6,064 research outputs found
Efficiency of thin film photocells
We propose a new concept for the design of high-efficiency photocells based
on ultra-thin (submicron) semiconductor films of controlled thickness. Using a
microscopic model of a thin dielectric layer interacting with incident
electromagnetic radiation we evaluate the efficiency of conversion of solar
radiation into the electric power. We determine the optimal range of parameters
which maximize the efficiency of such photovoltaic element.Comment: 5 pages, 3 figure
RF bifurcation of a Josephson junction: microwave embedding circuit requirements
A Josephson tunnel junction which is RF-driven near a dynamical bifurcation
point can amplify quantum signals. The bifurcation point will exist robustly
only if the electrodynamic environment of the junction meets certain criteria.
In this article we develop a general formalism for dealing with the non-linear
dynamics of Josephson junction embedded in an arbitrary microwave circuit. We
find sufficient conditions for the existence of the bifurcation regime: a) the
embedding impedance of the junction need to present a resonance at a particular
frequency , with the quality factor of the resonance and the
participation ratio of the junction satisfying , b) the drive
frequency should be low frequency detuned away from by more than
.Comment: Submitted to Phys. Rev. B, 12 pages, 6 figure
Absorption cross section in warped AdS_3 black hole revisited
We investigate the absorption cross section for minimal-coupled scalars in
the warped AdS_3 black hole. According to our calculation, the cross section
reduces to the horizon area in the low energy limit as usually expected in
contrast to what was previously found. We also calculate the greybody factor
and find that the effective temperatures for the two chiral CFT's are
consistent with that derived from the quasinormal modes. Observing the
conjectured warped AdS/CFT correspondence, we suspect that a specific sector of
the CFT operators with the desired conformal dimension could be responsible for
the peculiar thermal behaviour of the warped AdS_3 black hole.Comment: 16+1 pages, typos corrected, references and footnotes adde
Universal saturation of electron dephasing in three-dimensional disordered metals
We have systematically investigated the low-temperature electron dephasing
times in more than 40 three-dimensional polycrystalline impure
metals with distinct material characteristics. In all cases, a saturation of
the dephasing time is observed below about a (few) degree(s) Kelvin, depending
on samples. The value of the saturated dephasing time [] falls basically in the range 0.005 to 0.5 ns for
all samples. Particularly, we find that scales with the electron
diffusion constant as , with close to or
slightly larger than 1, for over two decades of from about 0.1 to 10
cm/s. Our observation suggests that the saturation behavior of
is universal and intrinsic in three-dimensional polycrystalline impure metals.
A complete theoretical explanation is not yet available.Comment: 4 pages, 3 eps figure
Kaluza-Klein Higher Derivative Induced Gravity
The existence and stability analysis of an inflationary solution in a
-dimensional anisotropic induced gravity is presented in this paper.
Nontrivial conditions in the field equations are shown to be compatible with a
cosmological model in which the 4-dimension external space evolves
inflationary, while, the D-dimension internal one is static. In particular,
only two additional constraints on the coupling constants are derived from the
abundant field equations and perturbation equations. In addition, a compact
formula for the non-redundant 4+D dimensional Friedmann equation is also
derived for convenience. Possible implications are also discussed in this
paper.Comment: 13 pages, typos/errors corrected, three additional appendices adde
U(1) spin liquids and valence bond solids in a large-N three-dimensional Heisenberg model
We study possible quantum ground states of the Sp(N) generalized Heisenberg
model on a cubic lattice with nearest-neighbor and next-nearest-neighbor
exchange interactions. The phase diagram is obtained in the large-N limit and
fluctuation effects are considered via appropriate gauge theories. In
particular, we find three U(1) spin liquid phases with different short-range
magnetic correlations. These phases are characterized by deconfined gapped
spinons, gapped monopoles, and gapless ``photons''. As N becomes smaller, a
confinement transition from these phases to valence bond solids (VBS) may
occur. This transition is studied by using duality and analyzing the resulting
theory of monopoles coupled to a non-compact dual gauge field; the condensation
of the monopoles leads to VBS phases. We determine the resulting VBS phases
emerging from two of the three spin liquid states. On the other hand, the spin
liquid state near J_1 \approx J_2 appears to be more stable against monopole
condensation and could be a promising candidate for a spin liquid state in real
systems.Comment: revtex file 12 pages, 17 figure
Understanding procurement methods in practice: an alternative perspective
The aim of this paper is to propose an agency-structure perspective on understanding how procurement methods are enacted in practice. It is argued that procurement methods manifest within a complex web of interconnections between various actors and the industrial structure. As an example, this paper focuses on the interrelations between the quangos' promotions of procurement initiatives and construction firms' responses to these initiatives. An understanding of such interrelations is achieved by integrating three sources of data regarding procurement developments. First, an analysis of the industry is considered as structural forces which influence procurement developments. This knowledge will be obtained through a literature review of the industry, including the overall economic conditions, the state of the industry itself, and the nature of construction demands. Second, the business path development of construction firms is regarded as responses to procurement initiatives. The method of case study will be applied to understand how construction firms' business path is related to procurement initiatives of time. Finally, a review of quango construction reports will be undertaken. Each report's key recommendations and impacts related to procurement initiatives will be identified. The three sources of data will then be plotted onto a timeline graph in order to form a more in-depth analysis. An assessment of the three sources of data at a chosen point/s for interconnections and causal relationships would then be undertaken. It is concluded that considering the interrelations between the three sources of data will offer a greater understanding of procurement methods in practice
Stability of a vacuum nonsingular black hole
This is the first of series of papers in which we investigate stability of
the spherically symmetric space-time with de Sitter center. Geometry,
asymptotically Schwarzschild for large and asymptotically de Sitter as
, describes a vacuum nonsingular black hole for and
particle-like self-gravitating structure for where a critical
value depends on the scale of the symmetry restoration to de Sitter
group in the origin. In this paper we address the question of stability of a
vacuum non-singular black hole with de Sitter center to external perturbations.
We specify first two types of geometries with and without changes of topology.
Then we derive the general equations for an arbitrary density profile and show
that in the whole range of the mass parameter objects described by
geometries with de Sitter center remain stable under axial perturbations. In
the case of the polar perturbations we find criteria of stability and study in
detail the case of the density profile
where is the density of de Sitter vacuum at the center, is de
Sitter radius and is the Schwarzschild radius.Comment: 18 pages, 8 figures, submitted to "Classical and Quantum Gravity
KP line solitons and Tamari lattices
The KP-II equation possesses a class of line soliton solutions which can be
qualitatively described via a tropical approximation as a chain of rooted
binary trees, except at "critical" events where a transition to a different
rooted binary tree takes place. We prove that these correspond to maximal
chains in Tamari lattices (which are poset structures on associahedra). We
further derive results that allow to compute details of the evolution,
including the critical events. Moreover, we present some insights into the
structure of the more general line soliton solutions. All this yields a
characterization of possible evolutions of line soliton patterns on a shallow
fluid surface (provided that the KP-II approximation applies).Comment: 49 pages, 36 figures, second version: section 4 expande
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