Evidence of vortices and mixed-state superconductivity in phosphorus-doped graphene. Part I (Nernst)

Abstract

We have observed phenomena in phosphorus-doped graphene films consistent with mixed-state superconductivity at temperatures as high as 260 K. This evidence includes transport, susceptibility and Nernst/Hall measurements as a function of the thermal gradient. This paper presents evidence of vortices and flux flow in the mixed state of phosphorus-doped graphene samples in the form of well-defined Nernst peaks being measurable up to 260 K.

Evidence of vortices and mixed-state superconductivity in phosphorus-doped graphene. Part II (Hysteresis)

“…The non-zero resistance and magnetic behavior of our graphene samples [14] indicate the presence of magnetic flux. For magnetic flux flow to occur, vortices must exist in the material.…”

“…It is important to notice that they may be due to thermal effects as well. In figure 9 we show the warming and cooling plots of delta T vs. T. By looking at the thermal hysteresis in comparison with their corresponding Nernst hysteresis plots it can be seen by inspection that Nernst peaks are not due to the variation of delta T with T. The slow rate of change of temperature with time [14] does not allow for a significant thermal difference across the graphene sample in the direction transverse to the gradient. In other words, the transverse temperature difference across the sample remains effectively constant and quite small (zero) as compared with the rate of change in temperature gradient.…”

“…The experimental setup used for the cyclic Nernst experiment involves the same fixture used in the simple Nernst effect discussed in part I [14]. A 1113 Gauss Neodymium disc magnet was used as source of perpendicular magnetic field.…”

“…This leads to partial screening of the antivortices until, at some point the thermal energy becomes insufficient to keep the material from screening or pinning the applied magnetic field. For a material with no interlayer stacks of vortices there are two energies involved; the vortex BKT transition and the H C1 where the bulk of the magnetic field is screened from the sample [14,[24][25][26][27][28][29]. Once the vortex BKT temperature is reached the Nernst voltage should undergo a steep transition and drop to a residual value set by the, still free, quasiparticles.…”