications doi: 10.1038/ncomms4048

Authors: N. Banerjee, C. B. Smiet, R. G. J. Smits, A. Ozaeta, F. S. Bergeret, M. G. Blamire, J. W. A. Robinson

Author(s): Jürgen Klinkhammer, Martin Schlipf, Fabian Craes, Sven Runte, Thomas Michely, and Carsten Busse

High-quality films of the ferromagnetic semiconductor EuO are grown on epitaxial graphene on Ir(111) and investigated in situ with scanning tunneling microscopy and spectroscopy. Electron scattering at defects leads to standing-wave patterns, manifesting the existence of a surface state in EuO. The ...

[Phys. Rev. Lett. 112, 016803] Published Tue Jan 07, 2014

Nature Nanotechnology 9, 64 (2014). doi:10.1038/nnano.2013.264

Authors: Jenny C. Oberg, M. Reyes Calvo, Fernando Delgado, María Moro-Lagares, David Serrate, David Jacob, Joaquín Fernández-Rossier & Cyrus F. Hirjibehedin

Magnetic skyrmions have the potential to provide solutions for low-power, high-density data storage and processing. One of the major challenges in developing skyrmion-based devices is the skyrmions' magnetic stability in confined helimagnetic nanostructures. Through a systematic study of equilibrium states, using a full three-dimensional micromagnetic model including demagnetisation effects, we demonstrate that skyrmionic textures are the lowest energy states in helimagnetic thin film nanostructures at zero external magnetic field and in absence of magnetocrystalline anisotropy. We also report the regions of metastability for non-ground state equilibrium configurations. We show that bistable skyrmionic textures undergo hysteretic behaviour between two energetically equivalent skyrmionic states with different core orientation, even in absence of both magnetocrystalline and demagnetisation-based shape anisotropies, suggesting the existence of Dzyaloshinskii-Moriya-based shape anisotropy. Finally, we show that the skyrmionic texture core reversal dynamics is facilitated by the Bloch point occurrence and propagation.

Author(s): D. Klar, B. Brena, H. C. Herper, S. Bhandary, C. Weis, B. Krumme, C. Schmitz-Antoniak, B. Sanyal, O. Eriksson, and H. Wende

The coupling of submonolayer coverages of Fe-phthalocyanine molecules on bare and oxygen-covered ferromagnetic Co(001) films was studied by x-ray-absorption spectroscopy, especially the x-ray magnetic circular dichroism, in combination with density functional theory. We observe that the magnetic mom...

[Phys. Rev. B 88, 224424] Published Mon Dec 30, 2013

Graphene was the first material predicted to realize a topological insulator (TI), but unfortunately the gap is unobservably small due to carbon's weak spin-orbital coupling (SOC). Based on first-principles calculations, we propose a stable sp-sp2 hybrid carbon network as a graphene analog whose electronic band structures in proximity of the Fermi level are characterized by Dirac cones. We demonstrate that this unique carbon framework has topologically nontrivial electronic structures with the Z2 topological invariant of v = 1 which is quite promising for hosting the quantum spin Hall effect (QSHE) in an experimentally accessible low temperature regime (<7 K). This provides a viable approach for searching for new TIs in 2D carbon allotropes.

Scientific Reports 3 doi: 10.1038/srep03532

Nature Nanotechnology 8, 899 (2013). doi:10.1038/nnano.2013.243

Authors: Naoto Nagaosa & Yoshinori Tokura

Nature Nanotechnology 8, 885 (2013). doi:10.1038/nnano.2013.263

Authors: V. Alek Dediu & Alberto Riminucci

Author(s): Pascal R. Ewen, Jan Sanning, Nikos L. Doltsinis, Matteo Mauro, Cristian A. Strassert, and Daniel Wegner

We have investigated the structural and electronic properties of phosphorescent planar platinum(II) complexes at the interface of Au(111) with submolecular resolution using combined scanning tunneling microscopy and spectroscopy as well as density functional theory. Our analysis shows that molecule-...

[Phys. Rev. Lett. 111, 267401] Published Fri Dec 27, 2013

The current dependence of the spin torque switching rate in a thermally activated region of an in-plane magnetized system was studied. The logarithm of the switching rate depended nonlinearly on current in the high-current region, I_{c} < I < I_{c}^{*}, where I_{c} and I_{c}^{*} are critical currents distinguishing the stability of the magnetization. We also found that the attempt frequency had a minimum around I_{c}, and that the attempt frequency at I_{c} was three orders of magnitude smaller than that at zero current, contrary to the assumption in previous analyses of experiments that it remains constant.

We present a comprehensive study of the adsorption of bis(phthalocyaninato)lutetium(III) (LuPc$_2$) on highly oriented pyrolytic graphite(0001) (HOPG). The growth and self-assembly of the molecular layers as well as the electronic structure has been investigated systematically using scanning tunnelling microscopy and scanning tunnelling spectroscopy combined with density functional theory (DFT) calculations and molecular mechanics simulations. We reveal that the adsorption of LuPc$_2$ leads to the formation of a square-like close-packed structure on the almost inert surface of HOPG, which is corroborated by simulations. Moreover, we observed a parallel orientation of the LuPc$_2$ molecules in the first monolayer, whereas in subsequent layers an increasing tilt out of the surface plane was found. Tip-sample distance-dependent tunnelling spectroscopy measurements allowed us to detect a shift in the energy positions of the peaks assigned to the lowest unoccupied molecular orbital toward the Fermi energy with decreasing tip-sample separation.

Pulse tube refrigerators are becoming more common, because they are cost efficient and demand less handling than conventional (wet) refrigerators. However, a downside of a pulse tube system is the vibration level at the cold-head, which is in most designs several micrometers. We implemented vibration isolation techniques which significantly reduced vibration levels at the experiment. These optimizations were necessary for the vibration sensitive Magnetic Resonance Force Microscopy experiments (MRFM) at milli-kelvin temperatures for which the cryostat is intended. With these modifications we show atomic resolution STM on graphite. This is promising for scanning probe microscopy applications at very low temperatures.

Nature advance online publication 22 December 2013. doi:10.1038/nature12927

Authors: F. Trinter, M. S. Schöffler, H.-K. Kim, F. P. Sturm, K. Cole, N. Neumann, A. Vredenborg, J. Williams, I. Bocharova, R. Guillemin, M. Simon, A. Belkacem, A. L. Landers, Th. Weber, H. Schmidt-Böcking, R. Dörner & T. Jahnke

In 1997, it was predicted that an electronically excited atom or molecule placed in a loosely bound chemical system (such as a hydrogen-bonded or van-der-Waals-bonded cluster) could efficiently decay by transferring its excess energy to a neighbouring species that would then emit a low-energy electron. This intermolecular Coulombic decay (ICD) process has since been shown to be a common phenomenon, raising questions about its role in DNA damage induced by ionizing radiation, in which low-energy electrons are known to play an important part. It was recently suggested that ICD can be triggered efficiently and site-selectively by resonantly core-exciting a target atom, which then transforms through Auger decay into an ionic species with sufficiently high excitation energy to permit ICD to occur. Here we show experimentally that resonant Auger decay can indeed trigger ICD in dimers of both molecular nitrogen and carbon monoxide. By using ion and electron momentum spectroscopy to measure simultaneously the charged species created in the resonant-Auger-driven ICD cascade, we find that ICD occurs in less time than the 20 femtoseconds it would take for individual molecules to undergo dissociation. Our experimental confirmation of this process and its efficiency may trigger renewed efforts to develop resonant X-ray excitation schemes for more localized and targeted cancer radiation therapy.

Article

The inverse spin-Hall effect is important in spintronics as it converts a spin current into charge current in nonmagnetic materials. Here, the authors show signatures of the inverse Rashba–Edelstein effect, a similar effect in two-dimensional interfacial states, in an Ag/Bi bilayer.

Nature Communications doi: 10.1038/ncomms3944

Authors: J. C. Rojas Sánchez, L. Vila, G. Desfonds, S. Gambarelli, J. P. Attané, J. M. De Teresa, C. Magén, A. Fert

Graphene was the first material predicted to realize a topological insulator (TI), but unfortunately the gap is unobservably small due to carbon's weak spin-orbital coupling (SOC). Based on first-principles calculations, we propose a stable sp-sp2 hybrid carbon network as a graphene analog whose electronic band structures in proximity of the Fermi level are characterized by Dirac cones. We demonstrate that this unique carbon framework has topologically nontrivial electronic structures with the Z2 topological invariant of v = 1 which is quite promising for hosting the quantum spin Hall effect (QSHE) in an experimentally accessible low temperature regime (<7 K). This provides a viable approach for searching for new TIs in 2D carbon allotropes.

Scientific Reports 3 doi: 10.1038/srep03532