Quantum simulator for the Hubbard model with long-range Coulomb interactions using surface acoustic waves

A practical experimental scheme for a quantum simulator of strongly correlated electrons is proposed. Our scheme employs electrons confined in a two dimensional electron gas in a GaAs/AlGaAs heterojunction. Two surface acoustic waves are then induced in the GaAs substrate, which create a two dimensional ``egg-carton'' potential. The dynamics of the electrons in this potential is described by a Hubbard model with long-range Coulomb interactions. The state of the electrons in this system can be probed via its conductance and noise properties. This allows the identification of a metallic or insulating state. Numerical estimates for the parameters appearing in the effective Hubbard model are calculated using the proposed experimental system. These calculations suggest that observations of quantum phase transition phenomena of the electrons in the potential array are within experimental reach.Comment: 5 pages, 5 figure

Alteration of U(VI)-Phases under oxidizing conditions

Uranium-(VI) phases are the primary alteration products of the UO 2 in spent nuclear fuel and the UO 2+x in natural uranium deposits. The U(VI)-phases generally form sheet structures of edge-sharing UO 2 2+ polyhedra. The complexity of these structures offers numerous possibilities for coupled-substitutions of trace metals and radionuclides. The incorporation of radionuclides into U(VI)-structures provides a potential barrier to their release and transport in a geologic repository that experiences oxidizing conditions. In this study, we have used natural samples of UO 2+x , to study the U(VI)-phases that form during alteration and to determine the fate of the associated trace elements

Behaviour of trace elements in arsenian pyrite in ore deposits

As-bearing pyrite is one of the main hosts for Au and other trace elements in epithermal, Carlin and mesothermal (orogenic) Au deposits. A review of our own and published SIMS, EMPA, LA-ICP-MS and PIXE analyses of pyrite from these deposits suggests that the solubility of Ag, Te, Hg, Sb and Pb in arsenian pyrite is controlled by As-content in a manner similar to that previously reported for Au by Reich et al., (2005). The trace elements can be divided into two groups that exhibit different solubility limits: i) Au, Ag, Te, Hg and Bi ii) Sb and Pb. HRTEM and HAADF-STEM observations reveal nanoparticles with compositions of Sb-As-Fe-Ni, Sb-Pb-Te, Pb-Bi, PbS and Ag in arsenian pyrite above the solubility limit. Most nanoparticles are between 5 and 200 nm, with some containing Pb reaching 500 nm. Pyrite from Carlin-type and epithermal deposits contains larger amounts of Sb and/or As than pyrite from higher-temperature orogenic gold/mesothermal deposits. This suggests that the solubility of trace elements in pyrite appears to decrease with increasing temperature

Radiation-induced decomposition of U(6) alteration phases of UO2

U{sup 6+}-phases are common alteration products of spent nuclear fuel under oxidizing conditions, and they may potentially incorporate actinides, such as long-lived {sup 239}Pu and {sup 237}Np, delaying their transport to the biosphere. In order to evaluate the ballistic effects of {alpha}-decay events on the stability of the U{sup 6+}-phases, we report, for the first time, the results of ion beam irradiations (1.0 MeV Kr{sup 2+}) for six different structures of U{sup 6+}-phases: uranophane, kasolite, boltwoodite, saleeite, carnotite, and liebigite. The target uranyl-minerals were characterized by powder X-ray diffraction and identification confirmed by SAED (selected area electron diffraction) in TEM (transmission electron microscopy). The TEM observation revealed no initial contamination of uraninite in these U{sup 6+} phases. All of the samples were irradiated with in situ TEM observation using 1.0 MeV Kr{sup 2+} in the IVEM (intermediate-voltage electron microscope) at the IVEM-Tandem Facility of Argonne National Laboratory. The ion flux was 6.3 x 10{sup 11} ions/cm{sup 2}/sec. The specimen temperatures during irradiation were 298 and 673 K, respectively. The Kr{sup 2+}-irradiation decomposed the U{sup 6+}-phases to nanocrystals of UO{sub 2} at doses as low as 0.006 dpa. The cumulative doses for the pure U{sup 6+}-phases, e.g., uranophane, at 0.1 and 1 million years (m.y.) are calculated to be 0.009 and 0.09 dpa using SRIM2003. However, with the incorporation of 1 wt.% {sup 239}Pu, the calculated doses reach 0.27 and {approx}1.00 dpa in ten thousand and one hundred thousand years, respectively. Under oxidizing conditions, multiple cycles of radiation-induced decomposition to UO{sub 2} followed by alteration to U{sup 6+}-phases should be further investigated to determine the fate of trace elements that may have been incorporated in the U{sup 6+}-phases

Quasicontinuum γ\gamma-decay of 91,92^{91,92}Zr: benchmarking indirect (n,γn,\gamma) cross section measurements for the ss-process

Nuclear level densities (NLDs) and $\gamma$-ray strength functions ($\gamma$SFs) have been extracted from particle-$\gamma$ coincidences of the $^{92}$Zr($p,p' \gamma$)$^{92}$Zr and $^{92}$Zr($p,d \gamma$)$^{91}$Zr reactions using the Oslo method. The new $^{91,92}$Zr $\gamma$SF data, combined with photonuclear cross sections, cover the whole energy range from $E_{\gamma} \approx 1.5$~MeV up to the giant dipole resonance at $E_{\gamma} \approx 17$~MeV. The wide-range $\gamma$SF data display structures at $E_{\gamma} \approx 9.5$~MeV, compatible with a superposition of the spin-flip $M1$ resonance and a pygmy $E1$ resonance. Furthermore, the $\gamma$SF shows a minimum at $E_{\gamma} \approx 2-3$~MeV and an increase at lower $\gamma$-ray energies. The experimentally constrained NLDs and $\gamma$SFs are shown to reproduce known ($n, \gamma$) and Maxwellian-averaged cross sections for $^{91,92}$Zr using the {\sf TALYS} reaction code, thus serving as a benchmark for this indirect method of estimating ($n, \gamma$) cross sections for Zr isotopes.Comment: 10 pages and 9 figure

Constraints on Hf and Zr mobility in high-sulfidation epithermal systems: formation of kosnarite, KZr2(PO4)3, in the Chaquicocha gold deposit, Yanacocha district, Peru

We report the first occurrence of Hf-rich kosnarite [K(Hf,Zr)2(PO4)3], space group R-3c, Z = 6, in the giant Chaquicocha high-sulfidation epithermal gold deposit in the Yanacocha mining district, Peru. Kosnarite crystals are small (<100 μm) and occur in 2–3-mm-thick veins that cut intensively silicified rocks. The paragenesis includes a first stage of As-free pyrite and quartz (plus gratonite and rutile), followed by trace metal-rich pyrite [(Fe,As,Pb,Au)S2] and secondary Fe sulfates. Kosnarite is associated with quartz and is clearly late within the paragenetic sequence. Electron microprobe analyses (EMPA) of kosnarite show relatively high concentrations of HfO2 and Rb2O (7.61 and 1.05 wt.%, respectively). The re-calculated chemical formulas of kosnarite vary from KΣ1.00(Zr1.93Na0.01Hf0.01Mn0.01)Σ1.96(P3.04O4)Σ3 to (K0.92Rb0.05Na0.03)Σ1.00(Zr1.81Hf0.19)Σ2.00 [(P2.98Si0.02As0.01)Σ3.01O4]Σ3, where Hf and Rb are most likely incorporated according to a coupled substitution of Hf4+ + Rb+ ⇔ Zr4+ + K+. Back-scattered electron (BSE) images and elemental mapping of kosnarite reveal that Hf and Rb are enriched in 2–10-μm-wide oscillatory and/or sector zones. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations of such zones reveal a pattern of alternating, 5–50-nm-thick, Hf-rich and Zr-rich nanozones. These high-resolution observations indicate that the incorporation of Hf does not appear to cause significant distortion in the kosnarite structure. Semiquantitative TEM-energy-dispersive X-ray spectrometry (EDS) analyses of the nano-layers show up to 22 wt.% of HfO2, which corresponds to 31 mol% of the hypothetical, KHf2(PO4)3, end-member. The presence of kosnarite in the advanced argillic alteration zone at Yanacocha is indicative of Hf and Zr mobility under highly acidic conditions and points towards an unforeseen role of phosphates as sinks of Zr and Hf in high-sulfidation epithermal environments. Finally, potentially new geochronological applications of highly insoluble vein kosnarite, including Rb-Sr dating, may provide further age constraints in pervasively altered areas where other isotopic systems might have been reset