14,774 research outputs found
A planar magneto-inductive lens for three-dimensional subwavelength imaging
A planar near-field magnetoinductive lens operating in the microwave range is
presented. The proposed device consists of two parallel planar arrays of
metallic broadside coupled (BC-) split ring resonators (SRRs), or BC-SRRs.
Power coming from a point-like source located in front of the lens is focused
into a receiver located in free space behind the device. This focus is clearly
removed from the back side of the lens, and has a size which is an order of
magnitude smaller than the free space wavelength of the incoming radiation. The
imaging properties of the device mainly relies on the excitation of
magnetoinductive surface waves on the BC-SRR arrays. By simply scaling the
BC-SRRs size, as well as the arrays periodicity, the frequency of operation of
the device can be tuned over a wide frequency range. Thus the proposed design
is potentially useful for many applications ranging from megahertzs to
terahertzs.Comment: 14 pages, 3 figures, submitted to Applied Physics Letter
Subband structure of II-VI modulation-doped magnetic quantum wells
Here we investigate the spin-dependent subband structure of newly-developed
Mn-based modulation-doped quantum wells. In the presence of an external
magnetic field, the s-d exchange coupling between carriers and localized d
electrons of the Mn impurities gives rise to large spin splittings resulting in
a magnetic-field dependent subband structure. Within the framework of the
effective-mass approximation, we self-consistently calculate the subband
structure at zero temperature using Density Functional Theory (DFT) with a
Local Spin Density Approximation (LSDA). We present results for the
magnetic-field dependence of the subband structure of shallow ZnSe/ZnCdMnSe
modulation doped quantum wells. Our results show a significant contribution to
the self-consistent potential due to the exchange-correlation term. These
calculations are the first step in the study of a variety of interesting
spin-dependent phenomena, e.g., spin-resolved transport and many-body effects
in polarized two-dimensional electron gases.Comment: 3 pages, 3 postscript figures, submitted to the proceedings of the
10th Brazilian Workshop on Semiconductor Physics (BWSP10
Experimental demonstration of a mu=-1 metamaterial lens for magnetic resonance imaging
In this work a mu=-1 metamaterial (MM) lens for magnetic resonance imaging
(MRI) is demonstrated. MRI uses surface coils to detect the radiofrequency(RF)
energy absorbed and emitted by the nuclear spins in the imaged object. The
proposed MM lens manipulates the RF field detected by these surface coils, so
that the coil sensitivity and spatial localization is substantially improved.
Beyond this specific application, we feel that the reported results are the
experimental confirmation of a new concept for the manipulation of RF field in
MRI, which paves the way to many other interesting applications.Comment: 9 pages, 3 figure
Confinement of two-dimensional excitons in a non-homogeneous magnetic field
The effective Hamiltonian describing the motion of an exciton in an external
non-homogeneous magnetic field is derived. The magnetic field plays the role of
an effective potential for the exciton motion, results into an increment of the
exciton mass and modifies the exciton kinetic energy operator. In contrast to
the homogeneous field case, the exciton in a non-homogeneous magnetic field can
also be trapped in the low field region and the field gradient increases the
exciton confinement. The trapping energy and wave function of the exciton in a
GaAs two-dimensional electron gas for specific circular magnetic field
configurations are calculated. The results show than excitons can be trapped by
non-homogeneous magnetic fields, and that the trapping energy is strongly
correlated with the shape and strength of the non-homogeneous magnetic field
profile.Comment: 9 pages, 12 figure
Many-body effects on the ringlike structures in two-subband wells
The longitudinal resistivity of two-dimensional electron gases
formed in wells with two subbands displays ringlike structures when plotted in
a density--magnetic-field diagram, due to the crossings of spin-split Landau
levels (LLs) from distinct subbands. Using spin density functional theory and
linear response, we investigate the shape and spin polarization of these
structures as a function of temperature and magnetic-field tilt angle. We find
that (i) some of the rings "break" at sufficiently low temperatures due to a
quantum Hall ferromagnetic phase transition, thus exhibiting a high degree of
spin polarization (%) within, consistent with the NMR data of Zhang
\textit{et al.} [Phys. Rev. Lett. {\bf 98}, 246802 (2007)], and (ii) for
increasing tilting angles the interplay between the anticrossings due to
inter-LL couplings and the exchange-correlation (XC) effects leads to a
collapse of the rings at some critical angle , in agreement with the
data of Guo \textit{et al.} [Phys. Rev. B {\bf 78}, 233305 (2008)].Comment: 4 pages, 3 figure
The whole mesh Deformation Model for 2D and 3D image segmentation
In this paper we present a novel approach for image segmentation using Active Nets and Active Volumes. Those solutions are based on the Deformable Models, with slight difference in the method for describing the shapes of interests - instead of using a contour or a surface they represented the segmented objects with a mesh structure, which allows to describe not only the surface of the objects but also to model their interiors. This is obtained by dividing the nodes of the mesh in two categories, namely internal and external ones, which will be responsible for two different tasks. In our new approach we propose to negate this separation and use only one type of nodes. Using that assumption we manage to significantly shorten the time of segmentation while maintaining its quality
Gauge invariance, background fields and modified Ward identities
In this talk the gauge symmetry for Wilsonian flows in pure Yang-Mills
theories is discussed. The background field formalism is used for the
construction of a gauge invariant effective action. The symmetries of the
effective action under gauge transformations for both the gauge field and the
auxiliary background field are separately evaluated. Modified Ward-Takahashi
and background field identities are used in my study. Finally it is shown how
the symmetry properties of the full theory are restored in the limit where the
cut-off is removed.Comment: 6 pages, to appear in the Proceedings of the 2nd Conference on the
Exact Renormalization Group, Rome 200
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