888 research outputs found
Green's Functions from Quantum Cluster Algorithms
We show that cluster algorithms for quantum models have a meaning independent
of the basis chosen to construct them. Using this idea, we propose a new method
for measuring with little effort a whole class of Green's functions, once a
cluster algorithm for the partition function has been constructed. To explain
the idea, we consider the quantum XY model and compute its two point Green's
function in various ways, showing that all of them are equivalent. We also
provide numerical evidence confirming the analytic arguments. Similar
techniques are applicable to other models. In particular, in the recently
constructed quantum link models, the new technique allows us to construct
improved estimators for Wilson loops and may lead to a very precise
determination of the glueball spectrum.Comment: 15 pages, LaTeX, with four figures. Added preprint numbe
High Efficiency Detection of Argon Scintillation Light of 128nm Using LAAPDs
The possibility of efficient collection and detection of vacuum ultraviolet
light as emitted by argon, krypton, and xenon gas is studied. Absolute quantum
efficiencies of large area avalanche photodiodes (LAAPDs) are derived at these
wavelengths. VUV light of wavelengths down to the 128nm of Ar emission is shown
to be detectable with silicon avalanche photodiodes at quantum efficiencies
above 42%. Flexible Mylar foil overcoated with Al+MgF is measured to have a
specular reflectivity of 91% at argon emission wavelength. Low-pressure
argon gas is shown to emit significant amounts of non-UV radiation. The average
energy expenditure for the creation of non-UV photons in argon gas at this
pressure is measured to be below 378 eV.Comment: 5 pages, 4 figures, Talk given at IEEE 2005 Nuclear Science Symposium
and Medical Imaging Conference, Puerto Ric
Non-perturbative Renormalization Constants using Ward Identities
We extend the application of vector and axial Ward identities to calculate
, and , coefficients that give the mass dependence of the
renormalization constants of the corresponding bilinear operators in the
quenched theory. The extension relies on using operators with non-degenerate
quark masses. It allows a complete determination of the improvement
coefficients for bilinears in the quenched approximation using Ward Identities
alone. Only the scale dependent normalization constants (or )
and are undetermined. We present results of a pilot numerical study using
hadronic correlators.Comment: 3 pages. Makefile and sources included. Talk presented at LATTICE98
(matrixelement
Kosterlitz-Thouless Universality in a Fermionic System
A new extension of the attractive Hubbard model is constructed to study the
critical behavior near a finite temperature superconducting phase transition in
two dimensions using the recently developed meron-cluster algorithm. Unlike
previous calculations in the attractive Hubbard model which were limited to
small lattices, the new algorithm is used to study the critical behavior on
lattices as large as . These precise results for the first time
show that a fermionic system can undergo a finite temperature phase transition
whose critical behavior is well described by the predictions of Kosterlitz and
Thouless almost three decades ago. In particular it is confirmed that the
spatial winding number susceptibility obeys the well known predictions of
finite size scaling for and up to logarithmic corrections the pair
susceptibility scales as at large volumes with for .Comment: Revtex format; 4 pages, 2 figure
Topological Phases in Neuberger-Dirac operator
The response of the Neuberger-Dirac fermion operator D=\Id + V in the
topologically nontrivial background gauge field depends on the negative mass
parameter in the Wilson-Dirac fermion operator which enters
through the unitary operator . We classify
the topological phases of by comparing its index to the topological charge
of the smooth background gauge field. An exact discrete symmetry in the
topological phase diagram is proved for any gauge configurations. A formula for
the index of D in each topological phase is derived by obtaining the total
chiral charge of the zero modes in the exact solution of the free fermion
propagator.Comment: 27 pages, Latex, 3 figures, appendix A has been revise
Quantum Link Models with Many Rishon Flavors and with Many Colors
Quantum link models are a novel formulation of gauge theories in terms of
discrete degrees of freedom. These degrees of freedom are described by quantum
operators acting in a finite-dimensional Hilbert space. We show that for
certain representations of the operator algebra, the usual Yang-Mills action is
recovered in the continuum limit. The quantum operators can be expressed as
bilinears of fermionic creation and annihilation operators called rishons.
Using the rishon representation the quantum link Hamiltonian can be expressed
entirely in terms of color-neutral operators. This allows us to study the large
N_c limit of this model. In the 't Hooft limit we find an area law for the
Wilson loop and a mass gap. Furthermore, the strong coupling expansion is a
topological expansion in which graphs with handles and boundaries are
suppressed.Comment: Lattice2001(theorydevelop), poster by O. Baer and talk by B.
Schlittgen, 6 page
State-recycling and time-resolved imaging in topological photonic lattices
Photonic lattices - arrays of optical waveguides - are powerful platforms for
simulating a range of phenomena, including topological phases. While probing
dynamics is possible in these systems, by reinterpreting the propagation
direction as "time," accessing long timescales constitutes a severe
experimental challenge. Here, we overcome this limitation by placing the
photonic lattice in a cavity, which allows the optical state to evolve through
the lattice multiple times. The accompanying detection method, which exploits a
multi-pixel single-photon detector array, offers quasi-real time-resolved
measurements after each round trip. We apply the state-recycling scheme to
intriguing photonic lattices emulating Dirac fermions and Floquet topological
phases. In this new platform, we also realise a synthetic pulsed electric
field, which can be used to drive transport within photonic lattices. This work
opens a new route towards the detection of long timescale effects in engineered
photonic lattices and the realization of hybrid analogue-digital simulators.Comment: Comments are welcom
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