On The Quest of Superconductivity at Room Temperature
1Division of Superconductivity and Magnetism, Institut für Experimentelle Physik II, Universität Leipzig, Germany,
2Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Santo André, São Paulo, Brazil,
3Division of Nuclear Solid State Physics, Institut für Experimentelle Physik II, Universität Leipzig, Germany,
4Institut für Mineralogie, Kristallographie und Materialwissenschaft, Fakultät für Chemie und Mineralogie, Universität Leipzig, Germany,
5Golden Bowerbird Pty Ltd., Mullumbimby, NSW, Australia,
6Present address: Leibniz Institute of Surface Modification, Physical Department, Leipzig, Germany.
Superconductivity is the phenomenon in nature where the electrical resistance of a conducting sample vanishes completely below a certain temperature which is known as “critical temperature (Tc)”. For low enough applied magnetic fields upon sample geometry, the phenomenon of flux expulsion (the Meissner effect) is observable, an effect of special importance for the physics of superconductivity. Due to its interesting characteristics, the phenomenon of superconductivity discovered by Kammerlingh Onnes in Leiden in 1911, is one of the most studied phenomena in experimental and theoretical solid state physics. It has important applications, like the generation of high magnetic fields using superconducting solenoids cooled at liquid He (4K) up to liquid nitrogen (77K) temperatures, or the use of extremely sensitive magnetic field sensors via the so-called Josephson effect. The higher the critical temperature, the easier is the use of superconducting devices, especially in microelectronic. The critical temperature of superconducting materials ranges between a few tens of mK to ~200K, this last critical temperature found recently in SxHy under very high pressures .
Among experts in low-temperature physics, in particular those with solid backgrounds on superconductivity, there exists a kind of unproven law regarding the (im)possibility to have superconductivity at room temperature, which means having a material with a critical temperature above 300K. In short, for most of the experts it is extremely difficult to accept that a room temperature superconductor would be possible at all, although there is actually no clear theoretical upper limit for Tc. This general (over)skepticism is probably the reason why, for more than 35 years, the work of Kazimierz Antonowicz  (on the superconducting-like behavior he observed on annealed graphite/amorphous carbon powders at room temperature ) was not taken seriously by the scientific community. Probably, the lack of easy reproducibility of the observed superconducting-like behavior and the vanishing of the amplitude of the signals within a few days  (added to the (over)skepticism of scientists) did not encourage them to look more carefully at those results. The work of Antonowicz on the room temperature superconductivity in carbon powders  was not cited in reviews discussing the possibility to reach superconductivity at room temperature, see, e.g., .
In the last 16 years, however, different measurements done in highly oriented pyrolytic graphite samples and graphite powders, see [5,6] for reviews, suggest that some kind of interfaces in the graphite structure may quite possibly be the origin for some of the measured signals. This may explain several aspects of this hidden superconductivity, like low reproducibility, time instability, small amount of superconducting mass and the difficulty to localize the superconducting phase(s).
Assuming that somewhere in graphite samples the room temperature superconductivity exists, the question arises: which is actually the critical temperature? This was the main question the work of Precker et al.  wanted to answer. For that purpose, the authors took natural crystals from Brazil and Sri Lanka mines. A reader would perhaps be surprised that in these days someone selects natural graphite crystals instead of highly pure and ordered pyrolytic graphite, so called HOPG, for research. The main reason to start with ordered natural crystals is that their several microns long interfaces are very well defined, see Fig. 1. The team in  also performed measurements with HOPG samples, whose results support those found in natural graphite crystals. Highly ordered natural graphite crystals of good quality were created during the earth's early evolution at temperature and pressure conditions unreachable in laboratories nowadays. Therefore, the well-defined stacking order phases (hexagonal and rhombohedral) and their interfaces shown in Fig.1 may contribute substantially to the metallic-like behavior of graphite [5,6].
Fig.1: (Click on the image to view with higher resolution) Scanning Transmission electron microscopy (STEM) pictures taken from three ~100nm thick lamellae from three different regions of a natural crystal from Brazil. The e-beam points always parallel to the graphene layers. The different colors mean different stacking ordered regions or regions with the same stacking order but rotated a certain angle around the c-axis. The c-axis is always normal to the graphene planes and interfaces. The picture (c) shows that there are regions in the same sample with no or much less interfaces density. The scale bars at the right bottom denotes 1 µm.
Detailed X-ray diffraction studies done in Ref. show that in all samples a mixture of hexagonal (ABAB…, the majority phase) and rhombohedral (ABCABCA…) stacking orders exist in bulk graphite samples, independently of the sample origin. These two phases as well as their twist around the c-axis are the reason for the different colors in the STEM pictures of Fig.1. There are experimental [5,6] as well as theoretical reasons  that indicate that the origin for the metallic, and also most probably the superconducting behavior of graphite, is localized at some of those interfaces. One of the reasons why one expects superconductivity at certain interfaces, e.g. between rhombohedral and hexagonal stacking order, is that the relation between energy and wave-vector for conduction electrons becomes dispersionless. In this case and following the common BCS theory of superconductivity, the superconducting critical temperature is proportional to the Cooper pairs interaction strength. Therefore, it is expected that Tc is much higher than in the case of a quadratic dispersion relation .
Coming back to the main question, i.e. the critical temperature of the hidden superconductivity in graphite samples with interfaces, two results obtained in Ref. and shown in Fig.2 resume the main evidence suggesting the existence of granular superconductivity below 350K in the measured crystal.
Figure 2(a) shows the temperature dependence of the resistance (a linear in temperature background is subtracted from the original data) around the transition. It is accompanied by the difference between the field cooled and zero field cooled magnetic moment that starts to increase at the lowest temperature onset of the transition in the resistance. Figure 2(b) shows the change in resistance for the same sample at 325K and after cooling it from 390K at zero field. The relatively large response of the resistance with field and the irreversibility are compatible with granular superconductivity; see also other results in [5,6].
Fig.2: (Click on the image to view with higher resolution) (a) The difference (left y-axis, red points) between the measured field cooled magnetic moment mFC and the zero field cooled mZFC vs. temperature at a field of 50 mT applied at 250K for a natural graphite crystal from Brazil. Right y-axis: Difference between the measured resistance and a linear in temperature background vs. temperature -- for a sample from the same batch at zero field. (b) Change of the resistance with field at a temperature of 325K after cooling it from 390K at zero field. The field was applied normal to the interfaces.
The observed remanence in the resistance indicates that magnetic flux remains trapped within certain regions of the graphite samples. The origin and characteristics of this trapped flux and its non-monotonous temperature behavior  have to be clarified in the future and using other experimental techniques. One should also clarify to what extent a magnetically ordered state could have some influence on the observed phenomena. The observed phenomenology in Ref. (see Fig.2) as well as in different studies done on graphite in the past [5,6] strongly suggest the existence of superconductivity. Although several details of the phenomenology, especially the large magnetic anisotropy of the effects in resistance, do not support magnetic order as a possible origin, one should not rule out yet the existence of unusual magnetic states at the graphite embedded interfaces, which are partially being studied theoretically nowadays, see, e.g., Ref..
 A. P. Drozdov, M. I. Eremets, I. A. Troyan, V. Ksenofontov, S. I. Shylin, "Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system", Nature, 525, 73–6. Abstract.
 Kazimierz Antonowicz (1914–2003) started in the 60s the carbon research at Nicolas Copernicus University (Torum, Poland) investigating the structural and electronic properties of different forms of carbon.
 K. Antonowicz, "Possible superconductivity at room temperature", Nature, 247, 358–60 (1974). Abstract; "The effect of microwaves on DC current in an Al–carbon–Al sandwich", Physica Status Solidi (a), 28, 497–502 (1975). Abstract.
 Arthur W. Sleight, "Room Temperature Superconductors", Accounts of Chemical Research, 28, 103-108 (1995). Abstract.
 Pablo Esquinazi, "Invited review: Graphite and its hidden superconductivity", Papers in Physics, 5, 050007 (2013). Abstract.
 P. Esquinazi, Y.V. Lysogorsky, "Experimental evidence for the existence of interfaces in graphite and their relation to the observed metallic and superconducting behavior", ed. P Esquinazi (Switzerland: Springer) pp 145-179 (2016), and refs. therein.
 Christian E Precker, Pablo D Esquinazi, Ana Champi, José Barzola-Quiquia, Mahsa Zoraghi, Santiago Muiños-Landin, Annette Setzer, Winfried Böhlmann, Daniel Spemann, Jan Meijer, Tom Muenster, Oliver Baehre, Gert Kloess, Henning Beth, "Identification of a possible superconducting transition above room temperature in natural graphite crystals", New Journal of Physics, 18, 113041 (2016). Abstract.
 T.T Heikkilä, G.E. Volovik, "Flat bands as a route to high-temperature superconductivity in graphite", ed. P Esquinazi (Switzerland: Springer) pp 123-143 (2016), and refs. therein.
 Betül Pamuk, Jacopo Baima, Francesco Mauri, Matteo Calandra, "Magnetic gap opening in rhombohedral stacked multilayer graphene from first principles", arXiv:1610.03445 [cond-mat.mtrl-sci].