ram seshadri group: research & publications

logorrhœa, logorrhea: Excessive volubility accompanying some forms of mental illness; also gen., an excessive flow of words, prolixity. So logorrhœic, logorrhœtic adjs. OED

"Another damned, thick, square, book! Always scribble, scribble, scribble, Eh Mr. Gibbon?" (William Henry, Duke of Gloucester, upon receiving the second volume from the author, 1781)

7. Wovon man nicht sprechen kann, darüber muß man schweigen (The last chapter of Wittgenstein's Logisch-Philosophische Abhandlung)

"What causes hesitation is the fact that, after all, Mr Wittgenstein manages to say a good deal about what cannot be said" (Russell, in his introduction to the above)

publications as PDF

By clicking on the DOI links in the PDF file, you may be able to download the paper (depending on your domain). If that does not work, please email.

some of our research areas (updated March 2008)

Insulating magnets, magnetic frustration, magnetocapacitance

	The spinels CoB2O4 with magnetic Co2+ ions on the diamond lattice A site can be frustrated because of competing near-neighbor (J1) and next-near neighbor (J2) interactions. We attempt to tune the relative strengths of these interactions by substitution on the non-magnetic B-site. The system we employ is CoAl2-xGaxO4, where Al is systematically replaced by the larger Ga, ostensibly on the B site. Shown here is the temperature dependence of the scaled zero-field cooled magnetic susceptibility of the different spinel samples. Data were acquired under a 100 Oe field. Plotted this way, samples with purely antiferromagnetic interactions are separated from samples with other kinds of interactions. Our goal is to drive complex magnetic ordering via frustration, and then to look coupling between magnetism and capacitance in the resulting ground states.
Personnel: Brent Melot, Jennifer Drews, Ivana Sabaj
Representative publications: Phys. Rev. B 76 (2007) 024409(1-6) & Phys. Rev. B 74 (2006) 024413(1-6)
Collaborators: Leon Balents, Gavin Lawes, Art Ramirez
Support: NSF Career

Composite magnets, interfaces

Exchange biased composites of ferromagnetic single-domain Ni nanoparticles embedded within large grains of MnO have been prepared by reduction of NixMn1-xO4 phases in flowing hydrogen. The Ni precipitates are 15-30 nm in extent, and the majority are completely encased within the MnO matrix. The manner in which the Ni nanoparticles are spontaneously formed imparts a high ferromagnetic-antiferromagnetic interface/volume ratio, which results in substantial exchange bias effects. Exchange bias fields of up to 100 Oe are observed, in cases where the starting Ni content x in the precursor phase is small. For particles of approximately the same size, the exchange bias leads to significant hardening of the magnetization, with the coercive field scaling nearly linearly with the exchange bias field.
Personnel: Daniel Shoemaker, Ivana Sabaj, Madeleine Grossman
Representative publications: J. Phys. C 20(2008) 195219(1-9)
Support: ACS PRF

Functional nanomaterials

	A systematic study of the effect of reduction on vanadium oxide nanoscrolls has been performed, resulting in the preparation of a range of interesting and potentially useful materials. The accompanying image displays (top) the electrical resistivity of a pressed pellet of VO2 nanoparticles, and (bottom) the results of thermodiffraction where the insulator-metal transition and the accompanying structural change is monitored by following the change in the cell volume of tetragonal VO2.
Personnel: Dr. Serena Corr, Madeleine Grossman
Representative publications:
Collaborators: Galen Stucky
Support: UC Discovery

Polar oxides: Size dependence of local structure

We have investigated the paraelectric-to-ferroelectric phase transition of various sizes of nanocrystalline barium titanate (BaTiO3) using temperature-dependent Raman spectroscopy and powder X-ray diffraction (XRD). Synchrotron X-ray scattering has been used to elucidate the room temperature structures of particles of different sizes using both Rietveld refinement and pair distribution function (PDF) analysis. The image shows PDF fits of the total X-ray scattering for the different particles. Circles correspond to the experimental PDFs and the fits are red lines through the data. The difference curves are displayed in each panel, and have been offset for clarity. Rietveld and PDF analysis suggest increased distortions with decreasing particle size, albeit in conjunction with a tendency to a cubic average structure. These results suggest that while structural distortions are robust to changes in particle size, what is affected is the coherency of the distortions, which is decreased in the smaller particles. Other related projects include an examination of the local structure of perovskite oxynitrides, and the effects of Nb-substitution on the B site of SrTiO3 and BaTiO3.
Personnel: Katharine Page
Representative publications: Chem. Mater. 19 (2007) 4037-4042 & J. Am. Chem. Soc. 130 (2008) 6955-6963
Support: NSF Graduate Student Fellowship to KP
Collaboration: Louis Brus, P. M. Woodward, T. Kolodiazhnyi

ZnO structure and polarization

	The neutron PDF analysis of Zn0.875Mg0.125O using a 2x2x1 supercell model containing Zn7MgO8 shows that the local geometry of MgO4 fragments concurs with the density functional theory (DFT) based structure relaxation of hexagonal wurtzite MgO. The MgO4 tetrahedra are markedly compressed along the c-axis, are are smaller in volume than ZnO4 units by around 6%. Mg atoms in Zn0.875Mg0.125O have a shorter bond to the c-axial oxygen than to the three lateral oxygens, which is distinct from the Zn coordination. These results have profound implications for the design of Polarization Field-Effect Transistors (PolFETs) and other devices, including 2DEGs at the interface between ZnO and Zn1-xMgxO. We find that in contrast to assumptions in the literature as well as DFT results, Mg-substitution decreases the static polarization of ZnO.
Personnel: Dr. Young-Il Kim
Representative publications: Phys. Rev. B 76 (2007) 115204(1-10) & Appl. Phys. Lett. 90 (2007) 101904(1-3)
Collaborators: David Clarke, Brad Chmelka
Support: NSF MRSEC

Catalytic materials

As possible substitute materials for platinum group metal heterogeneous catalysts, high surface area carbides of the early transition metals Mo and W are of great interest. We havve reported nanostructured, high surface area Mo2C and WC prepared by decomposing and carburizing ammonium paramolybdate and ammonium paratungstate in flowing 50%CH4/50%H2. Surface areas as high as 52 m2/g for Mo2C and 24 m2/g for WC were obtained, with both structures crystallizing in structures appropriate for catalytic activity. We have studied these materials using a combination of neutron diffraction Rietveld refinement, X-ray photoelectron spectroscopy, surface area measurements, and scanning transmission electron microscopy. In addition, we have used pair distribution function (PDF) analysis of the neutron total scattering data as a means of establishing the presence of graphitic carbon in the as-prepared materials. The accompanying images shows XRD pattterns of WC particles of different sizes. We are also extensively investigating perovskite and related oxides as hosts for noble (PGM) metals, as a way of reducing PGM use in oxidative catalysis.
Personnel: Jun Li, Joshua Kurzman
Representative publications: Chem. Mater. 19 (2007) 1418-1426 & J. Catal. 249 (2007) 349-358.
Support: DOE
Collaboration: Susannah Scott

Phosphors

	We are contributing to Solid State Lighting efforts at UCSB by researching new phosphor materials for white lighting applications. Here, a new yellow-emitting phosphor is reported that displays a peak in the excitation at 450 nm and a peak in the emission at 556 nm. When this phosphor is pumped by a blue InGaN light-emitting diode, (450 nm excitation) we obtain white light with color rendering index (Ra) between 81 and 85 and color temperatures between 4200 K to 5500 K, suggesting this material is competitive as a blue-pumped yellow phosphor for white lighting applications. A patent is pending on this new phosphor material.
Personnel: Dr. Won-Bin Im
Representative publications:
Collaborators: Steve DenBaars
Support: SSLEC