Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D14: Focus Session: Engineered Group IV Clathrates and Clusters |
Hide Abstracts |
Sponsoring Units: FIAP Chair: Jan Gryko, Jacksonville State University Room: LACC 403B |
Monday, March 21, 2005 2:30PM - 2:42PM |
D14.00001: A nuclear inelastic and nuclear forward scattering study of Eu$_{8}$Ga$_{16}$Ge$_{30}$ Rapha\"el P. Hermann, Veerle Keppens, Fernande Grandjean, Olaf Leupold, Rudolf R\"uffer, George S. Nolas, Gary J. Long The type-I filled germanium clathrates contain ``rattling" atoms in oversized atomic cages. This rattling provides the phonon glass behavior required for efficient thermoelectric materials. We have investigated the lattice dynamics in these compounds by nuclear inelastic scattering on $^{151}$Eu in Eu$_{8}$Ga$_{16}$Ge$_{30}$ and obtained the weighted partial phonon density of states for the europium guests. Einstein oscillator energies of 3.8$\pm$0.3 and 6.7$\pm$0.2 meV have been obtained for the europium guests in the larger and smaller cages, respectively. The nuclear forward scattering measurements have yielded information about the magnetic behavior of the europium guests and provide insight in the tunneling dynamics of the guest located in the larger cage. These results will be compared to resonant ultrasound spectroscopy measurements. [The European Synchrotron Radiation Facility is acknowledged for provision of synchrotron radiation facility at the beamline ID22n, the RUS work was supported by the National Science Foundation] [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D14.00002: Chemical synthesis of type I clathrates Jan Gryko, Sharon Brooks, Mary Millwood We report synthesis of type I clathrates in a metathesis reaction between alkali metal (Na, K, Rb, and Cs) silicides (or germanides) and ammonium halides. In this reaction, clathrates such as Na$_{8}$Si$_{46}$ are formed. The byproducts are alkali halides, ammonia, and hydrogen. The purity of the clathrate depends strongly on pressure and temperature, with the best product formed at t = 300 $^{o}$C and pressures 20 -- 30 atm. In the same conditions, lithium silicides form only amorphous/nanocrystalline silicon (or germanium). The most interesting aspect of this approach is synthesis of mixed, silicon/germanium clathrates. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D14.00003: Dilute Fe-doping and Magnetic Properties of Type-I Germanium Clathrates Yang Li, Joseph H. Ross, Jr. We have prepared samples of Ba$_{8}$Ge$_{30}$Ga$_{16-x}$Fe$_{x}$ with $x \leq$ 1. Fe substitutes on the type-I clathrate framework, with a lattice parameter decreasing with $x$. Ferromagnetic behavior was observed, with $T_{c}$ = 65 K for $x$ = 1, decreasing linearly to 58 K at $x$ = 0.2. Fe exhibits a low-spin moment: for example 1.9 $\mu _{B}$ per Fe for $x$ = 1, from a Curie fit above $T_{c}$. The field-cooled and zero-field-cooled magnetization exhibit a pronounced divergence below 60 K, and at 2 K the saturation magnetization is only 0.75 $\mu _{B}$ per Fe in a field of 7 T. These results suggests a noncollinear spin configuration. In ac susceptibility measurements, $\chi\prime$ and $\chi\prime\prime$ become frequency dependent below $T_{c}$, however with very little peak shift, rather different from the spin-glass behavior we previously observed in the more highly Fe-doped chiral clathrate. We conclude that the transition near 65 K is ferrimagnetic. At these doping levels, Fe occupancy is well below the percolation threshold. It is thus likely that the mechanism for magnetic ordering is conduction-electron mediation, as in diluted magnetic semiconductors. This work was supported by the Robert A. Welch Foundation, and by the NSF (DMR-0103455). [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D14.00004: NMR Study of Atomic Hopping in Type-I Sr-Ga-Ge Clathrate Weiping Gou, Yang Li, Ji Chi, Joseph H. Ross, Jr., M. Beekman, J. Martin, G.S. Nolas Type-I Sr$_{8}$Ga$_{16}$Ge$_{30}$ clathrate exhibits glass-like thermal conductivity at low temperatures, attributed to the ``rattling'' of Sr ions in the large cages of the clathrate lattice. We measured $^{71}$Ga NMR down to 1.9 K in order to study the low-temperature dynamics. $T_{2}$ measurements are indicative of motion at low temperatures, while lineshape broadening at low temperatures could be fit to an activated dynamics, with an activation barrier of 7 K. These changes are consistent with motional narrowing, however they imply a wide range of hopping timescales, including the ms range. This is orders of magnitude slower than tunneling rates implied by the known Sr-ion displacement parameters in a symmetric potential well. However, disorder-induced asymmetry can induce activated behavior and timescales consistent with the observations. To further investigate this behavior, we made a series of Carr-Purcell-Meiboom-Gill NMR measurements, which could be fit to a hopping model with an activation energy of 4.5 K, consistent with the lineshape result. Thus we understand the atomic hopping to be strongly influenced by disorder, presumably due to random Ga site occupation. This work was supported by the Robert A. Welch Foundation, Grant No. A-1526, and by the NSF (DMR-0103455). [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D14.00005: Roll of dimer formation in crystallization of clathrate I: from clusters to crystal Kazuo Tsumuraya, Takeharu Aoi, Toshihiko Ogura Silicon clathrates are metastable and have been synthesized only when alkaline or alkaline earth metal atoms coexist as electron donors or VIa or VIIa atoms coexist as electron acceptors. These guest atoms locate in the cages of the crystals. Now, however, the roll of the guest atoms is unclear in the crystallization processes. We investigate it through investigating the coagulation processes between the cage clusters. We use ab initio electronic structure analysis in real space and the one in momentum space. The formation of dimers between the guest atoms is found to be essential in the synthesis. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D14.00006: Guest displacements in silicon clathrate II Hiroyuki Takenaka, Kazuo Tsumuraya Silicon clathrates are one of the candidates of the high performance thermoelectric materials due to their rattling effect of atoms in the cages. The clathrate II consists of Si20 and Si28 cages and the clathrate I Si20 and Si24 cages. Thus the clathrate II may have lower thermal conductivity than clathrate I. We investigate the stable position of the guest Na atom in the Si28 cage and the effect of nearest coordinated Na atoms in Si20 or Si28 cages on the displacements of Na atom in Si28 cage using the ab inito planewave method with pseudopotentials. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D14.00007: Structural, transport, magnetic and thermal properties of type I \& II clathrates Invited Speaker: George Nolas Compounds with the clathrate-hydrate crystal structure possess interesting physical properties that are directly related to their structural and chemical properties. Several compositional and stoichiometric variations can be synthesized, particularly in type II clathrates, resulting in a rich assortment of interesting properties that are only now being brought to light, in part due to their interest for potential technological applications in thermoelectrics, opto-electronics and superconductivity. I will present an overview of the recent transport properties of these novel materials with an emphasis on structure-property relationships. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D14.00008: Superconductivity and electronic structures of Cu-doped Si and Ge clathrates Liu Yang, Ning Chen, Guohui Cao, Yang Li We present a joint experimental and theoretical study of the superconductivity and electronic structures in type-I Cu-doped silicon clathrates and germanium clathrates. The superconducting critical temperature in Ba$_{8}$Si$_{46-x}$Cu$_{x}$ decrease with copper content increasing. No evidence in Cu-free and Cu-doped germanium clathrates demonstrates a bulk superconducting transition down to 2 K. These results are corroborated by first-principles simulations calculated from the meaning density-functional theory with plane waves and pseudopotentials. The simulation of Cu-doped clathrates has shown that Cu doping result in a decrease of electronic density of states in Fermi level, which might explain Tc decrease with Cu-doping in the BCS frame. Comparing with the electronic structure of Si clathrate, there is a less density of states on Fermi level for the Ge clathrate, which also is explained as the reason for no bulk superconductivity occurring. The theoretical study is supported by experimental investigations. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D14.00009: Novel Silicon-Carbon Nanostructures: Electronic structure study on the stability of Si60C2n Clusters. A. Srinivasan, M.N. Huda, A.K. Ray The formalism of generalized gradient approximation to density functional theory has been used to study the electronic and geometric structures of Si$_{60}$C$_{2n }$fullerene-like nanostructures. In our previous work, we have shown that the additions of carbon atoms increase the stability of smaller silicon cages [1]. In this talk, we will present our results on the addition of two and four carbon atoms on the surface of the Si$_{60}$ cages by substitution as also inside the cage at various symmetry orientations. Full geometry optimizations have been performed using the Hay-Wadt basis set without any symmetry constraints using the Gaussian 03 suite of programs [2]. Binding energies, ionization potentials, electron affinities and the ``band'' gaps of the stable silicon-carbon fullerene like nanostructures will be presented and discussed in detail. In general, we find that the optimized silicon-carbon fullerene-like cages have increased stability compared to the bare Si$_{60 }$cage. Possibilities of adding larger carbon clusters to the Si$_{60 }$structure will also be discussed. *Work supported, in part, by the Welch Foundation, Houston, Texas (Grant No. Y-1525). [1] M. N. Huda and A. K. Ray, Phys. Rev. A \textbf{69}, 011201(R) (2004); Eur. Phys. J. D \textbf{31}, 63 (2004). [2] \textit{Gaussian 03}, M. J. Frisch \textit{et al}. Gaussian Inc., Pittsburgh, PA. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D14.00010: Electronic Structure of Pure Selenium and Tellurium Chains and Selenium Rings and with Impurities N. B. Maharjan, D. D. Paudyal, D. R. Mishra, S. Byahut, Hwa-Suck Cho, R. H. Scheicher (*) , Junho Jeong, T. P. Das (**) We have studied the electronic structures of pure chain-structured Selenium and Tellurium and with chalcogen impurities as well as ring-structured Selenium both pure and with Tellurium impurity atoms. The Hartree-Fock Cluster Theory procedure combined with many-body perturbation theory procedure has been used. The accuracy of the calculated electronic wave functions is tested by the investigation of $^{77}$Se and $^{125}$Te nuclear quadrupole interaction parameters. Good agreement is found with experiment for the pure systems. For the impurity systems, the agreement is reasonable but suggests the need for inclusion of more extensive relaxation around the impurity atoms. (*) Current Address: Dept. of Physics, Uppsala University, Sweden (**) Also: Dept. of Physics, University of Central Florida, Orlando, Florida [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D14.00011: Phonon Sidebands in Nanoscale Systems Connie Chang, James Sethna We calculate phonon sidebands for nanoscale systems in the Coulomb-Blockade regime where one electron at a time tunnels onto and off of the system. Using a combination of quantum chemistry and simple analytic calculations, we determine the relative strengths and the importance of environmental effects on the sidebands. Systems we have studied include the $C_{72}$ (linked buckyballs), short nanotubes, and polyacetylene. In addition, we study molecule size effects on the appearance of phonon sidebands. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700