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- Selected properties of some of the high- Tc
superconductors. dagger indicates a single-crystal measurement. The
coherence length quoted for ceramic sample is an average one. NR means
that the parameter was not reported in the cited reference.
- Tc
values for C8KHg as reported by several research
groups. There is almost universal agreement on Tc 1.9 K.
- Tc
values for C4KHg as reported by several research
groups. There is a great deal of scatter in the values, which seem to fall
primarily
into two groups, with most likely values being about 0.8 and 1.5 K.
dagger indicates a broad superconducting transition. ddagger
indicates a liquid-nitrogen quenched sample.[58]
- Parameters obtained from fits to the C4KHg
neutron diffraction data. NHg is the number of Hg atoms per 4
carbon atoms. Additional parameters for the hydrogenated sample: hydrogen
position = 3.9 Å; hydrogen/carbon ratio = 1.0. The parameters reported
for the hydrogenated sample are not well constrained since the residual in
this case is considerably higher than for the other diffraction patterns.
- Stoichiometries of the KHg-GIC's as determined from various
experiments. ND = not determined by this experiment; NR = not reported by
the authors of the cited reference.
- Transition temperatures Tc and reduced widths Delta Tc/Tc for a sampling of C4KHg specimens.
- Calculated values of kappa for selected C4KHg samples.
The Hc2(0) numbers come from linear extrapolation of the Hc2(T) data (see Section
), and so are independent of the
Hc2(theta) measurements.
- Hc(t) values for C4KHg obtained from
fits to Eqn.
. For comparison, Hc(t) calculated
from the specific heat data for a Tc = 1.53 K sample is also
included. The samples' Hc2(theta) data are shown in
Figs.
,
, and
.
- Residuals
for fits to Hc2(theta) using the AGL formula and the Tinkham
formula, both with and without type I behavior. The residual index cal R is defined in Eqn.
, the AGL formula is Eqn.
,
and the Tinkham formula is Eqn.
. Eqn.
shows
how each of these formulae was modified to account for possible type I
superconductivity.
- Parameters obtained from least-squares fits
of Eqn.
to C4KHg Hc2(T) data. The
individual specimens are labeled by the value of Tc obtained from
the zero-field sweeps. The other parameters, including the ``fit'' Tc, are obtained from the least-squares linear fits to the critical field
data that are shown in Figure
. The residual cal R is
calculated using Eqn.
. NR denotes that the parameter was not
reported in the cited reference.
- Comparison of the
anisotropy parameter epsilon as obtained from Hc2(T) and Hc2(theta) fits. The Hc2(T) epsilon numbers were obtained
from the ratio of the slopes. The Hc2(theta) numbers were
obtained from fits using Eqns.
(AGL) and
(Tinkham's formula), with type I superconductivity allowed for small
theta. In each case, the TF fits had lower residuals than the AGL fits
(see Table
). NA indicates that a TF fit was not
performed on this data; NR denotes
information that was not reported in the cited reference.
- Values of the KLB parameter r for the
GIC superconductors.
means that the value of r was calculated from
parameters in the cited references.
- Comparison for C4KHg of two different methods for
determination of Maki's alpha parameter.[262] The
orientation indicated is that of the applied magnetic field. In parentheses
it is noted which of the two halves of Eqn.
was used.
- Values of Lambdaep, the electron-phonon coupling
parameter, for GIC superconductors. Tc = 0.73 K[120] is used
for C4KHg since no transition was observed down to 0.8 K during the
specific-heat measurement.[8] Values of Lambdaep
for the KH-GIC's are gathered in Table
.
- A summary of the known differences between the pink and gold C4KHg. The numbers here are representative of a typical sample of its
type, although some variation was observed from sample to sample. Lambdaep is McMillan's electron-phonon coupling
parameter.[165] kappa is the Ginzburg-Landau parameter
discussed in Section
; kappa ;SPMlt; 1/sqrt2 indicates type I superconductivity. The density of states,
N(0), shown here was calculated in Section
from the Hc2(theta) data. NA means not applicable.
- Superconducting transition temperatures of the KH-GIC's and C8K. Two methods of preparation are possible: direct intercalation of KH
powder into graphite,[76] and intercalation of K into graphite
to form C8K, followed by hydrogen chemisorption.[78]
Single-phase specimens are difficult to obtain because of slow hydrogen
sorption kinetics.[231,78]
- Parameters relevant to superconductivity in the KH- and
RbH-GIC's. Adapted from Ref. [78]. The density of states has
been corrected for the electron-phonon coupling. dagger means a
calculated parameter; ? means not measured.
- Effect of
hydrogenation on Tc in KHg-GIC's. All the Tc's are for C4KHg, except for the blue 1.879 K sample, which was C8KHg.
dagger indicates the second hydrogenation of a previously
hydrogenated sample. * indicates that deuterium rather than
hydrogen was added. S indicates that the remeasurement of a
transition one year after hydrogenation.
- 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.
- Electronic
parameters for selected superconducting GIC's. The stage and phase of each
compound are listed in parentheses.
The
compounds are listed in order of increasing superconducting transition
temperature. Omegap is the unscreened plasma frequency.
- Two tables prepared by McRae and Marêché[166] which
list the c-axis resistivity and resistivity anisotropy of many GIC's, both
donors and acceptors. A== rhoc/rhoa.
Correct sources for these numbers are given in Ref. [166].
5em
Abbreviations and Symbols
Abbreviations and Symbols Used in This Work
- Hc1
- lower critical field
- Hc2
- upper critical field
- Hc3
- surface-nucleated critical field
- Hc
- thermodynamic critical field
- GIC
- graphite intercalation compound
- TMDC
- transition metal dichalcogenide
- xi
- superconducting coherence length
- Lambda
- superconducting penetration depth
- Lambdaep
- electron-phonon coupling parameter
- Lambdatr
- superconductivity cleanliness parameter
- kappa
- Ginzburg-Landau parameter; see Eqn.
- Ic
- c-axis lattice constant (i.e., multilayer period) for a GIC
- Tc
- zero-field superconducting critical temperature
- TMDCIC
- transition metal dichalcogenide intercalation compounds
- epsilon
- critical field anisotropy parameter == Hc2,|| ^c/Hc2, _|_ ^c
- t
- reduced temperature == T/Tc
- S/N
- superconductor/normal metal
- S/I
- superconductor/insulator
- S/M
- superconductor/magnetic material
- HOPG
- highly oriented pyrolytic graphite
- theta
- angle between the graphite c-axis and the applied field (See Figure
a.)
- fC
- charge transfer per carbon atom from the intercalant states
- SdH
- Shubnikov-de Haas
- DL
- Dresselhaus-Leung model[65]
- FS
- Fermi surface
- BR
- Blinowski-Rigaux model[27]
- HWHM
- half-width-at-half-maximum-intensity of a peak
- DPE
- differential paramagnetic effect
- AGL
- anisotropic Ginzburg-Landau model[244]
- Gamma
- linear specific heat coefficient (see Eqn.
)
- cal D
- demagnetization factor
- cal R
- residual quality-of-fit parameter
- r
- Klemm-Luther-Beasley dimensionality-crossover parameter[131]
- WHH
- Werthamer, Helfand and Hohenberg[262]
- KLB
- Klemm, Luther and Beasley[131]
- FS
- Fermi surface
- PC
- positive curvature
- T*
- dimensionality crossover temperature in layered superconductors
- BKC
- Biagi, Kogan, and Clem[22]
- alpha
- parameter which determines the importance of spin effects on Hc2(T)
- HP
- Pauli limiting field
- N(0)
- electronic density of states at the Fermi level
- N( EF)
- electronic density of states at the Fermi level
- TEM
- transmission electron microscopy
- RBS
- Rutherford backscattering spectrometry
- CDW
- charge-density wave
- thetaD
- the Debye temperature; characteristic energy of an acoustic phonon
- TE
- Einstein temperature; characteristic energy of an optic phonon
- TCDW
- charge-density wave onset temperature
- 2H, 1T, 4H b
- names of crystal structures of the transition metal dichalcogenides[90]
- CP
- heat capacity at constant pressure
1em
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alchaiken@gmail.com (Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995