Manipulating Andreev bound states in bilayer graphene Josephson junction

Andreev bound states (ABS) are supercurrent-carrying modes which are bounded in a normal conductor in proximity Josephson junction. ABSs manifest unique phase-dependent energy spectrums according to the interrelation of channel length (L) and superconducting coherence length (ξ_s) of the junction. Depending on the ratio of L to ξ_s, one can varies the number of ABS modes from one pair (L/ξ_s <!--[if gte msEquation 12]> lang=EN-US style='font-size:11.0pt;mso-bidi-font-size:10.0pt;font-family:"Cambria Math",serif;
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JA;mso-bidi-language:AR-SA'>« 1, short Josephson junction) to two or more pairs (L/ξ_s <!--[if gte msEquation 12]> lang=EN-US style='font-size:11.0pt;mso-bidi-font-size:10.0pt;font-family:"Cambria Math",serif;
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JA;mso-bidi-language:AR-SA'>» 1, long Josephson junction). Many efforts have been made to probe the spectrum of ABSs in short Josephson junction, however, there are very few experimental reports on ABSs in long Josephson junction so far. Here, we present the experimental observation of crossover from short to long Josephson coupling by tunneling spectroscopy. The tunnel probe is made of one-dimensional contact of Al superconducting electrode on one side of high-quality bilayer graphene (BLG) encapsulated with thin hexagonal boron nitride crystals. The sharp density of states of superconducting tunneling probe enables high spectral resolution in tunneling spectroscopy. Also, crossover from short to long Josephson junction can be achieved by modulating the charge carrier density of BLG without varying physical channel length. The result reveals that additional ABS modes appear gradually when the Fermi level approaches close enough to the charge neutral point of bilayer graphene, which implies transition of ABSs from short to long regime. Furthermore, replica of ABS resonance appears even above the superconducting gap at high carrier density regime. This work will give not only new degree of freedom to manipulate ABS mode numbers but also better understanding of underlying physics of long Josephson junction.