235 research outputs found
Density-operator theory of orbital magnetic susceptibility in periodic insulators
The theoretical treatment of homogeneous static magnetic fields in periodic
systems is challenging, as the corresponding vector potential breaks the
translational invariance of the Hamiltonian. Based on density operators and
perturbation theory, we propose, for insulators, a periodic framework for the
treatment of magnetic fields up to arbitrary order of perturbation, similar to
widely used schemes for electric fields. The second-order term delivers a new,
remarkably simple, formulation of the macroscopic orbital magnetic
susceptibility for periodic insulators. We validate the latter expression using
a tight-binding model, analytically from the present theory and numerically
from the large-size limit of a finite cluster, with excellent numerical
agreement.Comment: 5 pages including 2 figures; accepted for publication in Phys. Rev.
Residual internal stress in partially crystallized photothermorefractive glass: Evaluation by nuclear magnetic resonance spectroscopy and first principles calculations
In some circumstances, the mechanical and optical properties of multiphase brittle materials strongly depend on the level of residual micromechanical stresses that arise upon cooling due to thermal and elastic mismatch between the constituent phases. Here we study the residual internal stress in a partially crystallized oxyfluoride glass, best known as photothermorefractive (PTR) glass. This material is composed of a glass matrix with embedded nanosize sodium fluoride (NaF) crystals. Using both the Selsing model and solid-state nuclear magnetic resonance in combination with first principles calculations we found that the crystals are under a tensile stress field of approximately 610-800 MPa. For this stress level the estimated critical crystal diameter for spontaneous cracking is about 2300-1900 nm, which greatly exceeds the observed diameters of 7-35 nm. Hence no spontaneous cracking is expected for the PTR glasses. First principles calculations indicate that the stress induced change of the refractive index of the NaF crystals is about -0.08%, which agrees with the observed refractive index changes
Hyper-Raman scattering from vitreous boron oxide: coherent enhancement of the boson peak
Hyper-Raman scattering spectra of vitreous BO are reported and
compared to Raman scattering results. The main features are indexed in terms of
vibrations of structural units. Particular attention is given to the low
frequency boson peak which is shown to relate to out-of-plane librations of
BO boroxol rings and BO triangles. Its hyper-Raman strength is
comparable to that of cooperative polar modes. It points to a sizeable coherent
enhancement of the hyper-Raman signal compared to the Raman one. This is
explained by the symmetry of the structural units.Comment: 4 pages, 3 figure
Temporal Interferometry: A Mechanism for Controlling Qubit Transitions During Twisted Rapid Passage with Possible Application to Quantum Computing
In an adiabatic rapid passage experiment, the Bloch vector of a two-level
system (qubit) is inverted by slowly inverting an external field to which it is
coupled, and along which it is initially aligned. In twisted rapid passage, the
external field is allowed to twist around its initial direction with azimuthal
angle at the same time that it is inverted. For polynomial twist:
. We show that for , multiple avoided crossings
can occur during the inversion of the external field, and that these crossings
give rise to strong interference effects in the qubit transition probability.
The transition probability is found to be a function of the twist strength ,
which can be used to control the time-separation of the avoided crossings, and
hence the character of the interference. Constructive and destructive
interference are possible. The interference effects are a consequence of the
temporal phase coherence of the wavefunction. The ability to vary this
coherence by varying the temporal separation of the avoided crossings renders
twisted rapid passage with adjustable twist strength into a temporal
interferometer through which qubit transitions can be greatly enhanced or
suppressed. Possible application of this interference mechanism to construction
of fast fault-tolerant quantum CNOT and NOT gates is discussed.Comment: 29 pages, 16 figures, submitted to Phys. Rev.
Truncating first-order Dyson-Schwinger equations in Coulomb-Gauge Yang-Mills theory
The non-perturbative domain of QCD contains confinement, chiral symmetry
breaking, and the bound state spectrum. For the calculation of the latter, the
Coulomb gauge is particularly well-suited. Access to these non-perturbative
properties should be possible by means of the Green's functions. However,
Coulomb gauge is also very involved, and thus hard to tackle. We introduce a
novel BRST-type operator r, and show that the left-hand side of Gauss' law is
r-exact.
We investigate a possible truncation scheme of the Dyson-Schwinger equations
in first-order formalism for the propagators based on an instantaneous
approximation. We demonstrate that this is insufficient to obtain solutions
with the expected property of a linear-rising Coulomb potential. We also show
systematically that a class of possible vertex dressings does not change this
result.Comment: 22 pages, 4 figures, 1 tabl
Roles of the color antisymmetric ghost propagator in the infrared QCD
The results of Coulomb gauge and Landau gauge lattice QCD simulation do not
agree completely with continuum theory. There are indications that the ghost
propagator in the infrared region is not purely color diagonal as in high
energy region. After presenting lattice simulation of configurations produced
with Kogut-Susskind fermion (MILC collaboration) and those with domain wall
fermion (RBC/UKQCD collaboration), I investigate in triple gluon vertex and the
ghost-gluon-ghost vertex how the square of the color antisymmetric ghost
contributes. Then the effect of the vertex correction to the gluon propagator
and the ghost propagator is investigated.
Recent Dyson-Schwinger equation analysis suggests the ghost dressing function
finite and no infrared enhancement or . But the ghost
propagator renormalized by the loop containing a product of color antisymmetric
ghost is expected to behave as with
with , if the fixed point
scenario is valid. I interpret the solution should contain a
vertex correction. The infrared exponent of our lattice Landau gauge gluon
propagator of the RBC/UKQCD is and that of MILC is about
-0.7.
The implication for the Kugo-Ojima color confinement criterion, QCD effective
coupling and the Slavnov identity are given.Comment: 13 pages 10 figures, references added and revised. version to be
published in Few-Body System
On the infrared scaling solution of SU(N) Yang-Mills theories in the maximally Abelian gauge
An improved method for extracting infrared exponents from functional
equations is presented. The generalizations introduced allow for an analysis of
quite complicated systems such as Yang-Mills theory in the maximally Abelian
gauge. Assuming the absence of cancellations in the appropriately renormalized
integrals the only consistent scaling solution yields an infrared enhanced
diagonal gluon propagator in support of the Abelian dominance hypothesis. This
is explicitly shown for SU(2) and subsequently verified for SU(N), where
additional interactions exist. We also derive the most infrared divergent
scaling solution possible for vertex functions in terms of the propagators'
infrared exponents. We provide general conditions for the existence of a
scaling solution for a given system and comment on the cases of linear
covariant gauges and ghost anti-ghost symmetric gauges.Comment: 23 pages, 10 figures; version coincides with version published in
EPJ
Renormalization of the Yang-Mills theory in the ambiguity-free gauge
The renormalization procedure for the Yang-Mills theory in the gauge free of
the Gribov ambiguity is constructed. It is shown that all the ultraviolet
infinities may be removed by renormalization of the parameters entering the
classical Lagrangian and the local redefinition of the fields.Comment: 20 pages. Some explanations extended, one reference added. Final
version published in the journa
Dual Projection and Selfduality in Three Dimensions
We discuss the notion of duality and selfduality in the context of the dual
projection operation that creates an internal space of potentials. Contrary to
the prevailing algebraic or group theoretical methods, this technique is
applicable to both even and odd dimensions. The role of parity in the kernel of
the Gauss law to determine the dimensional dependence is clarified. We derive
the appropriate invariant actions, discuss the symmetry groups and their proper
generators. In particular, the novel concept of duality symmetry and
selfduality in Maxwell theory in (2+1) dimensions is analysed in details. The
corresponding action is a 3D version of the familiar duality symmetric
electromagnetic theory in 4D. Finally, the duality symmetric actions in the
different dimensions constructed here manifest both the SO(2) and
symmetries, contrary to conventional results.Comment: 20 pages, late
A Systematic Extended Iterative Solution for QCD
An outline is given of an extended perturbative solution of Euclidean QCD
which systematically accounts for a class of nonperturbative effects, while
allowing renormalization by the perturbative counterterms. Proper vertices
Gamma are approximated by a double sequence Gamma[r,p], with r the degree of
rational approximation w.r.t. the QCD mass scale Lambda, nonanalytic in the
coupling g, and p the order of perturbative corrections in g-squared,
calculated from Gamma[r,0] - rather than from the perturbative Feynman rules
Gamma(0)(pert) - as a starting point. The mechanism allowing the
nonperturbative terms to reproduce themselves in the Dyson-Schwinger equations
preserves perturbative renormalizability and is tied to the divergence
structure of the theory. As a result, it restricts the self-consistency problem
for the Gamma[r,0] rigorously - i.e. without decoupling approximations - to the
superficially divergent vertices. An interesting aspect of the scheme is that
rational-function sequences for the propagators allow subsequences describing
short-lived excitations. The method is calculational, in that it allows known
techniques of loop computation to be used while dealing with integrands of
truly nonperturbative content.Comment: 48 pages (figures included). Scope of replacement: correction of a
technical defect; no changes in conten
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