296 research outputs found
A new type of temperature driven reorientation transition in magnetic thin films
We present a new type of temperature driven spin reorientation transition
(SRT) in thin films. It can occur when the lattice and the shape anisotropy
favor different easy directions of the magnetization. Due to different
temperature dependencies of the two contributions the effective anisotropy may
change its sign and thus the direction of the magnetization as a function of
temperature may change. Contrary to the well-known reorientation transition
caused by competing surface and bulk anisotropy contributions the reorientation
that we discuss is also found in film systems with a uniform lattice
anisotropy. The results of our theoretical model study may have experimental
relevance for film systems with positive lattice anisotropy, as e.g. thin iron
films grown on copper.Comment: 7 pages, 4 figures, accepted for publication in EPJ
Non-collapsing renormalized QRPA with proton-neutron pairing for neutrinoless double beta decay
Using the renormalized quasiparticle random phase approximation (RQRPA), we
calculate the light neutrino mass mediated mode of neutrinoless double beta
decay of Ge76, Mo100, Te128 and Te130. Our results indicate that the simple
quasiboson approximation is not good enough to study the neutrinoless double
beta decay, because its solutions collapse for physical values of g_pp. We find
that extension of the Hilbert space and inclusion of the Pauli Principle in the
QRPA with proton-neutron pairing, allows us to extend our calculations beyond
the point of collapse, for physical values of the nuclear force strength. As a
consequence one might be able to extract more accurate values on the effective
neutrino mass by using the best available experimental limits on the half-life
of neutrinoless double beta decay.Comment: 15 pages, RevTex, 2 Postscript figures, to appear in Phys. Lett.
Self Consistent and Renormalized particle-particle RPA in a Schematic Model
The dynamical effects of ground state correlations for excitation energies
and transition strengths near the superfluid phase transition are studied in
the soluble two level pairing model, in the context of the particle-particle
self consistent Random Phase Approximation (SCRPA). Exact results are well
reproduced across the transition region, beyond the collapse of the standard
particle-particle Random Phase Approximation. The effects of two-body
correlation in the SCRPA are displayed explicitly.Comment: 11 pages, revtex, 3ps figures, to appear in Phys. Rev.
A large Hilbert space QRPA and RQRPA calculation of neutrinoless double beta decay
A large Hilbert space is used for the calculation of the nuclear matrix
elements governing the light neutrino mass mediated mode of neutrinoless double
beta decay of Ge76, Mo100, Cd116, Te128 and Xe136 within the proton-neutron
quasiparticle random phase approximation (pn-QRPA) and the renormalized QRPA
with proton-neutron pairing (full-RQRPA) methods. We have found that the
nuclear matrix elements obtained with the standard pn-QRPA for several nuclear
transitions are extremely sensitive to the renormalization of the
particle-particle component of the residual interaction of the nuclear
hamiltonian. Therefore the standard pn-QRPA does not guarantee the necessary
accuracy to allow us to extract a reliable limit on the effective neutrino
mass. This behaviour, already known from the calculation of the two-neutrino
double beta decay matrix elements, manifests itself in the neutrinoless
double-beta decay but only if a large model space is used. The full-RQRPA,
which takes into account proton-neutron pairing and considers the Pauli
principle in an approximate way, offers a stable solution in the physically
acceptable region of the particle-particle strength. In this way more accurate
values on the effective neutrino mass have been deduced from the experimental
lower limits of the half-lifes of neutrinoless double beta decay.Comment: 19 pages, RevTex, 1 Postscript figur
Zn(O, S) layers for chalcoyprite solar cells sputtered from a single target
A simplified Cu(In, Ga)(S, Se)2/Zn(O, S)/ZnO:Al stack for chalcopyrite thin-
film solar cells is proposed. In this stack the Zn(O, S) layer combines the
roles of the traditional CdS buffer and undoped ZnO layers. It will be shown
that Zn(O, S) films can be sputtered in argon atmosphere from a single mixed
target without substrate heating. The photovoltaic performance of the
simplified stack matches that of the conventional approach. Replacing the ZnO
target with a ZnO/ZnS target may therefore be sufficient to omit the CdS
buffer layer and avoid the associated complexity, safety and recycling issues,
and to lower production cost
Correlation and surface effects in Vanadium Oxides
Recent photoemission experiments have shown strong surface modifications in
the spectra from vanadium oxides as (V,Cr)_2O_3 or
(Sr,Ca)VO_3. The effective mass is enhanced at the surface and the coherent
part of the surface spectrum is narrowed as compared to the bulk. The
quasiparticle weight is more sensitive at the surface than in the bulk against
bandwidth variations. We investigate these effects theoretically considering
the single-band Hubbard model for a film geometry. A simplified dynamical
mean-field scheme is used to calculate the main features of the interacting
layer-dependent spectral function. It turns out that the experimentally
confirmed effects are inherent properties of a system of strongly correlated
electrons. The reduction of the weight and the variance of the coherent part of
the surface spectrum can be traced back to the reduced surface coordination
number. Surface correlation effects can be strongly amplified by changes of the
hopping integrals at the surface.Comment: to appear in PRB; 8 pages, 6 figure
Neutrinoless double beta decay within Self-consistent Renormalized Quasiparticle Random Phase Approximation and inclusion of induced nucleon currents
The first, to our knowledge, calculation of neutrinoless double beta decay
(-decay) matrix elements within the self-consistent
renormalised Quasiparticle Random Phase Approximation (SRQRPA) is presented.
The contribution from the momentum-dependent induced nucleon currents to
-decay amplitude is taken into account. A detailed nuclear
structure study includes the discussion of the sensitivity of the obtained
SRQRPA results for -decay of Ge to the parameters of
nuclear Hamiltonian, two-nucleon short-range correlations and the truncation of
the model space. A comparision with the standard and renormalized QRPA is
presented. We have found a considerable reduction of the SRQRPA nuclear matrix
elements, resulting in less stringent limits for the effective neutrino mass.Comment: 13 pages, 3 figures, 1 tabl
The Pauli principle, QRPA and the two-neutrino double beta decay
We examine the violation of the Pauli exclusion principle in the
Quasiparticle Random Phase Approximation (QRPA) calculation of the two-neutrino
double beta decay matrix elements, which has its origin in the quasi-boson
approximation. For that purpose we propose a new renormalized QRPA with
proton-neutron pairing method (full-RQRPA) for nuclear structure studies, which
includes ground state correlation beyond the QRPA. This is achieved by using of
renormalized quasi-boson approximation, in which the Pauli exclusion principle
is taken into account more carefully. The full-RQRPA has been applied to
two-neutrino double beta decay of , , and
. The nuclear matrix elements have been found significantly less
sensitive to the increasing strength of particle-particle interaction in the
physically interesting region in comparison with QRPA results. The strong
differences between the results of both methods indicate that the Pauli
exclusion principle plays an important role in the evaluation of the double
beta decay. The inclusion of the Pauli principle removes the difficulties with
the strong dependence on the particle-particle strength in the QRPA on
the two-neutrino double beta decay.Comment: Accepted for publication in Nucl. Phys. A, 22 pages, including 5
figures, LaTeX (using REVTeX and epsfig-style
MFI Type Zeolite Aggregates with Nanosized Particles via a Combination of Spray Drying and Steam-Assisted Crystallization (SAC) Techniques
This article belongs to the Special Issue Catalysis on Zeolites and Zeolite-Like Materials II
Fully-Renormalized QRPA fulfills Ikeda sum rule exactly
The renormalized quasiparticle-RPA is reformulated for even-even nuclei using
restrictions imposed by the commutativity of the phonon creation operator with
the total particle number operator. This new version, Fully-Renormalized QRPA
(FR-QRPA), is free from the spurious low-energy solutions. Analytical proof is
given that the Ikeda sum rule is fullfiled within the FR-QRPA.Comment: 9 page
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