120 research outputs found
A transition in the spectrum of the topological sector of theory at strong coupling
We investigate the strong coupling region of the topological sector of the
two-dimensional theory. Using discrete light cone quantization (DLCQ),
we extract the masses of the lowest few excitations and observe level
crossings. To understand this phenomena, we evaluate the expectation value of
the integral of the normal ordered operator and we extract the number
density of constituents in these states. A coherent state variational
calculation confirms that the number density for low-lying states above the
transition coupling is dominantly that of a kink-antikink-kink state. The
Fourier transform of the form factor of the lowest excitation is extracted
which reveals a structure close to a kink-antikink-kink profile. Thus, we
demonstrate that the structure of the lowest excitations becomes that of a
kink-antikink-kink configuration at moderately strong coupling. We extract the
critical coupling for the transition of the lowest state from that of a kink to
a kink-antikink-kink. We interpret the transition as evidence for the onset of
kink condensation which is believed to be the physical mechanism for the
symmetry restoring phase transition in two-dimensional theory.Comment: revtex4, 14 figure
Optical Absorption Characteristics of Silicon Nanowires for Photovoltaic Applications
Solar cells have generated a lot of interest as a potential source of clean
renewable energy for the future. However a big bottleneck in wide scale
deployment of these energy sources remain the low efficiency of these
conversion devices. Recently the use of nanostructures and the strategy of
quantum confinement have been as a general approach towards better charge
carrier generation and capture. In this article we have presented calculations
on the optical characteristics of nanowires made out of Silicon. Our
calculations show these nanowires form excellent optoelectronic materials and
may yield efficient photovoltaic devices
Constraints and Hamiltonian in Light-Front Quantized Field Theory
Self-consistent Hamiltonian formulation of scalar theory on the null plane is
constructed following Dirac method. The theory contains also {\it constraint
equations}. They would give, if solved, to a nonlinear and nonlocal
Hamiltonian. The constraints lead us in the continuum to a different
description of spontaneous symmetry breaking since, the symmetry generators now
annihilate the vacuum. In two examples where the procedure lacks
self-consistency, the corresponding theories are known ill-defined from
equal-time quantization. This lends support to the method adopted where both
the background field and the fluctuation above it are treated as dynamical
variables on the null plane. We let the self-consistency of the Dirac procedure
determine their properties in the quantized theory. The results following from
the continuum and the discretized formulations in the infinite volume limit do
agree.Comment: 11 pages, Padova University preprint DFPF/92/TH/52 (December '92
The Mass Operator in the Light-Cone Representation
I argue that for the case of fermions with nonzero bare mass there is a term
in the matter density operator in the light-cone representation which has been
omitted from previous calculations. The new term provides agreement with
previous results in the equal-time representation for mass perturbation theory
in the massive Schwinger model. For the DLCQ case the physics of the new term
can be represented by an effective operator which acts in the DLCQ subspace,
but the form of the term might be hard to guess and I do not know how to
determine its coefficient from symmetry considerations.Comment: Revtex, 8 page
Light-cone quantization of two dimensional field theory in the path integral approach
A quantization condition due to the boundary conditions and the
compatification of the light cone space-time coordinate is identified at
the level of the classical equations for the right-handed fermionic field in
two dimensions. A detailed analysis of the implications of the implementation
of this quantization condition at the quantum level is presented. In the case
of the Thirring model one has selection rules on the excitations as a function
of the coupling and in the case of the Schwinger model a double integer
structure of the vacuum is derived in the light-cone frame. Two different
quantized chiral Schwinger models are found, one of them without a
-vacuum structure. A generalization of the quantization condition to
theories with several fermionic fields and to higher dimensions is presented.Comment: revtex, 14 p
Light-Cone Quantization of Gauge Fields
Light-cone quantization of gauge field theory is considered. With a careful
treatment of the relevant degrees of freedom and where they must be
initialized, the results obtained in equal-time quantization are recovered, in
particular the Mandelstam-Leibbrandt form of the gauge field propagator. Some
aspects of the ``discretized'' light-cone quantization of gauge fields are
discussed.Comment: SMUHEP/93-20, 17 pages (one figure available separately from the
authors). Plain TeX, all macros include
The Zero Temperature Chiral Phase Transition in SU(N) Gauge Theories
We investigate the zero temperature chiral phase transition in an SU(N) gauge
theory as the number of fermions is varied. We argue that there exists a
critical number of fermions , above which there is no chiral symmetry
breaking or confinement, and below which both chiral symmetry breaking and
confinement set in. We estimate and discuss the nature of the phase
transition.Comment: 13 pages, LaTeX, version published in PR
A Comment on the Zero Temperature Chiral Phase Transition in Gauge Theories
Recently Appelquist, Terning, and Wijewardhana investigated the zero
temperature chiral phase transition in SU(N) gauge theory as the number of
fermions N_f is varied. They argued that there is a critical number of fermions
N^c_f, above which there is no chiral symmetry breaking and below which chiral
symmetry breaking and confinement set in. They further argued that that the
transition is not second order even though the order parameter for chiral
symmetry breaking vanishes continuously as N_f approaches N^c_f on the broken
side. In this note I propose a simple physical picture for the spectrum of
states as N_f approaches N^c_f from below (i.e. on the broken side) and argue
that this picture predicts very different and non-universal behavior than is
the case in an ordinary second order phase transition. In this way the
transition can be continuous without behaving conventionally. I further argue
that this feature results from the (presumed) existence of an infrared
Banks-Zaks fixed point of the gauge coupling in the neighborhood of the chiral
transition and therefore depends on the long-distance nature of the non-abelian
gauge force.Comment: 7 pages, 2 figure
Meson masses in large Nf QCD from the Bethe-Salpeter equation
We solve the homogeneous Bethe-Salpeter (HBS) equation for the scalar,
pseudoscalar, vector, and axial-vector bound states of quark and anti-quark in
large Nf QCD with the improved ladder approximation in the Landau gauge. The
quark mass function in the HBS equation is obtained from the Schwinger-Dyson
(SD) equation in the same approximation for consistency with the chiral
symmetry. Amazingly, due to the fact that the two-loop running coupling of
large Nf QCD is explicitly written in terms of an analytic function, large Nf
QCD turns out to be the first example in which the SD equation can be solved in
the complex plane and hence the HBS equation directly in the time-like region.
We find that approaching the chiral phase transition point from the broken
phase, the scalar, vector, and axial-vector meson masses vanish to zero with
the same scaling behavior, all degenerate with the massless pseudoscalar meson.
This may suggest a new type of manifestation of the chiral symmetry restoration
in large Nf QCD.Comment: 33 pages, 16 figures. Typos are corrected. Minor corrections and
references are added. Version to appear in Phys. Rev.
Anti-Periodic Boundary Conditions in Supersymmetric DLCQ
It is of considerable importance to have a numerical method for solving
supersymmetric theories that can support a non-zero central charge. The central
charge in supersymmetric theories is in general a boundary integral and
therefore vanishes when one uses periodic boundary conditions. One is therefore
prevented from studying BPS states in the standard supersymmetric formulation
of DLCQ (SDLCQ). We present a novel formulation of SDLCQ where the fields
satisfy anti-periodic boundary conditions. The Hamiltonian is written as the
anti-commutator of two charges, as in SDLCQ. The anti-periodic SDLCQ we
consider breaks supersymmetry at finite resolution, but requires no
renormalization and becomes supersymmetric in the continuum limit. In
principle, this method could be used to study BPS states. However, we find its
convergence to be disappointingly slow.Comment: 9pp, 2 figure
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