Conformal Antenna Array for Millimeter-Wave Communications: Performance Evaluation

In this paper, we study the influence of the radius of a cylindrical supporting structure on radiation properties of a conformal millimeter-wave antenna array. Bent antenna array structures on cylindrical surfaces may have important applications in future mobile devices. Small radii may be needed if the antenna is printed on the edges of mobile devices and in items which human beings are wearing, such as wrist watches, bracelets and rings. The antenna under study consists of four linear series-fed arrays of four patch elements and is operating at 58.8 GHz with linear polarization. The antenna array is fabricated on polytetrafluoroethylene substrate with thickness of 0.127 mm due to its good plasticity properties and low losses. Results for both planar and conformal antenna arrays show rather good agreement between simulation and measurements. The results show that conformal antenna structures allow achieving large angular coverage and may allow beam-steering implementations if switches are used to select between different arrays around a cylindrical supporting structure.Comment: Keywords: conformal antenna, millimeter-wave communications, patch antenna array. 11 pages, 10 figures, 1 tabl

Origin of low-temperature magnetic ordering in Ga1-xMnxN

By employing highly sensitive millikelvin SQUID magnetometry, the magnitude of the Curie temperature as a function of the Mn concentration x is determined for thoroughly characterized Ga1-xMnxN. The interpretation of the results in the frame of tight binding theory and of Monte Carlo simulations, allows us to assign the spin interaction to ferromagnetic superexchange and to benchmark the accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics of dilute magnetic insulators.Comment: 4+ pages, 3 figure

Strain and correlation of self-organized Ge_(1-x)Mn_x nanocolumns embedded in Ge (001)

We report on the structural properties of Ge_(1-x)Mn_x layers grown by molecular beam epitaxy. In these layers, nanocolumns with a high Mn content are embedded in an almost-pure Ge matrix. We have used grazing-incidence X-ray scattering, atomic force and transmission electron microscopy to study the structural properties of the columns. We demonstrate how the elastic deformation of the matrix (as calculated using atomistic simulations) around the columns, as well as the average inter-column distance can account for the shape of the diffusion around Bragg peaks.Comment: 9 pages, 7 figure

Experimental probing of exchange interactions between localized spins in the dilute magnetic insulator (Ga,Mn)N

The sign, magnitude, and range of the exchange couplings between pairs of Mn ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have been grown by metalorganic vapor phase epitaxy and characterized by secondary-ion mass spectroscopy; high-resolution transmission electron microscopy with capabilities allowing for chemical analysis, including the annular dark-field mode and electron energy loss spectroscopy; high-resolution and synchrotron x-ray diffraction; synchrotron extended x-ray absorption fine-structure; synchrotron x-ray absorption near-edge structure; infra-red optics and electron spin resonance. The results of high resolution magnetic measurements and their quantitative interpretation have allowed to verify a series of ab initio predictions on the possibility of ferromagnetism in dilute magnetic insulators and to demonstrate that the interaction changes from ferromagnetic to antiferromagnetic when the charge state of the Mn ions is reduced from 3+ to 2+.Comment: 12 pages, 14 figures; This version contains the detailed characterization of the crystal structure as well as of the Mn distribution and charge stat

Presentations: from Kac-Moody groups to profinite and back

We go back and forth between, on the one hand, presentations of arithmetic and Kac-Moody groups and, on the other hand, presentations of profinite groups, deducing along the way new results on both

Grit ingestion and size-related consumption of tubers by Graylag Geese

In herbivorous birds the processing rate of food is constrained by gizzard capacity. To enhance digestive processes, many species ingest grit to grind the food. Grit ingestion, however, may further limit the capacity of file gizzard. Graylag Geese (Anser anser) wintering in SW Spain fed mainly on Alkali Bulrush (Scirpus maritimus) tubers, showing a preference for small tubers. This preference may be due to a faster disintegration of small tubers than larger ones inside the gizzard. As larger tubers are likely coarser than smaller tubers, more grit would be necessary to process larger tubers. However, the ingestion of more grit to grind large tubers would be at the expense of ingesting additional tubers because of gizzard capacity limitations. Under these circumstances, there may be an inverse relationship between tuber size and amount of grit ingested to optimize food ingestion. Indeed, we found such a relationship. Grit facilitated the disintegration of tubers. This suggests that relying on some amount of grit to facilitate the grinding of food should outweigh the loss of gizzard capacity to the amount of food ingested.Peer Reviewe

Exchange bias in GeMn nanocolumns: the role of surface oxidation

We report on the exchange biasing of self-assembled ferromagnetic GeMn nanocolumns by GeMn-oxide caps. The x-ray absorption spectroscopy analysis of this surface oxide shows a multiplet fine structure that is typical of the Mn2+ valence state in MnO. A magnetization hysteresis shift |HE|~100 Oe and a coercivity enhancement of about 70 Oe have been obtained upon cooling (300-5 K) in a magnetic field as low as 0.25 T. This exchange bias is attributed to the interface coupling between the ferromagnetic nanocolumns and the antiferromagnetic MnO-like caps. The effect enhancement is achieved by depositing a MnO layer on the GeMn nanocolumns.Comment: 7 pages, 5 figure

Quantitative magneto-optical investigation of superconductor/ferromagnet hybrid structures

We present a detailed quantitative magneto-optical imaging study of several superconductor/ferromagnet hybrid structures, including Nb deposited on top of thermomagnetically patterned NdFeB, and permalloy/niobium with erasable and tailored magnetic landscapes imprinted in the permalloy layer. The magneto-optical imaging data is complemented with and compared to scanning Hall probe microscopy measurements. Comprehensive protocols have been developed for calibrating, testing, and converting Faraday rotation data to magnetic field maps. Applied to the acquired data, they reveal the comparatively weaker magnetic response of the superconductor from the background of larger fields and field gradients generated by the magnetic layer.Comment: 21 pages, including 2 pages of supplementary materia