647 research outputs found
CONFORMAL GRAVITY AND GRAVITATIONAL WAVES
No full text We consider monochromatic, plane gravitational waves in a conformally invariant theory of general relativity. We show that the simple, standard ansatz for the metric, usually that which is taken for the linearized theory of these waves, is reducible to the metric of Minkowski spacetime via a sequence of conformal and coordinate transformations. This implies that we have in fact, exact plane wave solutions. However they are simply coordinate/conformal artifacts. As a consequence, they carry no energy. </jats:p
Monochromatic plane-fronted waves in conformal gravity are pure gauge
No full textWe consider plane-fronted, monochromatic gravitational waves on a Minkowski background, in a conformally invariant theory of general relativity. By this we mean waves of the form: g(mu nu) = eta(mu nu) + epsilon(mu nu) F(k.x), where epsilon(mu nu) is a constant polarization tensor, and k(mu) is a lightlike vector. We also assume the coordinate gauge condition vertical bar g vertical bar(-1/4) partial derivative(tau)(vertical bar g vertical bar(1/4) g(sigma tau)) = 0 which is the conformal analog of the harmonic gauge condition g(mu nu)Gamma(sigma)(mu nu) = - vertical bar g vertical bar(-1/2)partial derivative(tau)(vertical bar g vertical bar(1/2) g(sigma tau)) = 0, where det[g(mu nu)] equivalent to g. Requiring additionally the conformal gauge condition g = -1 surprisingly implies that the waves are both transverse and traceless. Although the ansatz for the metric is eminently reasonable when considering perturbative gravitational waves, we show that the metric is reducible to the metric of Minkowski space-time via a sequence of coordinate transformations which respect the gauge conditions, without any perturbative approximation that epsilon(mu nu) be small. This implies that we have, in fact, exact plane-wave solutions; however, they are simply coordinate/conformal artifacts. As a consequence, they carry no energy. Our result does not imply that conformal gravity does not have gravitational wave phenomena. A different, more generalized ansatz for the deviation, taking into account the fourth-order nature of the field equation, which has the form g(mu nu) = eta(mu nu) + B-mu nu(n.x)G(k.x), indeed yields waves which carry energy and momentum [P. D. Mannheim, Gen. Relativ. Gravit. 43, 703 (2010)]. It is just surprising that transverse, traceless, plane-fronted gravitational waves, those that would be used in any standard, perturbative, quantum analysis of the theory, simply do not exist
Gravitationally coupled magnetic monopole and conformal symmetry breaking
No full textWe consider a Georgi-Glashow model conformally coupled to gravity. The conformally invariant action includes a triplet of scalar fields and SO(3) non-Abelian gauge fields. However, the usual mass term mu(22) is forbidden by the symmetry, and this role is now played by the conformal coupling of the Ricci scalar to the scalar fields. Spontaneous symmetry breaking occurs via gravitation. The spherically symmetric solutions correspond to localized solitons (magnetic monopoles) in asymptotically anti-de Sitter (AdS) spacetime and the metric outside the core of the monopole is found to be Schwarzschild-AdS. Though conformal symmetry excludes the Einstein-Hilbert term in the original action, it emerges in the effective action after spontaneous symmetry breaking and dominates the low-energy-long-distance regime outside the core of the monopole
Transverse, traceless, plane fronted, monochromatic plane waves in conformal gravity have zero-energy
No full textPlane fronted, monochromatic gravitational waves on a Minkowski background are considered in conformal gravity. Assuming the conformal gauge condition Det [gμ?] = - 1 and the conformal generalization of the harmonic gauge condition |g|-1/4?t(|g1/4|gst) = 0 implies that the waves are both transverse and traceless. We show, without any perturbative approximation that the metric is reducible to the metric of Minkowski space-time via additional coordinate transformations that preserve the gauge conditions. This implies that the waves are simply coordinate/conformal artifacts. As a consequence, they carry no energy
Spontaneous breaking of conformal invariance in theories of conformally coupled matter and Weyl gravity
No full textWe study the theory of Weyl conformal gravity with matter degrees of freedom in a conformally invariant interaction. Specifically, we consider a triplet of scalar fields and SO(3) non-Abelian gauge fields, i.e. the Georgi-Glashow model conformally coupled to Weyl gravity. We show that the equations of motion admit solutions spontaneously breaking the conformal symmetry and the gauge symmetry, providing a mechanism for supplying a scale in the theory. The vacuum solution corresponds to anti-de Sitter spacetime, while localized soliton solutions correspond to magnetic monopoles in asymptotically anti-de Sitter spacetime. This mechanism strengthens the reasons for considering conformally invariant matter-gravity theory, which has shown promising indications concerning the problem of missing matter in galactic rotation curves
Characterization of TMAH Silicon Etchant Using Ammomium Persulfate as an Oxidizing Agent
No full textAmong the silicon anisotropic etchants, tetramethyl ammonium hydroxide [TMAH] is of great interest due to the absence of metal ions. Therefore using TMAH solutions at low concentrations has the advantage of being more economical, both in terms of cost and time. Unfortunately the surface quality of the etched silicon is strongly influenced by the concentration of the solution i.e. at low concentrations (less than 15%), the etched surfaces are very often covered with pyramidal-shaped hillocks, thus producing a very rough surface finish. Ammonium Persulfate (NH4)2S2O8 can be added to TMAH to suppress hillock formation. We investigated the effects of this additive under different oxidizer addition conditions. The influence of TMAH concentration and etchant temperature was evaluated
Mehrsprachigkeit und multikulturelle Literatur. Multilingualism and Multicultural Literature
No full textApplication of Dual Doped TMAH Silicon Etchant in the Fabrication of a Micromachined Aluminium Flexing Beam Actuator
No full textOne of the main goals of micromechanical systems (MEMS) fabrication is microdevice integration with standard integrated circuit (IC) technologies, such as bipolar or the more prevalent complementary metal oxide semiconductor (CMOS) processes. To that end, it has been found that the anisotropic silicon etchant tetra-methyl ammonium-hydroxide (TMAH) can be effectively used in a post-processing step with CMOS-based fabrication by doping it with silicic acid to prevent the unwanted etching of exposed alluminum. Furthermore, the addition of ammonium persulfate to the TMAH/silicic acid solution enhances etch rate and surface quality. The final etching solution, called dual-doped TMAH, is a CMOS-compatible, highly selective to silicon over aluminum, and can therefore allow an aluminum layer to be used as an etch mask. In this paper, we utilize dual-doped TMAH towards the fabrication of a microstructure made entirely of aluminum. A flexing beam microactuar suspended over a bulk micromachined silicon cavity is presented for use as a magnetomete
A Micromachined Angled Hall Magnetic Field Sensor Using Novel In-Cavity Patterning
No full textThis paper will present a novel technique used in the fabrication of a three-dimensional magnetic field vector sensor based on an angled Hall plate structure. This new sensor design relies on simultaneously detecting all magnetic field vector components using Hall plates that are imbedded into the silicon sidewalls of a bulk micromachined cavity. A shadow mask technique was developed for the in-cavity patterning necessary for doping and metalization. This mask is made from standard thin film layers deposited onto a previously `un-etched` silicon surface. Therefore, the patterned layers also served as the etch mask to expose the silicon for anisotropic etching. This paper will concentrate on how an angled Hall device (AHD) can be fabricated using this technique
Dual-doped TMAH silicon etchant for microelectromechanical structures and systems applications
No full textTetra-methyl ammonium-hydroxide (TMAH), is an anisotropic silicon etchant that is gaining considerable use in silicon micromachining due to its excellent silicon etch rate, etch selectivity to masking layers, degree of anisotropy, and relatively low toxicity. However, a shortcoming of TMAH is that it aggressively etches exposed aluminium layers. A dual-doped low concentration TMAH solution is presented, which incorporates a silicate additive for aluminium passivation and an oxidizer additive for improving etch rate and surface quality. Using etch and under-etch experiments, the dual-doped 5 wt% TMAH solution is shown to have characteristics comparable to those of high concentration TMAH solutions with the added benefit of improved etch rate, smoother etched surfaces, and selectivity to silicon over aluminium. Such an etchant can find effective use in batch fabrication and intergration of microelectromechanical systems within the framework of standard foundry processe
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