3,113 research outputs found
Topological Order and the Quantum Spin Hall Effect
The quantum spin Hall (QSH) phase is a time reversal invariant electronic
state with a bulk electronic band gap that supports the transport of charge and
spin in gapless edge states. We show that this phase is associated with a novel
topological invariant, which distinguishes it from an ordinary insulator.
The classification, which is defined for time reversal invariant
Hamiltonians, is analogous to the Chern number classification of the quantum
Hall effect. We establish the order of the QSH phase in the two band
model of graphene and propose a generalization of the formalism applicable to
multi band and interacting systems.Comment: 4 pages RevTeX. Added reference, minor correction
Wagner’s Law and Peacock and Wiseman’s Displacement Effect in European Union Countries: A Panel Data Study
Wagner’s Law is the fi rst model of public expenditure in the history of public fi nance. It suggests that during the process of economic
development the share of public spending in national income tends to expand (Wagner, 1883). Nevertheless, Peacock and Scott in 2000 wrote
a paper entitled “The curious attraction of Wagner’s law,” explaining the reasons for why this (apparently) outworn theory is still studied by
modern economists. On the other hand, Keynes (1936) considered public spending as an exogenous factor to be used as a policy instrument to
infl uence growth. Moreover, Peacock and Wiseman (1961) presented the displacement effect, according to which during times of war tax rates
are increased to generate more revenues, sustaining the increase in defense spending. While Peacock and Wiseman (1979) surveys the literature
on public expenditure growth. This paper aims to analyze the relationship between public expenditure and aggregate income in European Union
countries, for the period 1980-2013, using panel data methodologies. After a brief introduction, a survey of the economic literature on this issue
is discussed. Then, panel data tests on stationarity, cross-dependence, cointegration, and causality are shown. Finally, some notes on policy
implications conclude the paper
Theoretical investigation of the evolution of the topological phase of BiSe under mechanical strain
The topological insulating phase results from inversion of the band gap due
to spin-orbit coupling at an odd number of time-reversal symmetric points. In
BiSe, this inversion occurs at the point. For bulk
BiSe, we have analyzed the effect of arbitrary strain on the
point band gap using Density Functional Theory. By computing the band structure
both with and without spin-orbit interactions, we consider the effects of
strain on the gap via Coulombic interaction and spin-orbit interaction
separately. While compressive strain acts to decrease the Coulombic gap, it
also increases the strength of the spin-orbit interaction, increasing the
inverted gap. Comparison with BiTe supports the conclusion that effects
on both Coulombic and spin-orbit interactions are critical to understanding the
behavior of topological insulators under strain, and we propose that the
topological insulating phase can be effectively manipulated by inducing strain
through chemical substitution
Mechanical properties of polyethylene terephthalate under selected conditions and methods of preparation Semiannual progress report, period ending 31 Oct. 1967
Mechanical properties of polyethylene terephthalate under selected conditions and methods of preparatio
Spin texture on the Fermi surface of tensile strained HgTe
We present ab initio and k.p calculations of the spin texture on the Fermi
surface of tensile strained HgTe, which is obtained by stretching the
zincblende lattice along the (111) axis. Tensile strained HgTe is a semimetal
with pointlike accidental degeneracies between a mirror symmetry protected
twofold degenerate band and two nondegenerate bands near the Fermi level. The
Fermi surface consists of two ellipsoids which contact at the point where the
Fermi level crosses the twofold degenerate band along the (111) axis. However,
the spin texture of occupied states indicates that neither ellipsoid carries a
compensating Chern number. Consequently, the spin texture is locked in the
plane perpendicular to the (111) axis, exhibits a nonzero winding number in
that plane, and changes winding number from one end of the Fermi ellipsoids to
the other. The change in the winding of the spin texture suggests the existence
of singular points. An ordered alloy of HgTe with ZnTe has the same effect as
stretching the zincblende lattice in the (111) direction. We present ab initio
calculations of ordered Hg_xZn_1-xTe that confirm the existence of a spin
texture locked in a 2D plane on the Fermi surface with different winding
numbers on either end.Comment: 8 pages, 8 figure
A Sustainable Approach to Delivering Programmable Peer-to-Peer Offline Payments
Payment apps and digital wallets are powerful tools used to exchange e-money via the internet. However, with the progressive disappearance of cash, there is a need for the digital equivalent of physical banknotes to guarantee the same level of anonymity of private payments. Few efforts to solve the double-spending problem exist in P2P payments (i.e., in avoiding the possibility of a payer retaining copies of digital coins in absence of a trusted third party (TTP)), and further research efforts are needed to explore options to preserve the privacy of payments, as per the mandates of numerous central bank digital currency (CBDC) exploratory initiatives, such as the digital euro. Moreover, generic programmability requirements and energetic impacts should be considered. In this paper, we present a sustainable offline P2P payment scheme to face the double-spending problem by means of a one-time program (OTP) approach. The approach consists of wiping the business logic out of a client’s app and allowing financial intermediaries to inject a certified payment code into the user’s device, which will execute (asynchronously and offline) at the time of payment. To do so, we wrap each coin in a program at the time of withdrawal. Then the program exploits the trusted execution environment (TEE) of modern smartphones to transfer itself from the payer to the payee via a direct IoT link. To confirm the validity of the approach, we performed qualitative and quantitative evaluations, specifically focusing on the energetic sustainability of the proposed scheme. Results show that our payment scheme is energetically sustainable as the current absorbed for sending one coin is, at most, ~1.8 mAh on an Apple smartphone. We advance the state-of-the-art because the scheme meets the programmability, anonymity, and sustainability requirements (at the same time)
Bioinformatics tools for marine biotechnology: A practical tutorial with a metagenomic approach
Background: Bioinformatics has pervaded all fields of biology and has become an indispensable tool for almost all research projects. Although teaching bioinformatics has been incorporated in all traditional life science curricula, practical hands-on experiences in tight combination with wet-lab experiments are needed to motivate students. Results: We present a tutorial that starts from a practical problem: finding novel enzymes from marine environments. First, we introduce the idea of metagenomics, a recent approach that extends biotechnology to non-culturable microbes. We presuppose that a probe for the screening of metagenomic cosmid library is needed. The students start from the chemical structure of the substrate that should be acted on by the novel enzyme and end with the sequence of the probe. To attain their goal, they discover databases such as BRENDA and programs such as BLAST and Clustal Omega. Students' answers to a satisfaction questionnaire show that a multistep tutorial integrated into a research wet-lab project is preferable to conventional lectures illustrating bioinformatics tools. Conclusion: Experimental biologists can better operate basic bioinformatics if a problem-solving approach is chosen
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