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Register a new userSpecular Interband Andreev Reflections in Graphene
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Electrons incident from a normal metal onto a superconductor are reflected back as holes - a process called Andreev reflection. In a normal metal where the Fermi energy is much larger than a typical superconducting gap, the reflected hole retraces the path taken by the incident electron. In graphene with ultra low disorder, however, the Fermi energy can be tuned to be smaller than the superconducting gap. In this unusual limit, the holes are expected to be reflected specularly at the superconductor-graphene interface due to the onset of interband Andreev processes, where the effective mass of the reflected holes change sign. Here we present measurements of gate modulated Andreev reflections across the low disorder van der Waals interface formed between graphene and the superconducting NbSe2. We find that the conductance across the graphene-superconductor interface exhibits a characteristic suppression when the Fermi energy is tuned to values smaller than the superconducting gap, a hallmark for the transition between intraband retro- and interband specular- Andreev reflections.
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Epitaxially grown, high quality semiconductor InSb nanowires are emerging material systems for the development of high performance nanoelectronics and quantum information processing and communication devices, and for the studies of new physical phenomena in solid state systems. Here, we report on measurements of a superconductor-normal conductor-superconductor junction device fabricated from an InSb nanowire with aluminum based superconducting contacts. The measurements show a proximity induced supercurrent flowing through the InSb nanowire segment, with a critical current tunable by a gate, in the current bias configuration and multiple Andreev reflection characteristics in the voltage bias configuration. The temperature dependence and the magnetic field dependence of the critical current and the multiple Andreev reflection characteristics of the junction are also studied. Furthermore, we extract the excess current from the measurements and study its temperature and magnetic field dependences. The successful observation of the superconductivity in the InSb nanowire based Josephson junction device indicates that InSb nanowires provide an excellent material system for creating and observing novel physical phenomena such as Majorana fermions in solid state systems.
Andreev reflection in graphene is special since it can be of two types- retro or specular. Specular Andreev reflection (SAR) dominates when the position of the Fermi energy in graphene is comparable to or smaller than the superconducting gap. Bilayer graphene (BLG) is an ideal candidate to observe the crossover from retro to specular since the Fermi energy broadening near the Dirac point is much weaker compared to monolayer graphene. Recently, the observation of signatures of SAR in BLG have been reported experimentally by looking at the enhancement of conductance at finite bias near the Dirac point. However, the signatures were not very pronounced possibly due to the participation of normal quasi-particles at bias energies close to the superconducting gap. Here, we propose a scheme to observe the features of enhanced SAR even at zero bias at a normal metal (NM)-superconductor (SC) junction on BLG. Our scheme involves applying a Zeeman field to the NM side of the NM-SC junction on BLG (making the NM ferromagnetic), which energetically separates the Dirac points for up-spin and down-spin. We calculate the conductance as a function of chemical potential and bias within the superconducting gap and show that well-defined regions of specular- and retro-type Andreev reflection exist. We compare the results with and without superconductivity. We also investigate the possibility of the formation of a p-n junction at the interface between the NM and SC due to a work function mismatch.
Hybrid superconductor/graphene (SC/g) junctions are excellent candidates for investigating correlations between Cooper pairs and quantum Hall (QH) edge modes. Experimental studies are challenging as Andreev reflections are extremely sensitive to junction disorder and high magnetic fields are required to form QH edge states. We fabricated low-resistance SC/g interfaces, composed of graphene edge contacted with NbN with a barrier strength of $Zapprox 0.4$, that remain superconducting under magnetic fields larger than $18$ T. We establish the role of graphenes Dirac band structure on zero-field Andreev reflections and demonstrate dynamic tunability of the Andreev reflection spectrum by moving the boundary between specular and retro Andreev reflections with parallel magnetic fields. Through the application of perpendicular magnetic fields, we observe an oscillatory suppression of the 2-probe conductance in the $ u = 4$ Landau level attributed to the reduced efficiency of Andreev processes at the NbN/g interface, consistent with theoretical predictions.
We have investigated electronic transport of few-layer-graphene (FLG) connected to superconducting electrodes. The device is prepared by mechanical exfoliation of graphite. A small mesa of FLG is placed on the surface of an insulating Alumina layer over silicon substrate, and is connected with two tungsten electrodes, separated by 2.5 microns, grown by focused ion beam. While tungsten electrodes are superconducting below 4 K, proximity induced superconductivity in FLG is observed below 1K with a large differential resistance drop at low bias. Signatures of multiple Andreev reflections are observed as peaks located at voltages corresponding to sub-multiple values of the superconducting gap of the electrodes.
It is known that the parity of reflection amplitude can either be even or odd under the mirror operation. Up to now, all the parities of reflection amplitude in the one-mode energy region are even under the mirror operation. In this paper, we give an example of odd parity for Andreev reflection (AR) in a three-terminal graphene-supercondutor hybrid systems. We found that the parity is even for the Andreev retroreflection (ARR) and odd for specular Andreev reflection (SAR). We attribute this remarkable phenomenon to the distinct topology of the band structure of graphene and the specular Andreev reflection involving two energy bands with different parity symmetry. As a result of odd parity of SAR, the SAR probability of a four-terminal system with two superconducting leads (two reflection interfaces) can be zero even when the system is asymmetric due to the quantum interference of two ARs.
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