Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J23: Metals: Actinides and Transport |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Duane Johnson, University of Illinois at Urbana-Champaign Room: Colorado Convention Center 110 |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J23.00001: Evolving Magnetism from self-damage in PuAm alloys S.K. McCall, M.J. Fluss, B.W. Chung, M.W. McElfresh, G.F. Chapline, R.G. Haire As a consequence of the unusual nature of plutonium's electronic structure, point- and extended-defects exhibit extraordinary properties. Low temperature magnetic susceptibility measurements on Pu and PuAm show that the magnetic susceptibility increases as a function of time, yet upon annealing the specimen returns to its initial value. This excess magnetic susceptibility arises from the $\alpha $-decay and U recoil damage cascades which produce vacancy and interstitials as point and extended defects and at low temperatures exceeds 10{\%} of the annealed value after about 1 month of damage accumulation. Isochronal annealing measurements of $\alpha $-Pu and stabilized $\delta $-Pu reveal that the damage is frozen in place below $\sim $30K and completely annealed away above 300K. The binary PuAm alloy follows a similar trend, but after warming to temperatures between 35 and 50K where defects are expected to begin moving, an enormous Curie like magnetic susceptibility arises with a Curie constant approximating 1 $\mu _{B}$/actinide atom. This large effective moment disappears after 60K as further annealing takes place. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J23.00002: Study of Phonons in $\delta $-Plutonium near the $\delta -\alpha $' Structural Phase Transition by X-ray Thermal Diffuse Scattering Ruqing Xu, Joe Wong, Paul Zschack, Hawoong Hong, Tai-Chang Chiang The 5f electrons in Pu can be either bonding or localized, depending sensitively on the temperature, pressure, and impurity doping. As a result, Pu displays a rich phase diagram involving a large number of phases with substantially different atomic volumes. In a recent report of the phonon dispersion curves of Ga-stablized $\delta $-Pu at room temperature and ambient pressure, a pronounced deepening of the TA[111] phonon branch near the L point was discovered. This phonon softening was suggested to be related to a lattice shearing mechanism that could lead to the structural phase transition from the fcc $\delta $ phase to the monoclinic $\alpha $' phase at about 170 K. Here we report our measurements of x-ray thermal diffuse scattering from a $\delta $-Pu crystal (with 0.6 wt{\%} Ga) at temperatures from 307K to 200K. The results show no further softening of the phonons near the L point as the sample temperature decreases. The implications regarding the relationship between the soft mode and the phase transition will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J23.00003: Lattice dynamics of the light actinides Johann Bouchet Despite general interest in $f$-electron elements, details about their phonon-dispersion relationships are very limited. But recently, a new hope has emerged with several works, using inelastic x-ray scattering, mostly on U and Pu. Nevertheless all this experimental issues show that theoretical works are needed to tackle the $f$ electrons systems elastic properties. Unfortunately these calculations are far from being straightforward. Theoretically, the most important problem comes from the difficulty to treat correctly the $f$-electrons and the relativistic effects needed in such heavy materials. The lattice dynamics of $\alpha$-uranium are known for almost 30 years, but until now any theory has been able to successfully reproduce these experimental data. Here we present the first \textit{ab-initio} phonon spectrum of $\alpha$-U[1]. We compare our spectrum obtained at 0 K with the neutron-scattering data obtained at room temperature with a particular attention to its anomalies. Then we predict the behavior of lattice dynamics of uranium as a function of pressure. We have also calculated the phonon spectra and the thermodynamic properties of Th, as the linear thermal expansion or specific heats[2], and the elastic properties of Th, Pa and U. [1]J. Bouchet submitted to Phys. Rev. Letter. [2]J. Bouchet, F. Jollet and G. Zerah, Phys. Rev. B 74 064637 [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J23.00004: Density-Functional Calculations of $\alpha $-Pu-Ga (Al) Alloys Alexander Landa, Per Soderlind, Levente Vitos At atmospheric pressure plutonium metal exhibits six crystal structures. The least dense phase ($\delta $-Pu) has a 25{\%} larger volume than the ground-state ($\alpha $-Pu) phase and is thermodynamically stable at temperatures between 593 and 736 K. In order to extend the stability of $\delta $-Pu to ambient temperatures, plutonium is alloyed with a small amount of so-called '$\delta $-stabilizers', for example, Ga or Al. The $\alpha $-phase has no equilibrium solubility with any of these $\delta $-stabilizers but upon cooling of the $\delta $-Pu-Ga (Al) alloys, under certain conditions, Ga (Al) atoms can be trapped in the $\alpha $-lattice causing an expansion. First-principles methods are employed to study the ground-state atomic volumes of $\alpha $-Pu-Ga (Al) alloys. It was shown that a random distribution of Ga (Al) atoms in the monoclinic lattice of $\alpha $-Pu results in a maximum expansion of this lattice. Any kind of ordering of Ga (Al) on the monoclinic lattice results in shrinking of the lattice constant while the ordered $\alpha _{8}$-(Pu-Ga (Al)) configuration yields the smallest lattice constant which is very close to that of pure $\alpha $-Pu. In addition, energetics of the ordered and disordered configurations is discussed. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J23.00005: The Elastic Moduli of Monoclinic and Orthorhombic Plutonium Albert Migliori, Jon B. Betts, C. Pantea, I. Mihut, C. Mielke, J.N. Mitchell Measurements were made of the bulk and shear moduli of high-purity polycrystalline Pu from 10K to 670K using resonant ultrasound spectroscopy. A simple dilatometer was employed to provide redundant detection of the phase transitions. We observed the expected phase transitions from monoclinic ($\alpha )$ to body centered monoclinic ($\beta )$ to orthorhombic ($\gamma )$ to face centered cubic ($\delta )$. Very accurate values were obtained for $\alpha $-Pu, $\beta $-Pu was very soft and difficult to analyze, as was $\delta $-Pu. Surprisingly, the $\gamma $-phase produced the high-Q resonances needed for accurate elastic modulus determination. We discuss also the unusual temperature dependences. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J23.00006: Magnetotransport properties and the Fermi surface of high-quality single crystal VB$_{2}$ A.B. Karki, D. Gautreaux, J.Y. Chan, N. Harrison, D. Browne, R.G. Goodrich, D.P. Young We have performed magnetotransport and dHvA measurements on high quality single crystals of VB$_{2}$ grown from a molten aluminum flux. At low temperature the magnetoresistance (MR) of VB$_{2}$ is very large ($\sim $1100{\%}) and is found to be extremely sensitive to sample quality. The field dependence of the MR is proportional to the applied field squared, as is expected from open orbits on the Fermi surface. In addition, we have performed full potential LAPW calculations with the WIEN2K band package using the GGA density functional to compute the bands and Fermi surface. The calculations suggest that the area of the Fermi surface is a strong function of the lattice constants. The results of the calculations will be compared to experiment. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J23.00007: Nernst-Ettingshausen effect in Bismuth across the quantum limit Kamran Behnia, Marie-Aude Measson, Yakov Kopelevich In elemental Bismuth, 10$^{5}$ atoms share a single itinerant electron. A magnetic field of the order of 10 T can confine electrons to the lowest Landau level. We report on the first study of metallic thermoelectricity in this regime. The thermoelectric response is mainly off-diagonal and peaks each time a Landau level hits the chemical potential. The associated maxima in the Ettingshausen coefficient saturates to a temperature-independent magnitude offering a notable constraint for theory. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J23.00008: Hall Effect and Magnetoresistance in cubic helimagnets J.F. DiTusa, C. Capan, A. Karki, D. Young, L. Pham, A.D. Bianchi, Z. Fisk, E. Thomas, J. Chan, G. Aeppli An anomalous contribution to Hall Effect is observed in a variety of ferromagnetic conductors ranging from simple metals and oxides to dilute magnetic semiconductors, manganites, ruthenates and spinel compounds. Its microscopic mechanism (intrinsic versus extrinsic) is still under investigation. FeGe and MnSi are itinerant helimagnets, due to the lack of inversion symmetry of the crystal structure. A large anomalous contribution to the Hall Effect is observed in these systems from 2K up to 300K, with no significant sample-to-sample dependence. Moreover, the transverse magnetoresistance has a positive orbital part at low temperatures that involves the same anomalous contribution to the Hall angle. These results establish, on purely experimental grounds, the essentially intrinsic nature of the anomalous Hall Effect, associated in recent theoretical models with a finite Berry phase acting as an effective magnetic field on charge carriers. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J23.00009: Quantum kinetics in layered systems Dmitrii Maslov, Dmitri Gutman Electron transport in strongly anisotropic materials exhibit several unusual properties.In particular, the resistivity across the layers (c-axis) has either an insulating-like or non-monotonic temperature dependences, whereas the resistivity along the layers is metallic. It is generally believed that when the scattering rate, 1/$\tau$, becomes larger than the tunneling rate between the layers, J, the nature of transport changes from coherent (band-like) to incoherent (tunneling-like). By using a Prange-Kadanoff--like quantum Boltzmann equation for electrons coupled to phonons, we show that there is no coherent/incoherent crossover for any value of J$\tau$, as long as the usual ``good metal'' condition is satisfied, i.e., E$_F\tau\gg$ 1. In other words, a strongly anisotropic metal is as ``coherent,'' as an isotropic one. The situation is changed in the presence of resonant tunneling centers between the layers. We show that the unusual behavior of the c-axis resistivity can be explained by inelastic resonant tunneling through such centers. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J23.00010: Giant Fluctuations of the Coulomb Drag Boris Narozhny, Adam Price, Alex Savchenko, David Ritchie Coulomb drag has been shown to provide information about electron-electron interactions not available from conventional conductance measurements, e.g. [1]. For mesoscopic conductors, a spectacular interference phenomenon is UCF. There has been a prediction that Coulomb drag should also show similar fluctuations: with decreasing temperature the fluctuations in the drag are expected to become larger than the average drag, resulting in a random change of the sign of the drag with varying carrier density [2]. Contrary to the UCF, the origin of these fluctuations involves both quantum interference and electron-electron interaction effects. Here we report the first observation of reproducible fluctuations of Coulomb drag in a double-layer GaAs structure, as a function of both the carrier density and magnetic field. Surprisingly, the observed fluctuations are almost four orders of magnitude larger than originally predicted in the theory, which considered diffusive transport of interacting electrons. We explain the observed enhancement by the fact that in ballistic transport, realised in our structures, the Coulomb drag probes the local properties of the system. The latter are expected to show much larger fluctuations than the global ones. [1] Gramila, Eisenstein, et al PRL 66, 1216 (1991). [2] Narozhny and Aleiner, PRL 84, 5383 (2000). [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J23.00011: Phase coherence of conduction electrons below the Kondo temperature Gassem M. Alzoubi, Norman O. Birge The scattering of conduction electrons by magnetic impurities is known as the Kondo effect. This effect has been the subject of theoretical and experimental investigations for several decades. Until very recently [1, 2], however, there was no theoretical expression for the temperature dependence of the inelastic scattering rate valid for temperatures $T $not too far below the Kondo temperature, $T_{K}$. We present experimental measurements of the phase decoherence rate, $\tau _{\phi }^{-1}$, of conduction electrons in disordered dilute AgFe Kondo wires [3]. We compare the temperature dependence of the magnetic scattering rate, $\gamma _{m}$, with a recent theory of dephasing by Kondo impurities [2]. A good agreement with theory is obtained for $T$/$T_{K }>$ 0.1. At lower $T$, $\gamma _{m}$ deviates from theory with a flatter $T$-dependence. \newline \newline [1] G. Zarand, L. Borda, J. von Delft, and N. Andrei, Phys.Rev. Lett. 93, 107204 (2004). \newline [2] T. Micklitz, A. Altland, T. A. Costi, A. Rosch, Phys.Rev. Lett. 96, 226601 (2006). \newline [3] G.M. Alzoubi and N.O. Birge, Phys.Rev. Lett. in press (2006). [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J23.00012: Pre-Exponential factor and hopping criterion in the Efros-Shklovskii regime Miguel Rodriguez, Bonalde Ismardo, Ernesto Medina We address the variable-range hopping regime in the range for which
the measured temperature $T$ is of the order of the characteristic
Efros-Shklovskii temperature $T_{ES}$. In such a range current
theories imply $r_{hop}/\xi<1$, where $r_{hop}$ is the hopping
length and $\xi$ is the localization length, clearly in
contradiction with the standard criterion for hopping conduction.
We consider impurity overlap wavefunctions of the form $\psi(r)
\propto r^{-n}\exp(-r/\xi)$ and include the preexponential factor
of the resistivity as a logarithmic correction in the Mott
optimization procedure. From the general expression derived, the
standard Efros-Shklovskii law is recovered for $T< |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J23.00013: Interaction Effects in a High-Mobility Two-Dimensional Electron Gas in a Nonquantizing Magnetic Field Tigran Sedrakyan, Eugene Mishchenko, Mikhail Raikh Two dimensional electron gas in a perpendicular nonquantizing magnetic field, $B$, is considered. We demonstrate \footnote{preprint cond-mat/0611111.} that the anomaly in the polarization operator, $\Pi(q)$, near $q=2k_F$, where $k_F$ is the Fermi momentum, gets smeared with $B$ in a peculiar fashion: slowly decaying modulation, periodic in $(2k_F-q)^{3/2}$, emerges. The period of modulation sets a spatial scale, $p_0^{-1}\propto B^{-2/3}$, which is much smaller than the Larmour radius, but much larger than the de Broglie wavelength. This scale manifests itself, {\em e.g.,} in lifting the periodicity of the Friedel oscillations, $\delta \rho (r)$ in magnetic field, namely we find that $\delta \rho (r)\propto \sin \left[2 k_F r-(p_0 r)^3/12\right]/r^2$. The corrections to the interaction-induced characteristics of the $2D$ gas, such as relaxation rate and the tunnel density of states, coming from the distances $\sim p_0^{-1}$, are shown to be strongly singular (as $B^{1/3}$) in magnetic field. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J23.00014: Electric-Field-Induced Hopping Conductivity in Polymers Steven Hart, J.R. Dennison, Jerilyn Brunson The resistivity of highly insulating polymers exhibits a dependence on electric field strength. Mott and Davis as well as Poole and Frankle describe theoretically the resistivity of disordered semiconductors, when subject to a changing electric field, in terms of hopping conductivity models. Although such models have often been applied to polymers, there is little direct experimental evidence to confirm the validity of these theories when applied to polymers. We present such results for a newly-developed block co-polymer Hytrel, a highly insulating material. The constant voltage resistivity test method has been used to study Hytrel for a range of electric fields approaching electrostatic breakdown. Previously taken preliminary measurements are suggestive that Hytrel validates hopping conductivity models. With additional data we consider whether the Hytrel results are consistent with existing models of electric-field induced hopping conductivity. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J23.00015: Dephasing in the semiclassical limit is system-dependent Philippe Jacquod We investigate dephasing in open ballistic chaotic systems in the limit of large system size to Fermi wavelength ratio, $L/\lambda_{\rm F} \gg 1$. Using the trajectory-based semiclassical theory, we calculate the weak localization correction $g^{\rm wl}$ to the conductance for a quantum dot coupled to (i) a dephasing voltage probe and (ii) an external closed quantum dot. In addition to the universal algebraic suppression $g^{\rm wl} \propto (1+\tau_{\rm D}/\tau_\phi)$ with the dwell time $\tau_{\rm D}$ through the cavity and the dephasing rate $\tau_\phi^{-1}$, we find an exponential suppression of weak localization by a factor $\propto \exp[-\tilde{\tau}/\tau_\phi]$, with a system-dependent $\tilde{\tau}$. In the dephasing probe model, $\tilde{\tau}$ coincides with the Ehrenfest time, $\tilde{\tau} \propto \ln [L/\lambda_{\rm F}]$, in both cases of perfectly and partially transparent dot-lead couplings. In contrast, when dephasing occurs due to the coupling to an external dot, $\tilde{\tau} \propto \ln [L/\xi]$ depends on the correlation length $\xi$ of the coupling potential instead of $\lambda_{\rm F}$. [Preview Abstract] |
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