Magnetic field suppression of Andreev conductance at superconductor-graphene interfaces

Studying the interplay between superconductivity and quantum magnetotransport in two-dimensional materials has been a topic of interest in recent years. Towards such a goal it is important to understand the impact of magnetic field on the charge transport at the superconductor-normal channel (SN) interface. Here we carried out a comprehensive study of Andreev conductance under weak magnetic fields using diffusive superconductor- graphene Josephson weak links. We observe that the Andreev conductance is suppressed even in magnetic fields far below the upper critical field of the superconductor. The suppression of Andreev conductance depends on and can be minimized by controlling the ramping of the magnetic field. We identify that the key factor behind this suppression is the reduction of the superconducting gap due to the piling of vortices on the superconducting contacts. In devices where superconducting gap at the superconductor-graphene interface is heavily reduced by proximity effect, the enlarged vortex cores overlap quickly with increasing magnetic field, resulting in a rapid decrease of the interfacial gap. However, in weak links with relatively large effective superconducting gap the AR conductance persists up to the upper critical field. Our results provide guidance to the study of quantum material-superconductor systems in presence of magnetic field, where 'survival' of induced superconductivity is critical.Comment: 22 pages, 4 figures. Typos corrected, reference adde

Bolometric response in graphene based superconducting tunnel junctions

We fabricate graphene-TiOx-Al tunnel junctions and characterize their radio frequency response. Below the superconducting critical temperature of Al and when biased within the superconducting gap, the devices show enhanced dynamic resistance which increases with decreasing temperature. Application of radio frequency radiation affects the dynamic resistance through electronic heating. The relation between the electron temperature rise and the absorbed radiation power is measured, from which the bolometric parameters, including heat conductance, noise equivalent power and responsivity, are characterized

Signatures of evanescent transport in ballistic suspended graphene superconductor junctions

In Dirac materials, the low energy excitations behave like ultra-relativistic massless particles with linear energy dispersion. A particularly intriguing phenomenon arises with the intrinsic charge transport behavior at the Dirac point where the charge density approaches zero. In graphene, a 2-D Dirac fermion system, it was predicted that charge transport near the Dirac point is carried by evanescent modes, resulting in unconventional pseudo-diffusive charge transport even in the absence of disorder. In the past decade, experimental observation of this phenomenon remained challenging due to the presence of strong disorder in graphene devices which limits the accessibility of the low carrier density regime close enough to the Dirac point. Here we report transport measurements on ballistic suspended graphene-Niobium Josephson weak links that demonstrate a transition from ballistic to pseudo-diffusive like evanescent transport below a carrier density of $(10^{10} cm^{-2})$.Approaching the Dirac point, the sub-harmonic gap structures due to multiple Andreev reflections display a strong Fermi energy-dependence and become increasingly pronounced, while the normalized excess current through the superconductor-graphene interface decreases sharply. Our observations are in qualitative agreement with the longstanding theoretical prediction for the emergence of evanescent transport mediated pseudo-diffusive transport in graphene.Comment: 17 pages, 3 figures. New version after peer review and publication.Typos correcte

Tuning Strain in Flexible Graphene Nanoelectromechanical Resonators

The structural flexibility of low dimensional nanomaterials offers unique opportunities for studying the impact of strain on their physical properties and for developing innovative devices utilizing strain engineering. A key towards such goals is a device platform which allows the independent tuning and reliable calibration of the strain. Here we report the fabrication and characterization of graphene nanoelectromechanical resonators(GNEMRs) on flexible substrates. Combining substrate bending and electrostatic gating, we achieve the independent tuning of the strain and sagging in graphene and explore the nonlinear dynamics over a wide parameter space. Analytical and numerical studies of a continuum mechanics model, including the competing higher order nonlinear terms, reveal a comprehensive nonlinear dynamics phase diagram, which quantitatively explains the complex behaviors of GNEMRs

Long-range ballistic transport of Brown-Zak fermions in graphene superlattices

In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 106 cm2 V−1 s−1 and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1 K. We also found negative bend resistance at 1/q fractions for electrical probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field

One-dimensional proximity superconductivity in the quantum Hall regime

Extensive efforts have been undertaken to combine superconductivity and the quantum Hall effect so that Cooper-pair transport between superconducting electrodes in Josephson junctions is mediated by one-dimensional (1D) edge states. This interest has been motivated by prospects of finding new physics, including topologically-protected quasiparticles, but also extends into metrology and device applications. So far it has proven challenging to achieve detectable supercurrents through quantum Hall conductors. Here we show that domain walls in minimally twisted bilayer graphene support exceptionally robust proximity superconductivity in the quantum Hall regime, allowing Josephson junctions operational in fields close to the upper critical field of superconducting electrodes. The critical current is found to be non-oscillatory, practically unchanging over the entire range of quantizing fields, with its value being limited by the quantum conductance of ballistic strictly-1D electronic channels residing within the domain walls. The described system is unique in its ability to support Andreev bound states in high fields and offers many interesting directions for further exploration