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Caillard D.,CNRS Toulouse Center for Materials Elaboration and Structural Studies
Acta Materialia | Year: 2014

We present a complete analysis of the velocity of individual screw dislocations in pure iron, as a function of stress and temperature, by means of in situ straining experiments in a transmission electron microscope. The results show a very pronounced deviation from the classical laws of thermodynamics, which is at the origin of the discrepancy between experimental and calculated deformation stresses at low temperature. This strongly supports the occurrence of large quantum effects proposed in a recent theoretical study. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source


Hawkes P.W.,CNRS Toulouse Center for Materials Elaboration and Structural Studies
Ultramicroscopy | Year: 2015

The progress of electron lens aberration correction from about 1990 onwards is chronicled. Reasonably complete lists of publications on this and related topics are appended.A present for Max Haider and Ondrej Krivanek in the year of their 65th birthdays.By a happy coincidence, this review was completed in the year that both Max Haider and Ondrej Krivanek reached the age of 65. It is a pleasure to dedicate it to the two leading actors in the saga of aberration corrector design and construction. They would both wish to associate their colleagues with such a tribute but it is the names of Haider and Krivanek (not forgetting Joachim Zach) that will remain in the annals of electron optics, next to that of Harald Rose. I am proud to know that both regard me as a friend as well as a colleague. © 2015 Elsevier B.V. Source


Caillard D.,CNRS Toulouse Center for Materials Elaboration and Structural Studies
Acta Materialia | Year: 2010

In situ straining experiments have been carried out in pure Fe, in order to determine the geometry and the kinetics of dislocation glide at room temperature. Straight screw dislocations glide slowly in {1 1 0} elemental slip planes, at a velocity proportional to their length, whereas curved non-screw parts are highly mobile. The exact loop shape can yield the local stress as well as the difference of core energy between pure screw and near-screw orientations. The velocity-stress dependence of screws has been measured at the scale of a single dislocation source, and compared with macroscopic activation areas. The results are discussed in terms of the kink-pair mechanism. © 2010 Acta Materialia Inc. Source


Caillard D.,CNRS Toulouse Center for Materials Elaboration and Structural Studies
Acta Materialia | Year: 2010

In situ straining experiments have been carried out at low temperature in pure Fe, in order to study the change of mechanism occurring at around 250 K. The local stress necessary to move individual screw dislocations is in good agreement with the macroscopic yield stress at various temperatures. In the lower temperature range, straight screw segments have a jerky motion in {1 1 0} planes, at variance from the steady motion observed near room temperature. The distributions of waiting times in locked positions, and jump distances, the temperature variation of the average jump distance, and the stress/temperature variation of the macroscopic activation areas, are inconsistent with the kink-pair mechanism observed above 250 K. They have been interpreted in terms of a locking-unlocking mechanism, already proposed in hexagonal-closed-packed metals. Under such conditions, the change of mechanism at 250 K can account for the surprisingly low value of the flow stress extrapolated to 0 K (much lower than the theoretical Peierls stress). © 2010 Acta Materialia Inc. Source


Koch M.,Fritz Haber Institute of the Max Planck Society | Ample F.,Institute of Materials Research and Engineering of Singapore | Joachim C.,Institute of Materials Research and Engineering of Singapore | Joachim C.,CNRS Toulouse Center for Materials Elaboration and Structural Studies | Grill L.,Fritz Haber Institute of the Max Planck Society
Nature Nanotechnology | Year: 2012

Graphene nanoribbons could potentially be used to create molecular wires with tailored conductance properties. However, understanding charge transport through a single molecule requires length-dependent conductance measurements and a systematic variation of the electrode potentials relative to the electronic states of the molecule. Here, we show that the conductance properties of a single molecule can be correlated with its electronic states. Using a scanning tunnelling microscope, the electronic structure of a long and narrow graphene nanoribbon, which is adsorbed on a Au(111) surface, is spatially mapped and its conductance then measured by lifting the molecule off the surface with the tip of the microscope. The tunnelling decay length is measured over a wide range of bias voltages, from the localized Tamm states over the gap up to the delocalized occupied and unoccupied electronic states of the nanoribbon. We also show how the conductance depends on the precise atomic structure and bending of the molecule in the junction, illustrating the importance of the edge states and a planar geometry. © 2012 Macmillan Publishers Limited. All rights reserved. Source

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