Highlights

Exfoliated graphene crystals are not perfectly flat but can deform out-of-plane due to intrinsic and extrinsic factors. Ripples and distortions are known to be the most important sources of electron scattering in graphene, greatly affecting its transport properties. Such corrugations are also a serious obstacle to carry out angle resolved photoemission (ARPES) for probing the material's electronic structure, since this technique demands atomically flat surfaces. Combining ARPES with microprobe low energy electron diffraction makes it possible to circumvent such limitations. By measuring independently the short range roughness of corrugated suspended graphene sheets, we can distinguish corrugation effects from intrinsic lifetime broadening in ARPES,

showing that the quasiparticle lifetime scales inversely with energy. This approach is expected to be useful for probing the band structure of a variety of corrugated 2D system. K.R. Knox et al., in APS Physics.

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Making angle-resolved photoemission measurements on corrugated monolayer crystals: Suspended exfoliated single-crystal graphene;
K. R. Knox, A.Locatelli, M.B. Yilmaz, D. Cvetko, T.O. Menteş, M.Á. Niño, P. Kim, A. Morgante, and R.M. Osgood, Jr.;
Phys. Rev. B 84, 115401 (2011).
doi: 10.1103/PhysRevB.84.115401

Spontaneous formation of periodic patterns is an example of nature's tendency towards order. A class of such structures is induced by surface stress, and has been widely observed on single crystal surfaces. It is well known that the formation of these equilibrium patterns is driven by a competition between interactions at different length scales. The forces in action, due to short-range near-neighbour and long-range dipolar interactions, are of the most general type resulting in very similar phenomena occurring also in magnetic and electrostatic systems. Here, we explore the possibility of controlling the thermal fluctuations by adding a second adspecies to an already stripe-forming system.

The idea is based on the slow dynamics in a high-density binary lattice gas, which leads to a tendency towards glassy behaviour. In particular, we implement this by dosing small amounts of oxygen on submonolayer Pd/W(110).

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Stress engineering at the nanometer scale: Two-component adlayer stripes;
T.O. Menteş, N. Stojic, A. Locatelli, L. Aballe, N. Binggeli, M.Á. Niño, M. Kiskinova, E. Bauer; EPL (Europhysics Letters), 94 (3), 38003 (2011).
10.1209/0295-5075/94/38003

Corrugations, lattice distortions and charge transfer from adsorbates are the most important sources of electron scattering in graphene. Here, we investigate corrugations in graphene exploiting the multi-technique capabilities offered by a low energy electron microscope (LEEM): real space imaging of the sample morphology over large surface areas (up to several tens micron), with lateral resolution of 10 nm and atomic depth sensitivity, micro-probe low energy electron diffraction (μ-LEED). The short-range roughness of graphene at length scales below 20 nm is quantified by diffraction line-shape analysis, depending on film thickness and interaction with the SiO₂ support.  Due to its reduced stiffness, single-layer graphene shows larger roughness than multi-layers. Because of the absence of an

interacting support, suspended graphene displays a smoother texture than supported graphene, resulting in a notable narrowing of diffraction spots. Our LEED data suggests that the corrugation in suspended graphene films is influenced by both extrinsic and intrinsic factors, and in particular by adsorbate load and temperature.

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Corrugation in Exfoliated Graphene: An Electron Microscopy and Diffraction Study;
A. Locatelli, K. R. Knox, D. Cvetko, T.O. Menteş, M.Á. Niño, S. Wang, M. B. Yilmaz, P. Kim, R. M. Osgood Jr., and A. Morgante;
ACS Nano, 4, 4879-4889 (2010).
10.1021/nn101116n

A tedious limitation for XPEEM? Space charge artifacts in PEEM have been so far observed in PEEM instruments when employing ultra-bright laser sources. In this work, we focus on artifacts in XPEEM imaging and spectroscopy when using high intensity standard synchrotron radiation produced by an undulator source. We demonstrate that Coulomb interactions between photoelectrons along the PEEM optics result in the degradation of both the microscope lateral and energy resolution, due to the combined action of the Loeffler and Boersch effects. At a flux of 2×10¹³ photons/s, the lateral resolution in XPEEM imaging with either core level or secondary electrons deteriorates to more than 50 nm. Fermi level broadening up to several hundred meV

and spectral shift to higher kinetic energies are observed at similar photon fluxes, which correspond to peak electron photocurrents of a few μA in our estimates. These effects might have severe implications for the emerging generation of aberration corrected PEEM instruments, as they impose a physical limitation on the best lateral and energy resolution that can be achieved in XPEEM

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Single layer epitaxial Graphene on Re has been studied using photoelectron spectroscopy and density functional calculations, exploring the complex relationship between corrugation in graphene and its thermal stability. LEEM and microprobe-LEED were here used to address the growth and structure of graphene. Most importantly, this study reports on a novel approach to determine the relationship between the corrugation and the thermal stability of epitaxial graphene grown on a strongly interacting substrate. According to density functional theory calculations, the C single layer grown on Re(0001) is strongly corrugated, with a buckling of 1.6 Å, yielding a simulated C 1s core level spectrum which is in excellent agreement with the experimental one. We found that corrugation is closely knit with the thermal stability of the C

network: C-C bond breaking is favored in the strongly buckled regions of the moiré cell, though it requires the presence of diffusing graphene layer vacancies. Our data shows that there is a close relationship between graphene corrugation and its thermal stability, a key achievement in sight of the potential high temperature applications of supported graphene.

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Thermal Stability of Corrugated Epitaxial Graphene Grown on Re(0001);
E. Miniussi, M. Pozzo, A. Baraldi, E. Vesselli, R. R. Zhan, G. Comelli, T.O. Menteş, M.Á. Niño, A. Locatelli, S. Lizzit, and D. Alfè;
Phys. Rev. Lett. 106, 216101 (2011).
10.1103/PhysRevLett.106.216101

One of the newly devised non-volatile memories is the so-called racetrack memory which is based on magnetic domain wall motion. For this purpose, various approaches have been put forward. Using pure diffusive spin currents, where the electrons diffuse without an associated net charge current, is a possible alternative approach. Its main advantage is that the generation of spin currents, that involves energy dissipation, can occur at a distant location from the device, which can thus be kept cool and still manipulated by the absorbed diffusive spin currents. In line with this approach nonlocal spin valve geometries have been developed, where pure spin currents are

generated across ferromagnetic - non-magnetic contacts. Here, we report the  first observation of domain wall depinning assisted by pure spin currents.

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Domain-Wall Depinning Assisted by Pure Spin Currents;
D. Ilgaz, J. Nievendick, L. Heyne, D. Backes, J. Rhensius, T.A. Moore, M.Á. Niño, A. Locatelli, T.O. Menteş, A. V. Schmidsfeld, A. V. Bieren, S. Krzyk, L. J. Heyderman, and M. Kläui;
Phys. Rev. Lett. 105, 076601 (2010).
10.1103/PhysRevLett.105.076601