968 research outputs found
Two performances of the "Return of Alpamis" : current performance-practice in the Uzbek oral epic of the Sheraban school
The present study attempts to clarify the issues of text composition and poetic style within the Uzbek oral poetic genre known as the doston (dastan). It focuses on four short oral poetic texts: two recordings of two identical sections of the Alpamis doston sung by a single bard (baxsi) in 1990 and in 1991. Due to the fact that researchers within Uzbekistan and other former Soviet republics of Central Asia have paid little attention to issues of "improvisation" and "memorization," even such a modest attempt at multiple recording can help to state the relevant questions more clearly. A close analysis of the four texts demonstrates how the techniques of oral composition intersect with poetic style. This analysis is aided at times by interviews with and explanations from the bard. In addition, these interviews and observations of the bard and his immediate environment reveal aspects of the baxsi profession that he viewed as having significance.Not
Singular order parameter interaction at nematic quantum critical point in two dimensional electron systems
We analyze the infrared behavior of effective N-point interactions between
order parameter fluctuations for nematic and other quantum critical electron
systems with a scalar order parameter in two dimensions. The interactions
exhibit a singular momentum and energy dependence and thus cannot be
represented by local vertices. They diverge for all N greater or equal 4 in a
collinear static limit, where energy variables scale to zero faster than
momenta, and momenta become increasingly collinear. The degree of divergence is
not reduced by any cancellations and renders all N-point interactions marginal.
A truncation of the order parameter action at quartic or any other finite order
is therefore not justified. The same conclusion can be drawn for the effective
action describing fermions coupled to a U(1) gauge field in two dimensions.Comment: 18 pages, 1 figur
Determinant Bounds and the Matsubara UV Problem of Many-Fermion Systems
It is known that perturbation theory converges in fermionic field theory at
weak coupling if the interaction and the covariance are summable and if certain
determinants arising in the expansion can be bounded efficiently, e.g. if the
covariance admits a Gram representation with a finite Gram constant. The
covariances of the standard many--fermion systems do not fall into this class
due to the slow decay of the covariance at large Matsubara frequency, giving
rise to a UV problem in the integration over degrees of freedom with Matsubara
frequencies larger than some Omega (usually the first step in a multiscale
analysis). We show that these covariances do not have Gram representations on
any separable Hilbert space. We then prove a general bound for determinants
associated to chronological products which is stronger than the usual Gram
bound and which applies to the many--fermion case. This allows us to prove
convergence of the first integration step in a rather easy way, for a
short--range interaction which can be arbitrarily strong, provided Omega is
chosen large enough. Moreover, we give - for the first time - nonperturbative
bounds on all scales for the case of scale decompositions of the propagator
which do not impose cutoffs on the Matsubara frequency.Comment: 29 pages LaTe
Spectral Function of 2D Fermi Liquids
We show that the spectral function for single-particle excitations in a
two-dimensional Fermi liquid has Lorentzian shape in the low energy limit.
Landau quasi-particles have a uniquely defined spectral weight and a decay rate
which is much smaller than the quasi-particle energy. By contrast, perturbation
theory and the T-matrix approximation yield spurious deviations from Fermi
liquid behavior, which are particularly pronounced for a linearized dispersion
relation.Comment: 6 pages, LaTeX2e, 5 EPS figure
Analysis of Shroud Options in Support of the Human Exploration of Mars
In support of the Mars Design Reference Architecture (DRA) 5.0, the NASA study team analyzed several shroud options for use on the Ares V launch vehicle.1,2 These shroud options included conventional "large encapsulation" shrouds with outer diameters ranging from 8.4 to 12.9 meters (m) and overall lengths of 22.0 to 54.3 meters, along with a "nosecone-only" shroud option used for Mars transfer vehicle component delivery. Also examined was a "multi-use" aerodynamic encapsulation shroud used for launch, Mars aerocapture, and entry, descent, and landing of the cargo and habitat landers. All conventional shroud options assessed for use on the Mars launch vehicles were the standard biconic design derived from the reference shroud utilized in the Constellation Program s lunar campaign. It is the purpose of this paper to discuss the technical details of each of these shroud options including material properties, structural mass, etc., while also discussing both the volume and mass of the various space transportation and surface system payload elements required to support a "minimum launch" Mars mission strategy, as well as the synergy, potential differences and upgrade paths that may be required between the Lunar and Mars mission shrouds
Renormalized perturbation theory for Fermi systems: Fermi surface deformation and superconductivity in the two-dimensional Hubbard model
Divergencies appearing in perturbation expansions of interacting many-body
systems can often be removed by expanding around a suitably chosen renormalized
(instead of the non-interacting) Hamiltonian. We describe such a renormalized
perturbation expansion for interacting Fermi systems, which treats Fermi
surface shifts and superconductivity with an arbitrary gap function via
additive counterterms. The expansion is formulated explicitly for the Hubbard
model to second order in the interaction. Numerical soutions of the
self-consistency condition determining the Fermi surface and the gap function
are calculated for the two-dimensional case. For the repulsive Hubbard model
close to half-filling we find a superconducting state with d-wave symmetry, as
expected. For Fermi levels close to the van Hove singularity a Pomeranchuk
instability leads to Fermi surfaces with broken square lattice symmetry, whose
topology can be closed or open. For the attractive Hubbard model the second
order calculation yeilds s-wave superconductivity with a weakly momentum
dependent gap, whose size is reduced compared to the mean-field result.Comment: 18 pages incl. 6 figure
GALEX FUV Observations of Comet C/2004 Q2 (Machholz): The Ionization Lifetime of Carbon
We present a measurement of the lifetime of ground state atomic carbon,
C(^3P), against ionization processes in interplanetary space and compare it to
the lifetime expected from the dominant physical processes likely to occur in
this medium. Our measurement is based on analysis of a far ultraviolet (FUV)
image of comet C/2004 Q2 (Machholz) recorded by the Galaxy Evolution Explorer
(GALEX) on 2005 March 1. The bright CI 1561 A and 1657 A multiplets dominate
the GALEX FUV band. We used the image to create high S/N radial profiles that
extended beyond one million km from the comet nucleus. Our measurements yielded
a total carbon lifetime of 7.1 -- 9.6 x 10^5 s (scaled to 1 AU). Which compares
favorably to calculations assuming solar photoionization, solar wind proton
change exchange and solar wind electron impact ionization are the dominant
processes occurring in this medium and that comet Machholz was embedded in the
slow solar wind. The shape of the CI profiles inside 3x10^5 km suggests that
either the CO lifetime is shorter than previously thought and/or a
shorter-lived carbon-bearing parent molecule, such as CH_4 is providing the
majority of the carbon in this region of the coma of comet Machholz.Comment: 26 pages, 6 figures, accepted for publication in the Astrophysical
Journa
Functional renormalization group approach to correlated fermion systems
Numerous correlated electron systems exhibit a strongly scale-dependent
behavior. Upon lowering the energy scale, collective phenomena, bound states,
and new effective degrees of freedom emerge. Typical examples include (i)
competing magnetic, charge, and pairing instabilities in two-dimensional
electron systems, (ii) the interplay of electronic excitations and order
parameter fluctuations near thermal and quantum phase transitions in metals,
(iii) correlation effects such as Luttinger liquid behavior and the Kondo
effect showing up in linear and non-equilibrium transport through quantum wires
and quantum dots. The functional renormalization group is a flexible and
unbiased tool for dealing with such scale-dependent behavior. Its starting
point is an exact functional flow equation, which yields the gradual evolution
from a microscopic model action to the final effective action as a function of
a continuously decreasing energy scale. Expanding in powers of the fields one
obtains an exact hierarchy of flow equations for vertex functions. Truncations
of this hierarchy have led to powerful new approximation schemes. This review
is a comprehensive introduction to the functional renormalization group method
for interacting Fermi systems. We present a self-contained derivation of the
exact flow equations and describe frequently used truncation schemes. Reviewing
selected applications we then show how approximations based on the functional
renormalization group can be fruitfully used to improve our understanding of
correlated fermion systems.Comment: Review article, final version, 59 pages, 28 figure
Fermion loops, loop cancellation and density correlations in two dimensional Fermi systems
We derive explicit results for fermion loops with an arbitrary number of
density vertices in two dimensions at zero temperature. The 3-loop is an
elementary function of the three external momenta and frequencies, and the
N-loop can be expressed as a linear combination of 3-loops with coefficients
that are rational functions of momenta and frequencies. We show that the
divergencies of single loops for low energy and small momenta cancel each other
when loops with permuted external variables are summed. The symmetrized N-loop,
i.e. the connected N-point density correlation function of the Fermi gas, does
not diverge for low energies and small momenta. In the dynamical limit, where
momenta scale to zero at fixed finite energy variables, the symmetrized N-loop
vanishes as the (2N-2)-th power of the scale parameter.Comment: 24 pages (including 3 EPS figures), LaTeX2e; submitted to Phys. Rev.
A packet of letters
1 volume, 19 pages. One of 45 numbered copies. The type of A Packet of Letters is 18 pt. Bodoni and was set by Mollohan Typesetting of West Warwick, Rhode Island. It was printed on Nideggen paper on a hand fed Vandercook Press. The binding is by Stuart Einhorn of Providence, R.I.A Packet of Letters was published in an edition of 45 ... --Colophon. Title and text in red and black on title page. Bound at the head of the text with white-coated metal spiral binding. Text is attached to a cloth cover binder with an off-center opening in front. A mounted paper label with title and author name is split to allow the cover opening. Library has copy no. 31 Keywords: woodcuts, literacy, World War II, letters from a son serving in the military during WWII to his mother.https://digitalcommons.risd.edu/specialcollections_artistsbooks/1214/thumbnail.jp
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