7,261 research outputs found
Cryogenic MMIC low noise amplifiers
Monolithic (MMIC) and discrete transistor (MIC) low noise amplifiers are compared on the basis of performance, cost, and reliability. The need for cryogenic LNA’s for future large microwave arrays for radio astronomy is briefly discussed and data is presented on a prototype LNA for the 1 to 10 GHz range along with a very wideband LNA for the 1 to 60 GHz range. A table of MMIC LNA and mixer designs under development for the frequencies up to 210 GHz is reported and data on cryogenic amplifiers in the 85 to 115 GHz is reviewed. The current status of the topics of transconductance fluctuations and cryogenic noise modeling will be briefly summarized
Squarepants in a Tree: Sum of Subtree Clustering and Hyperbolic Pants Decomposition
We provide efficient constant factor approximation algorithms for the
problems of finding a hierarchical clustering of a point set in any metric
space, minimizing the sum of minimimum spanning tree lengths within each
cluster, and in the hyperbolic or Euclidean planes, minimizing the sum of
cluster perimeters. Our algorithms for the hyperbolic and Euclidean planes can
also be used to provide a pants decomposition, that is, a set of disjoint
simple closed curves partitioning the plane minus the input points into subsets
with exactly three boundary components, with approximately minimum total
length. In the Euclidean case, these curves are squares; in the hyperbolic
case, they combine our Euclidean square pants decomposition with our tree
clustering method for general metric spaces.Comment: 22 pages, 14 figures. This version replaces the proof of what is now
Lemma 5.2, as the previous proof was erroneou
Molecular Gas Content of HI Monsters and Implications to Cold Gas Content Evolution in Galaxies
We present 12CO (J=1-0) observations of a sample of local galaxies
(0.04<z<0.08) with a large neutral hydrogen reservoir, or "HI monsters". The
data were obtained using the Redshift Search Receiver on the FCRAO 14 m
telescope. The sample consists of 20 HI-massive galaxies with M(HI)>3e10Msun
from the ALFALFA survey and 8 LSBs with a comparable M(HI) (>1.5e10Msun). Our
sample selection is purely based on the amount of neutral hydrogen, thereby
providing a chance to study how atomic and molecular gas relate to each other
in these HI-massive systems. We have detected CO in 15 out of 20 ALFALFA
selected galaxies and 4 out of 8 LSBs with molecular gas mass M(H2) of
(1-11)e9Msun. Their total cold gas masses of (2-7e10Msun make them some of the
most gas-massive galaxies identified to date in the Local Universe. Observed
trends associated with HI, H2, and stellar properties of the HI massive
galaxies and the field comparison sample are analyzed in the context of
theoretical models of galaxy cold gas content and evolution, and the importance
of total gas content and improved recipes for handling spatially differentiated
behaviors of disk and halo gas are identified as potential areas of improvement
for the modeling.Comment: 18 pages, 11 figures, 2 tables; Accepted for publication in MNRA
Supersymmetric Open Wilson Lines
In this paper we study Open Wilson Lines (OWL's) in the context of two
Supersymmetric Yang Mills theories. First we consider four dimensional N=2
Supersymmetric Yang Mills Theory with hypermultiplets transforming in the
fundamental representation of the gauge group, and find supersymmetric OWL's
only in the superconformal versions of these theories. We then consider four
dimensional N=4 SYM coupled to a three dimensional defect hypermultiplet. Here
there is a semi-circular supersymmetric OWL, which is related to the ray by a
conformal transformation. We perform a perturbative calculation of the
operators in both theories, and discuss using localization to compute them
non-perturbatively.Comment: 26 pages, 3 figure
Ladders for Wilson Loops Beyond Leading Order
We set up a general scheme to resum ladder diagrams for the quark-anti-quark
potential in N=4 super-Yang-Mills theory, and do explicit calculations at the
next-to-leading order. The results perfectly agree with string theory in
AdS(5)xS(5) when continued to strong coupling, in spite of a potential
order-of-limits problem.Comment: 18 pages, 5 figure
Comparing strings in AdS(5)xS(5) to planar diagrams: an example
The correlator of a Wilson loop with a local operator in N=4 SYM theory can
be represented by a string amplitude in AdS(5)xS(5). This amplitude describes
an overlap of the boundary state, which is associated with the loop, with the
string mode, which is dual to the local operator. For chiral primary operators
with a large R charge, the amplitude can be calculated by semiclassical
techniques. We compare the semiclassical string amplitude to the SYM
perturbation theory and find an exact agrement to the first two non-vanishing
orders.Comment: 16 pages, 4 figures, LaTeX; v2: typos corrected; v3: clarification of
boundary conditions at infinity adde
On the integrability of Wilson loops in AdS_5 x S^5: Some periodic ansatze
Wilson loops are calculated within the AdS/CFT correspondence by finding a
classical solution to the string equations of motion in AdS_5 x S^5 and
evaluating its action. An important fact is that this sigma-model used to
evaluate the Wilson loops is integrable, a feature that has gained relevance
through the study of spinning strings carrying large quantum numbers and
spin-chains. We apply the same techniques used to solve the equations for
spinning strings to find the minimal surfaces describing a wide class of Wilson
loops. We focus on different cases with periodic boundary conditions on the
AdS_5 and S^5 factors and find a rich array of solutions. We examine the
different phases that appear in the problem and comment on the applicability of
integrability to the general problem.Comment: LaTex, 49 pages, 8 figure
Quark--anti-quark potential in N=4 SYM
We construct a closed system of equations describing the quark--anti-quark
potential at any coupling in planar N=4 supersymmetric Yang-Mills theory. It is
based on the Quantum Spectral Curve method supplemented with a novel type of
asymptotics. We present a high precision numerical solution reproducing the
classical and one-loop string predictions very accurately. We also analytically
compute the first 7 nontrivial orders of the weak coupling expansion.
Moreover, we study analytically the generalized quark--anti-quark potential
in the limit of large imaginary twist to all orders in perturbation theory. We
demonstrate how the QSC reduces in this case to a one-dimensional Schrodinger
equation. In the process we establish a link between the Q-functions and the
solution of the Bethe-Salpeter equation.Comment: 31 pages, 1 figure; v2: minor correcton
Open string fluctuations in AdS_5xS^5 and operators with large R-charge
A semiclassical string description is given for correlators of Wilson loops
with local operators in N=4 SYM theory in the regime when operators carry
parametrically large R-charge. The OPE coefficients of the circular Wilson loop
in chiral primary operators are computed to all orders in the alpha' expansion
in AdS_5xS^5 string theory. The results agree with field-theory predictions.Comment: 16 pages, 2 figures; v2: five misprints correcte
- …