4,022 research outputs found
Three-dimensional electronic instabilities in polymerized solid A1C60
The low-temperature structure of A1C60 (A=K, Rb) is an ordered array of
polymerized C60 chains, with magnetic properties that suggest a non-metallic
ground state. We study the paramagnetic state of this phase using
first-principles electronic-structure methods, and examine the magnetic
fluctuations around this state using a model Hamiltonian. The electronic and
magnetic properties of even this polymerized phase remain strongly three
dimensional, and the magnetic fluctuations favor an unusual three-dimensional
antiferromagnetically ordered structure with a semi-metallic electronic
spectrum.Comment: REVTeX 3.0, 10 pages, 4 figures available on request from
[email protected]
Important role of alkali atoms in A4C60
We show that hopping via the alkali atoms plays an important role for the t1u
band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is
broadened by more than 40 % by the presence of the alkali atoms. The difference
between A4C60 and A3C60 is in particular due to the less symmetric location of
the alkali atoms in A4C60.Comment: 5 pages, revtex, 2 figures, submitted to Phys. Rev. B more
information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
Investigation of unconventional reconstruction and electronic properties on the Na2IrO3 surface
Na2IrO3 is an intriguing material for which spin-orbit coupling plays a key
role. Theoretical predictions, so far unverified, have been made that the
surface of Na2IrO3 should exhibit a clear signature of the quantum spin Hall
effect. We studied the surface of Na2IrO3 using scanning tunneling microscopy
and density-functional theory calculations. We observed atomic level resolution
of the surface and two types of terminations with different surface periodicity
and Na content. By comparing bias-dependent experimental topographic images to
simulated images, we determined the detailed atomistic structure of both
observed surfaces. One of these reveals a strong relaxation to the surface of
Na atoms from the subsurface region two atomic layers below. Such dramatic
structural changes at the surface cast doubt on any prediction of surface
properties based on bulk electronic structure. Indeed, using spatially resolved
tunneling spectroscopy we found no indication of the predicted quantum spin
Hall behavior
Commensurate and modulated magnetic phases in orthorhombic A1C60
Competing magnetically ordered structures in polymerized orthorhombic A1C60
are studied. A mean-field theory for the equilibrium phases is developed using
an Ising model and a classical Heisenberg model to describe the competition
between inter- and intra-chain magnetic order in the solid. In the Ising model,
the limiting commensurate one-dimensional and three-dimensional phases are
separated by a commensurate three-sublattice state and by two sectors
containing higher-order commensurate phases. For the Heisenberg model the
quasi-1D phase is never the equilibrium state; instead the 3D commensurate
phases exhibits a transition to a continuum of coplanar spiral magnetic phases.Comment: 11 pages REVTeX 3.0 plus 4 figures appende
The interdependencies between food and biofuel production in European agriculture - an application of EUFASOM
In the continuous quest to reduce anthropogenic emissions of carbon dioxide, the production and use of organically grown fuels in Europe has increased in importance in the recent past. However, the production of so-called biofuels is a direct competitor of agricultural food production for land, labor, water resources etc. with both land use options influencing each other depending on the respective boundary conditions defined by political regulations and economic considerations. In this study we will explore the economic and technical potentials of biofuels in Europe as well as the interdependencies between these two land use options for different economic incentives for biofuels using the European Forest and Agriculture Sector Optimization Model (EUFASOM). Key data on biodiesel and ethanol production have been gathered and are used for calibration of the model. The simulations extend until the year 2030, for which results are presented. Results indicate that moderate production targets of biofuels lead to an expansion of mainly the biodiesel production while more ambitious targets call for a focus on bioethanol. This has to do with the different levels of production efficiency depending on the production output. Growth of bioethanol feedstock is spread over entire Europe while the production of biodiesel feedstock occurs mainly in Central Europe.biodiesel, bioethanol, Europe, EUFASOM, modeling
Research in the effective implementation of guidance computers with large scale arrays Interim report
Functional logic character implementation in breadboard design of NASA modular compute
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure
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