125 research outputs found
Dynamical Mean-Field Theory within an Augmented Plane-Wave Framework: Assessing Electronic Correlations in the Iron Pnictide LaFeAsO
We present an approach that combines the local density approximation (LDA)
and the dynamical mean-field theory (DMFT) in the framework of the
full-potential linear augmented plane waves (FLAPW) method. Wannier-like
functions for the correlated shell are constructed by projecting local orbitals
onto a set of Bloch eigenstates located within a certain energy window. The
screened Coulomb interaction and Hund's coupling are calculated from a
first-principle constrained RPA scheme. We apply this LDA+DMFT implementation,
in conjunction with continuous-time quantum Monte-Carlo, to study the
electronic correlations in LaFeAsO. Our findings support the physical picture
of a metal with intermediate correlations. The average value of the mass
renormalization of the Fe 3d bands is about 1.6, in reasonable agreement with
the picture inferred from photoemission experiments. The discrepancies between
different LDA+DMFT calculations (all technically correct) which have been
reported in the literature are shown to have two causes: i) the specific value
of the interaction parameters used in these calculations and ii) the degree of
localization of the Wannier orbitals chosen to represent the Fe 3d states, to
which many-body terms are applied. The latter is a fundamental issue in the
application of many-body calculations, such as DMFT, in a realistic setting. We
provide strong evidence that the DMFT approximation is more accurate and more
straightforward to implement when well-localized orbitals are constructed from
a large energy window encompassing Fe-3d, As-4p and O-2p, and point out several
difficulties associated with the use of extended Wannier functions associated
with the low-energy iron bands. Some of these issues have important physical
consequences, regarding in particular the sensitivity to the Hund's coupling.Comment: 16 pages, 9 figures, published versio
Neutrino oscillations: Quantum mechanics vs. quantum field theory
A consistent description of neutrino oscillations requires either the
quantum-mechanical (QM) wave packet approach or a quantum field theoretic (QFT)
treatment. We compare these two approaches to neutrino oscillations and discuss
the correspondence between them. In particular, we derive expressions for the
QM neutrino wave packets from QFT and relate the free parameters of the QM
framework, in particular the effective momentum uncertainty of the neutrino
state, to the more fundamental parameters of the QFT approach. We include in
our discussion the possibilities that some of the neutrino's interaction
partners are not detected, that the neutrino is produced in the decay of an
unstable parent particle, and that the overlap of the wave packets of the
particles involved in the neutrino production (or detection) process is not
maximal. Finally, we demonstrate how the properly normalized oscillation
probabilities can be obtained in the QFT framework without an ad hoc
normalization procedure employed in the QM approach.Comment: LaTeX, 42 pages, 1 figure; v2: minor clarifications, matches
published version; v3: Corrected the discussion of the conditions under which
an oscillation probability can be sensibly defined in the QFT approach (sec.
5.2.4
Phase Transitions of Charged Scalars at Finite Temperature and Chemical Potential
We calculate the grand canonical partition function at the one-loop level for
scalar quantum electrodynamics at finite temperature and chemical potential. A
classical background charge density with a charge opposite that of the scalars
ensures the neutrality of the system. For low density systems we find evidence
of a first order phase transition. We find upper and lower bounds on the
transition temperature below which the charged scalars form a condensate. A
first order phase transition may have consequences for helium-core white dwarf
stars in which it has been argued that such a condensate of charged helium-4
nuclei could exist.Comment: 20 pages, 3 figures. Version accepted for publication in JHE
Field Theory for a Deuteron Quantum Liquid
Based on general symmetry principles we study an effective Lagrangian for a
neutral system of condensed spin-1 deuteron nuclei and electrons, at
greater-than-atomic but less-than-nuclear densities. We expect such matter to
be present in thin layers within certain low-mass brown dwarfs. It may also be
produced in future shock-wave-compression experiments as an effective fuel for
laser induced nuclear fusion. We find a background solution of the effective
theory describing a net spin zero condensate of deuterons with their spins
aligned and anti-aligned in a certain spontaneously emerged preferred
direction. The spectrum of low energy collective excitations contains two spin
waves with linear dispersions -- like in antiferromagnets -- as well as gapped
longitudinal and transverse modes related to the Meissner effect -- like in
superconductors. We show that counting of the Nambu-Goldstone modes of
spontaneously broken internal and space-time symmetries obeys, in a nontrivial
way, the rules of the Goldstone theorem for Lorentz non-invariant systems. We
discuss thermodynamic properties of the condensate, and its potential
manifestation in the low-mass brown dwarfs.Comment: 19 LaTeX pages; v2: 2 refs added, JHEP versio
On the Perturbative Stability of Quantum Field Theories in de Sitter Space
We use a field theoretic generalization of the Wigner-Weisskopf method to
study the stability of the Bunch-Davies vacuum state for a massless,
conformally coupled interacting test field in de Sitter space. We find that in
theory the vacuum does {\em not} decay, while in
non-conformally invariant models, the vacuum decays as a consequence of a
vacuum wave function renormalization that depends \emph{singularly} on
(conformal) time and is proportional to the spatial volume. In a particular
regularization scheme the vacuum wave function renormalization is the same as
in Minkowski spacetime, but in terms of the \emph{physical volume}, which leads
to an interpretation of the decay. A simple example of the impact of vacuum
decay upon a non-gaussian correlation is discussed. Single particle excitations
also decay into two particle states, leading to particle production that
hastens the exiting of modes from the de Sitter horizon resulting in the
production of \emph{entangled superhorizon pairs} with a population consistent
with unitary evolution. We find a non-perturbative, self-consistent "screening"
mechanism that shuts off vacuum decay asymptotically, leading to a stationary
vacuum state in a manner not unlike the approach to a fixed point in the space
of states.Comment: 36 pages, 4 figures. Version to appear in JHEP, more explanation
Mixing of Active and Sterile Neutrinos
We investigate mixing of neutrinos in the MSM (neutrino Minimal Standard
Model), which is the MSM extended by three right-handed neutrinos. Especially,
we study elements of the mixing matrix between three
left-handed neutrinos () and two sterile
neutrinos () which are responsible to the seesaw mechanism
generating the suppressed masses of active neutrinos as well as the generation
of the baryon asymmetry of the universe (BAU). It is shown that
can be suppressed by many orders of magnitude compared with
and , when the Chooz angle is large in the
normal hierarchy of active neutrino masses. We then discuss the neutrinoless
double beta decay in this framework by taking into account the contributions
not only from active neutrinos but also from all the three sterile neutrinos.
It is shown that and give substantial, destructive contributions
when their masses are smaller than a few 100 MeV, and as a results receive no stringent constraint from the current bounds on such decay.
Finally, we discuss the impacts of the obtained results on the direct searches
of in meson decays for the case when are lighter than pion
mass. We show that there exists the allowed region for with such
small masses in the normal hierarchy case even if the current bound on the
lifetimes of from the big bang nucleosynthesis is imposed. It is also
pointed out that the direct search by using and might miss such since the branching ratios can be
extremely small due to the cancellation in , but the search by
can cover the whole allowed region by improving the
measurement of the branching ratio by a factor of 5.Comment: 30 pages, 32 figure
Fluids in cosmology
We review the role of fluids in cosmology by first introducing them in
General Relativity and then by applying them to a FRW Universe's model. We
describe how relativistic and non-relativistic components evolve in the
background dynamics. We also introduce scalar fields to show that they are able
to yield an inflationary dynamics at very early times (inflation) and late
times (quintessence). Then, we proceed to study the thermodynamical properties
of the fluids and, lastly, its perturbed kinematics. We make emphasis in the
constrictions of parameters by recent cosmological probes.Comment: 34 pages, 4 figures, version accepted as invited review to the book
"Computational and Experimental Fluid Mechanics with Applications to Physics,
Engineering and the Environment". Version 2: typos corrected and references
expande
Skyrmion Multi-Walls
Skyrmion walls are topologically-nontrivial solutions of the Skyrme system
which are periodic in two spatial directions. We report numerical
investigations which show that solutions representing parallel multi-walls
exist. The most stable configuration is that of the square -wall, which in
the limit becomes the cubically-symmetric Skyrme crystal. There is
also a solution resembling parallel hexagonal walls, but this is less stable.Comment: 7 pages, 1 figur
Implications of Flavor Dynamics for Fermion Triplet Leptogenesis
We analyze the importance of flavor effects in models in which leptogenesis
proceeds via the decay of Majorana electroweak triplets. We find that depending
on the relative strengths of gauge and Yukawa reactions the asymmetry can
be sizably enhanced, exceeding in some cases an order of magnitude level. We
also discuss the impact that such effects can have for TeV-scale triplets
showing that as long as the asymmetry is produced by the dynamics of the
lightest such triplet they are negligible, but open the possibility for
scenarios in which the asymmetry is generated above the TeV scale by heavier
states, possibly surviving the TeV triplet related washouts. We investigate
these cases and show how they can be disentangled at the LHC by using Majorana
triplet collider observables and, in the case of minimal type III see-saw
models even through lepton flavor violation observables.Comment: 22 pages, 9 figures, extended discussion on collider phenomenology,
references added. Version matches publication in JHE
A Search for leptophilic Z_(l) boson at future linear colliders
We study the possible dynamics associated with leptonic charge in future
linear colliders. Leptophilic massive vector boson, Z_(l), have been
investigated through the process e^(+)e^(-) -> mu^(+)mu^(-). We have shown that
ILC and CLIC will give opportunity to observe Z_(l) with masses up to the
center of mass energy if the corresponding coupling constant g_(l) exceeds
10^(-3).Comment: 12 pages, 10 figure
- …