SPring-8, the large synchrotron radiation facility

Date: July 25, 2006  15:00 -

Place: HOUKOUKAN

Speaker: Dr. Zahir Islam ( ASP, Argonne National Laboratory )

Title: The tale of inhomogeneous "nanodomains" in yttrium barium copper oxide superconductors

Abstract: In the case of the high-T_c cuprates as charge carriers are introduced into a parent Mott insulator, the antiferromagnetic (AF) state is quickly destroyed, and superconductivity starts to emerge above a certain doping level. However, numerous bulk and spectroscopic measurements indicate that the electronic states of these cuprates are intrinsically inhomogeneous at various length scales. Despite the presence of inhomogeneities, the superconducting ground state exhibits remarkable robustness. A clear understanding of these inhomogeneous phases is important because they may play a crucial role in the overall phase diagram. In the case of YBa₂Cu₃O_6+x (YBCO), charge carriers are doped via oxygen stoichiometry variations. Systematic x-ray diffuse scattering experiments on YBCO revealed the presence of inhomogeneous short-range ordered lattice modulations characterized by a wavevector of the form q=(qx, 0, 0) where qx depends on oxygen-vacancy concentration. These modulations form along the shorter Cu-Cu bond direction with periodicity varying between 2 to ~10 unit cells, and they correspond to correlated atomic displacements of Ba, Cu, and O atoms, respectively. Correlation lengths of the modulations extend only a few unit cells in planes parallel to CuO₂ planes and a fraction of a unit cell in the perpendicular direction, respectively, forming distorted "nanodomains" which are distributed throughout the lattice. Theoretical work shows that these nanodomains are consistent with oxygen-vacancy ordered superstructure phases. Short-range order coexists with the AF ground state as well. Furthermore, "bowtie"-shape Huang diffuse scattering indicate characteristic strain at all compositions regardless of the average structure creating an intrinsically inhomogeneous lattice throughout the phase diagram. These nanodomains affect transport, thermodynamic, and spectroscopic measurements in significant ways. They imply that the treatment of high-T_c superconductivity in these complex oxides in terms of a single homogeneous phase may be too simplistic.

The APS is supported by Department of Energy, Office of Basic Energy Science, under Contract No. W-31-109-ENG-38.

Organizer: Mizuki (PHS 3902)