TY - JOUR
T1 - Cooperation of superexchange and polaron effects as pairing mechanism in high-Tc superconductors
AU - Kuramoto, Y.
AU - Watanabe, T.
N1 - Funding Information:
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.
PY - 1987/12
Y1 - 1987/12
N2 - Further discussion is given on our proposal that cooperation of superexchange and polaron effects causes the high-Tc superconductivity in some Cu oxides. The key observation has been that the polaron effect reduces the transfer energy, but does not reduce the superexchange. It is pointed out that the wave-function renormalization makes the pairing interaction much smaller than the estimate by a mean-field theory. In the present model the isotope effect shows various characteristics depending on the strength of the polaron effect, and for a particular strength giving the highest Tc, there is no isotope effect.
AB - Further discussion is given on our proposal that cooperation of superexchange and polaron effects causes the high-Tc superconductivity in some Cu oxides. The key observation has been that the polaron effect reduces the transfer energy, but does not reduce the superexchange. It is pointed out that the wave-function renormalization makes the pairing interaction much smaller than the estimate by a mean-field theory. In the present model the isotope effect shows various characteristics depending on the strength of the polaron effect, and for a particular strength giving the highest Tc, there is no isotope effect.
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U2 - 10.1016/0378-4363(87)90201-4
DO - 10.1016/0378-4363(87)90201-4
M3 - Article
AN - SCOPUS:0023578113
VL - 148
SP - 246
EP - 249
JO - Physica B: Physics of Condensed Matter & C: Atomic, Molecular and Plasma Physics, Optics
JF - Physica B: Physics of Condensed Matter & C: Atomic, Molecular and Plasma Physics, Optics
SN - 0378-4363
IS - 1-3
ER -