Determination of biaxial creep strength of T-111 tantalum alloy

Biaxial creep strength of T-111 tantalum alloy tubing in high temperature, high vacuum environmen

Random-phase-approximation-based correlation energy functionals: Benchmark results for atoms

The random phase approximation (RPA) for the correlation energy functional of density functional theory has recently attracted renewed interest. Formulated in terms of the Kohn-Sham (KS) orbitals and eigenvalues, it promises to resolve some of the fundamental limitations of the local density and generalized gradient approximations, as for instance their inability to account for dispersion forces. First results for atoms, however, indicate that the RPA overestimates correlation effects as much as the orbital-dependent functional obtained by a second order perturbation expansion on the basis of the KS Hamiltonian. In this contribution, three simple extensions of the RPA are examined, (a) its augmentation by an LDA for short-range correlation, (b) its combination with the second order exchange term, and (c) its combination with a partial resummation of the perturbation series including the second order exchange. It is found that the ground state and correlation energies as well as the ionization potentials resulting from the extensions (a) and (c) for closed sub-shell atoms are clearly superior to those obtained with the unmodified RPA. Quite some effort is made to ensure highly converged RPA data, so that the results may serve as benchmark data. The numerical techniques developed in this context, in particular for the inherent frequency integration, should also be useful for applications of RPA-type functionals to more complex systems.Comment: 11 pages, 7 figure

Mesoscopic Spin-Hall Effect in 2D electron systems with smooth boundaries

Spin-Hall effect in ballistic 2D electron gas with Rashba-type spin-orbit coupling and smooth edge confinement is studied. We predict that the interplay of semiclassical electron motion and quantum dynamics of spins leads to several distinct features in spin density along the edge that originate from accumulation of turning points from many classical trajectories. Strong peak is found near a point of the vanishing of electron Fermi velocity in the lower spin-split subband. It is followed by a strip of negative spin density that extends until the crossing of the local Fermi energy with the degeneracy point where the two spin subbands intersect. Beyond this crossing there is a wide region of a smooth positive spin density. The total amount of spin accumulated in each of these features exceeds greatly the net spin across the entire edge. The features become more pronounced for shallower boundary potentials, controlled by gating in typical experimental setups.Comment: 4 pages, 4 figures, published versio

Competition Between Fractional Quantum Hall Liquid, Bubble and Wigner Crystal Phases in the Third Landau Level

Magnetotransport measurements were performed in a ultra-high mobility GaAs/AlGaAs quantum well of density $\sim 3.0 \times 10^{11}$ $cm^{-2}$. The temperature dependence of the magnetoresistance $R_{xx}$ was studied in detail in the vicinity of $\nu={9/2}$. In particular, we discovered new minima in $R_{xx}$ at filling factor $\nu\simeq 4{1/5}$ and $4{4/5}$, but only at intermediate temperatures $80\lesssim T\lesssim 120$ mK. We interpret these as evidence for a fractional quantum Hall liquid forming in the N=2 Landau level and competing with bubble and Wigner crystal phases favored at lower temperatures. Our data suggest that a magnetically driven insulator-insulator quantum phase transition occurs between the bubble and Wigner crystal phases at T=0.Comment: Phys. Rev. Lett.93 266804 (2004

Colossal magnetoresistance in an ultra-clean weakly interacting 2D Fermi liquid

We report the observation of a new phenomenon of colossal magnetoresistance in a 40 nm wide GaAs quantum well in the presence of an external magnetic field applied parallel to the high-mobility 2D electron layer. In a strong magnetic field, the magnetoresistance is observed to increase by a factor of ~300 from 0 to 45T without the system undergoing any metal-insulator transition. We discuss how this colossal magnetoresistance effect cannot be attributed to the spin degree-of-freedom or localization physics, but most likely emanates from strong magneto-orbital coupling between the two-dimensional electron gas and the magnetic field. Our observation is consistent with a field-induced 2D-to-3D transition in the confined electronic system

Giant microwave photoresistance of two-dimensional electron gas

We measure microwave frequency (4-40 GHz) photoresistance at low magnetic field B, in high mobility 2D electron gas samples, excited by signals applied to a transmission line fabricated on the sample surface. Oscillatory photoresistance vs B is observed. For excitation at the cyclotron resonance frequency, we find an unprecedented, giant relative photoresistance (\Delta R)/R of up to 250 percent. The photoresistance is apparently proportional to the square root of applied power, and disappears as the temperature is increased.Comment: 4 pages, 3 figure

On the violation of a local form of the Lieb-Oxford bound

In the framework of density-functional theory, several popular density functionals for exchange and correlation have been constructed to satisfy a local form of the Lieb-Oxford bound. In its original global expression, the bound represents a rigorous lower limit for the indirect Coulomb interaction energy. Here we employ exact-exchange calculations for the G2 test set to show that the local form of the bound is violated in an extensive range of both the dimensionless gradient and the average electron density. Hence, the results demonstrate the severity in the usage of the local form of the bound in functional development. On the other hand, our results suggest alternative ways to construct accurate density functionals for the exchange energy.Comment: (Submitted on 27 April 2012