Dr.jens.brede

Nature Chemistry 5, 739 (2013). doi:10.1038/nchem.1704

Authors: Katsuaki Kawasumi, Qianyan Zhang, Yasutomo Segawa, Lawrence T. Scott & Kenichiro Itami

A grossly warped nanographene, C80H30, that incorporates five 7-membered rings and one 5-membered ring embedded in a hexagonal lattice has been synthesized, isolated and fully characterized. Experiments revealing how the properties of such a large graphene subunit are affected by multiple odd-membered-ring defects are also reported.

The Kondo effect arises due to the interaction between a localized spin and the electrons of a surrounding host. Studies of individual magnetic impurities by scanning tunneling spectroscopy have renewed interest in Kondo physics; however, a quantitative comparison with theoretical predictions remained challenging. Here we show that the zero-bias anomaly detected on an organic radical weakly coupled to a Au (111) surface can be described with astonishing agreement by perturbation theory as originally developed by Kondo 60 years ago. Our results demonstrate that Kondo physics can only be fully conceived by studying both temperature and magnetic field dependence of the resonance. The identification of a spin 1/2 Kondo system is of relevance not only as a benchmark for predictions for Kondo physics but also for correlated electron materials in general.

US sequester: Follow Obama's lead and take a pay cut

Nature 500, 7461 (2013). doi:10.1038/500151b

Author: Peter Foukal

In considering the impact of the US budget sequester on science (see, for example, Nature499, 147–148; 2013), I see no mention of salary reductions. A 5% reduction in the salaries of federally supported science staff, including administrative and agency

Author(s): M. Kofu, O. Yamamuro, T. Kajiwara, Y. Yoshimura, M. Nakano, K. Nakajima, S. Ohira-Kawamura, T. Kikuchi, and Y. Inamura

We report inelastic neutron scattering results on a rare-earth-based single-molecule magnet, the Tb-Cu dinuclear complex. By means of a high-resolution neutron chopper spectrometer, the details of the excitations were clarified. The magnetic excitations are clearly observed at ℏω=1.7 and 12.3 meV. T...

[Phys. Rev. B 88, 064405] Published Fri Aug 09, 2013

Spin-polarized currents delivered by a scanning tunneling microscope can be used to create and destroy spin whirlpools.

Authors: Niklas Romming, Christian Hanneken, Matthias Menzel, Jessica E. Bickel, Boris Wolter, Kirsten von Bergmann, André Kubetzka, Roland Wiesendanger

Article

Most new device concepts for random-access memory are based on inorganic spin filters, which need a permanent magnet to operate. Here, the authors exploit the chiral-induced spin selectivity effect in an organic spin filter to construct a novel type of memory device, which works without a permanent magnet.

Nature Communications doi: 10.1038/ncomms3256

Authors: Oren Ben Dor, Shira Yochelis, Shinto P. Mathew, Ron Naaman, Yossi Paltiel

Nature Nanotechnology 8, 575 (2013). doi:10.1038/nnano.2013.133

Authors: Stefan Wagner, Ferdinand Kisslinger, Stefan Ballmann, Frank Schramm, Rajadurai Chandrasekar, Tilmann Bodenstein, Olaf Fuhr, Daniel Secker, Karin Fink, Mario Ruben & Heiko B. Weber

The ground state of frustrated (antiferromagnetic) triangular molecular magnets is characterized by two total-spin $S =1/2$ doublets with opposite chirality. According to a group theory analysis [M. Trif \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 217201 (2008)] an external electric field can efficiently couple these two chiral spin states, even when the spin-orbit interaction (SOI) is absent. The strength of this coupling, $d$, is determined by an off-diagonal matrix element of the dipole operator, which can be calculated by \textit{ab-initio} methods [M. F. Islam \textit{et al.}, Phys. Rev. B \textbf{82}, 155446 (2010)]. In this work we propose that Coulomb-blockade transport experiments in the cotunneling regime can provide a direct way to determine the spin-electric coupling strength. Indeed, an electric field generates a $d$-dependent splitting of the ground state manifold, which can be detected in the inelastic cotunneling conductance. Our theoretical analysis is supported by master-equation calculations of quantum transport in the cotunneling regime. We employ a Hubbard-model approach to elucidate the relationship between the Hubbard parameters $t$ and $U$, and the spin-electric coupling constant $d$. This allows us to predict the regime in which the coupling constant $d$ can be extracted from experiment.

Atoms transferred to W(111) and W(110) tip apices from the Au(111) surface during tunneling and approach to mechanical contact experiments in STM are characterized in FIM at room temperature and at 158 K. The different activation energies for diffusion on the (111) and (110) tip planes and the experiment temperature are shown to be important factors controlling the extent of changes to the atomic structure of the tip. W(111) tips are much better suited to scanning probe studies which require the characterization of an atomically defined tip and subsequent verification of its integrity in FIM. The statistics of the observed spikes in the tunneling current when the tips are approached to Au(111) are interpreted using a simple model of adatoms diffusing through the STM junction.

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

Free-electron-like image potential states are observed in scanning tunneling spectroscopy on graphene quantum dots on Ir(111) acting as potential wells. The spectrum strongly depends on the size of the nanostructure as well as on the spatial position on top, indicating lateral confinement. Analysis ...

[Phys. Rev. Lett. 111, 056804] Published Wed Jul 31, 2013

Author(s): Wei Wu, N. M. Harrison, and A. J. Fisher

The magnetic properties and electronic structure of cobalt phthalocyanine (CoPc, spin-1/2) molecular chains have been studied using density functional theory with a hybrid exchange functional, over a wide range of chain geometries. Our theoretical results for the exchange interactions in the known p...

[Phys. Rev. B 88, 024426] Published Fri Jul 26, 2013

We demonstrate rotational excitation spectroscopy with the scanning tunneling microscope for physisorbed hydrogen and its isotopes hydrogen-deuterid and deuterium. The observed excitation energies are very close to the gas phase values and show the expected scaling with moment of inertia. Since these energies are characteristic for the molecular nuclear spin states we are able to identify the para and ortho species of hydrogen and deuterium, respectively. We thereby demonstrate nuclear spin sensitivity with unprecedented spatial resolution.

Author(s): S. Thiele, R. Vincent, M. Holzmann, S. Klyatskaya, M. Ruben, F. Balestro, and W. Wernsdorfer

We present the electrical readout of time trajectories obtained from an isolated nuclear spin. The device, a TbPc₂ single-molecule magnet spin transistor, detects the four different nuclear spin states of the Tb³⁺ ion with fidelities better than 69%, allowing us to measure individual relaxation time...

[Phys. Rev. Lett. 111, 037203] Published Wed Jul 17, 2013

Nature Materials. doi:10.1038/nmat3695

Authors: Wei Yang, Guorui Chen, Zhiwen Shi, Cheng-Cheng Liu, Lianchang Zhang, Guibai Xie, Meng Cheng, Duoming Wang, Rong Yang, Dongxia Shi, Kenji Watanabe, Takashi Taniguchi, Yugui Yao, Yuanbo Zhang & Guangyu Zhang

Hexagonal boron nitride (h-BN) has recently emerged as an excellent substrate for graphene nanodevices, owing to its atomically flat surface and its potential to engineer graphene’s electronic structure. Thus far, graphene/h-BN heterostructures have been obtained only through a transfer process, which introduces structural uncertainties due to the random stacking between graphene and h-BN substrate. Here we report the epitaxial growth of single-domain graphene on h-BN by a plasma-assisted deposition method. Large-area graphene single crystals were successfully grown for the first time on h-BN with a fixed stacking orientation. A two-dimensional (2D) superlattice of trigonal moiré pattern was observed on graphene by atomic force microscopy. Extra sets of Dirac points are produced as a result of the trigonal superlattice potential and the quantum Hall effect is observed with the 2D-superlattice-related feature developed in the fan diagram of longitudinal and Hall resistance, and the Dirac fermion physics near the original Dirac point is unperturbed. The macroscopic epitaxial graphene is in principle limited only by the size of the h-BN substrate and our synthesis method is potentially applicable on other flat surfaces. Our growth approach could thus open new ways of graphene band engineering through epitaxy on different substrates.

Nature Physics. doi:10.1038/nphys2671

Authors: M. P. Allan, F. Massee, D. K. Morr, J. Van Dyke, A. W. Rost, A. P. Mackenzie, C. Petrovic & J. C. Davis

The Cooper pairing mechanism of heavy fermionsuperconductors, long thought to be due to spin fluctuations, has not yet been determined. It is the momentum space (k-space) structure of the superconducting energy gap Δ(k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure Δ(k) for any heavy fermion superconductor. Bogoliubov quasiparticle interference imaging, a proven technique for measuring the energy gaps of superconductors with high critical temperatures, has recently been proposed as a new method to measure Δ(k) in heavy fermion superconductors, specifically CeCoIn5 (ref. ). By implementing this method, we detect a superconducting energy gap whose nodes are oriented along k∥(±1,±1)π/a0 directions. Moreover, for the first time in any heavy fermion superconductor, we determine the detailed structure of its multiband energy gaps Δi(k). For CeCoIn5, this information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that largest magnitude Δ(k) opens on a high- k band so that the primary gap nodes occur at unforeseen k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with dx2−y2 gap symmetry. Such quantitative knowledge of both the heavy band-structure and superconducting gap-structure will be critical in identifying the microscopic pairing mechanism of heavy fermion superconductivity.

Author(s): C. A. Mujica-Martinez, P. Nalbach, and M. Thorwart

We propose a design for molecular charge qubits based on π-conjugated block copolymers and determine their electronic structure as well as their vibrational active modes. By tuning the length of the oligomers, the tunnel coupling in the charge qubit and its decoherence properties due to molecular vi...

[Phys. Rev. Lett. 111, 016802] Published Wed Jul 03, 2013

Author(s): Z. G. Yu

We study carrier spin transport under a transverse magnetic field in organic structures. In organics, carriers are localized polarons and charge transport is via polaron hopping. Spin transport, however, can utilize the exchange coupling between localized polarons, which can be much faster than pola...

[Phys. Rev. Lett. 111, 016601] Published Tue Jul 02, 2013

Article

A lot of theoretical work on the Kondo effect has focused on spin 1/2 systems, but the characterization of a single-spin 1/2 atom or molecule in the weak coupling regime has been missing. Here, the authors close this gap with a scanning tunneling spectroscopy study of an organic radical on a gold surface.

Nature Communications doi: 10.1038/ncomms3110

Authors: Yong-hui Zhang, Steffen Kahle, Tobias Herden, Christophe Stroh, Marcel Mayor, Uta Schlickum, Markus Ternes, Peter Wahl, Klaus Kern

Author(s): Sumanta Bhandary, Barbara Brena, Pooja M. Panchmatia, Iulia Brumboiu, Matthias Bernien, Claudia Weis, Bernhard Krumme, Corina Etz, Wolfgang Kuch, Heiko Wende, Olle Eriksson, and Biplab Sanyal

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this pa...

[Phys. Rev. B 88, 024401] Published Mon Jul 01, 2013

We demonstrate a controllable surface-coordinated linear polymerization of long-chain poly(phenylacetylenyl)s that are self-organized into a “circuit-board” pattern on a Cu(100) surface. Scanning tunneling microscopy/spectroscopy (STM/S) corroborated by ab initio calculations, reveals the atomistic details of the molecular structure, and provides a clear signature of electronic and vibrational properties of the poly(phenylacetylene)s chains. Notably, the polymerization reaction is confined epitaxially to the copper lattice, despite a large strain along the polymerized chain that subsequently renders it metallic. Polymerization and depolymerization reactions can be controlled locally at the nanoscale by using a charged metal tip. This control demonstrates the possibility of precisely accessing and controlling conjugated chain-growth polymerization at low temperature. This finding may lead to the bottom-up design and realization of sophisticated architectures for molecular nano-devices.

Scientific Reports 3 doi: 10.1038/srep02102

The development of scanning probe microscopy repositioned modern physics. When Rohrer and Binnig first used electronic tunnelling effects to image atoms and quantum states they did more than pin down theoretical hypotheses to real-world observables; the scanning tunnelling microscope fed imaginations, prompting researchers to consider new directions and possibilities [1]. As Rohrer once commented, 'We could show that you can easily manipulate or position something small in space with an accuracy of 10 pm.... When you can do that, you simply have ideas of what you can do' [2]. The development heralded a cavalry of scanning probe techniques—such as atomic force microscopy (AFM) [3–5], scanning near-field optical microscopy (SNOM) [6–8] and Kelvin probe force microscopy (KPFM) [9, 10]—that still continue to bring nanomaterials and nanoscale phenomena into fresh focus. Not long after the development of scanning tunnelling microscopy, Binnig, Quate and Gerber collaborating in ...

Author(s): F. Donati, G. Fratesi, L. Ning, A. Brambilla, M. I. Trioni, A. Li Bassi, C. S. Casari, and M. Passoni

The electronic and magnetic properties of bulk Cr tips for scanning tunneling microscopy have been investigated by means of density functional theory (DFT) calculations and scanning tunneling spectroscopy (STS) measurements. Spin-resolved densities of states (DOS) were calculated for model tips, i.e...

[Phys. Rev. B 87, 235431] Published Wed Jun 26, 2013