An Embedded Evolutionary Controller to Navigate a Population of Autonomous Robots
Eduardo do Valle Simoes · 2008
Superconductor 112 measurements, have found that MgB 2 exhibits a rich multiple-band structure.These results are in agreement with band structure calculations and reveal strongly two-dimensional sp x p y (σ) bands, as well as three-dimensional p z ( ) bands.The identification of MgB 2 as a two gap superconductor has resulted in much research associated with the spectroscopy of this material.It has become generally accepted that the larger gap is associated with the 2D σ bands arising from the boron planes, which has the value of Δ σ ≅ 7.069 meV, while the 3D bands have a gap of Δ ≅ 2.70 meV (Bouquet et al., 2001;Kortus et al., 2001).MgB 2 has been fabricated in bulks, single crystals, thin films, tapes, and wires for different applications (Eisterer & Weber, 2009).In addition to the relatively high critical transition temperature, T c , and the simple crystal structure, MgB 2 possesses a large coherence length, high critical current density, and transparency of grain boundaries to current flow.The insitu route seems to be the most promising method to improve the upper critical field, H c2 , and the critical current density, J c , performance of MgB 2 .MgB 2 is a promising superconductor for high-magnetic field applications because of its already high J c .The grain boundaries in MgB 2 do not significantly degrade J c and even serve as pinning centers, which is different from the weak-link effects in high-T c superconductors.For single-gap dirty limit superconductors, the upper critical field H c2 (0) = 0.69T c (dH c2 /dT) Tc , and (d Hc2 /dT) Tc ∝ n , where n is the normal state resistivity (Werthame et al., 1966); therefore, H c2 increases with n , which can be achieved by adding impurities and defects into the superconductor.Gurevich pointed out that the two-band superconductor MgB 2 can be understood as a weakly-coupled bilayer in which two thin films corresponding to the σ and bands are in contact through Josephson coupling (Gurevich, 2007).Using the dirty-limit, weak-coupling, multiband Bardeen Cooper Schrieffer (BCS) model and taking into account both interband and intraband scattering by nonmagnetic impurities, Gurevich showed that the temperature dependence of H c2 (T) is influenced by whether the σ bands or bands are dirtier, making it very different from the temperature dependence in the one-band theory (Gurevich, 2003).The global H c2 (T) of the bilayer is dominated by the layer with the higher H c2 .If the layer is dirtier, it will have higher H c2 at low temperature, even though its T c is much lower.As a result, an upturn in the global H c2 (T) occurs at low temperature.H c2 (0) of MgB 2 can exceed 0.69T c (dH c2 /dT) Tc considerably because of the existence of the two bands.Considering the electron-phonon coupling effect, Gurevich argued that the strong coupling paramagnetic limit in MgB 2 can be as high as 130 T; thus, there is still room for further enhancement of H c2 by engineering the σand -band scattering (Gurevich, 2007).The high H c2 in MgB 2 is very attractive for high-magnetic-field applications.The H c2 behavior described by Gurevich has been observed in experimental results.For example, Braccini et al. observed different types of temperature dependence of H c2 , including the anomalous upturn at low temperature, reflecting different multiband scattering in thin film samples from various groups, with disorder introduced in different ways.The value of H c2 in carbondoped thin films has reached over 60 T at low temperature, approaching the BCS paramagnetic limit of 65 T (Braccini et al., 2005).The depairing current density, J d , is ~8.7 × 10 8 A/cm 2 for pure MgB 2 , as estimated from the Ginzburg-Landau (GL) formula: How to referenceIn order to correctly reference this scholarly work, feel free