8,787 research outputs found
Two-dimensional topological field theories as taffy
In this paper we use trivial defects to define global taffy-like operations
on string worldsheets, which preserve the field theory. We fold open and closed
strings on a space X into open strings on products of multiple copies of X, and
perform checks that the "taffy-folded" worldsheets have the same massless
spectra and other properties as the original worldsheets. Such folding tricks
are a standard method in the defects community; the novelty of this paper lies
in deriving mathematical identities to check that e.g. massless spectra are
invariant in topological field theories. We discuss the case of the B model
extensively, and also derive the same identities for string topology, where
they become statements of homotopy invariance. We outline analogous results in
the A model, B-twisted Landau-Ginzburg models, and physical strings. We also
discuss the understanding of the closed string states as the Hochschild
homology of the open string algebra, and outline possible applications to
elliptic genera.Comment: 61 pages, LaTeX; v2: typos fixe
Hall plateau diagram for the Hofstadter butterfly energy spectrum
We extensively study the localization and the quantum Hall effect in the
Hofstadter butterfly, which emerges in a two-dimensional electron system with a
weak two-dimensional periodic potential. We numerically calculate the Hall
conductivity and the localization length for finite systems with the disorder
in general magnetic fields, and estimate the energies of the extended levels in
an infinite system. We obtain the Hall plateau diagram on the whole region of
the Hofstadter butterfly, and propose a theory for the evolution of the plateau
structure with increasing disorder. There we show that a subband with the Hall
conductivity has separated bunches of extended levels, at least
for an integer . We also find that the clusters of the subbands with
identical Hall conductivity, which repeatedly appear in the Hofstadter
butterfly, have a similar localization property.Comment: 9 pages, 12 figure
Improvement of acoustic theory of ultrasonic waves in dilute bubbly liquids
The theory of the acoustics of dilute bubbly liquids is reviewed, and the dispersion relation is modified by including the effect of liquid compressibility on the natural frequency of the bubbles. The modified theory is shown to more accurately predict the trend in measured attenuation of ultrasonic waves. The model limitations associated with such high-frequency waves are discussed
Magnetotransport Properties of Antiferromagnetic YBa_2Cu_3O_6.25 Single Crystals
In-plane and out-of-plane magnetoresistivities (MR) of antiferromagnetic
YBa_2Cu_3O_6.25 single crystals were measured in magnetic fields H applied
along the (ab) plane. In-plane MR is a superposition of two components: The
first component is strongly in-plane anisotropic, changing sign from negative
when H is parallel to the electrical current I to positive when H is
perpendicular to I. The second component is positive, quadratic in H, and
isotropic in the (ab)-plane. The out-of-plane MR displays a fourfold symmetry
upon in-plane rotation of the magnetic field, with maxima along the easy axes
of antiferromagnetic spin ordering and minima along unfavorable directions of
spin orientation (45 degrees from the Cu-O-Cu bonds).Comment: 8 pages, 4 figure
Electron Interactions and Scaling Relations for Optical Excitations in Carbon Nanotubes
Recent fluorescence spectroscopy experiments on single wall carbon nanotubes
reveal substantial deviations of observed absorption and emission energies from
predictions of noninteracting models of the electronic structure. Nonetheless,
the data for nearly armchair nanotubes obey a nonlinear scaling relation as a
function the tube radius . We show that these effects can be understood in a
theory of large radius tubes, derived from the theory of two dimensional
graphene where the coulomb interaction leads to a logarithmic correction to the
electronic self energy and marginal Fermi liquid behavior. Interactions on
length scales larger than the tube circumference lead to strong self energy and
excitonic effects that compete and nearly cancel so that the observed optical
transitions are dominated by the graphene self energy effects.Comment: 4 page
GeV Emission from Prompt and Afterglow Phases of Gamma-Ray Bursts
We investigate the GeV emission from gamma-ray bursts (GRBs), using the
results from the Energetic Gamma Ray Experimental Telescope (EGRET), and in
view of the Gamma-ray Large Area Space Telescope (GLAST). Assuming that the
conventional prompt and afterglow photons originate from synchrotron radiation,
we compare an accompanying inverse-Compton component with EGRET measurements
and upper limits on GeV fluence, taking Klein-Nishina feedback into account. We
find that EGRET constraints are consistent with the theoretical framework of
the synchrotron self-Compton model for both prompt and afterglow phases, and
discuss constraints on microphysical parameters in both phases. Based on the
inverse-Compton model and using EGRET results, we predict that GLAST would
detect GRBs with GeV photons at a rate >~20 yr^{-1} from each of the prompt and
afterglow phases. This rate applies to the high-energy tail of the prompt
synchrotron emission and to the inverse-Compton component of the afterglow.
Theory predicts that in a large fraction of the cases where synchrotron GeV
prompt emission would be detected by GLAST, inverse-Compton photons should be
detected as well at high energies >~10 GeV. Therefore GLAST will enable a more
precise test of the high-energy emission mechanism. Finally, we show that the
contribution of GRBs to the flux of the extragalactic gamma-ray background
measured with EGRET is at least 0.01% and likely around 0.1%.Comment: 11 pages, 7 figures; accepted by Ap
Stochastic Liouville Equations for Femtosecond Stimulated Raman Spectroscopy
Electron and vibrational dynamics of molecules are commonly studied by
subjecting them to two interactions with a fast actinic pulse that prepares
them in a nonstationary state and after a variable delay period , probing
them with a Raman process induced by a combination of a broadband and a
narrowband pulse. This technique known as femtosecond stimulated Raman
spectroscopy (FSRS) can effectively probe time resolved vibrational resonances.
We show how FSRS signals can be modeled and interpreted using the stochastic
Liouville equations (SLE) originally developed for NMR lineshapes. The SLE
provides a convenient simulation protocol that can describe complex dynamics
due to coupling to collective coordinates at much lower cost that a full
dynamical simulation. The origin of the dispersive features which appear when
there is no separation of timescales between vibrational variations and
dephasing is clarified
Cluster decomposition, T-duality, and gerby CFT's
In this paper we study CFT's associated to gerbes. These theories suffer from
a lack of cluster decomposition, but this problem can be resolved: the CFT's
are the same as CFT's for disconnected targets. Such theories also lack cluster
decomposition, but in that form, the lack is manifestly not very problematic.
In particular, we shall see that this matching of CFT's, this duality between
noneffective gaugings and sigma models on disconnected targets, is a worldsheet
duality related to T-duality. We perform a wide variety of tests of this claim,
ranging from checking partition functions at arbitrary genus to D-branes to
mirror symmetry. We also discuss a number of applications of these results,
including predictions for quantum cohomology and Gromov-Witten theory and
additional physical understanding of the geometric Langlands program.Comment: 61 pages, LaTeX; v2,3: typos fixed; v4: writing improved in several
sections; v5: typos fixe
Effect of the superconducting wiggler on the DELSY beam dynamics
The project DELSY is being under development at JINR, Dubna, Russia. This
synchrotron radiation source is dedicated to the investigation on condensed
matter physics, atomic physics, biology, medicine, chemistry, micromechanics,
lithography and others. The storage ring DELSY is an electron storage ring with
the beam energy 1.2 GeV and 4 straight sections to accommodate accelerator
equipment and insertion devices. One of the straight sections is intended for a
10 T superconducting wiggler (wavelength shifter) and one for the undulator
with 150 periods and a magnetic field of 0.75 T. The wiggler will influence
many aspects of beam dynamics: linear motion, dynamic aperture, emittance,
damping times etc. The problem is rather serious for the DELSY machine because
the energy of the electron beam is small while the wiggler's magnetic field is
strong. In this paper we consider two models of the wiggler's magnetic field
with and without the focusing caused by the sextupolar field of the wiggler as
we need to develop the requirements to the wiggler design. We study the
influence of the 10 T wiggler on the beam dynamics in the DELSY storage ring
and propose a possible scheme to cure it. The combined work of the insertion
device is presented too.Comment: 17 pages, submitted to journal NIM
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