Electrically-Induced Mixed Valence Causes Resistive Switching in Copper Helical Metallopolymers

<div>Controlling the flow of electrical current at the nanoscale typically requires complex top-down approaches. Here we use a bottom-up approach to demonstrate resistive</div><div>switching within molecular wires that consist of double-helical metallopolymers and are constructed by self-assembly. When we expose the material to an electric field, we determine that approximately 25% of the copper atoms oxidise from Cu(I) to Cu(II) without rupture of the polymer chain. The ability to sustain such high level of oxidation is unprecedented in a copper-based molecule: it is made possible here by the double helix compressing in order to satisfy the new coordination geometry required by Cu(II).</div><div>This mixed-valence structure exhibits a 10⁴-fold increase in</div><div>conductivity, which is projected to last over 10 years. We explain the increase in conductivity as being promoted by the creation, upon oxidation, of partly filled d_z²</div><div>orbitals aligned along the mixed-valence copper array; the long-lasting nature of the change in conductivity is due to the structural rearrangement of the double-helix, which poses an energetic barrier to re-reduction. This work establishes helical metallopolymers as a new platform for controlling currents at the nanoscale.</div>

Electrically-Induced Mixed Valence Causes Resistive Switching in Copper Helical Metallopolymers

<div>Controlling the flow of electrical current at the nanoscale typically requires complex top-down approaches. Here we use a bottom-up approach to demonstrate resistive</div><div>switching within molecular wires that consist of double-helical metallopolymers and are constructed by self-assembly. When we expose the material to an electric field, we determine that approximately 25% of the copper atoms oxidise from Cu(I) to Cu(II) without rupture of the polymer chain. The ability to sustain such high level of oxidation is unprecedented in a copper-based molecule: it is made possible here by the double helix compressing in order to satisfy the new coordination geometry required by Cu(II).</div><div>This mixed-valence structure exhibits a 10⁴-fold increase in</div><div>conductivity, which is projected to last over 10 years. We explain the increase in conductivity as being promoted by the creation, upon oxidation, of partly filled d_z²</div><div>orbitals aligned along the mixed-valence copper array; the long-lasting nature of the change in conductivity is due to the structural rearrangement of the double-helix, which poses an energetic barrier to re-reduction. This work establishes helical metallopolymers as a new platform for controlling currents at the nanoscale.</div>

Reflections on the origin of the EMC effect

In the 35 years since the European Muon Collaboration announced the astonishing result that the valence structure of a nucleus was very different from that of a free nucleon, many explanations have been suggested. The first of the two most promising explanations is based upon the different effects of the strong Lorentz scalar and vector mean fields known to exist in a nucleus on the internal structure of the nucleon-like clusters which occupy shell model states. The second links the effect to the modification of the structure of nucleons involved in short-range correlations, which are far off their mass shell. We explore some of the methods which have been proposed to give complementary information on this puzzle, especially the spin-dependent EMC effect and the isovector EMC effect, both proposed by Cloët, Bentz and Thomas. It is shown that the predictions for the spin-dependent EMC effect, in particular, differ substantially within the mean-field and short-range correlation approaches. Hence, the measurement of the spin-dependent EMC effect at Jefferson Lab should give us a deeper understanding of the origin of the EMC effect and, indeed, of the structure of atomic nuclei.

Emil Fischer and the “art of chemical experimentation”

What did nineteenth-century chemists know? This essay uses Emil Fischer’s classic study of the sugars in 1880s and 90s Germany to argue that chemists’ knowledge was not primarily vested in the theories of valence, structure, and stereochemistry that have been the subject of so much historical and philosophical analysis of chemistry in this period. Nor can chemistry be reduced to a merely manipulative exercise requiring little or no intellectual input. Examining what chemists themselves termed the “art of chemical experimentation” reveals chemical practice as inseparable from its cognitive component, and it explains how chemists integrated theory with experiment through reason.