2,737 research outputs found
A twistor-like D=10 superparticle action with manifest N=8 world-line supersymmetry
We propose a new formulation of the Brink-Schwarz superparticle which
is manifestly invariant under both the target-space super-Poincar\'e group and
the world-line local superconformal group. This twistor-like construction
naturally involves the sphere as a coset space of the Lorentz
group. The action contains only a finite set of auxiliary fields, but they
appear in unusual trilinear combinations. The origin of the on-shell
fermionic symmetry of the standard Brink-Schwarz formulation is
explained. The coupling to a super-Maxwell background requires a new
mechanism, in which the electric charge appears only on shell as an integration
constant.Comment: 22pages, standard LATEX fil
A Twistor Formulation of the Non-Heterotic Superstring with Manifest Worldsheet Supersymmetry
We propose a new formulation of the type II superstring which is
manifestly invariant under both target-space supersymmetry and worldsheet
super reparametrizations. This gives rise to a set of twistor
(commuting spinor) variables, which provide a solution to the two Virasoro
constraints. The worldsheet supergravity fields are shown to play the r\^ole of
auxiliary fields.Comment: 21p., LaTe
Two-dimensional semiconducting nanostructures based on single graphene sheets with lines of adsorbed hydrogen atoms
It is shown that lines of adsorbed hydrogen pair atoms divide the graphene
sheet into strips and form hydrogen-based superlattice structures (2HG-SL). We
show that the forming of 2HG-SL drastically changes the electronic properties
of graphene from semimetal to semiconductor. The electronic spectra of "zigzag"
(n,0) 2HG-SL is similar to that of (n,0) carbon nanotubes and have a similar
oscillation of band gap with number n, but with non-zero minimal values. The
composite dual-periodic (n,0)+(m,0) 2HG-SLs of "zigzag" strips are analyzed,
with the conclusion that they may be treated as quasi-two-dimensional
heterostructures. We also suggest an experimental way of fabricating hydrogen
superlattices.Comment: 12 pages, 3 figure
Propionate and butyrate dependent bacterial sulfate reduction at extremely haloalkaline conditions and description of Desulfobotulus alkaliphilus sp. nov.
Evidence on the utilization of simple fatty acids by sulfate-reducing bacteria (SRB) at extremely haloalkaline conditions are practically absent, except for a single case of syntrophy by Desulfonatronum on acetate. Our experiments with sediments from soda lakes of Kulunda Steppe (Altai, Russia) showed sulfide production with sulfate as electron acceptor and propionate and butyrate (but not acetate) as an electron donor at a pH 10–10.5 and a salinity 70–180 g l−1. With propionate as substrate, a highly enriched sulfidogenic culture was obtained in which the main component was identified as a novel representative of the family Syntrophobacteraceae. With butyrate as substrate, a pure SRB culture was isolated which oxidized butyrate and some higher fatty acids incompletely to acetate. The strain represents the first haloalkaliphilic representative of the family Desulfobacteraceae and is described as Desulfobotulus alkaliphilus sp. nov
Continuous-wave room-temperature diamond maser
The maser, older sibling of the laser, has been confined to relative
obscurity due to its reliance on cryogenic refrigeration and high-vacuum
systems. Despite this it has found application in deep-space communications and
radio astronomy due to its unparalleled performance as a low-noise amplifier
and oscillator. The recent demonstration of a room-temperature solid- state
maser exploiting photo-excited triplet states in organic pentacene molecules
paves the way for a new class of maser that could find applications in
medicine, security and sensing, taking advantage of its sensitivity and low
noise. However, to date, only pulsed operation has been observed in this
system. Furthermore, organic maser molecules have poor thermal and mechanical
properties, and their triplet sub-level decay rates make continuous emission
challenging: alternative materials are therefore required. Therefore, inorganic
materials containing spin-defects such as diamond and silicon carbide have been
proposed. Here we report a continuous-wave (CW) room-temperature maser
oscillator using optically pumped charged nitrogen-vacancy (NV) defect centres
in diamond. This demonstration unlocks the potential of room-temperature
solid-state masers for use in a new generation of microwave devices.Comment: 7 pages, 4 figure
A PC parallel port button box provides millisecond response time accuracy under Linux
For psychologists, it is sometimes necessary to measure people's reaction times to the nearest millisecond. This article describes how to use the PC parallel port to receive signals from a button box to achieve millisecond response time accuracy. The workings of the parallel port, the corresponding port addresses, and a simple Linux program for controlling the port are described. A test of the speed and reliability of button box signal detection is reported. If the reader is moderately familiar with Linux, this article should provide sufficient instruction for him or her to build and test his or her own parallel port button box. This article also describes how the parallel port could be used to control an external apparatus
Why Don't We Have a Covariant Superstring Field Theory?
This talk deals with the old problem of formulatingn a covariant quantum
theory of superstrings, ``covariant'' here meaning having manifest Lorentz
symmetry and supersymmetry. The advantages and disadvantages of several
quantization methods are reviewed. Special emphasis is put on the approaches
using twistorial variables, and the algebraic structures of these. Some
unsolved problems are identified.Comment: 5 pages, Goteborg-ITP-94-24, plain te
Metal-semiconductor (semimetal) superlattices on a graphite sheet with vacancies
It has been found that periodically closely spaced vacancies on a graphite
sheet cause a significant rearrange-ment of its electronic spectrum: metallic
waveguides with a high density of states near the Fermi level are formed along
the vacancy lines. In the direction perpendicular to these lines, the spectrum
exhibits a semimetal or semiconductor character with a gap where a vacancy
miniband is degenerated into impurity levels.Comment: 4 pages, 3 figure
Superfield Theories in Tensorial Superspaces and the Dynamics of Higher Spin Fields
We present the superfield generalization of free higher spin equations in
tensorial superspaces and analyze tensorial supergravities with GL(n) and SL(n)
holonomy as a possible framework for the construction of a non-linear higher
spin field theory. Surprisingly enough, we find that the most general solution
of the supergravity constraints is given by a class of superconformally flat
and OSp(1|n)-related geometries. Because of the conformal symmetry of the
supergravity constraints and of the higher spin field equations such geometries
turn out to be trivial in the sense that they cannot generate a `minimal'
coupling of higher spin fields to their potentials even in curved backgrounds
with a non-zero cosmological constant. This suggests that the construction of
interacting higher spin theories in this framework might require an extension
of the tensorial superspace with additional coordinates such as twistor-like
spinor variables which are used to construct the OSp(1|2n) invariant
(`preonic') superparticle action in tensorial superspace.Comment: LaTeX, 30 pages, no figures. V2. Discussion on conventional
constraints extended, typos corrected, JHEP style, to appear in JHE
Superlattices Consisting of "Lines" of Adsorbed Hydrogen Atom Pairs on Graphene
The structures and electron properties of new superlattices formed on
graphene by adsorbed hydrogen molecules are theoretically described. It has
been shown that superlattices of the (n, 0) zigzag type with linearly arranged
pairs of H atoms have band structures similar to the spectra of (n, 0) carbon
nanotubes. At the same time, superlattices of the (n, n) type with a
"staircase" of adsorbed pairs of H atoms are substantially metallic with a high
density of electronic states at the Fermi level and this property distinguishes
their spectra from the spectra of the corresponding (n, n) nanotubes. The
features of the spectra have the Van Hove form, which is characteristic of each
individual superlattice. The possibility of using such planar structures with
nanometer thickness is discussed.Comment: 5 pages, 4 figure
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