437 research outputs found
The Flux-Phase of the Half-Filled Band
The conjecture is verified that the optimum, energy minimizing magnetic flux
for a half-filled band of electrons hopping on a planar, bipartite graph is
per square plaquette. We require {\it only} that the graph has
periodicity in one direction and the result includes the hexagonal lattice
(with flux 0 per hexagon) as a special case. The theorem goes beyond previous
conjectures in several ways: (1) It does not assume, a-priori, that all
plaquettes have the same flux (as in Hofstadter's model); (2) A Hubbard type
on-site interaction of any sign, as well as certain longer range interactions,
can be included; (3) The conclusion holds for positive temperature as well as
the ground state; (4) The results hold in dimensions if there is
periodicity in directions (e.g., the cubic lattice has the lowest energy
if there is flux in each square face).Comment: 9 pages, EHL14/Aug/9
Relationship Between Childhood Peer Rejection and Aggression and Adolescent Delinquency Severity and Type Among African American Youth
This prospective, longitudinal study examined peer rejection and aggression in childhood as predictors of the severity and type of delinquency during adolescence. Sociometric surveys were completed at third grade for a predominantly low-socioeconomic status, urban sample of African American boys and girls, and youth reports of delinquency were gathered at Grades 6, 8, and 10. Patterns of association between childhood peer rejection and aggression and delinquency severity varied by gender. For boys, the additive effect of childhood peer rejection and aggression was a strong predictor of more serious delinquency, whereas for girls only aggression predicted more serious delinquency. For boys, the combination of peer rejection and aggression was associated with felony assaults, and aggression was associated with a wide diversity of offenses during adolescence, whereas for girls only peer rejection predicted involvement in minor assault. Results are discussed in terms of the early starter pathway of antisocial behavior as it relates to peer rejection and aggression for boys, differing predictive patterns for girls, and implications for intervention with children with emotional and behavioral disorders.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
A linear RFQ ion trap for the Enriched Xenon Observatory
The design, construction, and performance of a linear radio-frequency ion
trap (RFQ) intended for use in the Enriched Xenon Observatory (EXO) are
described. EXO aims to detect the neutrinoless double-beta decay of Xe
to Ba. To suppress possible backgrounds EXO will complement the
measurement of decay energy and, to some extent, topology of candidate events
in a Xe filled detector with the identification of the daughter nucleus
(Ba). The ion trap described here is capable of accepting, cooling, and
confining individual Ba ions extracted from the site of the candidate
double-beta decay event. A single trapped ion can then be identified, with a
large signal-to-noise ratio, via laser spectroscopy.Comment: 18 pages, pdflatex, submitted to NIM
Fermion Masses and Mixing in Extended Technicolor Models
We study fermion masses and mixing angles, including the generation of a
seesaw mechanism for the neutrinos, in extended technicolor (ETC) theories. We
formulate an approach to these problems that relies on assigning right-handed
quarks and charged leptons to ETC representations that are conjugates
of those of the corresponding left-handed fermions. This leads to a natural
suppression of these masses relative to the quarks, as well as the
generation of quark mixing angles, both long-standing challenges for ETC
theories. Standard-model-singlet neutrinos are assigned to ETC representations
that provide a similar suppression of neutrino Dirac masses, as well as the
possibility of a realistic seesaw mechanism with no mass scale above the
highest ETC scale of roughly TeV. A simple model based on the ETC group
SU(5) is constructed and analyzed. This model leads to non-trivial, but not
realistic mixing angles in the quark and lepton sectors. It can also produce
sufficiently light neutrinos, although not simultaneously with a realistic
quark spectrum. We discuss several aspects of the phenomenology of this class
of models.Comment: 74 pages, revtex with embedded figure
Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model
Slave boson calculations have been carried out in the three-band tJ model for
the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode
phonons. Phonon-induced Van Hove nesting leads to a phase separation between a
hole-doped domain and a (magnetic) domain near half filling, with long-range
Coulomb forces limiting the separation to a nanoscopic scale. Strong
correlation effects pin the Fermi level close to, but not precisely at the Van
Hove singularity (VHS), which can enhance the tendency to phase separation. The
resulting dispersions have been calculated, both in the uniform phases and in
the phase separated regime. In the latter case, distinctly different
dispersions are found for large, random domains and for regular (static)
striped arrays, and a hypothetical form is presented for dynamic striped
arrays. The doping dependence of the latter is found to provide an excellent
description of photoemission and thermodynamic experiments on pseudogap
formation in underdoped cuprates. In particular, the multiplicity of observed
gaps is explained as a combination of flux phase plus charge density wave (CDW)
gaps along with a superconducting gap. The largest gap is associated with VHS
nesting. The apparent smooth evolution of this gap with doping masks a
crossover from CDW-like effects near optimal doping to magnetic effects (flux
phase) near half filling. A crossover from large Fermi surface to hole pockets
with increased underdoping is found. In the weakly overdoped regime, the CDW
undergoes a quantum phase transition (), which could be obscured
by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes,
esp. in Sect. 3, Figs 1-4,6 replace
Light-Ion-Induced Multifragmentation: The ISiS Project
An extensive study of GeV light-ion-induced multifragmentation and its
possible interpretation in terms of a nuclear liquid-gas phase transition has
been performed with the Indiana Silicon Sphere (ISiS)4 pi detector array.
Measurements were performed with 5-15 GeV/c p, pbar, and pion beams incident on
Au and 2-5 GeV He incident on Ag and Au targets.
Both the reaction dynamics and the subsequent decay of the heavy residues have
been explored. The data provide evidence for a dramatic change in the reaction
observables near an excitation energy of E*/A = 4-5 MeV per residue nucleon. In
this region, fragment multiplicities and energy spectra indicate emission from
an expanded/dilute source on a very short time scale (20-50 fm/c). These
properties, along with caloric curve and scaling-law behavior, yield a pattern
that is consistent with a nuclear liquid-gas phase transition.Comment: 67 pages, 44 figures, all included in tar fil
Genome-Wide Joint Meta-Analysis of SNP and SNP-by-Smoking Interaction Identifies Novel Loci for Pulmonary Function
Transitions of cardio-metabolic risk factors in the Americas between 1980 and 2014
Describing the prevalence and trends of cardiometabolic risk factors that are associated with non-communicable diseases (NCDs) is crucial for monitoring progress, planning prevention, and providing evidence to support policy efforts. We aimed to analyse the transition in body-mass index (BMI), obesity, blood pressure, raised blood pressure, and diabetes in the Americas, between 1980 and 2014
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