Papers in Physics, vol. 5, art. 050008 (2013)

Received: 10 October 2013, Accepted: 4 November 2013
Edited by: S. A. Grigera
Licence: Creative Commons Attribution 3.0
DOI: http://dx.doi.org/10.4279/PIP.050008

www.papersinphysics.org

ISSN 1852-4249

Commentary on “Graphite and its hidden superconductivity”

E. M. Forgan1∗

I write this comment on the article by Esquinazi
[1] as an expert on superconductivity but not as
an expert on graphite. I should also mention that
in 1986, a student asked me what I thought about
a paper written by Bednorz & Muller in Z. Phys.
After looking at it carefully, I commented that it
represented measurements on a mixed-phase sam-
ple, which had a resistivity ∼1000 times that of
copper at room temperature. The resistivity was
increasing as the temperature was lowered, i.e., be-
having in a non-metallic fashion. At ∼35 K, the
resistivity began to fall, but had not become “zero”
until ∼10 K. Note that “zero” on the scale of the
graph in the paper might just be the resistivity of
copper at room temperature. Hence I concluded
that there was no proof of superconductivity (such
as the Meissner effect) and I highlighted the word
“possible” in the title of the paper. However, other
workers were more “gullible” and attempted to re-
peat and extend this work. It turned out that the
phenomenon was very “democratic” and widely re-
producible (unlike the equally surprising reports
of “cold fusion” a few years later). So here I try
to discuss whether the proposed superconductiv-
ity in graphite at elevated temperatures is real or
not. One initial bibliographic comment may well
be relevant: the papers reporting signs of supercon-
ductivity in graphite have a very restricted group
of authors, suggesting that the phenomenon may

∗E-mail: ted.forgan@gmail.com

1 School of Physics & Astronomy, University of Birming-
ham, U.K.

not be “democratic”. Some workers have become
persuaded that the phenomenon is real, but they
have not yet convinced a much wider audience, who
probably feel that exceptional claims need excep-
tionally strong evidence.

It is clear from the discussion in section I of the
paper, and a review of the extensive literature, that
graphite is a complicated and sometimes irrepro-
ducible material. This is partly due to the weak in-
terlayer forces, which mean that it does not always
stack in an ideal ABAB hexagonal pattern. In ad-
dition, after the discovery of single-layer graphene,
we know that independent layers may exist with
extremely high mobility, conducting only in the
basal plane direction. Even without this compli-
cation, graphite is a highly anisotropic material:
this can easily cause difficulties in measuring trans-
port properties, since the anisotropy in resistivity
can give non-uniform current distributions. The
effect of magnetic field on electron motion is also
very anisotropic, with c-axis fields having strong ef-
fects on transport properties, and basal plane fields
having almost no effect. Furthermore, the diamag-
netic susceptibility is strong, very anisotropic and
temperature-dependent. This bulk property and
many others, such as the de Haas van Alphen effect
in large samples [2] have been understood in gen-
eral terms [3] as a consequence of a semi-metallic
band-structure [4] since ∼1960.

I now turn to the various sections of the paper.
In section II, there is an account of strong magne-
toresistance effects. Similar effects have also been
observed in bismuth [5] and have a very interesting
explanation [5] in terms of the semi-metallic prop-

050008-1



Papers in Physics, vol. 5, art. 050008 (2013) / E. M. Forgan

erties of graphite and bismuth, so there is no need
to propose a superconducting explanation for this.
In section III, a tiny hysteresis in magnetoresistance
is described. Two comments are relevant here: the
author notes that the sign of the hysteresis is oppo-
site to that expected for a superconductor and lim-
its himself to stating that the data provide “strik-
ing hints that granular superconductivity is at work
in some regions of these samples”. This is hardly
definitive proof. Section IV is headed “Direct ev-
idence for Josephson behavior”. This quotes data
from a recent publication from the author’s group
[6]. It is worth noting that these measurements
were made with very small currents, so that the
limit of measurement value is in the ohms region
or greater. In some cases [6], apparent negative
resistance values were observed. This can easily
occur (and has been observed by myself) in a lay-
ered material. The phenomenon arises from non-
uniform current flow enhanced by the resistivity
anisotropy, combined with voltage leads which ef-
fectively make contact at different positions along
the c-axis of the sample. It seems likely that these
curious results, and their current-dependence, arise
from non-ideal connections of the voltage and/or
current leads. Other odd features of the results,
such as sample-dependent noise at low tempera-
tures, and the fact that magnetic fields could in-
crease, decrease or have no effect on the voltages
observed, also cast great doubt on the Josephson
interpretation.

In section V, we have an account of some mag-
netic susceptibility measurements, such as those re-
ported in [7] on graphite “doped” with water. The
hysteresis loops reported in that paper correspond
to a maximum signal only . 1% of the c-axis sus-
ceptibility of graphite. The value of this suscepti-
bility, though relatively large, is < 0.001 (SI dimen-
sionless units). Hence if the width of the hysteresis
loop observed in these measurements corresponds
to a Meissner signal from superconductivity, then
this supposed superconductivity occupies a volume
fraction . 10−5. Esquinazi et al. contend that this
is consistent with superconductivity only present
at somewhat ill-defined interfaces; however, it also
means that one has to beware of artifacts. In re-
sponse to [7], a colleague repeated their measure-
ments as an undergraduate project [8]. Their clear
conclusion was that if the correct diamagnetic back-
ground slope (that obtained at large fields) is sub-

tracted, then the hysteresis corresponds to a tiny
ferromagnetic component. However, if a slightly
different background is chosen, the hysteresis loops
look somewhat like the response of a granular su-
perconductor. However, for a granular supercon-
ductor the hysteresis peaks should lie away from
the vertical axis in the bottom right/top left cor-
ners (see e.g. [9]) and this is contrary to what is
observed in graphite. Further evidence that this
hysteresis is not due to superconductivity may be
obtained from its temperature-dependence. We
see in [7] that the hysteresis at 300 K is essen-
tially the same as that at 5 K. We bear in mind
that by assumption the superconductivity is con-
fined to an atomic layer, and that the higher the
Tc of a superconductor the shorter the coherence
length. These two together ensure that thermal
fluctuations (which are already very noticeable at
T ∼ 100 K in cuprate materials) would be huge
for any room temperature graphite superconductiv-
ity [10]. Thermal fluctuations would greatly reduce
vortex pinning and magnetic irreversibility at room
temperature, contrary to what is observed. On
the other hand, a saturated ferromagnetic response
would be almost temperature-independent for tem-
peratures well below the Curie point. There are
further measurements [11] which appear to show
magnetic hysteresis (as a function of direction of
temperature sweep, not as a function of field) go-
ing to zero at 400 K. However, this temperature is
where the sweep direction changes, so the hystere-
sis with temperature is by definition zero at 400 K.
Once again the differences in the magnetic signals
are a tiny fraction of the total sample magnetiza-
tion. There are many possible reasons (both real
and due to experimental artifacts) why measure-
ments on a sample taken on heating and cooling
might disagree. Hence, the rather complicated re-
sults summarized in Esquinazi’s paper cannot con-
fidently be ascribed to (as yet not understood) su-
perconducting effects.

I cannot give an overriding simple explanation
for all the different results reported in Equinazi’s
paper, but neither can the author. In some cases
this is because the proposed superconductivity is
a “moving target”: sometimes with a Tc ∼ 25 K,
and sometimes Tc well above room temperature;
sometimes superconducting effects are suppressed
by magnetic field and sometimes enhanced at high
fields. In interpreting the evidence presented, the

050008-2



Papers in Physics, vol. 5, art. 050008 (2013) / E. M. Forgan

author has a tendency to jump to a superconduct-
ing interpretation, when others are perfectly pos-
sible. Unless and until graphite samples can be
produced which exhibit the Meissner effect for a
volume fraction of at least 1%, and which show
direct evidence of quantum coherence (hysteresis
which might arise from Josephson networks or from
other causes is not direct evidence), I expect that
the scientific community at large will not accept
that graphite exhibits high-temperature supercon-
ductivity.

[1] P Esquinazi, Graphite and its hidden super-
conductivity, Pap. Phys. 5, 050007 (2013).

[2] J W McClure, Band structure of graphite and
de Haas-van Alphen effect, Phys. Rev. 108,
612 (1957).

[3] J W McClure, Theory of diamagnetism of
graphite, Phys. Rev. 119, 606 (1960).

[4] J-C Charlier, X Gonze, J-P Michenaud, First-
principles study of the electronic properties of
graphite, Phys. Rev. B 43, 4579 (1991).

[5] X Du, S W Tsai, D L Maslov, A F Hebard,
Metal-insulator-like behavior in semimetallic
bismuth and graphite, Phys. Rev. Lett. 94,
166601 (2005).

[6] A Ballestar, J Barzola-Quiquia, T Scheike, P
Esquinazi, Josephson-coupled superconducting
regions embedded at the interfaces of highly
oriented pyrolytic graphite, New J. Phys. 15,
023024 (2013).

[7] T Scheike, W Bhlmann, P Esquinazi, J
Barzola-Quiquia, A Ballestar, A Setzer, Can
doping graphite trigger room temperature su-
perconductivity? Evidence for granular high-
temperature superconductivity in water-treated
graphite powder, Adv. Mater. 24, 5826 (2012).

[8] M Robson, P Diwell (unpublished). Super-
vised by E Blackburn, School of Physics &
Astronomy, University of Birmingham, U.K.
(2012).

[9] S Senoussi, C Aguillon, S Hadjoudj, The con-
tribution of the intergrain currents to the low
field hysteresis cycle of granular superconduc-
tors and the connection with the micro- and
macrostructures, Physica C 175, 215 (1991).

[10] A Gurevich, Challenges and opportunities for
applications on unconventional superconduc-
tors, Annu. Rev. Cond. Matter Phys., in press.

[11] T Scheike, P Esquinazi, A Setzer, W
Böhlmann, Granular superconductivity at
room temperature in bulk highly oriented py-
rolytic graphite samples, Carbon 59, 140
(2013).

050008-3