Abstract
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 gas 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 ~1010 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 long standing theoretical prediction for the emergence of evanescent transport mediated pseudo-diffusive transport in graphene.
Highlights
IntroductionEnergy-dependence near Dirac point, reflecting the crossover from ballistic to pseudo-diffusive charge carrier transmission
We find that the normalized excess current (IexcRN), which remains constant at high carrier densities, becomes suppressed rapidly at low carrier densities near the neutrality point (NP)
The devices used in this study are suspended graphene–Nb Josephson weak links fabricated on Si/SiO2 substrates
Summary
Energy-dependence near Dirac point, reflecting the crossover from ballistic to pseudo-diffusive charge carrier transmission. In realizing these predictions, the experimental work carried out so far[15,16,17,18,19], have been impeded by numerous technical challenges. We find that the normalized excess current (IexcRN), which remains constant at high carrier densities, becomes suppressed rapidly at low carrier densities near the NP Both observations are in contrast with previous experimental observations in disordered graphene superconductor junctions where both SHGS and IexcRN show no significant gate dependence[19,24,25].
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