We show that the wavefunctions form caustics in circular graphene p-n junctions which in the framework of geometrical optics can be interpreted with negative refractive index.

We study an accumulation mode Si/SiGe double quantum dot (DQD) containing a single electron that is dipole coupled to microwave photons in a superconducting cavity. Measurements of the cavity transmission reveal dispersive features due to the DQD valley states in Si. The occupation of the valley states can be increased by raising the temperature or applying a finite source-drain bias across the DQD, resulting in an increased signal. Using the cavity input-output theory and a four-level model of the DQD, it is possible to efficiently extract valley splittings and the inter- and intravalley tunnel couplings.

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We derive the boundary conditions for MoS2 and similar transition-metal dichalcogenide honeycomb (2H polytype) monolayers with the same type of k ·p Hamiltonian within the continuum model around the K points. In an effective 2-band description, the electron-hole symmetry breaking quadratic terms are also taken into account. We model the effect of the edges with a linear edge constraint method that has been applied previously to graphene. Focusing mainly on zigzag edges, we find that different reconstruction geometries with different edge-atoms can generally be described with one scalar parameter varying between 0 and 2π. We analyze the edge states and their dispersion relation in MoS2 in particular, and we find good agreement with the results of previous density functional theory calculations for various edge types. √ 3 2 N d a, where the lattice constant in MoS2 is a = |a1| = |a2| = 3.1565Å. 29 arXiv:1509.00184v2 [cond-mat.mes-hall]

We propose an implementation of a valley selective electronic Veselago lens in bilayer graphene. We demonstrate that in the presence of an appropriately oriented potential step, low-energy electrons radiating from a point source can be re-focused coherently within the same band. The phenomenon is due to the trigonal warping of the band structure that leads to a negative refraction index. We show that the interference pattern can be controlled by an external mechanical strain.

We present a theoretical study of electron wave functions in ballistic circular n-p junctions of bilayer graphene. Similarly to the case of a circular n-p junction of monolayer graphene, we find that (i) the wave functions form caustics inside the circular region, and (ii) the shape of these caustics are well described by a geometrical optics model using the concept of a negative refractive index. In contrast to the monolayer case, we show that the strong focusing effect is absent in the bilayer. We explain these findings in terms of the angular dependence of Klein tunneling at a planar n-p junction.

Devices formed from single molecules attached to noble-metal electrodes exhibit large conductance fluctuations, which inhibit their development as reproducible functional units. We demonstrate that single molecules with planar anchor groups attached carbon-based electrodes are more resilient to atomic-scale variation in the contacts and exhibit significantly-lower conductance fluctuations. We examine the conductance of a 2,6-dibenzylamino core-substituted naphthalenediimide (NDI) chromophore attached to carbon electrodes by either phenanthrene anchors or by more extended anchor groups, which include OPE spacers. We demonstrate for the more spatially-extended anchor groups, conductance fluctuations are significantly reduced. The current-voltage characteristic arising from long-range tunnelling, is found to be strongly non-linear with pronounced conductance suppression below a threshold voltage of approximately 2.5 volts.Combined experimental and theoretical studies have provided new insights into the interplay of molecular conformation, electronic structure and electrical conductance. 1 in single-molecule electronic devices. It is clear that measured conductance values depend on the atomic-scale contact geometry of the electrodes, 2 temperature, 3 the local environment of the system (vacuum or air, solvent, etc.) 4 and molecular features such as the extent of conjugation, 5,6 the nature of the terminal anchor groups (e.g. thiol, amine, carboxylic acid), 7 the detailed conformation 8 and tilt angle 9 of the molecule in the junction, and on quantum coherence and interference of electrons transiting the molecule 10,11 .

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