22,103 research outputs found
The un-structured student
SQL (Structured Query Language) is intended to liberate users from the complex syntax, complex semantics and complex memory management that would be required of a procedural approach to relational database manipulation, and so allow users to concentrate on problem-solving. However, students still have problems with language features, program concepts and the legacy of their prior learning. This paper aims to assist lecturers teaching SQL.We present typical student mistakes, attempt to explain why these mistakes arise, and propose possible remedies
Equilibration rates in a strongly coupled nonconformal quark-gluon plasma
We initiate the study of equilibration rates of strongly coupled quark-gluon
plasmas in the absence of conformal symmetry. We primarily consider a
supersymmetric mass deformation within gauge theory and use
holography to compute quasinormal modes of a variety of scalar operators, as
well as the energy-momentum tensor. In each case, the lowest quasinormal
frequency, which provides an approximate upper bound on the thermalization
time, is proportional to temperature, up to a pre-factor with only a mild
temperature dependence. We find similar behaviour in other holographic plasmas,
where the model contains an additional scale beyond the temperature. Hence, our
study suggests that the thermalization time is generically set by the
temperature, irrespective of any other scales, in strongly coupled gauge
theories.Comment: 6 pages, 7 figure
sQGP as hCFT
We examine the proposal to make quantitative comparisons between the strongly
coupled quark-gluon plasma and holographic descriptions of conformal field
theory. In this note, we calculate corrections to certain transport
coefficients appearing in second-order hydrodynamics from higher curvature
terms to the dual gravity theory. We also clarify how these results might be
consistently applied in comparisons with the sQGP.Comment: 13 page
Antiferromagnetic s-d exchange coupling in GaMnAs
Measurements of coherent electron spin dynamics in
Ga(1-x)Mn(x)As/Al(0.4)Ga(0.6)As quantum wells with 0.0006% < x < 0.03% show an
antiferromagnetic (negative) exchange bewteen s-like conduction band electrons
and electrons localized in the d-shell of the Mn2+ impurities. The magnitude of
the s-d exchange parameter, N0 alpha, varies as a function of well width
indicative of a large and negative contribution due to kinetic exchange. In the
limit of no quantum confinement, N0 alpha extrapolates to -0.09 +/- 0.03 eV
indicating that antiferromagnetic s-d exchange is a bulk property of GaMnAs.
Measurements of the polarization-resolved photoluminescence show strong
discrepancy from a simple model of the exchange enhanced Zeeman splitting,
indicative of additional complexity in the exchange split valence band.Comment: 5 pages, 4 figures and one action figur
Entanglement, Holography and Causal Diamonds
We argue that the degrees of freedom in a d-dimensional CFT can be
re-organized in an insightful way by studying observables on the moduli space
of causal diamonds (or equivalently, the space of pairs of timelike separated
points). This 2d-dimensional space naturally captures some of the fundamental
nonlocality and causal structure inherent in the entanglement of CFT states.
For any primary CFT operator, we construct an observable on this space, which
is defined by smearing the associated one-point function over causal diamonds.
Known examples of such quantities are the entanglement entropy of vacuum
excitations and its higher spin generalizations. We show that in holographic
CFTs, these observables are given by suitably defined integrals of dual bulk
fields over the corresponding Ryu-Takayanagi minimal surfaces. Furthermore, we
explain connections to the operator product expansion and the first law of
entanglement entropy from this unifying point of view. We demonstrate that for
small perturbations of the vacuum, our observables obey linear two-derivative
equations of motion on the space of causal diamonds. In two dimensions, the
latter is given by a product of two copies of a two-dimensional de Sitter
space. For a class of universal states, we show that the entanglement entropy
and its spin-three generalization obey nonlinear equations of motion with local
interactions on this moduli space, which can be identified with Liouville and
Toda equations, respectively. This suggests the possibility of extending the
definition of our new observables beyond the linear level more generally and in
such a way that they give rise to new dynamically interacting theories on the
moduli space of causal diamonds. Various challenges one has to face in order to
implement this idea are discussed.Comment: 84 pages, 12 figures; v2: expanded discussion on constraints in
section 7, matches published versio
Report on a Fossil Diatomaceous Deposit in Muscatine County, Iowa
Previous to the present year no fossil diatoms had been found in Iowa. On October 20, 1898, Prof. J. A. Udden, of Rock Island, Ill., while engaged in work for the Iowa Geological survey, found and sent to Prof. S. Calvin, of the State University, some diatomaceous earth. This material was taken from below the loess in Muscatine County, Iowa, and was turned over to the author for examination. It was of a dull, yellow color, composed of sand and decayed vegetable matter and a few diatoms
- …