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  • THE ELECTRONIC ALADDIN NEWSLETTER NO. 34

    October 2003


    1. Evidence Found for Band-like Nature of 5f Shell in USb2

    Heavy fermion materials have prompted twenty years of intense activity and even more intense debate on the proper model to describe the electronic structure near the Fermi level.  The term heavy fermion refers to material whose electronic properties suggest that the conduction electrons at the Fermi level have a very large effective mass. These materials are primarily  compounds with elements having unfilled 4f shells (rare earths) or 5f shells (actinides). The heavy fermion ground state often competes with an antiferromagnetic ground state. By studying both types of ground states in related materials one develops a better understanding of the behavior of f-electrons in the solid state.

    It is generally agreed that it is the f-electrons that are responsible for the unusual properties found in heavy fermion materials and closely related magnetic ground states.  The question is what is the proper description of the f electrons. One theory depicts the f-electrons as non-interacting impurities which are screened by a cloud of conduction electrons resulting in heavy masses but no magnetic moment. A competing ground state to the heavy fermion state is the antiferromagnetic state where the magnetic moments of each f-electron are arranged alternately in an ordered arrangement where the magnetic moment on adjacent sites cancel one another. An alternate description has f-electrons pictured as being central to the formation of well-defined, very narrow bands over a wide temperature range. Treating the f-electrons as bands rather than impurities in a solid has a profound effect on our interpretation of the electronic properties for these materials.

    A Los Alamos team led by John Joyce, and Cliff Olson, Ames Lab, have been investigating high quality single crystals of USb2  using high resolution ARPES techniques at SRC. They have recently discovered that there is a very narrow (less than 5meV natural width) state just below the Fermi level of uranium f-d character.  Away from the zone center the state disperses about 15 meV towards EF before splitting and moving deeper.  With increasing temperature the state changes, indicating a phase transition.  This transition was subsequently seen in bulk susceptibility and resistivity  measurements.  This is dramatic evidence of the band-like nature of the 5fs in this antiferromagnetic material.


    2. Nanoparticles Responsive to Environmental Changes

    Nanoscale materials and devices are the subject of wide research interest, particularly semiconductor nanoparticles, celebrated examples of quantum confinement used for color-tunable fluorescent dyes. It has been frequently observed that the crystal structure of nanoparticles of some materials is not the same as the structure observed in macroscopic samples. This is because particle size has consequences on the stability of structures, due to the large ratio of surface area to volume in nanoscale systems.

    Jill Banfield and members of her group at UC-Berkeley have now shown that changing the environment of surface atoms of zinc sulfide (ZnS) nanoparticles can drive a structural transformation in the whole particle at room temperature. Uncoated 3 nm ZnS nanoparticles synthesized in methanol are seen by x-ray diffraction (XRD) to possess considerable distortion. Extended x-ray absorption fine structure (EXAFS) measurements were made of ZnS nanoparticles coated with methanol, after thermal desorption of surface methanol, and after rewetting with methanol. Wide-angle x-ray scattering (WAXS) measurements were made of the nanoparticles after water was added to the original suspension in methanol.

    By manipulating the surface environment in these ways, and following the subsequent structural changes with synchrotron radiation techniques carried out at the SRC and other light sources, the group made two important observations: 1) the EXAFS measurements showed that methanol desorption causes a reversible structural modification - the original structure is regained when methanol is replaced; 2) wide-angle x-ray scattering (WAXS) measurements observed profound, irreversible structural transition associated with the addition of water, and probably due to surface interactions.These are believed to be the first surface-driven room temperature transitions observed in nanoparticles. The results indicate that nanoparticle structure is not kinetically trapped, but responsive to environmental changes. If alternative ligands or solvents can be found that stabilize alternative structures, there may be post-synthesis routes to uncommon structures. Given the strong influence of water, the structure and properties of environmental, technological, or extra-planetary nanoparticles may depend on the extent of hydration. In addition, any nanoscale material or component may be susceptible to unpredictable structural change if exposed to water, or other adsorbates. The results were published in the August 28th edition of Nature. (Nature 424, 1025, 2003.)

     


    3. 2003 SRC Open House Attracts New Visitors

    On Sunday, September 14th, 191 members of the general public, including 60 children, attended the 2003 SRC Open House. Visitors came from over 18 surrounding communities to participate in demonstrations and activities, listen to presentations about the SRC, and hear researchers talk about their work. Nearly twenty staff members and Users volunteered their time to make the Open House a great success.

    Similar to previous years (2000, 2001, and 2002), visitors were assigned tour times and, while waiting for their tour, participated in demonstrations and activities related to the science on how our facility works and how research is conducted by Users.  Older students had the opportunity to fill out a worksheet about the SRC and the youngest visitors could color in an activity sheet.

    The tour consisted of 6 stops, each taking about 10 minutes.  During the first two stops, visitors learned about the SRC, properties of light and electrons, and the kinds of research that goes on at the SRC.  The next three stops around the ring involved presentations from researchers on their work.  This year they included:  1) "Tomorrow's Technology Today” by Don Thielman of the Center for NanoTechnology; 2) “Chronic Wasting Disease” by Chris Johnson, a UW Scientist; and 3) “Mud in Microbes” by Lynn Diener and Will Bleam, UW Scientists.  The last stop was an “experts” table where visitors were given the opportunity to ask more questions and fill out an evaluation form. Comments about the Open House included: “I am so glad we have this in our community,” “Really appreciate all the people taking their Sunday to give us some insights as to what SRC is all about,” and “Very interesting, well done and geared to kids very nicely. Thank you!”  The next Open House will take place in the Spring of 2005.


    4. SRI Conference Review

    The 8th International Synchrotron Radiation Instrumentation Conference (SRI 2003) was held at the Yerba Buena Conference Center in downtown San Francisco August 25-29, 2003.  The conference was well attended with almost 600 papers presented by representatives of the worldwide synchrotron community. There were daily plenary sessions followed by parallel talk sessions and a poster session. The SRC presented five posters during the conference.   

    The hot topics in sources were free electron lasers (FELs), and X-ray FELs along with much talk about advances to insertion devices.  Spectromicroscopy, condensed matter physics, MCD, coherence, ultrafast experiments, and environmental studies were topics that generated a great deal of discussion as they are recognized as some of the most important research areas in the synchrotron community. All of these topics are related to creating the smallest spot size possible so that smaller and smaller parts of the sample can be investigated.  Less than 0.1 micron is routine.  This is accomplished through a variety of methods including zone plates, K-B pairs, and capillaries. 

    There were many new beamline designs presented as talks and posters. World-wide there are several being built for microscopy, including the VLS-PGM at SRC.  New types of mirrors, imaging systems, zone plates, and optical test equipment also were presented.  The conference ended with tours on Friday to ALS and SLAC.


    5. SRC Users’ Meeting to be Held on October 25, 2003

    The 36th annual SRC Users' Meeting will take place on Saturday, October 25th. John Joyce of Los Alamos National Laboratory will be the program chair. The deadline for early registration is October 15th. The program includes oral sessions, the Users' business meeting, a poster session, a Saturday night banquet, the Aladdin Lamp Award presentation and an award for best poster. The winner will receive an award at the end of the meeting on Saturday. All Users are strongly encouraged to present a poster of their current research and to submit an abstract about their recent work at the SRC. You must be present at the meeting to have a poster displayed.  Registration for the Users Meeting is available online on the SRC website ( www.src.wisc.edu/meetings/SRC_UM2003/ ) or can be sent by fax or mail.


    6. Notes:

    Congratulations to Jason Crain who was awarded the prestigious NRC Postdoctoral Fellowship to work at NIST. Jason was a graduate student at the UW-Madison Department of Physics under the direction of Franz Himpsel.

    Emeritus Professor and SRC Education Director James Taylor was appointed as Interim Director of the Antarctic Astronomy and Astrophysics Research Institute (A3RI) by Dean Cadwallader on June 15, 2003. A3RI is the umbrella organization for IceCube and AMANDA, the research projects designed to detect neutrinos in the South polar ice cap. The IceCube project is the largest NSF funded project at UW-Madison.