Observation of Composite Particles in Graphene by ARPES*
Eli Rotenberg, LBNL
Abstract
The lifetime of electrons and holes in doped graphene is important both as a practical matter for new devices as well as to answer fundamental questions about the many-body interactions among graphene’s effectively massless carriers. Here we report angle-resolved photoemission measurements of quasi-freestanding graphene and show that the well-known conical, linear bands crossing at the Dirac energy do not represent the actual excitation spectrum of doped graphene. In reality, the spectrum is split into two components, consisting of an ordinary quasiparticle band, and a dispersing “plasmaronic” band consisting of a core hole bound to a plasmon. As a result, the single Dirac crossing of the valence and conduction bands is replaced by 3 Dirac crossings: between ordinary holes, between one hole and one plasmaronic band, and between plasmaronic bands.
*in collaboration with A. Bostwick, F. Speck, Th. Seyller, K. Horn, M. Polini, R. Asgara, A. H. Macdonald
Micro-scale characterisation of deformation and distortion in ductile (poly)crystals by synchrotron X-ray beams*
Alexander Korsunsky, Oxford University
Abstract
Most engineering structural metallic alloys are used in polycrystalline form. The nature of the mechanical response of these systems is complex and hierarchical, spanning a range of scales. Lattice strains, distortions and defects (notably, dislocations) nucleate, interact, pile-up at grain boundaries and self-organise at the (sub)micron scale. Individual grains experience strong interactions with their neighbours and geometric features (cracks, notches). Groups of grains sharing common orientation find themselves embedded within large ensembles possessing certain statistical properties (size distributions, preferred orientation, etc.) Ultimately, the macroscopic properties of grain aggregates are determined by this hierarchy of interactions. Notably, while collective properties such as stiffness are relatively well represented by averages, strength properties associated with fracture, fatigue crack propagation, creep, and damage show a strong dependence on the local microscopic conditions of the “weakest link”.
The ongoing improvements in the spatial resolution of X-ray imaging and tomography, and the availability of micro-focused X-ray beams open up a number of opportunities for the study of structure and deformation at (sub)micron scales. Fundamental questions concerning the scale-dependence and strain gradient effects in solids can now be tackled by the combination of synchrotron X-ray methods and suitably refined deformation modelling.
In this study a range of methodologies and experimental configurations will be presented that have allowed us to develop improved insight into the physical mechanisms of plastic deformation in ductile metallic alloys. Examples will be presented of white beam energy-dispersive diffraction, micro-beam Laue diffraction, scanning micro-beam diffraction topography, high resolution reciprocal space mapping, and imaging. Connections will be established with advanced numerical models of polycrystal deformation using strain gradient plasticity and discrete dislocation dynamics modelling.
*with Brian Abbey, Felix Hofmann, Igor Dolbnya, Steve Collins, Mengyin Xie, Xu Song