24 research outputs found
How Do Fermions Behave on a Random Lattice?
Comparing random lattice, naive and Wilson fermions in two dimensional
abelian background gauge field, we show that the doublers suppressed in the
free field case are revived for random lattices in the continuum limit unless
gauge interactions are implemented in a non--invariant way.Comment: updated contribution to LAT92 conference; UM-P-92/90 and OZ-92/33; 4
pages; shar archive LaTex document with figures included, requires
espcrc2.sty fil
Correlations and Binding in 4D Dynamical Triangulation
We study correlations on the euclidean spacetimes generated in Monte Carlo
simulations of the model. In the elongated phase, curvature correlations appear
to fall off like a fractional power. Near the transition to the crumpled phase
this power is consistent with 4. We also present improved data of our
computations of the binding energy of test particles.Comment: 4 pages for proceedings Lattice '95; latex, espcrc2.sty and
postscript figure files packed with uufiles; corrected packing, contents of
paper unchange
Evolutionary Algorithms Applied to Landau-Gauge Fixing
Current algorithms used to put a lattice gauge configuration into Landau
gauge either suffer from the problem of critical slowing-down or involve an
additional computational expense to overcome it. Evolutionary Algorithms (EAs),
which have been widely applied to other global optimisation problems, may be of
use in gauge fixing. Also, being global, they should not suffer from critical
slowing-down as do local gradient based algorithms. We apply EA's and also a
Steepest Descent (SD) based method to the problem of Landau Gauge Fixing and
compare their performance.Comment: LATTICE98(algorithms), 3 pages, 6 figure
More on random-lattice fermions
The lattice fermion determinants, in a given background gauge field, are
evaluated for two different kinds of random lattices and compared to those of
naive and wilson fermions in the continuum limit. While the fermion doubling is
confirmed on one kind of lattices, there is positive evidence that it may be
absent for the other, at least for vector interactions in two dimensions.
Combined with previous studies, arbitrary randomness by itself is shown to be
not a sufficient condition to remove the fermion doublers.Comment: 3 pages, uuencoded compress postscript, contributed poster at the
Lattice '94 Symposiu
Monte Carlo Simulations with Complex-Valued Measure
A simulation method based on the RG blocking is shown to yield statistical
errors smaller than that of the crude MC using absolute values of the original
measures. The new method is particularly suitable to apply to the sign problem
of indefinite or complex-valued measures. We demonstrate the many advantages of
this method in the simulation of 2D Ising model with complex-valued
temperature.Comment: 3 pages, 3 Postscript figures, submitted to Nucl.Phys.B.Proc.Sup
Simulations with Complex Measure
A method is proposed to handle the sign problem in the simulation of systems
having indefinite or complex-valued measures. In general, this new approach,
which is based on renormalisation blocking, is shown to yield statistical
errors smaller than the crude Monte Carlo method using absolute values of the
original measures. The improved method is applied to the 2D Ising model with
temperature generalised to take on complex values. It is also adapted to
implement Monte Carlo Renormalisation Group calculations of the magnetic and
thermal critical exponents.Comment: 18 pages, 13 Postscript figures, submitted to and revised for
Nucl.Phys.B. Two figures are colour, but monochrome versions of these have
also been include
Monte Carlo Simulations with Indefinite and Complex-Valued Measures
A method is presented to tackle the sign problem in the simulations of
systems having indefinite or complex-valued measures. In general, this new
approach is shown to yield statistical errors smaller than the crude Monte
Carlo using absolute values of the original measures. Exactly solvable,
one-dimensional Ising models with complex temperature and complex activity
illustrate the considerable improvements and the workability of the new method
even when the crude one fails.Comment: 10 A4 pages, postscript (140K), UM-P-93-7
A proof of the Geroch-Horowitz-Penrose formulation of the strong cosmic censor conjecture motivated by computability theory
In this paper we present a proof of a mathematical version of the strong
cosmic censor conjecture attributed to Geroch-Horowitz and Penrose but
formulated explicitly by Wald. The proof is based on the existence of
future-inextendible causal curves in causal pasts of events on the future
Cauchy horizon in a non-globally hyperbolic space-time. By examining explicit
non-globally hyperbolic space-times we find that in case of several physically
relevant solutions these future-inextendible curves have in fact infinite
length. This way we recognize a close relationship between asymptotically flat
or anti-de Sitter, physically relevant extendible space-times and the so-called
Malament-Hogarth space-times which play a central role in recent investigations
in the theory of "gravitational computers". This motivates us to exhibit a more
sharp, more geometric formulation of the strong cosmic censor conjecture,
namely "all physically relevant, asymptotically flat or anti-de Sitter but
non-globally hyperbolic space-times are Malament-Hogarth ones".
Our observations may indicate a natural but hidden connection between the
strong cosmic censorship scenario and the Church-Turing thesis revealing an
unexpected conceptual depth beneath both conjectures.Comment: 16pp, LaTeX, no figures. Final published versio
Entangled quantum heat engines based on two two-spin systems with Dzyaloshinski-Moriya anisotropic antisymmetric interaction
We construct an entangled quantum heat engine (EQHE) based on two two-spin
systems with Dzyaloshinski-Moriya (DM) anisotropic antisymmetric interaction.
By applying the explanations of heat transferred and work performed at the
quantum level in Kieu's work [PRL, 93, 140403 (2004)], the basic thermodynamic
quantities, i.e., heat transferred, net work done in a cycle and efficiency of
EQHE are investigated in terms of DM interaction and concurrence. The validity
of the second law of thermodynamics is confirmed in the entangled system. It is
found that there is a same efficiency for both antiferromagnetic and
ferromagnetic cases, and the efficiency can be controlled in two manners: 1.
only by spin-spin interaction J and DM interaction D; 2. only by the
temperature T and concurrence C. In order to obtain a positive net work, we
need not entangle all qubits in two two-spin systems and we only require the
entanglement between qubits in a two-spin system not be zero. As the ratio of
entanglement between qubits in two two-spin systems increases, the efficiency
will approach infinitely the classical Carnot one. An interesting phenomenon is
an abrupt transition of the efficiency when the entanglements between qubits in
two two-spin systems are equal
Two-photon resonance absorption in four-level systems
An unconventional perturbation treatment of the theory of two- photon absorption in a four-level atomic system close to two-photon resonance is developed on the basis of the optical Bloch equations. The results for the longtime populations of the intermediate and upper states are valid in low- and medium-intensity regimes. In both regimes the upper state population shows a lorentzian resonance centred on zero two-photon detuning with a width equal to the natural linewidth of the upper state. The intermediate state population shows a similar behaviour in the intermediate regime only, with no significant dependence on two-photon detuning in the low-intensity regime. However, in both regimes the population of the virtual intermediate state is at least comparable to that of the resonant upper state. The perturbation theory results have been checked by direct numerical calculations based on the Bloch equations. Numerical calculations confirm the presence of a high-intensity regime characterized by power shifts, broadening and saturation effects