Studies of Lattice Modulations and Lattice Distortions in Complex Oxides and High-Tc Films

We have carried out synchrotron X-ray scattering studies of the layered LaSrMn₂O₇ manganite compounds and the high-Tc YBa₂Cu₃O_7-x (YBCO) superconducting compounds.

In the LaSrMn₂O₇ manganite compounds, diffuse X-ray scattering studies have revealed directly the formation of Jahn-Teller-driven polarons associated with oxygen displacements around the Mn³⁺ ions. These collapse below Tc with the onset of double-exchange-driven ferromagnetism, while above Tc they form a polaron-liquid with stripe-like short-range order.

In the YBa₂Cu₃O_7-x compounds, we see short-range order due to oxygen vacancy ordering in the chains which induces stripe-like lattice modulations of period 5a along the a-axis in the CuO₂ planes and two neighboring planes as the temperature is lowered, but which appears to saturate below Tc. Similar modulations are also seen in the optimally doped YBCO compound, but with period 4a, which condense around short-range ordered regions of oxygen vacancies on the chains. These modulated regions, which occupy approximately 10% of the sample volume are much "softer" elastically than the rest of the crystal and have a Debye-Waller factor which varies more rapidly with temperature. In addition, at low temperature the existence of Huang diffuse scattering around the Bragg reflections (as in the case of the manganites above) reveals the existence of anisotropic strains. These results confirm that even the optimally doped YBCO superconductors are intrinsically inhomogeneous in the superconducting phase. We plan to study other high-Tc superconductors to see whether similar inhomogeneities exist (e.g. "stripes" or other types of modulations).

We have also been studying, using X-ray reflectivity, the surface structure of deposited films of YBCO, LSCO and other cuprate superconductors. A surprising result is that there is always a surface layer on these films , several unit cells in depth, with much lower electron density than the bulk material. We are further investigating what kind of defect structure could be responsible for this.

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Publications

High-resolution polarization analysis study of long-range magnetic order in cuprates.
Z. Islam, D. Haskel, J. C. Lang, G. Srajer, X. Liu, S. K. Sinha and B. W. Veal
Phys. Rev. B 71, 212506 (2005). DOI: 10.1103/PhysRevB.71.212506

A four-unit cell superstructure in an optimally-doped YBa₂Cu₃O_6.92 superconductor
Z. Islam, X. Liu, S. K. Sinha, J. C. Lang, S. C. Moss, D. Haskel, G. Srajer, P. Wochner, D. R. Lee, D. R.Haeffner, and U. Welp
Phys. Rev. Lett. 93, 157008 (2004). DOI: 10.1103/PhysRevLett.93.157008

Synchrotron studies of lattice distortions and lattice modulations in complex oxides
S. K. Sinha
Zeitschrift fuer Kristallographie 219, 143 (2004). DOI: 10.1524/zkri.219.3.143.29096

Polaronic orbital polarization in La_1.2Sr_1.8Mn₂O₇
B. J. Campbell, S. K. Sinha, R. Osborn, S. Rosenkranz, L. Vasiliu- Doloc, J. F. Mitchell, Z. Islam, O. H. Seeck and J. W. Lynn
Phys. Rev. B 67, 020409 (2003). DOI: 10.1103/PhysRevB.67.020409

X-Ray Diffraction Study of Lattice Modulations in an Underdooped YBa₂Cu₃O_6+x Superconductor
Z. Islam, S. K. Sinha, D. Haskell, J. C. Lang, G. Srajer, B. W. Veal, D. R. Haeffner and H. A. Mook
Phys. Rev. B 66, 092501 (2002).DOI: 10.1103/PhysRevB.66.092501

Structure of nanoscale polaron correlations in La_1.2Sr_1.8Mn₂O₇
B. J. Campbell, R. Osborn, D. N. Argyriou, L. Vasiliu- Doloc, J. F. Mitchell, S. K. Sinha, U. Ruett, C. D. Ling, Z.Islam and J. W. Lynn
Phys. Rev.B 65, 014427 (2002). DOI: 10.1103/PhysRevB.65.014427

Spin Correlations and magnetoresistance in the bilayer Manganite La_1.2Sr_1.8Mn₂O₇
S. Rosenkranz, R. Osborn, L. Vasiliu-Doloc, J. W. Lynn, S. K. Sinha,and J. F. Mitchell
Physica B 312,763 (2002). DOI: 10.1016/S0921-4526(01)01224-8

Neutron Scattering Investigation of Magnetic Bilayer Correlations in La_1.2Sr_1.8Mn₂O₇
R. Osborn, S. Rosenkranz, D. N. Argyriou, L. Vasilliu-Doloc, J. W. Lynn, S. K. Sinha, J. F. Mitchell, K. E. Gray and S. D. Bader
Phys. Rev. Lett. 81, 3964, (1998). DOI: 10.1103/PhysRevLett.81.3964

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