Louisiana State University Physics & Astronomy

Interplay of spin, charge, orbital, and lattice degrees of freedom has continued to bring in dividends in the study of strongly correlated-electron systems for decades. 3d systems (e.g. manganites, cuprates) have been a fertile ground for high-T_c superconductivity, CMR effects, polaronic order, and spin-charge stripes. Currently, 4d electron systems (e.g. ruthenates), are proving to be equally exciting. In this talk, X-ray scattering studies of short and long-range order in “high-profile” oxides are presented.

In the case of cuprates, numerous experiments indicate that they are “inhomogeneous” even in their superconducting state. Although the origin of these inhomogeneities may well be electronic instabilities such as spin-charge stripes, a general consensus remains elusive. Systematic x-ray diffuse scattering studies of YBa₂Cu₃O_6+x (YBCO) and
Bi₂Sr₂CaCu₂O_8+δ (BSCCO) reveal lattice modulations on various length scales. In the case of YBCO, as charge carriers are tuned via O-stoichiometry variations, short-range ordered modulations characterized by a wavevector of the form
q=(q_x, 0, 0) are observed. These modulations correspond to coupled atomic displacements of Ba, Cu, and O atoms, respectively, correlated over several unit cells associated with O-vacancy ordered "nanodomains". They form well above room temperature and persist down to the lowest temperature studied. Furthermore, "bowtie"-shape Huang scattering indicates characteristic strain at all compositions, signifying an intrinsically inhomogeneous lattice in both YBCO and (La_1-xSr_x)₂CuO₄. The role of lattice distortions in modulating pair potential and exchange interactions has been highlighted recently. Perhaps the interplay between electronic inhomogeneities and lattice modulation is inevitable in cuprate superconductors.

Recent studies of bilayer (Sr_xCa_1-x)₃Ru₂O₇ revealed new modulations appearing at compositions with unconventional quantum critical behavior. These modulations are transversely polarized and quasi-3D ordered in that they are fully coherent in the RuO₂ planes with correlations at least one unit cell in extent in the direction normal to the planes. Possible roles of such modulations in electronic phase diagram are discussed.
Applications of magnetic fields to perturb the systems can have profound implications on our understanding of their phase diagram. Progress made using existing capabilities and future directions are briefly summarized.

*Use of the Advanced Photon Source is supported by the U. S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

BIO - Z. Islam

Employment History:

2006-Present -- Physicist, X-Ray Science Division, Advanced Photon Source
2001-2006 -- Assistant Physicist, X-Ray Science Division, Advanced Photon Source

Postdoctoral Fellow, Argonne National Laboratory
Postdoctoral Fellow, Argonne National Laboratory & Northern Illinois University

1995-1999 -- Research Assistant, Iowa State University
1993-1995 -- Teaching Assistant, Iowa State University
1991-1993 -- Departmental physics and mathematics tutor, Bemidji State University

Education:

1990-93 Department of Physics, Bemidji State University (BSU), Bemidji, Minnesota
B. S. in Physics, Summa Cum Laude
1993-99 Department of Physics & Astronomy, Iowa State University (ISU), Ames, Iowa
Ph. D. in Experimental condensed matter physics (Primary Advisor: Alan I. Goldman;
Co-Advisor: Paul C. Canfield)
Thesis: The interplay of long-range magnetic order and single-ion anisotropy in rare
earth nickel germanides

Research Interests:

My general research interest is the study of structural, charge, magnetic, and orbital ordering phenomena in oxides and rare-earth compounds. My primary strength lies in innovative use and development of x-ray scattering and spectroscopic techniques in such studies. At present, I am keen on studies of materials in high magnetic fields. I have developed 30-45 Tesla pulsed-field instrumentation for x-ray studies of materials at the APS. These studies may include novel ordering in oxides and meta-magnets, multi-ferroics, field-induced structural transitions, geometrically frustrated systems, etc.