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Register a new userMechanically Controlled Quantum Interference in Graphene Break Junctions
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The ability to detect and distinguish quantum interference signatures is important for both fundamental research and for the realization of devices including 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-Perot 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 (MCBJs) made from a single layer of graphene. We find that their electrical conductance displays pronounced oscillations at room temperature, with amplitudes that modulate over an order of magnitude as a function of sub-nanometer 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 quantum-interference and lattice-commensuration effects of two graphene layers that slide across each other. Our results provide direct experimental observation of Fabry-Perot-like interference of electron waves that are partially reflected/transmitted at the edges of the graphene bilayer overlap region.
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Recent observations of destructive quantum interference in single-molecule junctions confirm the role played by quantum effects in the electronic conductance properties of molecular systems. We show here that the destructive interference can be turned ON or OFF within the same molecular system by mechanically controlling its conformation. Using a combination of ab-initio calculations and single-molecule conductance measurements, we demonstrate the existence of a quasi-periodic destructive quantum interference pattern along the breaking traces of {pi}-{pi} stacked molecular dimers. The detection of these interferences, which are due to opposite signs of the intermolecular electronic couplings, was only made possible by a combination of wavelet transform and higher-order statistical analysis of single-breaking traces. The results demonstrate that it is possible to control the molecular conductance over a few orders of magnitudes and with a sub-angstrom resolution by exploiting the subtle structure-property relationship of {pi}-{pi} stack dimers. These large conductance changes may be beneficial for the design of single-molecule electronic components that exploit the intrinsic quantum effects occurring at the molecular scale.
Graphene quantum dots (QDs) are intensively studied as platforms for the next generation of quantum electronic devices. Fine tuning of the transport properties in monolayer graphene QDs, in particular with respect to the independent modulation of the tunnel barrier transparencies, remains challenging and is typically addressed using electrostatic gating. We investigate charge transport in back-gated graphene mechanical break junctions and reveal Coulomb blockade physics characteristic of a single, high-quality QD when a nanogap is opened in a graphene constriction. By mechanically controlling the distance across the newly-formed graphene nanogap, we achieve reversible tunability of the tunnel coupling to the drain electrode by five orders of magnitude, while keeping the source-QD tunnel coupling constant. These findings indicate that the tunnel coupling asymmetry can be significantly modulated with a mechanical tuning knob and has important implications for the development of future graphene-based devices, including energy converters and quantum calorimeters.
We report the first experimental study of the quantum interference correction to the conductivity of bilayer graphene. Low-field, positive magnetoconductivity due to the weak localisation effect is investigated at different carrier densities, including those around the electroneutrality region. Unlike conventional 2D systems, weak localisation in bilayer graphene is affected by elastic scattering processes such as intervalley scattering. Analysis of the dephasing determined from the magnetoconductivity is complemented by a study of the field- and density-dependent fluctuations of the conductance. Good agreement in the value of the coherence length is found between these two studies.
We investigate the influence of gauge fields induced by strain on the supercurrent passing through the graphene-based Josephson junctions. We show in the presence of a constant pseudomagnetic field ${bf B}_S$ originated from an arc-shape elastic deformation, the Josephson current is monotonically enhanced. This is in contrast with the oscillatory behavior of supercurrent (known as Fraunhofer pattern) caused by real magnetic fields passing through the junction. The absence of oscillatory supercurrent originates from the fact that strain-induced gauge fields have opposite directions at the two valleys due to the time-reversal symmetry. Subsequently there is no net Aharonov-Bohm effect due to ${bf B}_S$ in the current carried by the bound states composed of electrons and holes from different valleys. On the other hand, when both magnetic and pseudomagnetic fields are present, Fraunhofer-like oscillations as function of the real magnetic field flux are found. We find that the Fraunhofer pattern and in particular its period slightly change by varying the strain-induced gauge field as well as the geometric aspect ratio of the junction. Intriguingly, the combination of two kinds of gauge fields results in two special fingerprint in the local current density profile: (i) strong localization of the Josephson current density with more intense maximum amplitudes; (ii) appearance of the inflated vortex cores - finite regions with almost diminishing Josephson currents - which their sizes increases by increasing ${bf B}_S$. These findings reveal unexpected interference signatures of strain-induced gauge fields in graphene SNS junctions and provide unique tools for sensitive probing of the pseudomagnetic fields.
The unusual electronic properties of single-layer graphene make it a promising material system for fundamental advances in physics, and an attractive platform for new device technologies. Graphenes spin transport properties are expected to be particularly interesting, with predictions for extremely long coherence times and intrinsic spin-polarized states at zero field. In order to test such predictions, it is necessary to measure the spin polarization of electrical currents in graphene. Here, we resolve spin transport directly from conductance features that are caused by quantum interference. These features split visibly in an in-plane magnetic field, similar to Zeeman splitting in atomic and quantum dot systems. The spin-polarized conductance features that are the subject of this work may, in the future, lead to the development of graphene devices incorporating interference-based spin filters.
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