Specific heat evidence for strong coupling in YBa sub 2 Cu sub 3 O sub 7 Page: 2 of 6
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Specific Heat Evidence for Strong Coupling in YBa2Cu307
J E Gordon*, R A Fisher, S Kamin*, and N E Phillips
MCSD, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA
Specific heat data for YBa2Cu3O7 are consistent with y - 15 3 mJ/mole.K2
and 2Ao/kTc - 6.8 0.6. These results indicate that strong-coupling ef-
fects are present but that the coupling is unlikely to be predominantly
a conventional electron-phonon interaction.
In this note an interpretation of specific heat, C, measurements on
YBa2Cu3O7 is applied to an analysis of data in the vicinity of Tc to obtain
information about the strength of the coupling responsible for the
superconductivity. Correlations among several sample-dependen- parameters
have been interpreted (Phillips et al 1990) as showing that typical samples
are only 30-90% superconducting; that the volume fraction of
superconductivity, fs, can be determined from the discontinuity, AC(Tc)'
at the critical temperature, Tc; and that the Sommerfeld constant cor the
fully normal state, y, is -16 mJ/mole.K , a value close to that calculated
(Massidda et al 1987; Krakauer et al 1988) for the bare density of states,
'bs, indicating that electron-phonon enhancement effects are not
significant. Thus, if electron-phonon coupling is responsible for the
superconductivity, YBa2Cu307 should be a weakly-coupled BCS superconductor
with B - AC(Tc)/yTc - 1.43. However, such a conclusion is clearly at odds
with y - 16 mJ/mole.K2 and the measured values of &C(Td .
Figure 1 shows C/T vs T for a YBa2Cu307 sample made by the citrate pyrolysis
technique. An entropy-conserving construction gives AC(Tc )/T - 64
mJ/mole.K2, -83% of the value expected for a fully superconducting sample
(Phillips et al 1990; see also Junod et al 1990), i.e., fs - 0.83. With y
- 16 md/mole.K2 and B - &C(Tc)/fs7Tc - 5.5 -- almost 4 times the BCS weak-
coupling value and clearly indicative of strong-coupling. The dotted and
solid curves in Fig. 1 represent, respectively, approximations to Cn and Cs'
the normal- and the 83%-superconducting-state values of C. Cn = C1 + 7T,
where C1, the lattice contribution, is assumed to consist of dilatation and
harmonic terms. Cn was obtained by interpolation between the data above 96K
and the data in the region 62-65K, the region expected to include the
temperature, Tx, at which Cs - Cn. Over this interpolation interval the
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Gordon, J. E.; Fisher, R. A.; Kamin, S. & Phillips, N. E. Specific heat evidence for strong coupling in YBa sub 2 Cu sub 3 O sub 7, article, August 1, 1990; California. (https://digital.library.unt.edu/ark:/67531/metadc1206481/m1/2/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.