1,916 research outputs found
Flat-lens focusing of electrons on the surface of a topological insulator
We propose the implementation of an electronic Veselago lens on the
conducting surface of a three-dimensional topological insulator (such as
Bi2Te3). The negative refraction needed for such a flat lens results from the
sign change of the curvature of the Fermi surface, changing from a circular to
a snowflake-like shape across a sufficiently large electrostatic potential
step. No interband transition (as in graphene) is needed. For this reason, and
because the topological insulator provides protection against backscattering,
the potential step is able to focus a broad range of incident angles. We
calculate the quantum interference pattern produced by a point source,
generalizing the analogous optical calculation to include the effect of a
noncircular Fermi surface (having a nonzero conic constant).Comment: 6 pages, 6 figure
Accounting for the Future: Reducing the Differences between International and Domestic Accounting Standards and Becoming a Better Global Citizen While Maintaining Autonomy
In recent times, the FASB and the IASB have heavily discussed bringing the United States on board to utilize the same IFRS accounting standards that much of the rest of the world uses. At this present time, IFRS adoption in the United States is no longer actively being considered, and alternative means of convergence have become the preferred method for supporting globalization. This paper posits that in order to serve a worldwide business community, the principles-based approach is more conducive to the flexible modern environment. The practices currently utilized to jointly-research and co-develop accounting standards by both the FASB and the IASB are instrumental in promoting efficiency and increased perspective as new, joint standards are instituted. A continued movement toward principles or objectives-based accounting as previously suggested by the SEC is suggested as the best way to unify the United States with the rest of the world within the accounting field
Plume-lithosphere interaction : geochemical evidence from upper mantle and lower crusal Xenoliths from the Kerguelen Islands
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1999This study is a geochemical investigation ofthe evolution of the Kerguelen plume, on the
basis of upper mantle and lower crustal xenoliths. Ultramafic xenoliths include harzburgites
predominant, a lherzolite, dunites and pyroxenites, whereas lower crustal xenoliths are
cumulate gabbros recrystallized under granulite facies conditions. On the basis of the whole
rock major element characteristics and trace element abundance patterns in clinopyroxenes,
the harzburgites were found to be residues of extensive melting at high pressures within the
Kerguelen plume. These were then recrystallized at low pressures and metasomatized by
plume generated melts. Details of the metasomatic process were determined from trace
element variations in clinopyroxene in connection to texture. This demonstrated that meltrock
reaction and the precipitation of new clinopyroxenes occurred by metasomatic
carbonatitic melts.
It was also found that some of the harzburgites had distinctly unradiogenic Os isotopic
compositions and were identified as originating from the sub-Gondwanaland lithosphere.
On the basis of major and trace element compositions, the granulite xenoliths were found to
be originally gabbroic cumulates formed from plume-derived basaltic melts emplaced at the
base of the crust by underplating and subsequently recrystallized isobarically under
granulite conditions. The Sr, Nd and Os isotopic compositions of the peridotite and
granulite xenoliths demonstrate that the Kerguelen plume is isotopically heterogeneous and
displays a temporal progression toward more enriched Sr and Nd isotopic compositions
from the Ninetyeast Ridge to granulite xenoliths to Kerguelen basalts and Heard Island
basalts.This research was supported by the National Geographic Society 4629-9 1, the National
Science Foundation EAR-9219158 and OPP-9417806, and William Van Alan Clark
Senior Scientist Chair to Nobu Shimizu. I was also supported by a Cecil and Ida Green
Fellowship and a Education Graduate Research Fellowship
Anyonic interferometry without anyons: How a flux qubit can read out a topological qubit
Proposals to measure non-Abelian anyons in a superconductor by quantum
interference of vortices suffer from the predominantly classical dynamics of
the normal core of an Abrikosov vortex. We show how to avoid this obstruction
using coreless Josephson vortices, for which the quantum dynamics has been
demonstrated experimentally. The interferometer is a flux qubit in a Josephson
junction circuit, which can nondestructively read out a topological qubit
stored in a pair of anyons --- even though the Josephson vortices themselves
are not anyons. The flux qubit does not couple to intra-vortex excitations,
thereby removing the dominant restriction on the operating temperature of
anyonic interferometry in superconductors.Comment: 7 pages, 3 figures; Added an Appendix on parity-protected
single-qubit rotations; problem with Figure 3 correcte
Scattering formula for the topological quantum number of a disordered multi-mode wire
The topological quantum number Q of a superconducting or chiral insulating
wire counts the number of stable bound states at the end points. We determine Q
from the matrix r of reflection amplitudes from one of the ends, generalizing
the known result in the absence of time-reversal and chiral symmetry to all
five topologically nontrivial symmetry classes. The formula takes the form of
the determinant, Pfaffian, or matrix signature of r, depending on whether r is
a real matrix, a real antisymmetric matrix, or a Hermitian matrix. We apply
this formula to calculate the topological quantum number of N coupled dimerized
polymer chains, including the effects of disorder in the hopping constants. The
scattering theory relates a topological phase transition to a conductance peak,
of quantized height and with a universal (symmetry class independent) line
shape. Two peaks which merge are annihilated in the superconducting symmetry
classes, while they reinforce each other in the chiral symmetry classes.Comment: 8 pages, 3 figures, this is the final, published versio
Design and Fabrication of the NASA Decoupler Pylon for the F-16 Aircraft
The NASA Decoupler Pylon is a passive means of suppressing wing-store flutter. The feasibility of demonstrating this concept on the F-16 aircraft was established through model wind tunnel tests and analyses. As a result of these tests and studies a ship set of Decoupler Pylons was designed and fabricated for a flight test demonstration on the F-16 aircraft. Basic design criteria were developed during the analysis study pertaining to pylon pitch stiffness, alignment system requirements, and damping requirements. A design was developed which utilized an electrical motor for the pylon alignment system. The design uses a four pin, two link pivot design which results in a remote pivot located at the center of gravity of the store when the store is in the aligned position. The pitch spring was fabricated from a tapered constant stress cantilevered beam. The pylon has the same external lines as the existing production pylon and is designed to use a MAU-12 ejection rack which is the same as the one used with the production pylon. The detailed design and fabrication was supported with a complete ground test of the pylon prior to shipment to NASA
Quantized conductance at the Majorana phase transition in a disordered superconducting wire
Superconducting wires without time-reversal and spin-rotation symmetries can
be driven into a topological phase that supports Majorana bound states. Direct
detection of these zero-energy states is complicated by the proliferation of
low-lying excitations in a disordered multi-mode wire. We show that the phase
transition itself is signaled by a quantized thermal conductance and electrical
shot noise power, irrespective of the degree of disorder. In a ring geometry,
the phase transition is signaled by a period doubling of the magnetoconductance
oscillations. These signatures directly follow from the identification of the
sign of the determinant of the reflection matrix as a topological quantum
number.Comment: 7 pages, 4 figures; v3: added appendix with numerics for long-range
disorde
Coulomb-assisted braiding of Majorana fermions in a Josephson junction array
We show how to exchange (braid) Majorana fermions in a network of
superconducting nanowires by control over Coulomb interactions rather than
tunneling. Even though Majorana fermions are charge-neutral quasiparticles
(equal to their own antiparticle), they have an effective long-range
interaction through the even-odd electron number dependence of the
superconducting ground state. The flux through a split Josephson junction
controls this interaction via the ratio of Josephson and charging energies,
with exponential sensitivity. By switching the interaction on and off in
neighboring segments of a Josephson junction array, the non-Abelian braiding
statistics can be realized without the need to control tunnel couplings by gate
electrodes. This is a solution to the problem how to operate on topological
qubits when gate voltages are screened by the superconductor
Dessins, their delta-matroids and partial duals
Given a map on a connected and closed orientable surface, the
delta-matroid of is a combinatorial object associated to which captures some topological information of the embedding. We explore how
delta-matroids associated to dessins d'enfants behave under the action of the
absolute Galois group. Twists of delta-matroids are considered as well; they
correspond to the recently introduced operation of partial duality of maps.
Furthermore, we prove that every map has a partial dual defined over its field
of moduli. A relationship between dessins, partial duals and tropical curves
arising from the cartography groups of dessins is observed as well.Comment: 34 pages, 20 figures. Accepted for publication in the SIGMAP14
Conference Proceeding
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