Andreev Reflections in NbN/Graphene Junctions under Large Magnetic Fields

Abstract

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 Z ≈ 0.4, that remain superconducting under magnetic fields larger than 18 T. We establish the role of graphene’s 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 ν = 4 Landau level attributed to the reduced efficiency of Andreev processes at the NbN/g interface, consistent with theoretical predictions.

Entanglement of Nambu Spinors and Bell Inequality Test Without Beam Splitters

“…And (ii), the superconductor causes Andreev reflection [40,41] between electrons and holes and introduces the other rotation U S (θ ) = exp{−iθ τ x /2} about the x-axis by an angle θ = 2∆L s /( v) determined by the length L s and the effective pair potential ∆ induced by the superconductor [42]. Very recently, such coherent electron-hole conversion in the chiral edge states has been implemented in several experiments [43][44][45][46][47]. The detection of the Nambu spinor along the polarization direction n means that the initial states |±n should yield the expectation values of ±1 or explicitly, ±n|U † τ z U | ± n = ±1 with U = U S (θ )U G (ϕ ) the combined rotational operation.…”

“…Assuming experimentally realizable parameters for the setup [see caption of Fig. 2], that can be achieved by modern electron-beam lithography techniques [8][9][10][11][12][43][44][45][46][47], we plot B as a function of ∆ and δV g in Fig. 2(a).…”

“…We specify the experimental procedures in detail. The polarization direction n j of the spinor detector can be adjusted by the gate voltage V gj and the Andreev reflection at the superconductor SC j [43][44][45][46][47]. It is convenient to place the four vectors n j , ñj in Eq.…”

“…1(c)] and change the azimuthal angles ϕ j by V gj . This corresponds to an Andreev reflection with 50% probability, which can be realized by tuning the magnetic field or a large backgate [43][44][45][46][47]. To confirm the correct setting, one can exert a large potential V 0 at the central point contact to pinch off the connection between the edge channels on both sides.…”