13,366 research outputs found

### Regulating the infrared by mode matching: A massless scalar in expanding spaces with constant deceleration

In this paper we consider a massless scalar field, with a possible coupling
$\xi$ to the Ricci scalar in a $D$ dimensional FLRW spacetime with a constant
deceleration parameter $q=\epsilon-1$, $\epsilon=-{\dot{H}}/{H^2}$. Correlation
functions for the Bunch-Davies vacuum of such a theory have long been known to
be infrared divergent for a wide range of values of $\epsilon$. We resolve
these divergences by explicitly matching the spacetime under consideration to a
spacetime without infrared divergencies. Such a procedure ensures that all
correlation functions with respect to the vacuum in the spacetime of interest
are infrared finite. In this newly defined vacuum we construct the coincidence
limit of the propagator and as an example calculate the expectation value of
the stress energy tensor. We find that this approach gives both in the
ultraviolet and in the infrared satisfactory results. Moreover, we find that,
unless the effective mass due to the coupling to the Ricci scalar $\xi R$ is
negative, quantum contributions to the energy density always dilute away
faster, or just as fast, as the background energy density. Therefore, quantum
backreaction is insignificant at the one loop order, unless $\xi R$ is
negative. Finally we compare this approach with known results where the
infrared is regulated by placing the Universe in a finite box. In an
accelerating universe, the results are qualitatively the same, provided one
identifies the size of the Universe with the physical Hubble radius at the time
of the matching. In a decelerating universe however, the two schemes give
different late time behavior for the quantum stress energy tensor. This happens
because in this case the length scale at which one regulates the infrared
becomes sub-Hubble at late times.Comment: 55 pages, 6 figure

### Dynamics of modulated and composite aperiodic crystals: the signature of the inner polarization in the neutron coherent inelastic scattering

We compare within an unifying formalism the dynamical properties of modulated
and composite aperiodic (incommensurate) crystals. We discuss the concept of
inner polarization and we define an inner polarization parameter beta that
distinguishes between different acoustic modes of aperiodic crystals. Although
this concept has its limitations, we show that it can be used to extract
valuable information from neutron coherent inelastic scattering experiments.
Within certain conditions, the ratio between the dynamic and the static
structure factors at various Bragg peaks depends on beta. We show how the
knowledge of beta for modes of an unknown structure can be used to decide
whether the structure is composite or modulated. Furthermore, the same
information can be used to predict scattered intensity within unexplored
regions of the reciprocal space, being thus a guide for experiment

### Bridging Two Ways of Describing Final-State Interactions in A(e,e'p) Reactions

We outline a relativistic and unfactorized framework to treat the final-state
interactions in quasi-elastic A(e,e'p) reactions for four-momentum transfers
Q$^{2} \gtrsim 0.3$ (GeV/c)$^{2}$. The model, which relies on the eikonal
approximation, can be used in combination with optical potentials, as well as
with the Glauber multiple-scattering method. We argue that such a model can
bridge the gap between a typical ``low'' and ``high-energy'' description of
final-state interactions, in a reasonably smooth fashion. This argument is made
on the basis of calculated structure functions, polarization observables and
nuclear transparencies for the target nuclei $^{12}$C and $^{16}$O.Comment: revised versio

### Comment on "Magnetic quantum oscillations of the conductivity in layered conductors"

We discuss the recent theory of Gvozdikov [Phys. Rev. B 70, 085113 (2004)]
which aims at explaining the Shubnikov-de Haas oscillations of the longitudinal
resistivity \rho_zz observed in the quasi-two-dimensional organic compound
\beta''-(BEDT-TTF)_2SF_5CH_2CF_2SO_3.
We point out that the self-consistent equations of the theory yielding the
longitudinal resistivity and the magnetic field dependence of the chemical
potential have been incorrectly solved. We show that the consideration of the
self-consistent Born approximation (which determines the relaxation rate in
Gvozdikov's paper) leads in fact to the complete absence of the longitudinal
conductivity \sigma_{zz} at leading order in high magnetic fields.Comment: 4 pages, no figur

### On Critical Exponents and the Renormalization of the Coupling Constant in Growth Models with Surface Diffusion

It is shown by the method of renormalized field theory that in contrast to a
statement based on a mathematically ill-defined invariance transformation and
found in most of the recent publications on growth models with surface
diffusion, the coupling constant of these models renormalizes nontrivially.
This implies that the widely accepted supposedly exact scaling exponents are to
be corrected. A two-loop calculation shows that the corrections are small and
these exponents seem to be very good approximations.Comment: 4 pages, revtex, 2 postscript figures, to appear in Phys.Rev.Let

### Heat buffers improve capacity and exploitation degree of geothermal energy sources

This research focuses on the role of heat buffers to support optimal use of combinations of traditional and renewable heat sources like geothermal heat for greenhouse heating. The objective was to determine the contribution of heat buffers to effective new combinations of resources that satisfy greenhouse heat, carbon dioxide and electricity demand at minimum cost. Tank buffers, basement buffers and aquifers were considered as short and long term buffers. Simulations were carried out for a 10ha sweet pepper and a 30ha tomato greenhouse (15ha intensively lighted). Standard heating systems based on central boiler and co-generation were used as a reference and compared with combinations of boilers, co-generators, geothermal heat and heat buffer strategies. Crop production and greenhouse climate were simulated and resource demand determined for normal greenhouse operation. A linear programming algorithm was used to apply resources and equipment available to the model at minimum cost. Results show that heat buffers help to reduce the required capacity of a geothermal heat source, and increase both the utilisation degree of the source and the cover percentage of greenhouse heat demand. The technically most feasible solution for long term buffering was the basement buffer which allows high buffer volumes without loss of useful space and heat loss contributes to greenhouse heating, however this solution was economically not feasible. Also the deep aquifer was a good option, however exploitation risks and manageability are potential problems. Integration of geothermal heat with other sources resulted in the best solutions that were both technically and economically feasible. Simulation showed at gas price level 30¿ct.m-3, that geothermal heat was cheaper than central boiler and even co-generation heat when hours of operation exceed 1000h.y-1. Instead of using large buffers, peak loads can also be covered by central boilers. Simulated solutions reduced gas consumption with 60 to 95%

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