196 research outputs found
Scalar-Tensor theories from Plebanski gravity
We study a modification of the Plebanski action, which generically
corresponds to a bi-metric theory of gravity, and identify a subclass which is
equivalent to the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories.
In this manner, scalar-tensor theories are displayed as constrained BF
theories. We find that in this subclass, there is no need to impose reality of
the Urbantke metrics, as also the theory with real bivectors is a scalar-tensor
theory with a real Lorentzian metric. Furthermore, while under the former
reality conditions instabilities can arise from a wrong sign of the scalar mode
kinetic term, we show that such problems do not appear if the bivectors are
required to be real. Finally, we discuss how matter can be coupled to these
theories. The phenomenology of scalar field dark matter arises naturally within
this framework.Comment: 21 page
BEITRĂGE ZUR CHEMIE DER HETEROCYCLISCHEN, PSEUDOBASISCHEN AMINOCARBINOLE, XIX. VERGLEICHENDE UV-SPEKTROSKOPISCHE UNTERSUCHUNG DES COTARNINS UND EINIGER STRUKTURELL VERWANDTER VERBINDUNGEN
BEITRĂGE ZUR CHEMIE DER HETEROZYKLISCHEN PSEUDOBASISCHEN AMINOKARBINOLE, XV. DER DISSOZIATIONSGRAD DES KOTARNINS UND EINIGER ANALOGEN VERBINDUNGEN
Atomistic interpretation of the interface transfer coefficients for interdiffusion in AB binary phase separating system
Killing spinor space-times and constant-eigenvalue Killing tensors
A class of Petrov type D Killing spinor space-times is presented, having the
peculiar property that their conformal representants can only admit Killing
tensors with constant eigenvalues.Comment: 11 pages, submitted to CQ
On the asymmetry of the forward and reverse martensitic transformations in shape memory alloys
Differential Scanning Calorimetric, DSC, runs taken during martensitic phase
transformations in shape memory alloys, often look differently during cooling
and heating. Similar asymmetry is observed e.g. for the numbers of hits or the
critical exponents of energy and amplitude distributions (\epsilon and \alpha,
respectively) in acoustic emission measurements. It is illustrated that, in
accordance with empirical correlations, the above asymmetry of acoustic noises
can be classified into two groups: the relative changes of the exponents during
cooling and heating (\gamma \epsilon=(\epsilon h-\epsilon c)/\epsilon c as well
as \gamma \alpha=(\alpha h-\alpha c)/\alpha c)) are either positive or
negative. For positive \gamma values the number of hits and the total energy of
acoustic emission are larger for cooling, and the situation is just the reverse
for negative asymmetry. Our interpretation is based on the different ways of
relaxation of the elastic strain energy during cooling as well as heating. It
is illustrated that if the relaxed fraction of the total elastic strain energy
(which would be stored without relaxations) during cooling is larger than the
corresponding relaxed fraction during heating, then the asymmetry is positive.
Magnetic emission noises, accompanied with martensitic phase transformations in
ferromagnetic alloys, show similar asymmetry than those observed for thermal
(DSC) and acoustic noises and depends on the constant external magnetic field
too
Laser-induced optical changes in amorphous multilayers
It is shown that the well-known blue-shift of the fundamental absorption edge
in as-deposited compositionally modulated amorphous Si/Ge and As6Se94/Se80Te20
multilayers (with periods of 4-8 nm) is further enhanced due to the thermal or
laser-induced intermixing of adjacent layers. The laser-induced intermixing
process, as supported by experiments and model calculations, can be attributed
to both the local heating and photo-effects in As6Se94/Se80Te20 multilayers,
while only the thermal effects were observed for Si/Ge multilayers. Structural
transformations, based on this enhanced interdiffusion, provides good
capability for spatially patterning optoelectronic devices and digital
information recording
ON THE DETERMINATION OF THE EQUILIBRIUM SURFACE SEGREGATION ISOTHERMS FORM KINETIC MEASUREMENTS
Abstract. Thirty years ago, LagĂŒes and Domange [1] studying dissolution and segregation kinetics, introduced the s urface local equilibrium hypothesis. This hypothesis is based on the idea that the kinetics (diffusion) is faster in the near surface region (selvedge) than in the bulk and thus it can be assumed that the atoms just below the selvedge can be in equilibrium with the atoms of the deposit. In this contribution, based on our recent work [2], using three dimensional kinetic Monte Carlo (KMC) and one dimensional kinetic mean filed (KMF) models, we show that the local equilibrium can be violated even in simple cases when complex surface phenomena (defects, relaxation, size effects, etc.) are not taken into account. Furthermore we illustrate that, even if the above hypothesis is obeyed, the determination of the surface segregation isotherms from kinetics can lead to uncertain result because of the restricted validity of Fick's first equation on the nanoscale
Influence of hydrogen on the structural stability of annealed ultrathin Si/Ge amorphous layers
Semiconductor structures based on Si and Ge are generally submitted to hydrogenation because H passivates the dangling bonds of Si and Ge. By this way the devices prepared from those semiconductors, e.g., solar cells, have much better electrical properties. However, H stability is still a critical issue. In fact, there is wide evidence that H is very unstable against illumination as well as heat treatment. It has been seen that H out effuses from the samples under such treatments. As this causes unsaturation of the dangling bonds the electrical properties worsen significantly. In this work we will show that in the case of ultrathin Si/Ge amorphous layers the H thermal instability also affects the structural stability even up to the micrometric scale depending on the H content. Such type of structure can also be used to prepare SiGe alloys by mixing the layers with heat treatments. The samples were amorphous multilayers (MLs) of alternating ultrathin (3 nm) layers of Si and Ge deposited by sputtering on (100) oriented Si substrate. The total thickness of the MLs was 300 nm. The samples were hydrogenated by introducing H in the sputter chamber with flow rates varying from 0.8 to 6 ml/min. The MLs underwent different heat treatments, from the one at 350 ?C for 1 h up to the one at 250 ?C for 0.5 h + 450 ?C for 5 h. The samples were analysed by AFM, TEM, energy filtering TEM and Small-Angle X-Ray Diffraction (SAXRD). AFM showed that upon annealing the structure of the samples degrades with formation of surface bumps whose size increases by increasing the annealing temperature and/or time, for the same H content, or by increasing the H content for the same annealing conditions. For high H content and/or annealing conditions AFM showed that the bumps have blown up giving rise to craters. This suggests that H was released from its dangling bonds to Si and Ge and formed H bubbles in the MLs because of the energy supplied by the annealing. Additional energy for the break of the Si-H and Ge-H bonds could be the one supplied by the recombination of thermally generated carriers associated with the band gap fluctuations caused by the not uniform distribution of H in the MLs. The first sites of H accumulation are very likely nanocavities certainly present in the amorphous MLs. By TEM it has been seen that layer intermixing occurred which could be the first step of H bubbles formation. SAXRD measurements as well as TEM energy filtering maps for Si and Ge showed that Si and Ge interdiffusion took place in an asymmetric way as Si was seen to diffuse to the Ge layers whereas Ge did not diffuse to the Si layers. This might be due to the higher density of free dangling bonds in the Ge layers created by annealing because the binding energy of the Ge-H bond is smaller than the one of the Si-H bond
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