The unanticipated outcome of the Tc measurements was that no
superconductivity could be detected in either alpha-phase or mixed
alpha + beta-phase samples.[36] By using Equation ,
it was determined that the superconducting effective area that would give a
signal-to-noise ratio of one corresponded to about 2% of the area of the
CsBi polycrystals. Therefore, from the inductive measurements it can be
said with some confidence that no more than 2% of these samples is
superconducting within the temperature range from 4.2 to 0.5 K.
The temperature region above liquid helium temperature was only searched briefly for a transition at a Tc higher than that reported by Lagrange et al. It is safe to conclude that a higher-temperature transition is not present, though, since an applied magnetic field of 23 Tesla did not produce any kind of superconducting transition. (The highest critical field known is well under 1 T.[120])
In addition to the null result from the inductive measurements, no interesting features in the resistivity were detected at low temperature. The compounds appeared to exhibit typical metallic behavior ( drho/dT ;SPMgt; 0). The measured resistivity at 4.2 K was (8 ± 2) muOmega-cm in one stage 1 alpha-phase sample, and (30 ± 7) muOmega-cm in a stage 1u alpha + beta sample. These values are somewhat higher than the 5 K resistivity values quoted by Lagrange et al., who measured their resisitivity values inductively.[168] An applied magnetic field produced only traces consistent with magnetoresistive behavior, nothing like a superconducting-normal transition. [See discussion below.]
The MIT superconductivity measurements are obviously in conflict
with those described previously[146], as are those of several
other groups who also failed to find
superconductivity.[270,223] In the most extensive set of
measurements to date, Stang and coworkers in Berlin failed to find
superconductivity in stage 1 alpha-phase CsBi-GIC's down to 50 mK even
though they had sensitivity to a transition in about 0.1% of the sample
volume.[223] Even the Nancy group has had difficulty in
reproducing the Tc values they reported.[144] A summary
of all Tc measurements on the MBi-GIC's to date is reported in
Table .
Table: Superconducting transition temperatures of the MBi-GIC's
and related alloys. For Ref.[36], the volume fraction column
actually contains the effective areal fraction of superconductivity, as
explained in Section . NR=Not reported. NA=Not applicable.
The irreproducibility of the Tc measurements in the
CsBi-GIC's would be more surprising were it not for the wide range of
transition temperatures found for other superconducting
GIC's.[64] In C4KHg, as was discussed previously, fairly
subtle variations in intercalation conditions have a large effect on
superconductivity. The wide range of Tc values reported for C4KHg is thought to be due to the presence of the beta phase [see
Section ] in low- Tc samples. One possible
explanation for the lack of superconductivity in the non-superconducting
CsBi-GIC's is therefore that they possess minority phases which suppress
superconductivity. Alternatively, as Lagrange has
suggested,[145] it seems plausible that the differences found
in the superconducting behavior in the CsBi experiments could be
attributable to small differences in in-plane ordering or in
stoichiometry. This type of sensitivity to ordering and/or stoichiometry
has also been considered as an explanation for the wide range of Tc
values reported for C4KHg.[206]
On the other hand, there is another simple line of reasoning which
explains the discrepancy among the various sets of experiments. The
alternative explanation, also discussed by Stang and
coworkers,[223] is that the superconducting transitions reported
by Lagrange et al. were actually transitions of inclusions of the
alloy CsBi2. This possibility is suggested by the presence of
inclusions in the CsBi-GIC's which could not be completely removed by
repeated cleaving. The inclusions were imaged during TEM studies which
showed that they appeared throughout the samples prepared both here at MIT
and at the University of Kentucky.[222] An electron micrograph of
an inclusion is shown in Figure .
Figure: An
electron micrograph showing intercalant inclusions (bright regions) in a C4CsBix alpha + beta-phase polycrystal grown here at MIT. The
magnification for this micrograph is indicated by the 100 nm scale bar.
[Micrograph prepared by J. Speck, MIT.]
In the case of the MIT samples, the inclusions presumably had the
approximate composition of the starting alloy, Cs5Bi4, a
composition which is not superconducting. As would be expected, therefore,
no inclusion-derived superconducting signal was seen in the MIT CsBi-GIC's.
However, as shown by the phase diagram in Figure , Cs5Bi4 is adjacent to CsBi2, a phase which is superconducting.
The implication is that a starting alloy with a composition between Cs5Bi4 and CsBi2 (such as CsBi) should be a mixture of
superconducting and non-superconducting material. One would therefore
expect that a GIC made from such a starting alloy would have some
superconducting inclusions. Inclusion-derived superconductivity was
apparently seen by Stang et al..[223] They observed that
superconductivity was present only in small volume fractions in the
CsBi-GIC's, consistent with inclusion-derived transitions. Further, Stang
and coworkers found that Tc was about 4.7 K almost independent of
stage and phase. [See Table
.] The only processing variable
which seemed to impact Tc was the starting alloy composition, which
presumably determines what fraction of the inclusions are superconducting.
A plot of Tc versus starting alloy composition is displayed in
Figure
.
Figure: A plot of superconducting transition temperature Tc for
C4CsBix versus starting alloy Bi/Cs ratio. bigotimes, MIT
data from Ref. [36]; bigtriangleup, University of Kentucky data
from Ref. [270]; and bigcirc, Freie Universität Berlin data
from Ref. [223]. The X are alloy (not GIC) data from the
CRC Handbook. The presence of downarrow means that the nearest point
represents an upper bound on Tc. Data from the University of Nancy
is not included because precise starting alloy compositions are not given
for their samples.
The implication of the MIT and Berlin work[223] is that superconductivity in contact-intercalated GIC's can be mimicked by inclusions of a superconducting intercalant phase. In this interpretation, the superconductivity observed by Lagrange and colleagues in resistivity measurements was due to a network of CsBi2 through the sample, either adsorbed on the surface or interconnected through the bulk. (Bulk interconnections could perhaps be provided by Daumas-Herold domains.) This superconducting alloy would completely short out any normal-state transport due to the GIC itself. In this interpretation, the University of Nancy low-temperature resistivity measurements do not measure a property intrinsic to the GIC, but to the adsorbed alloy. Then the reason that the MIT measurements give a higher value of rhoa at 4.2 K than the Nancy measurements is that the MIT measurements give rhoa, GIC while the Nancy ones give rhoa, alloy. Interpretation of these results would be more certain if it were known exactly what starting alloy compositions the French used and whether they cleaved their samples before performing the transport measurements.
In the end, one cannot make a firm statement as to whether the
CsBi-GIC's are superconducting or not. Circumstantial evidence strongly
indicates that the answer is no, but considering the long history of
variability in the Tc's of GIC's, it is wise to be cautious. After
the discovery of superconductivity in C8K at 0.55 K in 1965, a
transition was not observed again until 1978 at a much lower temperature of
0.139 K,[140] with several unsuccessful attempts to reproduce the
original experiment in intervening years.[196] Considering
that it is not possible to tell whether the non-superconducting samples
made outside Nancy are similar in every detail to those the superconductors
synthesized there, an authoritative statement ruling out superconductivity
in MBi-GIC's cannot be made. This is especially true for the KBi-and
RbBi-GIC's, which have not yet been intensively studied outside Nancy. The
best one can do at this time is say that available evidence does not
support the identification of the MBi-GIC's as superconductors. A possible
explanation why these compounds are not superconducting is discussed in Section .