16,287 research outputs found
Bosonic versus fermionic pairs of topological spin defects in monolayered high-T_c superconductors
The energy associated with bosonic and fermionic pairs of topological spin
defects in doped antiferromagnetic quantum spin-1/2 square lattice is estimated
within a resonating valence bond scenario, as described by a t-t'-J-like model
Hamiltonian, plus a t-perpendicular, responsible of a three-dimensional
screening of the electrostatic repulsion within the bosonic pairs. For
parameters appropriate for monolayered high-T_c superconductors, both fermionic
and bosonic pairs show x^2-y^2 symmetry. We find a critical value of doping
such that the energy of the bosonic pairs goes below twice the energy of two
fermionic pairs at their Fermi level. This finding could be related to the
onset of high-T_c superconductivity.Comment: 10 pages, 6 figures. To be published in Phys. Rev.
Between-Site Variation in Suitability of \u3ci\u3eSalix Cordata\u3c/i\u3e as a Host for \u3ci\u3eAltica Subplicata\u3c/i\u3e (Coleoptera: Chrysomelidae)
To investigate local adaptation of insect herbivore populations to host plant populations, willow flea beetles (Altica subplicata) were collected from two distant sites in northern Michigan (Grass Bay, GB; Pte. Aux Chenes, PAC) and reared on host plants (Salix cordata) collected from each of the sites. Larval development (measured by molt frequency and length of larval stage) was significantly faster on PAC plants than on GB plants but did not differ for the two beetle populations. For both populations of beetles, mean pupal weight was also greater on PAC plants than on GB plants. Thus, there was no evidence for adaptation of beetle populations to local host plant populations. The greater performance of A. subplicata on PAC plants most likely resulted from a lower trichome density on leaves of plants from that site
Exponential localization of hydrogen-like atoms in relativistic quantum electrodynamics
We consider two different models of a hydrogenic atom in a quantized
electromagnetic field that treat the electron relativistically. The first one
is a no-pair model in the free picture, the second one is given by the
semi-relativistic Pauli-Fierz Hamiltonian. We prove that the no-pair operator
is semi-bounded below and that its spectral subspaces corresponding to energies
below the ionization threshold are exponentially localized. Both results hold
true, for arbitrary values of the fine-structure constant, , and the
ultra-violet cut-off, , and for all nuclear charges less than the
critical charge without radiation field, . We obtain
similar results for the semi-relativistic Pauli-Fierz operator, again for all
values of and and for nuclear charges less than .Comment: 37 page
Two-site dynamical mean field theory for the dynamic Hubbard model
At zero temperature, two-site dynamical mean field theory is applied to the
Dynamic Hubbard model. The Dynamic Hubbard model describes the orbital
relaxation that occurs when two electrons occupy the same site, by using a
two-level boson field at each site. At finite boson frequency, the appearance
of a Mott gap is found to be enhanced even though it shows a metallic phase
with the same bare on-site interaction in the conventional Hubbard model.
The lack of electron-hole symmetry is highlighted through the quasi-particle
weight and the single particle density of states at different fillings, which
qualitatively differentiates the dynamic Hubbard model from other conventional
Hubbard-like models.Comment: 13 pages, 15 figure
ECONOMIC FEASIBILITY OF USING BRUSH CONTROL TO ENHANCE OFF-SITE WATER YIELD
A feasibility study of brush for off-site water yield was undertaken in 1998 on the North Concho River near San Angelo, Texas. Subsequently, studies were conducted on eight additional Texas watersheds. Economic analysis was based on estimated control costs of the different options compared to the estimated rancher benefits of brush control. Control costs included initial and follow-up treatments required to reduce brush canopy to between 3 and 8%, and maintain it at the reduced level for 10 years. The state cost-share was estimated by subtracting the present value of rancher benefits from the present value of the total cost of the control program. The total cost of additional water was determined by dividing the total state cost-share if all eligible acreage were enrolled by the total added water estimated to result from the brush control program. This procedure resulted in present values of total control costs per acre ranging from 159.45. Rancher benefits, based on the present value of the improved net returns to typical cattle, sheep, goat, and wildlife enterprises, ranged from 52.12 per acre. Present values of the state cost-share per acre ranged from 138.85. The cost of added water estimated for the eight watersheds ranged from 204.05 per acre-foot averaged over each watershed.Resource /Energy Economics and Policy,
Hyperfine splitting in non-relativistic QED: uniqueness of the dressed hydrogen atom ground state
We consider a free hydrogen atom composed of a spin-1/2 nucleus and a
spin-1/2 electron in the standard model of non-relativistic QED. We study the
Pauli-Fierz Hamiltonian associated with this system at a fixed total momentum.
For small enough values of the fine-structure constant, we prove that the
ground state is unique. This result reflects the hyperfine structure of the
hydrogen atom ground state.Comment: 22 pages, 3 figure
WHIZARD 2.2 for Linear Colliders
We review the current status of the WHIZARD event generator. We discuss, in
particular, recent improvements and features that are relevant for simulating
the physics program at a future Linear Collider.Comment: Talk presented at the International Workshop on Future Linear
Colliders (LCWS13), Tokyo, Japan, 11-15 November 201
Ground States in the Spin Boson Model
We prove that the Hamiltonian of the model describing a spin which is
linearly coupled to a field of relativistic and massless bosons, also known as
the spin-boson model, admits a ground state for small values of the coupling
constant lambda. We show that the ground state energy is an analytic function
of lambda and that the corresponding ground state can also be chosen to be an
analytic function of lambda. No infrared regularization is imposed. Our proof
is based on a modified version of the BFS operator theoretic renormalization
analysis. Moreover, using a positivity argument we prove that the ground state
of the spin-boson model is unique. We show that the expansion coefficients of
the ground state and the ground state energy can be calculated using regular
analytic perturbation theory
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