• Trastoy, J.
• Villegas, Javier
• Kidambi, P., R.
• Hofmann, S.
• Sander, A.
• Seneor, Pierre
• Godel, F.
• Dlubak, B.
• Zhang, X., P.
• Bergeret, F., S.
• Zatko, V.
• Humbert, Vincent
• Ulysse, C.
• Seurre, K.
• Bercioux, D.
• Perconte, D.

Abstract

Recent experiments have shown that proximity with high-temperature superconductors induces unconventional superconducting correlations in graphene. Here we demonstrate that those correlations propagate hundreds of nanometer, allowing for the unique observation of d-wave Andreev pair interferences in YBa2Cu3O7-graphene devices that behave as a Fabry-Pérot cavity. The interferences show as a series of pronounced conductance oscillations analogous to those originally predicted by de Gennes-Saint-James for conventional metal-superconductor junctions. The present work is pivotal to the study of exotic directional effects expected for nodal superconductivity in Dirac materials. The superconducting proximity effect in graphene has attracted much interest since the pioneering experiments [1]. This roots down to the graphene's electronic structure, which strongly affects the underlying mechanisms: Andreev reflection and coherent propagation of electron-hole pairs [2]. A distinctive feature is the strong dependence of the proximity behavior on the graphene?s doping level, which dramatically changes the Andreev reflection [3, 4] as compared to metals. Other unique features include gate [5] or magnetic-field [6, 7] driven transitions from bulk to edge transport. Studies on graphene have also paved the way for understanding the proximity effect in other Dirac materials, such as topological insulators [8-11]. Experiments have mostly focused on conventional low critical temperature (T C) superconductors with s-wave pairing. Despite early theoretical studies showing that d-wave (high-T C) superconductors should lead to novel directional effects [12, 13] and exotic pairing [14, 15], evidence for unconventional superconductivity in graphene has been found only recently [16, 17]. Scanning electron tunneling microscopy (STM) of graphene on Pr 2−x Ce x CuO 4 (PCCO) revealed a superconduct-ing gap [16] and spectral features suggesting (p-wave) superconductivity induced in graphene. Experiments on YBa 2 Cu 3 O 7 (YBCO) and chemical-vapor-deposited (CVD) graphene devices by some of us [17] showed trans-FIG. 1. Interferences in a proximitized cavity (a) An electron Andreev-reflected at the SC/cavity interface propagates as a hole back to the other end, where it is normal-reflected towards the SC/cavity interface to undergo again Andreev-reflection (AR). The lower AR is the time reversal process of the upper one. (b) The electron can also be normal-reflected at the SC/cavity, travel back to the other cavity interface to be again normal-reflected. This process results in Fabry-Pérot resonances. parent superconductor-graphene interfaces and clear evidence of Andreev reflection. Interestingly, we also found that the Andreev electron-holes pair transmission can be modulated by a back-gate voltage, through a mechanism analogous to the Klein tunneling [18]. However, neither the STM experiments nor ours on solid-state devices probed the length scale over which the unconventional correlations penetrate into graphene. Here we demonstrate the long-range propagation of arXiv:2002.10173v2 [cond-mat.supr-con] 8 Sep 2020

Topics

• impedance spectroscopy
• experiment
• chemical vapor deposition
• scanning tunneling microscopy
• superconductivity
• superconductivity
• critical temperature