2,095 research outputs found
KMS conditions for 4-point Green functions at finite temperature
We study the 4-point function in the Keldysh formalism of the closed time
path formulation of real time finite temperature field theory.
We derive the KMS conditions for these functions and discuss the number of
4-point functions that are independent. We define a set of `physical' functions
which are linear combinations of the usual Keldysh functions. We show that
these functions satisfy simple KMS conditions. In addition, we consider a set
of integral equations which represent a resummation of ladder graphs. We show
that these integral equations decouple when one uses the physical functions
that we have defined. We discuss the generalization of these results to QED.Comment: 17 pages in Revtex with 2 figure
A General Expression for Symmetry Factors of Feynman Diagrams
The calculation of the symmetry factor corresponding to a given Feynman
diagram is well known to be a tedious problem. We have derived a simple formula
for these symmetry factors. Our formula works for any diagram in scalar theory
( and interactions), spinor QED, scalar QED, or QCD.Comment: RevTex 11 pages with 10 figure
Generation of electron spin polarization in disordered organic semiconductors
The generation mechanisms of electron spin polarization (ESP) of charge
carriers (electrons and holes, called "doublets") in doublet-doublet
recombination and triplet-doublet quenching in disordered organic
semiconductors are analyzed in detail. The ESP is assumed to result from
quantum transitions between the states of the spin Hamiltonian of the pair of
interacting particles. The value of the ESP is essentially determined by the
mechanism of relative motion of particles. In our work we have considered the
cage and free diffusion models. The effect of possible attractive
spin-independent interactions between particles is also analyzed. Estimation
with obtained formulas shows that the proposed mechanisms can lead to a fairly
strong ESP much larger than the thermal one (at room temperatures)Comment: 10 pages, 3 figure
Magnetic field effects on electron-hole recombination in disordered organic semiconductors
Characteristic properties of magnetic field effects on spin selective
geminate and bulk electron-hole polaron pair (PP) recombination are analyzed in
detail within the approach based on the stochastic Liouville equation. Simple
expressions for the magnetic field (B) dependence of recombination yield and
rate are derived within two models of relative PP motion: free diffusion and
diffusion in the presence of well (cage). The spin evolution of PPs is
described taking in account the relaxation induced by hyperfine interaction,
anisotropic part of the Zeeman interaction induced, as well as -mechanism. A large variety of the -dependences of the recombination yield
and rate is obtained depending on the relative weights of
above-mentioned mechanisms. The proposed general method and derived particular
formulas are shown to be quite useful for the analysis of recent experimental
results.Comment: 12 pages, 3 figure
A conceptual design for the attitude control and determination system for the Magnetosphere Imager spacecraft
This paper presents a conceptual design for the attitude control and determination (ACAD) system for the Magnetosphere Imager (Ml) spacecraft. The MI is a small spin-stabilized spacecraft that has been proposed for launch on a Taurus-S expendable launch vehicle into a highly-ellipdcal polar Earth orbit. Presently, launch is projected for 1999. The paper describes the MI mission and ACAD requirements and then proposes an ACAD system for meeting these requirements. The proposed design is low-power, low-mass, very simple conceptually, highly passive, and consistent with the overall MI design philosophy, which is faster-better-cheaper. Still, the MI ACAD system is extremely robust and can handle a number of unexpected, adverse situations on orbit without impacting the mission as a whole. Simulation results are presented that support the soundness of the design approach
Shear viscosity in theory from an extended ladder resummation
We study shear viscosity in weakly coupled hot theory using the CTP
formalism . We show that the viscosity can be obtained as the integral of a
three-point function. Non-perturbative corrections to the bare one-loop result
can be obtained by solving a decoupled Schwinger-Dyson type integral equation
for this vertex. This integral equation represents the resummation of an
infinite series of ladder diagrams which contribute to the leading order
result. It can be shown that this integral equation has exactly the same form
as the Boltzmann equation. We show that the integral equation for the viscosity
can be reexpressed by writing the vertex as a combination of polarization
tensors. An expression for this polarization tensor can be obtained by solving
another Schwinger-Dyson type integral equation. This procedure results in an
expression for the viscosity that represents a non-perturbative resummation of
contributions to the viscosity which includes certain non-ladder graphs, as
well as the usual ladders. We discuss the motivation for this resummation. We
show that these resummations can also be obtained by writing the viscosity as
an integral equation involving a single four-point function. Finally, we show
that when the viscosity is expressed in terms of a four-point function, it is
possible to further extend the set of graphs included in the resummation by
treating vertex and propagator corrections self-consistently. We discuss the
significance of such a self-consistent resummation and show that the integral
equation contains cancellations between vertex and propagator corrections.Comment: Revtex 40 pages with 29 figures, version to appear in Phys. Rev.
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