Evidence of superconductivity in doped graphite and graphene

Abstract

We have observed evidence of superconductivity at temperatures in the vicinity of 260 K in phosphorus-doped graphite and graphene. This evidence includes transport current, magnetic susceptibility, Hall effect and (pancake) vortex state measurements. All of these measurements indicate a transition which is that of a type II superconductor with no type I phase until below the limits of our measurement capabilities. Vortex states are inferred from periodically repeated steps in the resistance versus temperature characteristics of highly oriented pyrolytic graphite and exfoliated doped multilayer graphene. Magnetic susceptibility measurements have shown results qualitatively similar to those expected (and experimentally observed by others) for ultra-thin (thickness ≪λL) films. The magnetization is negative for field-cooled and zero-field-cooled measurements. The magnetization for field cooled and zero-field-cooled measurements begin to diverge at approximately 260 K. Hall effect measurements show a sign reversal in the Hall voltage as the temperature is reduced from 300 K to 78 K.

Study of Charge Carrier Transport in Graphene and Graphite as Two Dimensional and Quasi-Two Dimensional Materials and Their Interfaces

“…From the previous work of Larkins et al [56], the consecutive Resistance versus Temperature (R vs. T) measurements of a peeled thin-film of the sample # 023 had shown large and abrupt drops of resistance at high temperatures for a superconductor ( Figure 2.18). Therefore, the sample # 023 was selected to be one of the samples used in this work.…”

“…Magnetization measurements on thin-film superconductors where the penetration depth is many times greater than the film's thickness have shown that the magnetization is negative and has a "valley" or quasi-parabolic shape as a function of temperature and/or applied field. This is quite different from what can be seen form thick conventional superconductors where pancake vortices are not formed, and the applied magnetic field is expelled from the bulk of the superconductors [56,57,236]. In AC susceptibility measurements [55 -57], the primary differences between thin films where the thickness is considerably less than the magnetic penetration depth and materials with a unity ratio of thickness to penetration depth are…”

“…Thus, this research which is the continuation of [56], the phosphorus-implanted samples would absolutely be considered.…”

“…Bombarding graphene with argon (Ar) via ion implantation can cause atomic vacancy defects, and doping graphene with a chemical such as boron (B), nitrogen (N), or phosphorus (P) can cause the atoms around the border regions of the honeycomb structure of graphene to localize. Then the graphene can be responsible for magnetism[55,56,208,209].The work of Larkins et al shows that boron-doped HOPG samples show no sign of possible superconductivity at any temperature down to below 20 K [55]. On the other hand, phosphorus-implanted HOPG samples exhibit a deviation from the expected monotonic rise in resistance as temperature goes down at some point above 100 K. Their following work [56] was able to observe a response consistent with the presence of magnetic field flux vortices in phosphorus-implanted and in phosphorus-doped multilayer graphene.…”

“…The layer numbers indicate the number of multilayers peeled from the host sample, i.e. layer 7 would be the 7 th layer exfoliated from that sample[56]......................................................143 ~ 5 K and the onset of the vortex BKT transition at T ~ 80 K. ..................................................................................................................................145 6.3: Raman spectra for a film on Kapton tape exfoliated from a phosphorous dopedwhile-grown CVD graphene sample. The peak ratios give a thickness of approximately 5 monolayers.…”