Long-Range Propagation and Interference of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline">mml:mrowmml:mid -wave Superconducting Pairs in Graphene (original) (raw)
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Observation of long-range quantum interferences of d-wave Andreev pairs in graphene
arXiv (Cornell University), 2020
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.
Cooper-pair propagation and superconducting correlations in graphene
Physical Review B, 2007
We investigate the Cooper-pair propagation and the proximity effect in graphene under conditions in which the distance L between superconducting electrodes is much larger than the width W of the contacts. In the case of undoped graphene, supercurrents may exist with a spatial decay proportional to W 2 /L 3 . This changes upon doping into a 1/L 2 behavior, opening the possibility to observe a supercurrent over length scales above 1 µm at suitable doping levels. We also show that there is in general a crossover temperature T * ∼ vF /kBL that marks the onset of the strong decay of the supercurrent, and that corresponds to the scale below which the Cooper pairs are not disrupted by thermal effects during their propagation.
Dirac Fermions and Conductance Oscillations in s- and d-Wave Superconductor-Graphene Junctions
Physical Review Letters, 2007
We investigate quantum transport in a normal/superconductor graphene heterostructure, including the possibility of an anisotropic pairing potential in the superconducting region. We find that under certain circumstances, the conductance displays an undamped, oscillatory behaviour as a function of applied bias voltage. Also, we investigate how the conductance spectra are affected by a d-wave pairing symmetry. These results combine unusual features of the electronic structure of graphene with the unconventional pairing symmetry found for instance in high-Tc superconductors. PACS numbers: 74.45.+c, 74.78.Na Graphene is a monoatomic layer of graphite with a honeycomb lattice structure [1]. The electronic properties of graphene display several intriguing features, such as a sixpoint Fermi surface and Dirac-like low-energy energy dispersion around the Fermi-points. Condensed matter systems with such 'relativistic' electronic structure properties constitute fascinating examples of low-energy emergent symmetries (in this case Lorentz-invariance). Another example where precisely this occurs is in one-dimensional interacting fermion systems, where phenomena like breakdown of Fermi-liquid theory and spin-charge separation take place. Graphene features certain similarities to, but also important differences from, the nodal Dirac fermions emerging in the low-energy sector of the pseudogap phase of d-wave superconductors such as the high-T c cuprates. When Lorentz-invariance emerges in the low-energy sector of higher-dimensional condensed matter systems, it is bound to attract much interest from a fundamental physics point of view.
Physical Review B, 2012
We investigate the superconducting proximity effect through graphene in the long diffusive junction limit, at low and high magnetic field. The interface quality and sample phase coherence lead to a zero resistance state at low temperature, zero magnetic field, and high doping. We find a striking suppression of the critical current near graphene's charge neutrality point, which we attribute to specular reflexion of Andreev pairs at the interface of charge puddles. This type of reflexion, specific to the Dirac band structure, had up to now remained elusive. At high magnetic field the use of superconducting electrodes with high critical field enables the investigation of the proximity effect in the Quantum Hall regime. Although the supercurrent is not directly detectable in our two wire configuration, interference effects are visible which may be attributed to the injection of Cooper pairs into edge states.
Graphene Superconducting Quantum Interference Device
2010
Graphene can support Cooper pair transport when contacted with two superconducting electrodes, resulting in the well-known Josephson effect. By depositing aluminum/palladium electrodes in the geometry of a loop onto a single graphene sheet, we fabricate a two junction dc superconducting quantum interference device (SQUID).~ Not only an the supercurrent in this device be increased by moving the electrostatic gate away from the Dirac point, but it can also be modulated periodically by an applied magnetic field---a ...
Subgap states in two-dimensional spectroscopy of graphene-based superconducting hybrid junctions
Physical review, 2019
The two-dimensional nature of graphene makes it an ideal platform to explore proximity-induced unconventional planar superconductivity and the possibility of topological superconductivity. Using Green's functions techniques, we study the transport properties of a finite size ballistic graphene layer placed between a normal state electrode and a graphene lead with proximity-induced unconventional superconductivity. Our microscopic description of such a junction allows us to consider the effect of edge states in the graphene layer and the imperfect coupling to the electrodes. The tunnel conductance through the junction and the spectral density of states feature a rich interplay between graphene's edge states, interface bound states formed at the graphene-superconductor junction, Fabry-Pérot resonances originated from the finite size of the graphene layer, and the characteristic Andreev surface states of unconventional superconductors. Within our analytical formalism, we identify the separate contribution from each of these subgap states to the conductance and density of states. Our results show that graphene provides an advisable tool to determine experimentally the pairing symmetry of proximity-induced unconventional superconductivity.
Theory of superconductivity for Dirac electrons in graphene
Journal of Experimental and Theoretical Physics, 2010
Phonon exchange induced superconducting pairing of effectively ultrarelativistic electrons in graphene is investigated. The Eliashberg equation obtained for describing pairing in the Cooper channel with allowance for delayed interaction are matrix equations with indices corresponding to the valence and con duction bands. The equations are solved in the high doping limit, in which pairing is effectively a single band process, and in the vicinity of a critical quantum point of underdoped graphene for a value of the coupling constant for which pairing is an essentially multiband process. For such cases, analytic estimates are obtained for the superconducting transition temperature of the system. It is shown that the inclusion of dynamic effects makes it possible to determine the superconducting transition temperature, as well as the critical coupling constant for underdoped graphene, more accurately than in the static approximation of the BCS type. Esti mates of the constants of electron interaction with the scalar optical phonon mode in graphene indicate that an appreciable superconducting transition temperature can be attained under a high chemical doping level of graphene.
Superconducting quantum interference devices with graphene junctions
Bulletin of the American Physical Society, 2017
A series of five dc superconducting quantum interference devices ͑SQUIDs͒ have been operated as microstrip amplifiers at frequencies ranging from 2.2 to 7.4 GHz. In these devices, the signal is connected between the SQUID washer and coil, which acts as a microstrip resonator. The gain measured at 4.2 K ranged from 12Ϯ1 to 6Ϯ1 dB. The noise temperature of three devices at 4.2 K in the frequency range 2.2-4 GHz was between 1 and 2 K, and the saturation temperature was between 150 and 250 K. Applications of these devices include readout for axion detectors, and intermediate-frequency amplifiers for superconductor-insulator-superconductor and hot-electron bolometer mixers.