11,640 research outputs found
Superconductivity from Undressing
Photoemission experiments in high cuprates indicate that quasiparticles
are heavily 'dressed' in the normal state, particularly in the low doping
regime. Furthermore these experiments show that a gradual undressing occurs
both in the normal state as the system is doped and the carrier concentration
increases, as well as at fixed carrier concentration as the temperature is
lowered and the system becomes superconducting. A similar picture can be
inferred from optical experiments. It is argued that these experiments can be
simply understood with the single assumption that the quasiparticle dressing is
a function of the local carrier concentration. Microscopic Hamiltonians
describing this physics are discussed. The undressing process manifests itself
in both the one-particle and two-particle Green's functions, hence leads to
observable consequences in photoemission and optical experiments respectively.
An essential consequence of this phenomenology is that the microscopic
Hamiltonians describing it break electron-hole symmetry: these Hamiltonians
predict that superconductivity will only occur for carriers with hole-like
character, as proposed in the theory of hole superconductivity
Natural Resource Information System, remote sensing studies
Potential applications of remote sensing data were reviewed, and available imagery was interpreted to provide input to a demonstration data base. A literature review was conducted to determine the types and qualities of imagery required to satisfy identified data needs. Ektachrome imagery available over the demonstration areas was reviewed to establish the feasibility of interpreting cultural features, range condition, and timber type. Using the same imagery, a land use map was prepared for the demonstration area. The feasibility of identifying commercial timber areas using a density slicing technique was tested on multispectral imagery available for a portion of the demonstration area
Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates
Experimental evidence indicates that the superconducting transition in high
cuprates is an 'undressing' transition. Microscopic mechanisms giving
rise to this physics were discussed in the first paper of this series. Here we
discuss the calculation of the single particle Green's function and spectral
function for Hamiltonians describing undressing transitions in the normal and
superconducting states. A single parameter, , describes the strength
of the undressing process and drives the transition to superconductivity. In
the normal state, the spectral function evolves from predominantly incoherent
to partly coherent as the hole concentration increases. In the superconducting
state, the 'normal' Green's function acquires a contribution from the anomalous
Green's function when is non-zero; the resulting contribution to
the spectral function is for hole extraction and for hole
injection. It is proposed that these results explain the observation of sharp
quasiparticle states in the superconducting state of cuprates along the
direction and their absence along the direction.Comment: figures have been condensed in fewer pages for easier readin
Self-consistency in the Projected Shell Model
The Projected Shell Model is a shell model theory built up over a deformed
BCS mean field. Ground state and excited bands in even-even nuclei are obtained
through diagonalization of a pairing plus quadrupole Hamiltonian in an angular
momentum projected 0-, 2-, and 4-quasiparticle basis. The residual
quadrupole-quadrupole interaction strength is fixed self-consistently with the
deformed mean field and the pairing constants are the same used in constructing
the quasiparticle basis. Taking as an example, we calculate
low-lying states and compare them with experimental data. We exhibit the effect
of changing the residual interaction strengths on the spectra. It is clearly
seen that there are many bandheads whose energies can
only be reproduced using the self-consistent strengths. It is thus concluded
that the Projected Shell Model is a model essentially with no free parameters.Comment: 13 pages, 10 figures, submitted to Nuclear Physics
Why political disagreements over how the world works may be easier to solve than those over goals.
Politics is all about disagreement – disagreement over what government should be trying to achieve, but also about how the world works and how that affects the pursuit of shared goals. But what makes disagreements over beliefs different from disagreements over goals? In a new study, Alexander V. Hirsch writes that beliefs may be able to be changed when new information is introduced, and so disagreements driven by differing beliefs should result in more policy experimentation compared to disagreement driven by differing goals
Design of aircraft turbine fan drive gear transmission system
The following basic types of gear reduction concepts were studied as being feasible power train systems for a low-bypass-ratio, single-spool, geared turbofan engine for general aircraft use: (1) single-stage external-internal reduction, (2) gears (offset shafting), (3) multiple compound idler gear system (concentric shafting), and (4) star gear planetary system with internal ring gear final output member (concentric shafting-counterrotation). In addition, studies were made of taking the accessories drive power off both the high-speed and low-speed shafting, using either face gears or spiral bevel gears. Both antifriction and sleeve-type bearings were considered for the external-internal and star-planet reduction concepts
Polarization and Campaign Spending in Elections
We develop a Downsian model of electoral competition in which candidates with both policy and office-motivations use a mixture of platforms and campaign spending to gain the median voter’s support. The unique equilibrium involves randomizing over both platforms and spending, and exhibits the following properties – (i) ex-ante uncertainty in platforms, spending, and the election winner, (ii) platform divergence, (iii) inefficiency in spending and outcomes, (iv) polarization, and (v) voter extremism. We also show that platform polarization and campaign spending move in tandem, since spending is used by candidates to gain support for extreme platforms. Factors that contribute to both phenomena include the candidates’ desire for extreme platforms, and their ability to translate campaign spending into support for them. The latter insight generates new hypotheses about the potential causes of both rising polarization and spending
After-School Programs for High School Students: An Evaluation of After School Matters
Evaluates outcomes for teens in Chicago's After School Matters apprenticeship-like program, finding statistically significant benefits on some measures of youth development and reduced problem behaviors but not in job skills or school performance
- …