The ability to detect and distinguish quantum interference signatures is important for both fundamental research and for the realization of devices such as electron resonators, interferometers and interference-based spin filters. Consistent with the principles of subwavelength optics, the wave nature of electrons can give rise to various types of interference effects, such as Fabry-Pérot resonances, Fano resonances and the Aharonov-Bohm effect. Quantum interference conductance oscillations have, indeed, been predicted for multiwall carbon nanotube shuttles and telescopes, and arise from atomic-scale displacements between the inner and outer tubes. Previous theoretical work on graphene bilayers indicates that these systems may display similar interference features as a function of the relative position of the two sheets. Experimental verification is, however, still lacking. Graphene nanoconstrictions represent an ideal model system to study quantum transport phenomena due to the electronic coherence and the transverse confinement of the carriers. Here, we demonstrate the fabrication of bowtie-shaped nanoconstrictions with mechanically controlled break junctions made from a single layer of graphene. Their electrical conductance displays pronounced oscillations at room temperature, with amplitudes that modulate over an order of magnitude as a function of subnanometre displacements. Surprisingly, the oscillations exhibit a period larger than the graphene lattice constant. Charge-transport calculations show that the periodicity originates from a combination of the quantum interference and lattice commensuration effects of two graphene layers that slide across each other. Our results provide direct experimental observation of a Fabry-Pérot-like interference of electron waves that are partially reflected and/or transmitted at the edges of the graphene bilayer overlap region.

Spin-crossover (SCO) molecules are versatile magnetic switches with applications in molecular electronics and spintronics. Downscaling devices to the single-molecule level remains, however, a challenging task since the switching mechanism in bulk is mediated by cooperative intermolecular interactions. Here, we report on electron transport through individual Fe-SCO molecules coupled to few-layer graphene electrodes via π-π stacking. We observe a distinct bistability in the conductance of the molecule and a careful comparison with density functional theory (DFT) calculations allows to associate the bistability with a SCO-induced orbital reconfiguration of the molecule. We find long spin-state lifetimes that are caused by the specific coordination of the magnetic core and the absence of intermolecular interactions according to our calculations. In contrast with bulk samples, the SCO transition is not triggered by temperature but induced by small perturbations in the molecule at any temperature. We propose plausible mechanisms that could trigger the SCO at the single-molecule level.

We present a density-functional comparative study of the electronic properties and the structural trends of the late isoelectronic 3d ͑Fe, Co, Ni͒ and 4d ͑Ru, Rh, Pd͒ free-standing transition metal clusters of 13 and 23 atoms. Different types of structures have been analyzed: compact arrangements such as icosahedral shape, open arrangements such as the cubic ones, and structures reminiscent of the crystal lattices. We have calculated total-energy differences between the structural and the spin isomers, as well as the electronic charge and spin-moment distribution within the clusters. We have found that some structural trends correlate with electronic trends, which are consistent with the spectrum of the single atoms in the d n s 1 configuration ͑n =7,8,9 for Fe and Ru, Co and Rh, and Ni and Pd, respectively͒. We have also found that magnetism plays a role in the Fe clusters. The exceptions found in some clusters, which depart from the structural trend obtained for their respective 3d or 4d family, illustrate the complexity of the bond formation in transition metal systems in which the itinerant but localized d electrons coexist with the delocalized sp electrons in the valence states.

We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K ͑M 1 ͒; and 1 alkali, alkaline earth or 3d / 4d transition metal atom ͑M 2 ͒ plus two to five ͑BH 4 ͒ − groups, i.e., M 1 M 2 ͑BH 4 ͒ 2-5 , using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M 1 ͑Al/ Mn/ Fe͒͑BH 4 ͒ 4 , ͑Li/ Na͒Zn͑BH 4 ͒ 3 , and ͑Na/ K͒͑Ni/ Co͒͑BH 4 ͒ 3 alloys are found to be the most promising, followed by selected M 1 ͑Nb/ Rh͒͑BH 4 ͒ 4 alloys.

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