Incipient Berezinskii-Kosterlitz-Thouless transition in two-dimensional coplanar Josephson junctions

Abstract

Superconducting hybrid junctions are revealing a variety of novel effects. Some of them are due to the special layout of these devices, which often use a coplanar configuration with relatively large barrier channels and the possibility of hosting Pearl vortices. A Josephson junction with a quasi ideal two-dimensional barrier has been realized by growing graphene on SiC with Al electrodes. Chemical Vapor Deposition offers centimeter size monolayer areas where it is possible to realize a comparative analysis of different devices with nominally the same barrier. In samples with a graphene gap below 400 nm, we have found evidence of Josephson coherence in presence of an incipient BerezinskiiKosterlitz-Thouless transition. When the magnetic field is cycled, a remarkable hysteretic collapse and revival of the Josephson supercurrent occurs. Similar hysteresis are found in granular systems and are usually justified within the Bean Critical State model (CSM). We show that the CSM, with appropriate account for the low dimensional geometry, can partly explain the odd features measured in these junctions.

Hysteretic Vortex-Matching Effects in High- Tc Superconductors with Nanoscale Periodic Pinning Landscapes Fabricated by He Ion-Beam Projection

“…Remarkably strong flux gradients have been reported in BSCCO crystals within an area that was patterned with a blind hole array, but not in the neighbouring pristine parts of the crystal [12]. Recently, a hysteretic behavior of the Josephson supercurrent in coplanar Al-graphene-Al junctions upon cycling the magnetic field has been reported [40].…”

Thermal transport driven by charge imbalance in graphene in a magnetic field close to the charge neutrality point at low temperature: Nonlocal resistance

“…Hence we are at the limit of validity for our approach, because, in this range of values, the Dirac model gives up by turning into the more appropriate Fermi Liquid model. In passing, we note that, at these temperatures, similar, but p doped, graphene/SiC samples have been used as barrier for Al/graphene/Al superconducting Josephson junctions, showing a remarkable hysteretic loop of the critical current when the magnetic field is cycled and an Aslamazov-Larkin (AL) paraconductivity temperature dependence consistent with a Fermi Liquid picture 47,48 .…”

What happens in Josephson junctions at high critical current densities

“…The curves relative to the Al-graphene (sheet)-Al JJ, reported in Fig. 1(e) as a function of temperature, also point to overdamped behavior [11,12]. Here J cw = 10 4 − A/cm.…”

“…Here J cw = 10 4 − A/cm. In the case of graphene barrier the supercurrent is superimposed on a small resistance, which has been correlated to an incipient Berezinskii-Kosterlitz-Thouless (BKT) transition [11]. Vortex bundles break above the BKT temperature and move in the barrier adding a resistive channel.…”

“…In all these cases we present curves with and without hysteresis to encompass nominally the relevant limits in the phase dynamics. [11,12] in (e) and PbIn-graphene-PbIn in (f) (adapted from Ref. 13).…”