We have performed several experiments to explore the role of local lattice in the cuprate superconductors. The Cu K-edge x-ray absorption fine structure (EXAFS) results provide a direct evidence for the Jahn-Teller polaronic distortions of the CuO2 lattice in these materials. The Jahn-Teller polarons in the cuprates are found to be associated with Q2 mode which is degenerate with the Q3 Jahn-Teller mode. It is discussed that the Jahn-Teller polarons get ordered in the stripes. The temperature dependent Cu K-edge X-ray absorption near edge structure (XANES) measurements reveal a particular change in the local lattice at the charge stripe ordering temperature in different superconducting cuprate systems. This response of the local lattice to the stripe charge ordering has been used to study oxygen isotope effect (16O yields 18O) in the La1.94Sr0.06CuO4 superconducting system. The results show that the isotope substitution introduces a large increase to the onset temperature of the charge-stripe ordering by approximately 60 K, providing a compelling evidence for the vital role of the electron-lattice interactions to be an important ingredient for the charge-stripe ordering in the high Tc cuprate superconductors.
Experiments are enlightening that the high Tc superconductivity in cuprate perovskites takes place in a particular phase showing segregation of localized and itinerant charge carriers. We have studied topological features of the Fermi surface of Bi2Sr2CaCu2O8+d (Bi2212) superconductor to investigate implication of charge segregation in the system. For the purpose we have used constant initial state angle-scanning photoemission supported by the energy distribution curves. The resulting Fermi surface measured over an extended Brillouin zone shows broken segments with partial gaps around ((pi) ,O) and shadow bands around (0.5(pi) ,0.5(pi) ) and equivalent locations. In addition data provide an evidence for new electronic states at the Fermi surface due to a one dimensional band with a small dispersion and small kF along the ((pi) ,O) direction. We argue that the depression of photointensity around ((pi) ,O) and equivalent locations with well-defined kF and one dimensional band are related with charge ordering in the CuO2 plane along the (-(pi) ,(pi) ) direction and spin fluctuations along the ((pi) ,O) direction.
We have measured the amplitude of the 1D lattice modulation forming the stripe structure in the CuO2 plane of Bi2Sr2Ca2Cu2O8+y single crystals by EXAFS. The period of the 1D modulation of the CuO2 plane has been measured by Cu anomalous diffraction. The large anharmonic content of the 1D modulation clearly shows the formation of stripes of undistorted LTO lattice. The large amplitude of the local structure modulation in plane indicates the formation of sizable potential barrier due to the linear domain walls in the plane. The size of the stripes L is such that the Fermi level is tuned to the maximum of the density of states formed by the second subband. The tuning of the Fermi level at the shape resonance of the superlattice of quantum wires is proposed to be the mechanism for the amplification of the critical temperature in high Tc cuprate superconductors.
We have found that the structure of the CuO2 plane of Bi2Sr2CaCu2O8+y (Bi2212) is anisotropic. Experimental investigation of the Cu site configurations in the CuO2 plane of Bi2Sr2CaCu2O8+y (Bi2212) by XANES and EXAFS shows that the quasi 2D Fermi liquid is confined in a superlattice of quantum stripes of width L running in the x direction. We have found that the recently proposed mechanism to rise the critical temperature by the confinement of an electron gas in superlattices and tuning the Fermi level to a shape resonance is verified. In fact the component of the Fermi wavevector kFy in the y direction perpendicular satisfies the condition kFy equals 2(pi) /L in Bi2212. This resonance gives an amplification of the critical temperature Tc by a factor of order of 5 in comparison with the critical temperature of the homogeneous CuO2 plane estimated to be about 15 - 20 K.
Conference Committee Involvement (1)
Instruments, Methods, and Missions for Astrobiology IX
14 August 2006 | San Diego, California, United States
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