Low-Frequency Imaginary Impedance at the Superconducting Transition of 2H-NbSe2

The superconducting transition leads to a sharp resistance drop in a temperature interval that can be a small fraction of the critical temperature Tc. A superconductor exactly at Tc is thus very sensitive to all kinds of thermal perturbation, including the heat dissipated by the measurement current. We show that the interaction between electrical and thermal currents leads to a sizable imaginary impedance at frequencies of the order of tens of hertz at the resistive transition of single crystals of the layered material 2H-NbSe2.We explain the result using models developed for transition-edge sensors. By measuring under magnetic fields and at high currents, we find that the imaginary impedance is strongly influenced by the heat associated with vortex motion and out-of-equilibrium quasiparticles

Atomic layer deposition of a MgO barrier for a passivated black phosphorus spintronics platform

We demonstrate a stabilized black phosphorus (BP) 2D platform thanks to an ultrathin MgO barrier, as required for spintronic device integration. The in-situ MgO layer deposition is achieved by using a large-scale atomic layer deposition process with high nucleation density. Raman spectroscopy studies show that this layer protects the BP from degradation in ambient conditions, unlocking in particular the possibility to carry out usual lithographic fabrication steps. The resulting MgO/BP stack is then integrated in a device and probed electrically, confirming the tunnel properties of the ultrathin MgO contacts. We believe that this demonstration of a BP material platform passivated with a functional MgO tunnel barrier provides a promising perspective for BP spin transport devices

Long-range propagation and interference of d-wave superconducting pairs in graphene

Resumen del trabajo presentado a la Conference Low dimensional superconducting hybrids for novel quantum functionalities, celebrada en Paris (Francia) del 12 al 14 de octubre de 2021.Peer reviewe

Superconducting proximity effect in dd-wave cuprate/ graphene heterostructures

Superconducting proximity effects in graphene have received a great deal of attention for over a decade now. This has unveiled a plethora of exotic effects linked to the specificities of graphene's electronic properties. The vast majority of the related studies are based on conventional, low-temperature superconducting metals with isotropic $s$-wave pairing. Here we review recent advances made on the less studied case of unconventional high-temperature superconducting cuprates. These are characterized by an anisotropic $d$-wave pairing, whose interplay with Dirac electrons yields very rich physics and novel proximity behaviours. We provide a theoretical analysis and summarize the experiments reported so far. These unveil hints of proximity-induced unconventional pairing and demonstrate the gate-tunable, long-range propagation of high-temperature superconducting correlations in graphene. Finally, the fundamental and technological opportunities brought by the theoretical and experimental advances are discussed, together with the interest in extending similar studies to other Dirac materials.Comment: Short review on d-wave proximity in graphene and other 2D Dirac materials, 30 pages and 12 figure

Loading and compression of a single two-dimensional Bose gas in an optical accordion

http://arxiv.org/abs/1611.07681International audienceThe experimental realization of two-dimensional (2D) Bose gases with a tunable interaction strength is an important challenge for the study of ultracold quantum matter. Here we report on the realization of an optical accordion creating a lattice potential with a spacing that can be dynamically tuned between 11 and 2 μm. We show that we can load ultracold 87Rb atoms into a single node of this optical lattice in the large spacing configuration and then decrease nearly adiabatically the spacing to reach a strong harmonic confinement with frequencies larger than ωz/2π=10 kHz. Atoms are trapped in an additional flat-bottom in-plane potential that is shaped with a high resolution. By combining these tools we create custom-shaped uniform 2D Bose gases with tunable confinement along the transverse direction and hence with a tunable interaction strength

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

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 nanometers, allowing for the unique observation of d-wave Andreev-pair interferences in YBa2Cu3O7-graphene devices that behave as a Fabry-Perot 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 demonstration is pivotal to the study of exotic directional effects expected for nodal superconductivity in Dirac materials

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

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 effect is markedly influenced by graphene's unique electronic structure, both in fundamental and technologically relevant ways. These include an unconventional form1,2 of the 'leakage' mechanism- the Andreev reflection3-and the potential of supercurrent modulation through electrical gating4. Despite the interest of high-temperature superconductors in that context5,6, realizations have been exclusively based on low-temperature ones. Here we demonstrate a gate-tunable, high-temperature superconducting proximity effect in graphene. Notably, gating effects result fromthe perfect transmission of superconducting pairs across an energy barrier-a form of Klein tunnelling7,8, up to nowobserved only for non-superconducting carriers9,10- and quantum interferences controlled by graphene doping. Interestingly, we find that this type of interference becomesdominant without the need of ultraclean graphene, in stark contrast to the case of low-temperature superconductors11. These results pave the way to a new class of tunable, high-temperature Josephson devices based on large-scale graphene