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Selected properties of some of the high
T_{c} superconductors. dagger indicates a
singlecrystal measurement. The coherence length quoted for
ceramic sample is an average one. NR means that the
parameter was not reported in the cited reference.

T_{c} values for C_{8}KHg
as reported by several research groups. There is almost
universal agreement on T_{c} 1.9 K.

T_{c} values for C_{4}KHg
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 liquidnitrogen quenched sample.[58]

Parameters obtained from fits to the
C_{4}KHg neutron diffraction data. N_{Hg}
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 KHgGIC's as
determined from various experiments. ND = not determined by
this experiment; NR = not reported by the authors of the
cited reference.

Transition temperatures T_{c} and
reduced widths Delta T_{c}/T_{c} for
a sampling of C_{4}KHg specimens.

Calculated values of kappa for
selected C_{4}KHg samples. The H_{c2}(0)
numbers come from linear extrapolation of the
H_{c2}(T) data (see Section
), and so are independent of the
H_{c2}(theta) measurements.

H_{c}(t) values for
C_{4}KHg obtained from fits to Eqn.
. For comparison, H_{c}(t) calculated from the
specific heat data for a T_{c} = 1.53 K sample is
also included. The samples' H_{c2}(theta)
data are shown in Figs. ,
, and .

Residuals for fits to
H_{c2}(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 leastsquares
fits of Eqn. to C_{4}KHg
H_{c2}(T) data. The individual specimens are
labeled by the value of T_{c} obtained from the
zerofield sweeps. The other parameters, including the
``fit'' T_{c}, are obtained from the leastsquares
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 H_{c2}(T) and
H_{c2}(theta) fits. The H_{c2}(T)
epsilon numbers were obtained from the ratio of the
slopes. The H_{c2}(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 C_{4}KHg 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 Lambda_{ep},
the electronphonon coupling parameter, for GIC
superconductors. T_{c} = 0.73 K[120] is used for C_{4}KHg
since no transition was observed down to 0.8 K during the
specificheat measurement.[8] Values of
Lambda_{ep} for the KHGIC's are gathered in
Table .

A summary of the known differences
between the pink and gold C_{4}KHg. The numbers
here are representative of a typical sample of its type,
although some variation was observed from sample to sample.
Lambda_{ep} is McMillan's electronphonon
coupling parameter.[165] kappa is the
GinzburgLandau parameter discussed in Section
; kappa ;SPM_{l}t; 1/sqrt2 indicates
type I superconductivity. The density of states, N(0),
shown here was calculated in Section
from the H_{c2}(theta) data. NA means not
applicable.

Superconducting transition temperatures
of the KHGIC's and C_{8}K. Two methods of
preparation are possible: direct intercalation of KH powder
into graphite,[76]
and intercalation of K into graphite to form
C_{8}K, followed by hydrogen chemisorption.[78] Singlephase specimens
are difficult to obtain because of slow hydrogen sorption
kinetics.[231,78]

Parameters relevant to superconductivity
in the KH and RbHGIC's. Adapted from Ref. [78]. The density of states has
been corrected for the electronphonon coupling.
^{dagger} means a calculated parameter; ?
means not measured.

Effect of hydrogenation on T_{c}
in KHgGIC's. All the T_{c}'s are for
C_{4}KHg, except for the blue 1.879 K sample, which
was C_{8}KHg. ^{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 MBiGIC'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.
Omega_{p} is the unscreened plasma
frequency.

Two tables prepared by McRae and
Marêché[166] which list the caxis
resistivity and resistivity anisotropy of many GIC's, both
donors and acceptors. A==
rho_{c}/rho_{a}. Correct sources for
these numbers are given in Ref. [166].
5em
Abbreviations and Symbols
Abbreviations and Symbols Used in This Work

H_{c1}

lower critical field

H_{c2}

upper critical field

H_{c3}

surfacenucleated critical field

H_{c}

thermodynamic critical field

GIC

graphite intercalation compound

TMDC

transition metal dichalcogenide

xi

superconducting coherence length

Lambda

superconducting penetration depth

Lambda_{ep}

electronphonon coupling parameter

Lambda_{tr}

superconductivity cleanliness parameter

kappa

GinzburgLandau parameter; see Eqn.

I_{c}

caxis lattice constant (i.e., multilayer period) for a
GIC

T_{c}

zerofield superconducting critical temperature

TMDCIC

transition metal dichalcogenide intercalation compounds

epsilon

critical field anisotropy parameter == H_{c2,
^c}/H_{c2, __ ^c}

t

reduced temperature == T/T_{c}

S/N

superconductor/normal metal

S/I

superconductor/insulator

S/M

superconductor/magnetic material

HOPG

highly oriented pyrolytic graphite

theta

angle between the graphite caxis and the applied field
(See Figure a.)

f_{C}

charge transfer per carbon atom from the intercalant
states

SdH

Shubnikovde Haas

DL

DresselhausLeung model[65]

FS

Fermi surface

BR

BlinowskiRigaux model[27]

HWHM

halfwidthathalfmaximumintensity of a peak

DPE

differential paramagnetic effect

AGL

anisotropic GinzburgLandau model[244]

Gamma

linear specific heat coefficient (see Eqn.
)

cal D

demagnetization factor

cal R

residual qualityoffit parameter

r

KlemmLutherBeasley dimensionalitycrossover
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 H_{c2}(T)

H_{P}

Pauli limiting field

N(0)

electronic density of states at the Fermi level

N( E_{F})

electronic density of states at the Fermi level

TEM

transmission electron microscopy

RBS

Rutherford backscattering spectrometry

CDW

chargedensity wave

theta_{D}

the Debye temperature; characteristic energy of an
acoustic phonon

T_{E}

Einstein temperature; characteristic energy of an optic
phonon

T_{CDW}

chargedensity wave onset temperature

2H, 1T, 4H _{b}

names of crystal structures of the transition metal
dichalcogenides[90]

C_{P}

heat capacity at constant pressure
1em
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alchaiken@gmail.com
(Alison Chaiken)
Wed Oct 11 22:59:57 PDT 1995