3,513 research outputs found
The heavy fermion damping rate puzzle
: We examine again the problem of the damping rate of a moving heavy fermion
in a hot plasma within the resummed perturbative theory of Pisarski and
Braaten. The ansatz for its evaluation which relates it to the imaginary part
of the fermion propagator pole in the framework of a self-consistent approach
is critically analyzed. As already pointed out by various authors, the only way
to define the rate is through additional implementation of magnetic screening.
We show in detail how the ansatz works in this case and where we disagree with
other authors. We conclude that the self-consistent approach is not
satisfactory.Comment: 17 page
Sum rule of the correlation function
We discuss a sum rule satisfied by the correlation function of two particles
with small relative momenta. The sum rule, which results from the completeness
condition of the quantum states of the two particles, is first derived and then
we check how it works in practice. The sum rule is shown to be trivially
satisfied by free particle pair, and then there are considered three different
systems of interacting particles. We discuss a pair of neutron and proton in
the s-wave approximation and the case of the so-called hard spheres with the
phase shifts taken into account up to l=4. Finally, the Coulomb system of two
charged particles is analyzed.Comment: 18 pages, 18 figures, revised, to appear in Phys. Rev.
Remarks on transient photon production in heavy ion collisions
In this note, we discuss the derivation of a formula that has been used in
the literature in order to compute the number of photons emitted by a hot or
dense system during a finite time. Our derivation is based on a variation of
the standard operator-based -matrix approach. The shortcomings of this
formula are then emphasized, which leads to a negative conclusion concerning
the possibility of using it to predict transient effects for the photon rate.Comment: 13 page
Sequential bilateral central retinal vein occlusions in a cystic fibrosis patient with hyperhomocysteinemia and hypergamma-globulinemia
PURPOSE: To report a case of sequential bilateral central retinal vein occlusions in a cystic fibrosis patient with hyperhomocysteinemia and hypergamma-globulinemia over 6 years of follow up. METHODS: Observational case report of one patient. RESULTS: A 31 year-old male with a history of cystic fibrosis presented with a central retinal vein occlusion (CRVO) in his left eye, followed by a CRVO in his right eye 4 years later. His medical workup was significant for elevated levels of homocysteine and gamma-globulins, which coincided with initiation of intravenous immunoglobulin (IVIG) proceeding his second CRVO. CONCLUSIONS: We describe a case of sequential bilateral central retinal vein occlusions in a cystic fibrosis patient with hyperhomocysteinemia and hypergamma-globulinemia over 6 years of follow up and discuss the important role of these risk factors in retinal venous occlusive disease
Equivalence Theorems for Pseudoscalar Coupling
By a unitary transformation a rigorous equivalence theorem is established for the pseudoscalar coupling of pseudoscalar mesons (neutral and charged) to a second-quantized nucleon field. By the transformation the linear pseudoscalar coupling is eliminated in favor of a nonlinear pseudovector coupling term together with other terms. Among these is a term corresponding to a variation of the effective rest mass of the nucleons with position through its dependence on the meson potentials. The question of the connection of the nonlinear pseudovector coupling with heuristic proposals that such a coupling may account for the saturation of nuclear forces and the independence of single nucleon motions in nuclei is briefly discussed. The new representation of the Hamiltonian may have particular value in constructing a strong coupling theory of pseudoscalar coupled meson fields. Some theorems on a class of unitary transformations of which the present transformation is an example are stated and proved in an appendix.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86126/1/PhysRev.87.1061-RKO.pd
Intrinsic and Rashba Spin-orbit Interactions in Graphene Sheets
Starting from a microscopic tight-binding model and using second order
perturbation theory, we derive explicit expressions for the intrinsic and
Rashba spin-orbit interaction induced gaps in the Dirac-like low-energy band
structure of an isolated graphene sheet. The Rashba interaction parameter is
first order in the atomic carbon spin-orbit coupling strength and first
order in the external electric field perpendicular to the graphene plane,
whereas the intrinsic spin-orbit interaction which survives at E=0 is second
order in . The spin-orbit terms in the low-energy effective Hamiltonian
have the form proposed recently by Kane and Mele. \textit{Ab initio} electronic
structure calculations were performed as a partial check on the validity of the
tight-binding model.Comment: 5 pages, 2 figures; typos corrected, references update
Applications of Automata and Graphs: Labeling-Operators in Hilbert Space I
We show that certain representations of graphs by operators on Hilbert space
have uses in signal processing and in symbolic dynamics. Our main result is
that graphs built on automata have fractal characteristics. We make this
precise with the use of Representation Theory and of Spectral Theory of a
certain family of Hecke operators. Let G be a directed graph. We begin by
building the graph groupoid G induced by G, and representations of G. Our main
application is to the groupoids defined from automata. By assigning weights to
the edges of a fixed graph G, we give conditions for G to acquire fractal-like
properties, and hence we can have fractaloids or G-fractals. Our standing
assumption on G is that it is locally finite and connected, and our labeling of
G is determined by the "out-degrees of vertices". From our labeling, we arrive
at a family of Hecke-type operators whose spectrum is computed. As
applications, we are able to build representations by operators on Hilbert
spaces (including the Hecke operators); and we further show that automata built
on a finite alphabet generate fractaloids. Our Hecke-type operators, or
labeling operators, come from an amalgamated free probability construction, and
we compute the corresponding amalgamated free moments. We show that the free
moments are completely determined by certain scalar-valued functions.Comment: 69 page
Neural Network Model for Apparent Deterministic Chaos in Spontaneously Bursting Hippocampal Slices
A neural network model that exhibits stochastic population bursting is
studied by simulation. First return maps of inter-burst intervals exhibit
recurrent unstable periodic orbit (UPO)-like trajectories similar to those
found in experiments on hippocampal slices. Applications of various control
methods and surrogate analysis for UPO-detection also yield results similar to
those of experiments. Our results question the interpretation of the
experimental data as evidence for deterministic chaos and suggest caution in
the use of UPO-based methods for detecting determinism in time-series data.Comment: 4 pages, 5 .eps figures (included), requires psfrag.sty (included
The Importance of Boundary Conditions in Quantum Mechanics
We discuss the role of boundary conditions in determining the physical
content of the solutions of the Schrodinger equation. We study the
standing-wave, the ``in,'' the ``out,'' and the purely outgoing boundary
conditions. As well, we rephrase Feynman's prescription as a
time-asymmetric, causal boundary condition, and discuss the connection of
Feynman's prescription with the arrow of time of Quantum
Electrodynamics. A parallel of this arrow of time with that of Classical
Electrodynamics is made. We conclude that in general, the time evolution of a
closed quantum system has indeed an arrow of time built into the propagators.Comment: Contribution to the proceedings of the ICTP conference "Irreversible
Quantum Dynamics," Trieste, Italy, July 200
Charge Order in the Falicov-Kimball Model
We examine the spinless one-dimensional Falicov-Kimball model (FKM) below
half-filling, addressing both the binary alloy and valence transition
interpretations of the model. Using a non-perturbative technique, we derive an
effective Hamiltonian for the occupation of the localized orbitals, providing a
comprehensive description of charge order in the FKM. In particular, we uncover
the contradictory ordering roles of the forward-scattering and backscattering
itinerant electrons: the latter are responsible for the crystalline phases,
while the former produces the phase separation. We find an Ising model
describes the transition between the phase separated state and the crystalline
phases; for weak-coupling we present the critical line equation, finding
excellent agreement with numerical results. We consider several extensions of
the FKM that preserve the classical nature of the localized states. We also
investigate a parallel between the FKM and the Kondo lattice model, suggesting
a close relationship based upon the similar orthogonality catastrophe physics
of the associated single-impurity models.Comment: 39 pages, 6 figure
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