Correlated local distortions of the TlO layers inTl2Ba2

Abstract

We have used the XAFS (x-ray-absorption fine structure) technique to investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic T12BaqCuO"with a superconducting transition temperature T, =60 K. Our results clearly show that the 0(1), 0(2), Cu, and Ba atoms are at their ideal sites as given by the diffraction measurements, while the Tl and 0(3) atoms are more disordered than suggested by the average crystal structure. The Tl-Tl distance at 3.5 A, between the T10 layers does not change, but the Tl-Tl distance at 3.9 A within the T10 layer is not observed and the Tl-Ba and Ba-Tl peaks are very broad. The shorter Tl-0(3) distance in the T10 layer is about 2.33 A, significantly shorter than the distance calculated with both the Tl and 0(3) atoms at their ideal 4e sites ( x = y =0 or -). A model based on these results shows that the Tl atom is displaced along the (110) directions from its ideal site by about 0.11 A; the displacements of neighboring Tl atoms are correlated, with a local antiferroelectriclike order. The 0(3) atom is shifted from the 4e site by about 0.53 A roughly along the (100) directions. A comparison of the Tl Liii-edge XAFS spectra from three samples, with T =60, 76, and 89 K, shows that the 0 environment around the Tl atom is sensitive to T, while the Tl local displacement is insensitive to T and the structural symmetry. These conclusions are compared with other experimental results and the implications for charge transfer and superconductivity are discussed.

Ultrasonic Velocity Studies on Tl2Ba2Ca2Cu3O10 Superconductor

“…In the thallium-based superconductors, short range correlated Tl and O ordering in the Tl-O planes have been proposed in the Tl 2 Ba 2 CuO 6−δ (Tl2201) 24,25 and Tl 2 Ba 2 CaCu 2 O 8−δ (Tl2212) 26 materials. Ultrasonic method may provide further insight into the nature of the ordering process and the superconductivity in the Tl-based cuprates.…”

A survey of the Van Hove scenario for high-tc superconductivity with special emphasis on pseudogaps and striped phases

“…Thus for instance, in YBCO, the chain ordering splits the degeneracy of the X and Y point VHS's. In Tl-1201 and Tl-1212, the structural distortions have been analyzed in detail [613]. The O-atoms show a very large displacement from their nominal positions, and this distortion breaks the symmetry of the two in-(CuO 2 -)plane O's, thereby lifting the VHS degeneracy.…”

Does the local structure play a role in high temperature superconductivity?

“…Here we briefly describe our study of the Tl layer region; a more detailed discussion is found in Ref. [5].…”