Dr.jens.brede

Nature Chemistry 6, 41 (2014). doi:10.1038/nchem.1804

Authors: Takashi Kumagai, Felix Hanke, Sylwester Gawinkowski, John Sharp, Konstantinos Kotsis, Jacek Waluk, Mats Persson & Leonhard Grill

The rate of an intramolecular hydrogen transfer reaction in a single porphycene molecule resting on a copper surface can be controlled by placing a copper adatom close to it. Cooperativity effects are also observed in rows of porphycene molecules, where the reaction rate of each individual molecule depends on the precise tautomer state of its neighbours.

Nature Chemistry 6, 4 (2014). doi:10.1038/nchem.1839

Author: Stuart Cantrill

There has been recent controversy about the magnitude of spin-flipping in the heavy metal Pt, characterized by the spin-diffusion length, lsf(Pt) We propose a resolution of this controversy, and also present evidence for the importance of a phenomenon neglected in prior studies of transport across sputtered Ferromagnetic/Pt (F/Pt) interfaces, spin-flipping at the interface. The latter is characterized by an interface spin-flipping parameter, delta(Co/Pt) that specifies the probability P = [1 - exp(-delta)] of a conduction electron flipping its spin direction as it traverses a Co/Pt interface. From studies of the Current-Perpendicular-to-Plane (CPP) Resistances and Magnetoresistances of sputtered ferromagnetically coupled Co/Pt multilayers by themselves, and embedded within Py-based Double Exchange-biased Spin-Valves, we derive values at 4.2K of delta(Co/Pt) = 0.9 (+0.5/-0.2), the interface specific resistance, AR*(Co/Pt) = 0.74 +/- 0.15 fohm-m(2). and the interface spin-scattering asymmetry, gamma(Co/Pt) = 0.58 +/- 0.12. This value of delta(Co/Pt) is much larger than ones previously found for interfaces involving Co but not Pt. To derive delta requires knowledge of the spin-diffusion length, lsf(Pt), for our sputtered Pt. We derive lsf(Pt) from separate measurements. Combining our results with those from others, we find that lsf(Pt) for Pt is approximately proportional to the inverse resistivity, 1/rho(Pt).

Article

Ginzburg–Landau theory provides a powerful framework for describing the behaviour of conventional superconductors without detailed microscopic information about them. Bao et al. construct a similar framework for describing spin superconductivity, a recently proposed analogue of conventional superconductivity.

Nature Communications doi: 10.1038/ncomms3951

Authors: Zhi-qiang Bao, X.C. Xie, Qing-feng Sun

Author(s): C. Tonnoir, A. Kimouche, J. Coraux, L. Magaud, B. Delsol, B. Gilles, and C. Chapelier

We report a new way to strongly couple graphene to a superconductor. The graphene monolayer has been grown directly on top of a superconducting Re(0001) thin film and characterized by scanning tunneling microscopy and spectroscopy. We observed a moiré pattern due to the mismatch between Re and graph...

[Phys. Rev. Lett. 111, 246805] Published Wed Dec 11, 2013

In scanning tunneling experiments on semiconductor surfaces, the energy scale within the tunneling junction is usually unknown due to tip-induced band bending. Here, we experimentally recover the zero point of the energy scale by combining scanning tunneling microscopy with Kelvin probe force spectroscopy. With this technique, we revisit shallow acceptors buried in GaAs. Enhanced acceptor-related conductance is observed in negative, zero, and positive band-bending regimes. An Anderson-Hubbard model is used to rationalize our findings, capturing the crossover between the acceptor state being part of an impurity band for zero band bending, and the acceptor state being split off and localized for strong negative or positive band bending, respectively.

Author(s): Pavel F. Bessarab, Valery M. Uzdin, and Hannes Jónsson

The effect of hydrogen adsorption on the magnetic properties of an Fe₃ cluster immersed in a Cu(111) surface has been calculated using density functional theory and the results used to parametrize an Alexander-Anderson model which takes into account the interaction of d electrons with itinerant elec...

[Phys. Rev. B 88, 214407] Published Tue Dec 10, 2013

We investigate the role of the substrate for the strength of the electon phonon coupling in Li-decorated graphene. We find that the interaction with a $h$-BN substrate leads to a significant enhancement from $\lambda_0=0.62$ to $\lambda_1=0.67$, which corresponds to a $25\%$ increase of the transition temperature from $T_{c0}=10.33$ K to $T_{c1}=12.98$ K. The superconducting gaps amount to 1.56 meV (suspended) and 1.98 meV (supported). These findings open up a new route to enhanced superconducting transition temperatures in graphene-based materials by substrate engineering.

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-substrate coupling and lateral intermolecular interactions are weak. While the ligand orbitals remain essentially unchanged upon contact to the substrate, we found modified electronic behavior at the Pt atom due to local hybridization and charge transfer to the substrate. Thus, this novel class of phosphorescent molecules exhibits well-defined and tunable interaction with its local environment.

The presence of topological defects in magnetic media often leads to normal modes with zero frequency (zero modes). Such modes are crucial for long-time behavior, describing, for example, the motion of a domain wall as a whole. Conventional numerical methods to calculate the spin-wave spectrum in magnetic media are either inefficient or they fail for systems with zero modes. We present a new efficient computational scheme that reduces the magnetic normal-mode problem to a generalized Hermitian eigenvalue problem also in the presence of zero modes. We apply our scheme to several examples, including two-dimensional domain walls and Skyrmions, and show how the effective masses that determine the dynamics can be calculated directly. These systems highlight the fundamental distinction between the two types of zero modes that can occur in spin systems, which we call special and inertial zero modes. Our method is suitable for both conservative and dissipative systems. For the latter case, we present a perturbative scheme to take into account damping, which can also be used to calculate dynamical susceptibilities.

Author(s): Lei Shen (沈雷), Minggang Zeng, Yunhao Lu, Ming Yang, and Yuan Ping Feng

A two-step doping process, magnetic followed by charge or vice versa, is required to produce massive topological surface states (TSS) in topological insulators for many physics and device applications. Here, we demonstrate simultaneous magnetic and hole doping achieved with a single dopant, carbon, ...

[Phys. Rev. Lett. 111, 236803] Published Thu Dec 05, 2013

Author(s): F. Donati, Q. Dubout, G. Autès, F. Patthey, F. Calleja, P. Gambardella, O. V. Yazyev, and H. Brune

We report on the magnetic properties of single Co atoms on graphene on Pt(111). By means of scanning tunneling microscopy spin-excitation spectroscopy, we infer a magnetic anisotropy of K=-8.1  meV with out-of-plane hard axis and a magnetic moment of 2.2μ_B. Co adsorbs on the sixfold graphene hollow ...

[Phys. Rev. Lett. 111, 236801] Published Tue Dec 03, 2013

Nature Nanotechnology 8, 883 (2013). doi:10.1038/nnano.2013.282

The nucleation and manipulation of individual skyrmions in magnetic nanostructures will be essential in any future skyrmionic device.

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

Authors: Naoto Nagaosa & Yoshinori Tokura

Spin switching of organometallic complexes by ferromagnetic surfaces is an important topic in the area of molecular nanospintronics. Moreover, graphene has been shown as a 2D surface for physisorption of molecular magnets and strain engineering on graphene can tune the spin state of an iron porphyrin (FeP) molecule from S = 1 to S = 2. Our ab initio density functional calculations suggest that a pristine graphene layer placed between a Ni(111) surface and FeP yields an extremely weak exchange interaction between FeP and Ni whereas the introduction of defects in graphene shows a variety of ferromagnetic and antiferromagnetic exchange interactions. Moreover, these defects control the easy axes of magnetization, strengths of magnetic anisotropy energies and spin-dipolar contributions. Our study suggests a new way of manipulating molecular magnetism by defects in graphene and hence has the potential to be explored in designing spin qubits to realize logic operations in molecular nanospintronics.

Scientific Reports 3 doi: 10.1038/srep03405

The ability of atomic hydrogen to chemisorb on graphene makes the latter a promising material for hydrogen storage. Based on scanning tunneling microscopy techniques, we report on site-selective adsorption of atomic hydrogen on convexly curved regions of monolayer graphene grown on SiC(0001). This system exhibits an intrinsic curvature owing to the interaction with the substrate. We show that at low coverage hydrogen is found on convex areas of the graphene lattice. No hydrogen is detected on concave regions. These findings are in agreement with theoretical models which suggest that both binding energy and adsorption barrier can be tuned by controlling the local curvature of the graphene lattice. This curvature-dependence combined with the known graphene flexibility may be exploited for storage and controlled release of hydrogen at room temperature making it a valuable candidate for the implementation of hydrogen-storage devices.

Potential for spin-based information processing in a thin-film molecular semiconductor

Nature 503, 7477 (2013). doi:10.1038/nature12597

Authors: Marc Warner, Salahud Din, Igor S. Tupitsyn, Gavin W. Morley, A. Marshall Stoneham, Jules A. Gardener, Zhenlin Wu, Andrew J. Fisher, Sandrine Heutz, Christopher W. M. Kay & Gabriel Aeppli

Organic semiconductors are studied intensively for applications in electronics and optics, and even spin-based information technology, or spintronics. Fundamental quantities in spintronics are the population relaxation time (T1) and the phase memory time (T2): T1 measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and T2 measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, T1 and T2 are respectively 59 ms and 2.6 μs, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 μs and 1 μs, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range. T2 is more than two orders of magnitude greater than the duration of the spin manipulation pulses, which suggests that copper phthalocyanine holds promise for quantum information processing, and the long T1 indicates possibilities for medium-term storage of classical bits in all-organic devices on plastic substrates.

Author(s): David Nobis, Marco Potenz, Daniel Niesner, and Thomas Fauster

Time-, angle-, and energy-resolved two-photon photoelectron spectroscopy is used to study image-potential states of weakly bound monolayer graphene on the noble-metal surfaces Pt(111), Au/Ir(111), Au/Ni(111), and Ir(100). Binding energies are increased relative to the substrates, and lifetimes are s...

[Phys. Rev. B 88, 195435] Published Wed Nov 27, 2013

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

A long-standing challenge in spintronics is the development of a stable, processable and tunable organic magnetic semiconductor. We reveal, through first-principles calculations, that a p-electron organic molecular magnet, lithium phthalocyanine (LiPc), can display surprisingly strong antiferromagne...

[Phys. Rev. B 88, 180404] Published Tue Nov 26, 2013

Author(s): F. Romeo and R. Citro

We study a spin valve with a triplet superconductor spacer intercalated between two ferromagnets with noncollinear magnetizations. We show that the magnetoresistance of the triplet spin valve depends on the relative orientations of the d vector, characterizing the superconducting order parameter, an...

[Phys. Rev. Lett. 111, 226801] Published Mon Nov 25, 2013

By first principles calculations, we demonstrate that when decorated with photochromic molecules, it is possible to use light to reversibly control the magnetic properties of a nanoscale magnetic system. The combination of a graphene-based magnetic system and a photochromic azobenzene molecule is chosen as a model system. The {\it trans} and {\it cis} isomers of the azobenzene molecule that can be converted between each other by means of photoexcitations are found to have drastically different effects on the magnetic properties of the system. The results may pave the way for the future design of light controllable molecular-scale spintronic devices.

Author(s): Sampsa K. Hämäläinen, Mark P. Boneschanscher, Peter H. Jacobse, Ingmar Swart, Katariina Pussi, Wolfgang Moritz, Jouko Lahtinen, Peter Liljeroth, and Jani Sainio

We have studied the incommensurate moiré structure of epitaxial graphene grown on iridium(111) by dynamic low-energy electron diffraction [LEED I(V)] and noncontact atomic force microscopy (AFM) with a CO-terminated tip. Our LEED I(V) results yield the average positions of all the atoms in the surfa...

[Phys. Rev. B 88, 201406] Published Fri Nov 22, 2013

Author(s): M. Casartelli, S. Casolo, G. F. Tantardini, and R. Martinazzo

We investigate the details of the electronic structure in the neighborhoods of a carbon atom vacancy in graphene by employing magnetization-constrained density-functional theory on periodic slabs, and spin-exact, multireference, second-order perturbation theory on a finite cluster. The picture that ...

[Phys. Rev. B 88, 195424] Published Fri Nov 22, 2013

Nature Physics. doi:10.1038/nphys2796

Authors: A. V. Ievlev, S. Jesse, A. N. Morozovska, E. Strelcov, E. A. Eliseev, Y. V. Pershin, A. Kumar, V. Ya. Shur & S. V. Kalinin

The paper outlines the historical development of spin in physics from about 1920 to the present day. It aims to provide the student with an accurate chronology of important developments, both scientific and technical.