Fluxon dynamics by microwave surface resistance measurements in MgB2

Field-induced variations of the microwave surface resistance, Rs(H), have been investigated in high-density ceramic MgB2. At low temperatures, several peculiarities of the Rs(H) curves cannot be justified in the framework of models reported in the literature. We suggest that they are ascribable to the unconventional vortex structure in MgB2, related to the presence of two gaps. On the contrary, the results near Tc can be accounted for by the Coffey and Clem model, with fluxons moving in the flux-flow regime, provided that the anisotropy of the upper critical field is taken into due account.Comment: 6 pages, 4 figure

Model Retrieval by 3D Sketching in Immersive Virtual Reality

We describe a novel method for searching 3D model collections using free-form sketches within a virtual environment as queries. As opposed to traditional Sketch Retrieval, our queries are drawn directly onto an example model. Using immersive virtual reality the user can express their query through a sketch that demonstrates the desired structure, color and texture. Unlike previous sketch-based retrieval methods, users remain immersed within the environment without relying on textual queries or 2D projections which can disconnect the user from the environment. We show how a convolutional neural network (CNN) can create multi-view representations of colored 3D sketches. Using such a descriptor representation, our system is able to rapidly retrieve models and in this way, we provide the user with an interactive method of navigating large object datasets. Through a preliminary user study we demonstrate that by using our VR 3D model retrieval system, users can perform quick and intuitive search. Using our system users can rapidly populate a virtual environment with specific models from a very large database, and thus the technique has the potential to be broadly applicable in immersive editing systems

Carrier Transport in Magnesium Diboride: Role of Nano-inclusions

Anisotropic-gap and two-band effects smear out the superconducting transition (Tc) in literature reported thermal conductivity of MgB2, where large electronic contributions also suppress anomaly-manifestation in their negligible phononic-parts. Present thermal transport results on scarcely explored specimens featuring nano-inclusions exhibit a small but clear Tc-signature, traced to relatively appreciable phononic conduction, and its dominant electronic-scattering. The self-formed MgO as extended defects strongly scatter the charge carriers and minutely the phonons with their longer-mean-free-path near Tc. Conversely, near room temperature, the shorter-dominant-wavelength phonon's transport is hugely affected by these nanoparticles, undergoing ballistic to diffusive crossover and eventually entering the Ioffe-Regel mobility threshold regime.Comment: 14 pages, 4 figures, 28 reference

Volcanic deformation and flank instability due to magmatic sources and frictional rheology: the case of Mount Etna

The overall picture of Mount Etna deformation emerging since a couple of decades of geodetic surveys shows effects of magma accumulation, characterized by inflation/deflation cycle, accompanied by a sliding instability of the southeast flank, whose manifestation is an increase in the horizontal deformation away from the volcano summit. This is a very interesting case to test whether advanced models, taking into account topography, internal structure and frictional rheology, may contribute to a better understanding of the complex interplay among mechanical response, magmatic activity and gravitational load occurring in a volcanic system. Using finite element numerical models we make predictions of surface displacements associated with a simple expansion source and with a dike like vertical discontinuity. A new methodology is developed to initalize the lithostatic stress field according to the material and geometrical complexities of the models considered. Our results show that, while an amplification of the horizontal displacement can be easily obtained up to a maximum distance of 10 km from the source, we have not been able to find any onfiguration to extend further this signal. For the case of Mount Etna this suggests that the large horizontal displacements observed in the east flank along the coast cannot be directly related to magma accumulation below the volcano's summit

Josephson effect in MgB_2 break junctions

We present the first observation of the DC and AC Josephson effect in MgB_2 break junctions. The junctions, obtained at 4.2 K in high-quality, high-density polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson effect. By irradiating the junctions with microwaves we observe clear Shapiro steps spaced by the ideal $\Delta V$ value. The temperature dependence of the DC Josephson current and the dependence of the height of the steps on the microwave power are obtained. These results are a direct prove for the existence of pairs with charge 2e in this new metallic superconductor and give evidence of the superconductor-normal metal-superconductor weak link character of these junctions.Comment: 4 RevTEX pages, 4 eps figure

Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements

In a type-II superconductor the generation of higher harmonics in the magnetic response to an alternating magnetic field is a consequence of the non-linearity in the I-V relationship. The shape of the current-voltage (I-V) curve is determined by the current dependence of the thermal activation energy U(J) and is thus related to the dynamical regimes governing the vortex motion. In order to investigate the vortex dynamics in MgB2 bulk superconductors we have studied the fundamental (chi1) and third (chi3) harmonics of the ac magnetic susceptibility. Measurements have been performed as a function of the temperature and the dc magnetic field, up to 9 T, for various frequencies and amplitudes of the ac field. We show that the analysis of the behaviour in frequency of chi3(T) and chi3(B) curves can provide clear information about the non-linearity in different regions of the I-V characteristic. By comparing the experimental curves with numerical simulations of the non-linear diffusion equation for the magnetic field we are able to resolve the crossover between a dissipative regime governed by flux creep and one dominated by flux flow phenomena.Comment: to be published in "Horizons in Superconductivity Research" (Nova Science Publishers, Inc., NY, 2003

Mixing realities for sketch retrieval in Virtual Reality

Users within a Virtual Environment often need support designing the environment around them with the need to find relevant content while remaining immersed. We focus on the familiar sketch-based interaction to support the process of content placing and specifically investigate how interactions from a tablet or desktop translate into the virtual environment. To understand sketching interaction within a virtual environment, we compare different methods of sketch interaction, i.e., 3D mid-air sketching, 2D sketching on a virtual tablet, 2D sketching on a fixed virtual whiteboard, and 2D sketching on a real tablet. The user remains immersed within the environment and queries a database containing detailed 3D models and replace them into the virtual environment. Our results show that 3D mid-air sketching is considered to be a more intuitive method to search a collection of models; while the addition of physical devices creates confusion due to the complications of their inclusion within a virtual environment. While we pose our work as a retrieval problem for 3D models of chairs, our results are extendable to other sketching tasks for virtual environments

Mixing Modalities of 3D Sketching and Speech for Interactive Model Retrieval in Virtual Reality

Sketch and speech are intuitive interaction methods that convey complementary information and have been independently used for 3D model retrieval in virtual environments. While sketch has been shown to be an effective retrieval method, not all collections are easily navigable using this modality alone. We design a new challenging database for sketch comprised of 3D chairs where each of the components (arms, legs, seat, back) are independently colored. To overcome this, we implement a multimodal interface for querying 3D model databases within a virtual environment. We base the sketch on the state-of-the-art for 3D Sketch Retrieval, and use a Wizard-of-Oz style experiment to process the voice input. In this way, we avoid the complexities of natural language processing which frequently requires fine-tuning to be robust. We conduct two user studies and show that hybrid search strategies emerge from the combination of interactions, fostering the advantages provided by both modalities