Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H23: Optical and Spectroscopic Properties and Nano |
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Sponsoring Units: FIAP DCMP Chair: Stefano Curtarolo, Duke University Room: 325 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H23.00001: Photoemission core level shifts and phonon broadening for Cs film on Cu(100) Xubing Zhou, Kevin Koch, J.L. Erskine Department of Physics, University of Texas at Austin, Austin, TX 78712. We have measured Cs 5p photoemission core level broadening data for Cs film on Cu(100) at different temperatures and have obtained bulk-atom and surface-atom values of the zero-temperature phonon width and the effective Debye temperature, which governs the temperature dependence of the broadening. The coupling constant $C$ for alkali metals in the phonon-broadening theory of Hedin and Rosengren[1] is about 30{\%} higher in the surface than in the bulk. This result is compared with experiment results on other alkali metals[2]. We also studied the temperature dependent binding energy shifts for the Cs 5p core level peaks and our data fit the lattice expansion theory very well except at temperature higher than 220 K. The high temperature deviations are proved to be caused by thermal evaporation of Cs film. [1] L. Hedin and A. Rosengren, J. Phys. F \textbf{7}, 1339 (1977). [2] D.M. Riffe and G.K. Wertheim, Phys. Rev. B 61, 2302 (2000). [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H23.00002: Auger-Photoelectron Coincidence Spectroscopy measurement of the secondary electron distribution from 0 eV to 81 eV, created by the MVV Auger transition in Cu (100) K. Shastry, S. Mukherjee, A.H. Weiss, S.L. Hulbert, R.A. Bartynski In conventional spectroscopic measurements, low energy Auger lines are superimposed upon a large background due to secondary electrons that arise from loss processes that are unrelated to the Auger process. Here we present the results of measurements in which Auger-Photoelectron coincidence techniques were used to eliminate background unrelated to the Auger process and obtain the energy distribution of electrons emitted as a result of the M$_{23}$VV transition in Cu (100) over the full range of emitted energies (0 eV -- 81 eV). The measurements revealed a well formed Auger peak at $\sim $60eV accompanied by a low energy tail (LET) associated with the MVV transition. The LET extends to 0 eV and has a broad maximum at $\sim $ 6eV. The integrated intensity of the LET was $\sim $ 6 times larger than that of the Auger peak itself. The origin of the LET will be discussed in terms of extrinsic mechanisms in which electrons from the peak lose energy as they propagate to the sample surface, as well as intrinsic mechanisms in which multi-electron Auger processes distribute the energy gained by the filling of the core-hole to multiple electrons. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H23.00003: Ultrafast Spectroscopy on Solids at FLASH David Bernstein, Yves Acremann, Andreas Scherz, Martin Beye, Alexander F\"{o}hlisch, William Schlotter, Torbin Beeck, Florian Sorgenfrei, Annette Pietzsch, Wilfried Wurth, Joachim St\"{o}hr X-ray/VUV free electron laser (FEL) facilities such as FLASH, LCLS, and the European X-FEL open the door to a wide variety of exciting experiments in x-ray physics. Due to the random stochastic processes governing FEL radiation and the difficulties in tuning an FEL, it has not been clear whether spectroscopy could be done using such sources. Here we demonstrate the feasibility of doing near edge x-ray absorption fine structure (NEXAFS) spectroscopy on solids. Samples consisting of LaMnO and Al films, respectively, were lithographically fabricated on thin silicon nitride membranes. Ultrafast femtosecond pulses of radiation from the FLASH FEL were dispersed by the monochromator grating at beamline PG2 and impinged upon the samples. Absorption was measured in transmission using a Ce:YAG crystal and imaged by an intensified CCD. The incident intensity was measured through a blank nitride membrane next to the sample. By tuning the FEL to the La N-edge ($\sim{}$102eV) and the Al L-edge ($\sim{}$72eV), respectively, we take an entire NEXAFS absorption spectrum in each shot. Spectra are calculated using many shots in order to reduce statistical uncertainties. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H23.00004: STM study of CeTe$_{3}$: contribution of the subsurface lattice Aleksandra Tomic, Josh Veazey, Zsolt Rak, Christos Malliakas, Mercouri Kanatzidis, S. D. Mahanti, Stuart Tessmer We have studied the nature of the surface charge distribution in CeTe$_{3}$ with scanning tunneling microscopy (STM). At 77 K, the STM topography and Fourier transform show both the atomic lattice of surface Te atoms arranged in a square net and the CDW modulations oriented at 45 degrees with respect to the Te net. In addition, we observe peaks in the Fourier transform that we attribute to atoms lying below the surface Te net. We discuss the possibility of both subsurface Ce and Te as giving rise to this signal; density functional theory calculations indicate that the subsurface Ce atom gives a more significant contribution to the overall tunneling current. [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H23.00005: Effect of a Fe substitutional impurity on the geometric and electronic structure of Au$_{13}$ cluster Ghazal Shafai, Talat Rahman We have carried out spin-polarized density functional theory calculations based on the pseudopotential method to determine the changes in the characteristics of the Au$_{13}$ cluster when one Au atom is replaced by Fe. For a pure Au$_{13}$ cluster, the 2D geometry is the lowest-energy isomer, followed closely by a flake structure, while the icosahedron is higher in energy by 2.98 eV and is not stable since it is found to undergo Mackay transition to form a cuboctahedron. When a surface or central Au atom is replaced by Fe, we find dramatic changes in the energy ordering of these nanoparticles, since Fe tries to move inwards so as to be highly coordinated. In fact the distorted icosahedron and a biplanar structure obtain the lowest energy. The structure of the Fe-centered icosahedron is slightly distorted (Jahn-Teller distortion), so that the degeneracy on two bands near Fermi level is removed. The lowest energy isomer in this study has the highest magnetic moment (3.98 $\mu _{B})$ in comparison with that of the other isomers. The magnetic moment of the icosahedron with an Fe atom at the center is 3.1 $\mu _{B}$, which is in in agreement with previous findings. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H23.00006: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H23.00007: Size-dependent crystallinity of nano-Pt/$\gamma $-Al$_{2}$O$_{3}$ Long Li, L.-L. Wang, S. I. Sanchez, J. H. Kang, Q. Wang, Z. Zhang, A. I. Frenkel, R. G. Nuzzo, D. D. Johnson, J. C. Yang Metallic platinum nanoparticles (NP) on $\gamma $-Al$_{2}$O$_{3 }$powders were synthesized with a size range from sub- to several nanometers. High-resolution transmission electron microscopy (HRTEM) studies revealed a size-dependent crystallinity of the Pt NPs, where Pt NPs with size $<$1 nm had a disordered structure, Pt NPs with size $>$2.5 nm all showed a crystalline structure. For Pt NPs with sizes between 1.1 and 2.4 nm, a transition zone exists in which $\sim $85{\%} of NPs appeared disordered and $\sim $15{\%} ordered. X-ray absorption spectroscopy (XAS) measurements support this result where increasing-disorder distribution of Pt-Pt bond lengths was noted with decreasing Pt nanoparticle size. A search for ground state structure of Pt37/$\gamma $-Al$_{2}$O$_{3}$ (100) with density functional theory (DFT) showed that the disordered structure is energetically more favorable than the ordered close-packed structures by 1.53 eV at the size of 1.1 nm. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H23.00008: Size Distribution of Nano-Crystallites in Non-Crystalline Binary Alloys Yong W. Kim, Andrew Abraham, Jerry Kim The factors that affect thermophysical property determination for non-crystalline metallic alloys include non-uniformities in compositional and morphological property within a specimen. A series of measurements have shown that a specimen's thermal history leads to a spatial profile of elemental composition that differ from one specimen to another for the same alloy.(See Y.W. Kim, \textit{Int. J. Thermophysics }\textbf{28, }732 (2007), and references therein.) In order to develop a theoretical model for temperature dependence of thermophysical properties, we consider thermal dissociation of nano-crystallites within a randomly close-packed (RCP) medium. Once the size distribution of the nano-crystallites has been established at a given temperature, a set of coupled dissociation equations can be solved at all other temperatures. Transport properties can then be computed over a range of temperature. In this paper we present the size distribution of crystallites in a bed of RCP spheres. Two different size spheres are mixed at several compositions to simulate non-crystalline binary alloys in 2-D. The distribution is found to be peaked at a crystallite size specific to a given alloy composition. This work is support in part by the NSF-DMR(Metals). [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H23.00009: Irreversibity in Cooling and Heating Processes in the Magnetocaloric MnAs and Alloys A.L. Lima Sharma, S. Gama, A.A. Coelho Irreversibility of adiabatic processes in the magnetocaloric MnAs and alloys are presented here. We used a differential scanning calorimeter in order to record the heat flux as a function of the temperature and applied field for MnAs, Mn$_{0.994}$Fe$_{0.006}$As and Mn$_{0.994}$Cu$_{0.006}$As. From the measured heat flux, we extracted the latent heat and entropy associated to cooling and heating processes. In the cooling curve, we observed that $S^{c}_{Mn} > S^{c}_{Fe} > S^{c}_{Cu}$, the index $c$ refers to cooling process, similarly, for the heating process: $S^{h}_{Fe}$\textit{ $\approx $ S}$^{h}_{Mn} > S^{h}_{Cu}.$ On the doped samples, the thermomagnetic behavior is compatible with a scenary where Zener's $p-d $exchange mechanism dominates, i.e. the interaction range is weaker but long ranged, because the extended valence hole states mediate the ferromagnetic interaction. The difference of the entropy obtained from cooling and heating process was found to be as high as 37{\%}. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H23.00010: Semiconductor behavior in bcc Cr$_{1-x}$Al$_{x}$ thin films Z. Boekelheide, F. Hellman Cr$_{1-x}$Al$_{x}$, with x = 0.15-0.25, has semiconducting electronic properties: extremely high resistivity and a negative temperature coefficient of resistance, along with a gap in the infrared reflectivity. This is unusual for an alloy of two metals, but similar behavior has been observed in Fe$_{2}$VAl and has been attributed to a hybridization-induced band gap. In bulk, Cr$_{1-x}$Al$_{x}$ is known to be inhomogeneous, with two crystal phases coexisting: one, a disordered bcc solid solution, and the other, called the ``X-phase'', with a microstructure consisting of 1-3nm domains. Because of this inhomogeneity, it was previously not known which phase was responsible for the semiconducting behavior. We used epitaxial thin film growth techniques to preferentially nucleate the bcc phase and study the effect of crystal structure on the electronic properties. We found that films grown epitaxially on MgO (100) substrates, expected to grow preferentially in the bcc phase, have high resistivity like bulk Cr$_{1-x}$Al$_{x}$, while polycrystalline films grown on amorphous SiO$_{2}$ have lower resistivity. This suggests that the semiconducting behavior is intrinsic to the bcc structure. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H23.00011: Exciton Recombination in Nanometer-Wide GaN/AlN Quantum Wells Zhenwen Pan, Madalina Furis, Alexander N. Cartwright, William J. Schaff In nitride semiconductor heterostructures, the presence of very strong built-in electric fields, oriented perpendicular to the semiconductor layers, dramatically impacts the electronic states, excitonic recombination, and photoluminescence in these materials. The origins of these fields lie in the non-centro- symmetric character and the strong piezoelectricity of the heterostructures. We investigated electronic states in the presence of strong built-in fields ($\sim$ 5 MV/cm) in very narrow, nanometer-wide GaN/AlN quantum wells via time-resolved photoluminescence spectroscopy. We find that the strong confinement ($\sim$ 2eV in the conduction band) leads to significant overlap in the electron and hole wavefunctions, even in the presence of large built-in fields. The temperature dependence of radiative lifetimes and emission energies indicates the band-edge recombination contributions (i. e. excitonic and/or shallow -acceptor pair) dominate the PL spectrum. Wells narrower than 3 monolayers exhibit temperature- independent emission and 1 ns radiative lifetimes. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H23.00012: Enhancement of Subband Effective Mass in Ag/Ge(111) Thin Film Quantum Wells Shu-Jung Tang, Wen-Kai Chang, Yu-Mei Chiu, Hsin-Yi Chen, Cheng-Maw Cheng, Ku-Ding Tsuei, Tom Miller, Tai-Chang Chiang Subband dispersions of quantum-well states in Ag films on Ge(111) have been determined by angle-resolved photoemission. The effective masses of the subbands at the zone center increase substantially for decreasing film thicknesses. This peculiar behavior is attributed to a kinetic constraint for standing wave formation governed by a momentum-dependent phase shift function. No evidence is found for in-plane electron localization within the confined geometry. [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H23.00013: Structure and Electrical Properties of Electrochemical Anodized Valve Metal Wenbin Fan, David Kirkwood, Jiwei Lu, Stuart Wolf Localized electrochemical anodization has been used to prepare metal tunnel junctions. The room-temperature I-V characteristics of anodized Vanadium, Tantalum and Titanium ultra small wires were studied. The nonlinear I-V curves indicate they behave as tunnel junctions. The resistance and tunneling characteristics are strongly determined by the details of anodization process. The High Resolution Transmission Electron Microscope (HRTEM) is used to explore the structure of the anodized Ti and V films and we found there are some crystaline grains on the bottom of the V film and in the center of Ti film. The grain size and the distance between two grains are changed by how the anodization process was terminated. Low temperature electrical properties of anodized films will be reported. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H23.00014: Direct writing of hafnium diboride metallic nanostructures on silicon (100) surfaces using a UHV-STM Wei Ye, Pamela Martin, Navneet Kumar, John Abelson, Greg Girolomi, Angus Rockett, Joseph Lyding The patterning of metallic nanostructures on surfaces is of great interest in fabricating nanoelectronics and quantum devices. In this work, we deposited HfB$_{2}$ nanostructures on silicon surfaces from Hf(BH$_{4})_{4}$ by electron beam induced deposition (EBID). At positive sample bias, the electron beam from a STM probe initiates the local CVD by the decomposition of Hf(BH$_{4})_{4}$ under STM tip. By repeatedly scanning STM tip along a specific path, well-defined HfB$_{2 }$nanostructures can be directly written onto the surface. Scanning tunneling spectroscopy was used to characterize the electronic properties of the nanostructures. We have achieved 4 nm linewidths and complete selectivity relative to adjacent H-Si(100) regions. The thickness of the nanostructures is controlled by the exposing time to the electron beam from STM tip, while the width is controlled only by the geometry of the tip apex and the sample-tip separation. STS data confirm that the HfB$_{2}$ nanostructures deposited are pure metallic, indicating minimum contaminations in the nanostructures, which we attribute to the carbon-free nature of the CVD precursor. To our knowledge this is the first demonstration of sub-5 nm metallic nanostructures in a STM/CVD experiment. [Preview Abstract] |
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