Tunable Klein-like tunnelling of high-temperature superconducting pairs into graphene

Abstract

Superconductivity can be induced in a normal material via the 'leakage' of superconducting pairs of charge carriers from an adjacent superconductor. This so-called proximity e ect is markedly influenced by graphene's unique electronic structure, both in fundamental and technologically relevant ways. These include an unconventional form 1,2 of the 'leakage' mechanismthe Andreev reflection 3 -and the potential of supercurrent modulation through electrical gating 4 . Despite the interest of high-temperature superconductors in that context 5,6 , realizations have been exclusively based on low-temperature ones. Here we demonstrate a gate-tunable, high-temperature superconducting proximity e ect in graphene. Notably, gating e ects result from the perfect transmission of superconducting pairs across an energy barrier-a form of Klein tunnelling 7,8 , up to now observed only for non-superconducting carriers 9,10 -and quantum interferences controlled by graphene doping. Interestingly, we find that this type of interference becomes dominant without the need of ultraclean graphene, in stark contrast to the case of low-temperature superconductors 11 . These results pave the way to a new class of tunable, high-temperature Josephson devices based on large-scale graphene.Superconductivity is induced in a normal metal (N) in contact with a superconductor (S) via the Andreev reflection (AR) 3 : an electron entering S from N pairs to another electron to form a Cooper pair, leaving a hole-like quasiparticle that is transmitted back into N. Electron and hole coherently propagate with parallel opposite wavevectors, carrying superconducting correlations into N. This mechanism allows supercurrent flow and Josephson coupling across S-N-S junctions 12 .S-N proximity devices that can be greatly tuned by electrostatic doping are one of the main technological prospects of induced superconductivity in graphene 4,13-16 . Several specific mechanisms allow for that. Besides the density-of-states narrowing at the Dirac point, around which the junction's resistance increases, subtler effects may play a role. For example, the unusual possibility that the AR-involved electron and hole reside in different bandsconduction and valence-results in a specular Andreev reflection 1 (SAR) in which electron and hole wavevectors are mirror-like. SAR can occur if the graphene's Fermi energy E F is lower than the superconducting energy gap ∆, while for E F > ∆ the conventional (intra-band) AR takes place 1 . Thus, an AR to SAR crossover can be driven by shifting E F through a gate voltage, which dramatically changes the S-N interface conductance 2 .In the present experiments, tunability results from a different mechanism that involves Klein tunnelling-that is, the reflectionless transmission of electrons across a high energy barrier 7-10 . Beyond single electrons, here we observe Klein-like tunnelling of Andreev electron-hole pairs that carry superconducting correlations from the high-temperature superconductor YBa 2 Cu 3 O 7 (YBCO) into graphene. That effect i...

Long-Range Propagation and Interference of d -wave Superconducting Pairs in Graphene

“…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 revealed a superconducting gap [16] and spectral features suggesting (p-wave) superconductivity induced in graphene.…”

“…Scanning electron tunneling microscopy (STM) of graphene on Pr 2-x Ce x CuO 4 revealed a superconducting gap [16] and spectral features suggesting (p-wave) superconductivity induced in graphene. Experiments on YBa 2 Cu 3 O 7 (YBCO) and chemical-vapordeposited (CVD) graphene devices by some of us [17] showed transparent superconductor-graphene interfaces and clear evidence of Andreev reflection. Interestingly, we also found that the Andreev electron-hole pair transmission can be modulated by a back-gate voltage, through a mechanism analogous to the Klein tunneling [18].…”

Identifying the graphene d-wave superconducting symmetry by an anomalous splitting zero-bias conductance peak

“…However, until recently, the fabrication of graphene/YBCO contacts and their superconducting proximity effect have been reported, where both the temperature dependence and the current bias dependence of resistance for the contacts are investigated [19], showing the indication of the tunneling of Cooper pairs from YBCO to the graphene. The ZBCP and so-called Klein-like tunneling of Andreev electron-hole pairs that carry superconducting correlations from the high-T c SC YBCO into graphene have also been experimentally observed very recently [20], as in the usual Klein tunneling with the reflectionless transmission of electrons across a high energy barrier. Moreover, the latest experimental findings show the realization of the intrinsic unconventional superconductivity in a 'magic angle' twisted bilayer which is created by stacking two sheets of graphene that are twisted relative to each other by a small angle [21].…”

“…Finally, we comment the execution of the present graphene-based FM/RSOC/I/d-wave SC junction. The proximity-induced d-wave superconductivity in graphene could be attained by YBCO films grown on SrTiO 3 [19,20]. As the YBCO films own very high T c , one can study the proximity effect of graphene above the liquid nitrogen temperature, which favors the easily experimental performance.…”

“…However, the spin-triplet AR in the graphenebased d-wave SC junctions has not been studied so far. It has been demonstrated that, the anisotropic d-wave pair symmetry could be merged with the relativistic nodal fermions of graphene and the anomaly of the ZBCP as an experimental signature of the high-T c superconducting proximity effect in graphene was also exhibited [20,13]. Since tunneling conductance spectra of the graphene-based FM/I/d-wave SC junctions have been investigated [14][15][16][17], it is highly desirable to study the ZBCP of graphene-based FM/RSOC/I/d-wave SC multilayered structure so as to find whether or not the triplet states induced by the interplay between RSOC and FM possess some new features on the ZBCP compared with the one without RSOC?…”