COOPERATION OF SUPEREXCHANGE AND POLARON EFFECTS AS PAIRING MECHANISM IN HIGH-Tc SUPERCONDUCTORS.

被引：0
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作者：
Kuramoto, Y. [1 ]
Watanabe, T. [1 ]
机构：
[1] Tohoku Univ, Sendai, Jpn, Tohoku Univ, Sendai, Jpn
来源：
Physica B: Physics of Condensed Matter & C: Atomic, Molecular and Plasma Physics, Optics | 1987年 / 148卷 / 1-3期
关键词：
The present study has a few implications to interpret experimental results. First; we address to the density of states measured by photoemission [9] and Bremsstrahlung [10] spectroscopies or by specific heat [11]. The spectroscopies show that the density of states at the Fermi level is very small. On the other hand the coefficient y of the term y T in the specific heat is suggested to be of order 10 mJ/mol K 2 [11]. The specific heat is proportional to the quasi-particle density of states p(O)/a r at the Fermi level. But spectroscopies measure the density of states of real electrons. Hence the enhancement 1/af does not appear. Furthermore since the energy resolution of the spectroscopies is much larger than the phonon energies; the polaron effect should hardly appear in the spectrum [12]. Thus the large discrepancy in the measured density of states shows the importance of the wave-function renormalization as well as the polaron effect. Secondly; we note that the magnetic susceptibility X of high-T c superconductors depends only weakly on temperature above T c [13; 14]. This occurs not only when the Coulomb correlation is weak but when the superexchange is much larger than the temperature. In the latter case X is proportional to j-1 [15]. The magnitude X 10 -4 emu/mol in Sr0.2Lal.8CuO4 [13] suggests J-10 3 K; which is consistent with the estimate using J = 2t2/U; t-0.5 eV and U-5 eV. Similar magnitude of X per Cu ion has been reported [14] for Ba2YCu3OT_ ~. The absence of isotope effects in Ba2YCu3OT_ ~ [16]s uggests that t-/opt is realized in this material. In summary we have shown that the present pairing mechanism leads to high value of T c in the nearly half-filled band of the 2D system; and that isotope effects have various appearances depending on the strength of the polaron effect. We have also pointed out the importance of the wave-function renormalization in understanding the disparity between spectroscopic and thermal quantities. This work is supported by a Grant-in-Aid for Scientific Research from the Ministry of Education; Science and Culture of Japan;
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