November 1, 2005

NSLS 2005 Annual Users’ Meeting Workshop

Spectroscopic Studies of Nanoscaled Systems

A workshop titled "Spectroscopic Studies of Nanoscaled Systems" was held on May 25, 2005, as part of the National Synchrotron Light Source 2005 Annual Users' Meeting. This workshop was intended to be a forum on the connection between nanoscience and the physics of interacting electron systems, particularly the occurrence of electronic ordering at the nanoscale in oxides, both in extended crystals and in artificial heterostructures. Our hope was that this session might feed the discussion concerning the correlated electron thrust area at Brookhaven Lab's planned Center for Functional Nanomaterials.

The day opened with a presentation on quantum confinement effects in thin films by Tai Chiang from the University of Illinois. Chiang's group carried out angle-resolved photoemission measurements on thin films of lead and determined a direct correlation between the thermal stability of layers of different thickness and the binding energy of the highest occupied orbital. This stability arises from the confinement of the electron in the direction perpendicular to the film and turns out to be periodic in the layer thickness. This periodicity can be thought of as a one-dimensional analogue of the periodic table.

The next speaker was Seamus Davis from Cornell University, who presented scanning tunneling spectroscopy (STS) measurements of the copper-oxide superconductor Ca_2-xNa_xCuO₂Cl₂. In an earlier study this material was reputed to contain an electronic (i.e. Wigner) crystal, and the current presentation contained additional measurements showing spectral weight modulations at a large binding energy (~ 200 meV). These measurements spurred a continuation of the debate over the definition of the term "Wigner crystal" and what aspect of STS measurements may be thought of as charge order.

After Davis, Ali Yazdani from Princeton University presented similar measurements on underdoped Bi₂Sr₂CaCu₂O_8+δ with trace quantities of zinc. These STS measurements were carried out in the pseudogap state, i.e. T_c < T < T*. Yazdani observed a "checkerboard" pattern that appears to be electronic in origin, disappears when the sample is cooled into the superconducting state, and was speculated to arise from some competing order responsible for the pseudogap. What relation these measurements have to Davis' Wigner crystal, or to the stripe phases seen with neutron scattering, is still a mystery.

After a lunch break, Girsh Blumberg of Bell Laboratories presented Raman measurements of very lightly doped La₂CuO_4+y (LCO), as well as the spin ladder material Sr₁₄Cu₂₄O₄₁, to characterize the nature of the inhomogeneity in these systems. From his measurements he concludes that the inhomogeneity in LCO is actually two dimensional ("giraffe disorder"), rather than one-dimensional ("zebra disorder").

The discussion turned from intrinsic-occurring order to artificial order with a presentation on quantum wires of underdoped YBa₂Cu₃O_6+y by Dale Van Harlingen of the University of Illinois. The Van Harlingen group carried out four-probe transport measurements on such wires and observed quasi-random "telegraph" noise in the IV characteristics. This noise was seen only at temperatures around the pseudogap temperature T*, and was speculated to arise from stripe domains that exist above T_c and thermally switch at random times. Such switching is not seen in optimally doped wires.

A major topic of discussion at the workshop was whether the physics of oxides differs substantially if patterned into nanostructures. Insight into this subject was provided by Antonio Castro-Neto of Boston University, who gave a theoretical perspective on the edge states that might arise at the surface of a (1,0,0) terminated doped Mott insulator. In a study based on density matrix renormalization group techniques, carried out in collaboration with Stephen White of the University of California at Irvine, Castro-Neto concluded that static stripes form near a terminated surface and, with increasing U, spread out from the bulk and move to the surface. These effects may be important for the properties of transition metal oxide devices.

The session closed with a presentation by Satoshi Okamoto of Columbia University on the subject of electronic reconstruction in oxides with reduced dimensions, such as at surfaces and interfaces in artificial structures. Okamoto carried out a computational study of the interface between LaTiO₃ (a Mott insulator) and SrTiO₃ (a band insulator), and found that the interface between the two is, in fact, highly metallic and exhibits orbital ordering. The precise form of the orbital ordering depends on the thickness of the interface and the geometry of the structure. Such effects may have already been seen by groups at Bell Labs and in Korea.

ACKNOWLEDGEMENTS
The workshop was sponsored by the Users' Executive Committee of the National Synchrotron Light Source and Gammadata Scienta, the maker of Scienta electron analyzers for angle-resolved photoemission experiments. Brookhaven National Laboratory is supported by the U.S. DOE Contract No. DE-AC02-98CH10886.

FOR MORE INFORMATION
Peter Abbamonte
National Synchrotron Light Source
Brookhaven National Laboratory
Upton, NY Email: abbamonte@bnl.gov

Peter Johnson
Department of Physics
Brookhaven National Laboratory
Upton, NY
Email: pdj@bnl.gov