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Specific heat measurements of high temperature superconductors between 1.5 K and 100 K using pulsed differential calorimetry

Experiments have shown that, in La2CuO4 (T - structure), an anomaly exists in the low frequency region of the phonon spectrum - namely, a peak in the density of states near an energy of 6 meV. The introduction of Sr, as a light impurity into this lattice, results in a decrease of this peak and the transfer of part ofthe phonon density of states to a higher frequency region. The introduction of Ba impurity, whose mass is insignificantly different from the La mass, also causes a reduction in this peak and this is probably due to its larger ion radius which changes the force constants of binding within the lattice. The introduction of Nd into the La2CuO4 lattice is interesting since Nd differs very little from La, in terms of both its mass and its ion radius, but nevertheless all of the compounds with the Nd displayed a decrease of the peak at 6meV and a transfer of part of their phonon density of states to higher frequencies when compared with theLa2CuO4 compound which was used as a standard in all of these experiments.Analysis of the experiments, in which La plays the role of a substitutionalimpurtity in the Nd2CuO4 lattice, i.e. where the heat capacities of Nd2-xLaxCuO4 and Nd1.3La0.7CuO4 were compared with each other, shows that the introduction of the La impurity even into the T'-structure lattice also produces a peak at ~ 6meV in the low frequency region of the phonon spectrum.

Finally, therefore, it may be concluded that in the cuprates (La2CuO4 , La2-xMxCuO4 and Nd2-xLaxCuO4) in the lattice of which La atoms exist, an anomaly isformed in the acoustic region of the phonon spectrum near 6meV and La atoms are responsible for this anomaly. It is supposed that this is connected with thespecific properties of the interaction of the La atoms in the medium, which is due to the peculiarity of electron shell construction of this element: La atoms areweakly bonded with the nearest atoms of oxygen and hence create soft dynamical configuration which form soft oscillation modes in the phonon spectrum.

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University of Sussex
BN1 9QH Brighton
United Kingdom
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