Specific heat of the superconductor Yba2Cu4O8 from 1.5 to 300K

Physica B 165&166 (1990) 1335-1336 North-Holland

SPECIFIC HEAT OF THE SUPERCONDUCTOR YBa2Cu40S FROM 1.5 TO 300K A. JUNOD, T. GRAF, D. SANCHEZ, G. TRISCONE and J. MULLER Departement de physique de la matiere condensee, Universite de Geneve CH-12ll Geneve 4, Switzerland The specific heat of selected ceramic samples of the SlK-superconductor YBa2Cu40S prepared under an oxygen pressure of S5 bar is investigated. For the most homogeneous sample, the coefficient of the "linear" term at T<
3. RESULTS 3.1. Specific heat at low temperatures Contrarily to 123, 124 samples never show low-temperature upturns in the CiT vs T plots (Fig. I), but they rather tend to follow the expression C-AT n with 1
2. PREPARATION The ceramic samples were prepared under an oxygen pressure of S5 bar at 1000·C. The extensive characterization included optical micrographs, X-ray diffraction, EDAX, density, resistivity, AC susceptibility, field cooling Meissner effect, critical current density, magnetization and normal-state susceptibility experiments. Details are given in Refs (1-3). We focus here on the specific heat of the most homogeneous sample (code lCO-3).

we find a coefficient ~* that has a value (per copper unit) identical to that of good quality 123, i.e. ~*-1.2 mJ/(K 2mole-Cu). The initial Debye temperature, 8(0)=350K, is smaller than that of oxidized 123, and close to that of reduced 123.

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FIGURE 1. Specific heat CIT vs T2 , 1.4-22K. Inset: expanded scale, 1.4-5.5K. 1 mole-IS gat.

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3.2 Specific heat at Tc ' The superconducting transition of the most homogeneous 124 sample is as sharp as that of good 123 ceramics in resistivity, AC susceptibility and Meissner experiments. The fraction of Meissner flux expulsion, -4~X, is also similar (Table I, Fig. 2). The specific heat jump at Tc ' 6C/T c ' is well defined (Fig. 2), but is about 4 times smaller than that of 123. The correlation observed among 123 samples between -4~X and· 6C/T c (4) does not hold for 124; samples with IS and 55% Meissner effect have almost identical specific heat jumps, 15 and 16 mJ/(K2mo l e ) (3). 3.3. Specific heat at high temperatures. The specific heat and the Debye temperature 8 of 124 and 123 are compared in Fig. 3. The dips in 8(T) at 22K reflect the first peak in the phonon DOS near 10-12 meV. At room temperature, the raw Debye temperature is constant for 124 but decreases for 123. This difference can be explained by the contribution of an electron term that is larger for 123 than for 124. We attempted a separation of electron and phonon contributions by means of a least squares fit of all C data but the transition region (7S-SSK). T~e electron contribution was repre-

Elsevier Science Publishers B.V. (North-Holland)

A. Junod, T. Graf, D. Sanchez, G. Triscone, J. Muller

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FIGURE 2. Specific heat CIT vs T near Tc ' Inset: FC Meissner susceptibility (full squares) and AC susceptibility (full line, adjusted scale).

sented by 3~T3Tc-2 below Tc ' and by ~T above Tc . A geometrical series of 4 Einstein peaks located at frequencies Wk-rkwl with weights adding up to 3N, represented the phonon DOS. The fitted parameters were~, r, wI and three independent weights. We obtained ~-12 mJ/(K 2mole), to be compared with 28 for 123 in the same conditions. Fig. 4 shows Cp.C(lattice) where C(lattice) is deduced from the fit. The anomaly at Tc is more sharply peaked than mean-field models would predict, indicating the presence of fluctuations. 4. CONCLUSION Three measurements, namely the normal-state susceptibility (1,2), the specific heat jump at Tc ' and the Sommerfeld constant at room temperature, suggest consistently that the EDOS is much smaller for 124 than for 123, although the critical temperatures differ by only 13%. This sets a constraint for theories that would relate quantitatively Tc with the EDOS. The low EDOS and the low resistivity (p(100K)=60~Ocm, p(O)< 10~Ocm (1)) explain why the slope of the critical field is small for 124. From several points of view (T c ' hole concentration, positive slope of the susceptibility, EDOS (4), Debye temperature), 124 is closer to YBa2Cu306.8_6.85 than to YBa2Cu307' An essential difference is the absence of oxygen disorder in 124, that leads to sharp transitions. REFERENCES (1) G. Triscone, T. Graf, A. Junod, D. Sanchez, 0. Brunner, D. Cattani and J. Muller, submitted to Physica C (1990). (2) G. Triscone, T. Graf, A. Junod, D. Sanchez and J. Muller, this volume. (3) A. Junod, D. Eckert, T. Graf, E. Kaldis, J. Karpinski, S. Rusiecki, D. Sanchez, G. Triscone and J. Muller, submitted to Physica C (1990). (4) A. Junod, D. Eckert, T. Graf, G. Triscone and J. Muller, Physica C162-l64 (1989) 1401.

Temperature (K)

FIGURE 3. Specific heat C and Debye temperature 6 vs T (1-300K) for 124 (full line) and 123 (dotted curve).

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Temperature (K) FIGURE 4. Electron specific heat for 124 (lower curve) and 123 (upper curve). Full scale: 3~% of the total specific heat at 300K.

TABLE I. A comparison of selected data for 124 (1,3) and 123 (3). S, entropy; H, enthalpy. Phase Sample code ~* [mJ/(K 2mol] 6(0) [K] 6(298) [K) Cp (298) [J/K/gat] S(298) [J/K/gatJ H(298)-H(0) [J/gat] Tc (calorimetric) [K] lIC/T c [mJ/(K 2mol)) ~ [mJ/(K 2mol] -41rX (FC Meissner) [- J Vat [A3/atomJ

"124" lCO-3 4.9±0.2 350±10 545±10 21.3 24.3 3850 81. 9 -16 -12 0.55 13 .485

"123" J465b 4.3±0.5 437±15 21. 9 24.8 3950 90.0 67 -28 0.65 13.327