Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S04: Actinides, Valence Fluctuation, Heavy Fermions, and metals |
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Sponsoring Units: DCMP Chair: Amy Liu, Georgetown University Room: BCEC 107C |
Thursday, March 7, 2019 11:15AM - 11:27AM |
S04.00001: Entropy changes and caloric effects driven by charge transfer: Application to YbInCu4 Nilson De Oliveira, Amós Troper In this work, we propose a new physical mechanism based on charge transfer to produce large entropy and caloric effects even in non magnetic solid materials. In order to carry out the calculations, we use a two subband model in which a strongly correlated narrow band is coupled with a wide conduction electrons band. In the model, the charge transfer is controlled by the energy bandwidth and the hybridization parameter. The entropy and the caloric functions are calculated using the standard relations [1]. |
Thursday, March 7, 2019 11:27AM - 11:39AM |
S04.00002: High field magnetostriction and magnetization of uranium monoantimonide single crystals Xiaxin Ding, Keshav Shrestha, Daniel Antonio, Franziska Weickert, Marcelo Jaime, Neil Harrison, Myron B Salamon, Tomasz Durakiewicz, James L Smith, Krzysztof Gofryk Uranium monoantimonide (USb) crystallizes in the cubic NaCl-type crystal structure and is known to order antiferromagnetically with a triple-k magnetic structure, with a wave vector (1,0,0), below TN = 213 K. IIn addition, the extent of hybridization that occurs between the uranium's 5f electrons with conduction electrons and its role for magnetism is not fully understood. During the talk we will present results of our recent magnetization and magnetostriction measurements of high-quality single crystals of USb at pulsed magnetic fields up to 65 T. The magnetization measurements help us to reveal the energy scales that are important in USb. In addition, the magnetostriction measurements performed in wide temperature range provides better microscopic understanding of the spin-lattice coupling and its strength in this material. We will discuss implications of these results. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S04.00003: Pressure-Driven 5f Localized-Itinerant Transition and Valence Fluctuation in Californium Li Huang, Haiyan Lu A combination of the density functional theory and the single-site dynamical mean-field theory is employed to study the pressure dependence of electronic structure for cubic phase californium. We predict that its 5f electrons could undergo an orbital-selective localized-itinerant transition under moderate pressure. The volume contraction causes remarkable charge redistribution and valence fluctuation behaviors, which are the driving forces of the divalent-trivalent transition. Additionally, we find that the angular momentum coupling mechanism is hardly affected by pressure. The 5f orbital occupancy is well described by the intermediate coupling scheme. |
Thursday, March 7, 2019 11:51AM - 12:03PM |
S04.00004: Effective Coulomb interaction in actinides from linear response approach Ruizhi Qiu, Bingyun Ao, Li Huang The effective on-site Coulomb interaction (Hubbard U) between 5f electrons in actinide metals (Th-Cf) is calculated with the framework of density-functional theory (DFT) using linear response approach. The U values seldom rely on the exchange-correlation functional, spin-orbital coupling, and magnetic states, but depend on the lattice volume and actinide element. Along the actinide series, the Coulomb parameter U of α-phase first decreases slowly, followed by a jump in the vicinity of Pu and then a monotonous increase. For light actinides, the lattice volume has a sizeable influence on U while the localization of 5f electrons is almost constant. But for transplutonium metals, U is almost independent of the lattice volume but the electronic localization increases rapidly. The calculated lattice parameters from DFT+U with the Coulomb parameters as input are in better agreement with the experimental values than those from DFT within local density approximation or Perdew-Burke-Ernzerhof approximation for solids (PBEsol). In particular, the agreement between PBEsol+U and experiment is remarkable. |
Thursday, March 7, 2019 12:03PM - 12:15PM |
S04.00005: Unifying DFT+U approach for plutonium modeling Boris Dorado, Bernard Amadon, Marc Torrent, Francois Bottin, Johann Bouchet The theoretical description of the plutonium phase diagram still represents a major challenge to modern electronic structure methods. For the δ phase, a comprehensive knowledge has gradually been building up and the correct description of spectroscopic, magnetic, structural properties could be obtained using the DMFT approach. Effective Coulomb interactions in Pu were calculated very recently with DMFT and were found to be around 1 eV, which is much smaller than the "commonly accepted value" of 4 eV. It was also shown that the Hunds exchange, together with the spin-orbit coupling, yields an improved description of all phases, thus opening the way to a unifying theory of Pu. |
Thursday, March 7, 2019 12:15PM - 12:27PM |
S04.00006: Crystal structure and thermal stability of uranium nitride Xiaofang Wang, Ruilong Yang, Qifa Pan, Yin Hu, Kezhao Liu Uranium nitrides are among the most promising fuels for Generation IV nuclear reactors, but until now, very little has been known about their thermal stability properties under nonequilibrium conditions. In this work, thermal decomposition of nitrogen-rich uranium nitride (denoted as UN2−x) under ultrahigh-vacuum (UHV) conditions was investigated by thermal desorption spectroscopy (TDS). It has been shown that the nitrogen TDS spectrum consists of two peaks at about 723 and 1038 K. The X-ray diffraction, scanning electron microscopy, and X-ray photoelectron microscopy results indicate that UN2−x (UN2 phase) decomposed into the α-U2N3 phase in the first step and the α-U2N3 phase decomposed into the UN phase in the second step. Further studies are needed to fully understand the thermal decomposition kinetics of the UN2−x film, and work is in progress. |
Thursday, March 7, 2019 12:27PM - 12:39PM |
S04.00007: Plutonium in high magnetic fields. Mark Wartenbe, Paul H Tobash, Neil Harrison, John Singleton, Laurel Winter, Jessica Hebert High magnetic field studies of plutonium have been limited up until now. The complex behavior of plutonium’s 5f electrons are thought to be responsible for the elements unusual behavior, making it possibly the strangest element in the periodic table. Here we review data from a new set of various experiments involving high magnetic field and extremes of temperature. High magnetic fields are especially effective in probing electronic states and is the highlighted method of this study. We present the data in the context of gaining in site into the 5f electrons physics of the material. |
Thursday, March 7, 2019 12:39PM - 12:51PM |
S04.00008: Atomistic Modelling of Nano-Indentation and Scratching of Nuclear Graphite Kenny Jolley, Ben Maerz, Houzheng Wu, Roger Smith Nuclear graphite, is a synthetic graphite typically manufactured by isostatic pressure moulding. It is used in the core structural components of the UK's nuclear reactors. This material contains many interesting atomic and mesoscale features. Recent TEM imaging shows that in-plane boundaries between misaligned basal planes within the graphite crystallites occur. Our previous work describes this “crazy paving” structure, which consists of nearly perfect slabs of graphite laminae, with nearly parallel c-axis. These slabs range in size from 100-1000 nm, with a thickness of ∼30 nm. |
Thursday, March 7, 2019 12:51PM - 1:03PM |
S04.00009: Ab-Initio Simulation of Fission Product Diffusion in Graphite James McHugh, Kenny Jolley, Pavlos Mouratidis, Malcolm Heggie, Patrick Briddon Graphite has been used for neutron moderation from the beginning of the nuclear reactor era. While research activities have abated over the years, there is renewed interest in graphite motivated by its use in Very High Temperature Reactors (VHTRs) and Molten Salt Reactors (MSRs). Retention of the activated fission products is paramount during normal operating and accident conditions, and a mechanistic understanding of the bonding and diffusion properties of fission products is imperative for predicting the release rates and designing appropriate barriers. |
Thursday, March 7, 2019 1:03PM - 1:15PM |
S04.00010: Magnetostriction and magnetic torque measurements of uranium monosulfide single crystals Narayan Poudel, Marcelo Jaime, Franziska Weickert, Jason R Jeffries, Krzysztof Gofryk The family of actinide compounds AX, where A = actinides and X = N, P, As, Sb, S, Se, and Te have drawn great interest in research of actinide compounds because of their simple structure and rich magnetic properties. Recent study of UO2 and UN revealed the effectiveness in probing the magneto-structural coupling in actinides by high magnetic field magnetostriction. Uranium monosulfide (US) crystalizes in simple cubic rock salt structure and exhibits the highest ferromagnetic transition temperature in uranium monochalcogenides (TC = 177 K) with easy axis along [111] direction. At the magnetic phase transition the crystal structure undergoes a rhombohedral distortion along one of the [111] directions with a large magnetic anisotropy. This might imply that strong magneto-elastic interactions play important role in this material. In this talk, our recent measurements of the magnetostriction and magnetic torque of uranium monosulfide single crystals will be presented. |
Thursday, March 7, 2019 1:15PM - 1:27PM |
S04.00011: Extreme Fermi surface smearing in a maximally disordered concentrated solid solution Stephen Dugdale, Hannah Robarts, Thomas E Millichamp, Daniel A Lagos, Jude Laverock, David Billington, Jonathan Duffy, Daniel O'Neill, Sean Giblin, Jonathan Taylor, Grazyna Kontrym-Sznajd, Malgorzata Samsel-Czekala, Hongbin Bei, Sai Mu, German Samolyuk, George Malcolm Stocks The impact of extreme chemical disorder on the Fermi surface of the equiatomic alloy Ni0.25Fe0.25Co0.25Cr0.25 was probed by high-resolution X-ray Compton scattering. Such experiments probe the Fermi surface via the occupied momentum states, meaning that such measurements are ideally suited to revealing the electronic structure of such disordered alloys. The smearing of the Fermi surface is rather strong, reaching up to 40% of 2π/a, but nevertheless a Fermi surface can be clearly identified in the experimental data and the extent of the smearing and its variation on and across different sheets has been investigated. By connecting this smearing with the coherence length of the quasiparticle states, estimates of the electronic mean-free-path and residual resistivity have been made. It is found that the mean-free paths are in the range 0.7 - 0.9 nm. |
Thursday, March 7, 2019 1:27PM - 1:39PM |
S04.00012: Ballistic effects in squares and crosses of the ultra-pure delafossite PtCoO2 Philippa McGuinness, Elina Zhakina, Veronika Sunko, Markus Koenig, Seunghyun Khim, Andrew Mackenzie The delafossite PtCoO2 is a layered oxide material with a single-band, hexagonal Fermi surface. This metal is extremely conductive, with a room temperature resistivity of 1.8 μOhmcm and a low-temperature mean free path of 5 μm [1]. Using focused ion beam microstructuring, we have created PtCoO2 square and cross devices which demonstrate strong ballistic behavior when their dimensions are similar to this mesoscopic length scale. These ballistic signals, in a weaker form, persist to structure sizes of at least 50 μm. Novel magnetoresistance effects, not observed in materials with circular Fermi surfaces, are also present within these devices, suggesting an influence of the hexagonal Fermi surface on ballistic transport. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S04.00013: Optical properties of the delafossite PdCoO2 Christopher Homes, Seunghyun Khim, Andrew P. Mackenzie The delafossite PdCoO2 is remarkable for its extremely low in-plane residual resistivity at low temperature [1]. The in-plane optical properties have been determined over a wide frequency range at a variety of temperatures. At 295 K the reflectance of this material is extremely high (>99%) before encountering a sharp plasma edge at ~ 6000 cm-1, above which it decreases rapidly. The real part of the optical conductivity reveals that the free-carrier (intraband) response falls well below the interband excitations, allowing the plasma frequency to be determined from the f-sum rule, ωp ~ 33500 cm-1; this value is in good agreement with first principle results. The Drude model for the optical properties of a metal may be used to estimate the free-carrier scattering rate, 1/τ ~ 80 cm-1 at 5 K, which is well above the transport estimate of 1/τ < 0.2 cm-1; this suggests that the scattering rate is strongly renormalized with frequency, and that the optical value is an average of these scattering rates. Structure is observed in the region of the expected infrared-active modes; however, the location and strength of these features suggests that some are not simple vibrations. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S04.00014: Controlled introduction of defects to delafossite metals by electron irradiation Veronika Sunko, Philippa McGuinness, Elina Zhakina, Marcin Konczykowski, Jérémie Lefèvre, Jörg Sichelschmidt, Andrew Mackenzie The delafossite metals PdCoO2 and PtCoO2 are among the highest conductivity materials known. At room temperature the mean free path is at least twice as long as those of silver or copper, and at low temperature mean free paths of tens of microns are observed in as-grown single crystals, enabling searches for unusual regimes of ballistic and hydrodynamic electrical transport. In parallel with that class of experiments, we wish to understand the microscopic mechanisms for the existence of these extraordinary mean free paths. Somewhat paradoxically, one route to obtaining more insight is to deliberately introduce point defects and study their effect on the electrical transport properties. We report on a systematic study in which both bulk single crystals and focused ion beam sculpted mesostructures are irradiated with 0.8 - 2.5 MeV electrons and their transport properties studied both in- and ex-situ. |
Thursday, March 7, 2019 2:03PM - 2:15PM |
S04.00015: Local Analysis of Disorder Driven Insulator-to-Metal Transition in the 2D Mott-Hubbard Model. Joseph Szabo, Kyungmin Lee, Nandini Trivedi, Jared O'Neal We show that increasing the number of disordered sites drives an insulator-to-metal transition in 2D Mott-Hubbard insulator. In our numerical approach we treat the spatial disorder exactly and use real-space Hartree-Fock mean-field decomposition for electron-electron interactions. These numerical methods provide insight into the localization physics as it relates to the interplay between disorder and interactions. Our results reveal an asymmetric closing of the Mott-gap and formation of a pseudogap in the density of states arising from inhomogeneous disorder potential mediated screening of the Hubbard interaction. Global lattice transport properties exhibit a transition from an insulating, gapped optical conductivity to an ungapped, metallic phase. A local picture consisting of magnetic order, compressibility, and charge mobility evolves toward a paramagnetic, metallic state with increasing disorder number due to correlations with regions of greater effective disorder. We present these local measurements to explain and provide avenues to characterize disorder induced transitions in ongoing experimental efforts. |
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