257 research outputs found
Magnetic-field dependence of the spin states of the negatively charged exciton in GaAs quantum wells
We present high-field (<50 T) photoluminescence measurements of the binding energy of the singlet and triplet states of the negatively charged exciton in a 200-Angstrom quantum well. Comparing our data with those of other groups and with theoretical predictions we clearly show how the singlet, "bright" and "dark" triplet states may be identified according to the high-field dependence of their binding energies. We demonstrate that a very consistent behavior of the binding energy in a magnetic field has been observed in quantum wells of different widths by different groups and conclude that the triplet state found in this, as well as nearly all other experiments, is undoubtedly the bright triplet. By combining our data with that in the literature we are able to present the generic form of the binding energy of the spin states of the charged exciton in a magnetic field, which reveals the predicted singlet to dark triplet ground state transition at about 20 T
Influence of the sample geometry on the vortex matter in superconducting microstructures
The dependence of the vortex penetration and expulsion on the geometry of
mesoscopic superconductors is reported. Hall magnetometry measurements were
performed on a superconducting Al square and triangle. The stability of the
vortex patterns imposed by the sample geometry is discussed. The
field-temperature diagram has been reconstructed showing the transitions
between states with different vorticity. We have found that the vortex
penetration is only weakly affected by the vortex configuration inside the
sample while the expulsion is strongly controlled by the stability of the
vortex patterns. A qualitative explanation for this observation is given.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.
Nucleation of Superconductivity in a Mesoscopic Loop of Finite Width
The normal/superconducting phase boundary Tc has been calculated for
mesoscopic loops, as a function of an applied perpendicular magnetic field H.
While for thin-wire loops and filled disks the Tc(H) curves are well known, the
intermediate case, namely mesoscopic loops of finite wire width, have been
studied much less. The linearized first Ginzburg-Landau equation is solved with
the proper normal/vacuum boundary conditions both at the internal and at the
external loop radius. For thin-wire loops the Tc(H) oscillations are perfectly
periodic, and the Tc(H) background is parabolic (this is the usual Little-Parks
effect). For loops of thicker wire width, there is a crossover magnetic field
above which Tc(H) becomes quasi-linear, with the period identical to the Tc(H)
of a filled disk (i.e. pseudoperiodic oscillations). This dimensional
transition is similar to the 2D-3D transition for thin films in a parallel
field, where vortices start penetrating the material as soon as the film
thickness exceeds the temperature dependent coherence length by a factor 1.8.
For the presently studied loops, the crossover point is controlled by a similar
condition. In the high field '3D' regime, a giant vortex state establishes,
where only a surface superconducting sheath near the sample's outer radius is
present.Comment: 7 pages text, 2 EPS figures, uses LaTeX's elsart.sty, proceedings of
the First Euroconference on "Vortex Matter in Superconductors", held in Crete
(18-24 september 1999
Self-assembled hexagonal double fishnets as negative index materials
We show experimentally the successful use of colloidal lithography for the
fabrication of negative index metamaterials in the near-infrared wavelength
range. In particular, we investigated a specific implementation of the widely
studied double fishnet metamaterials, consisting of a gold-silica-gold layer
stack perforated by a hexagonal array of round holes. Tuning of the hole
diameter allows us to tailor these self-assembled metamaterials both as single-
({\epsilon} < 0) and double ({\epsilon} < 0 and {\mu} < 0) negative
metamaterials
Nanoengineered magnetic-field-induced superconductivity
The perpendicular critical fields of a superconducting film have been
strongly enhanced by using a nanoengineered lattice of magnetic dots (dipoles)
on top of the film. Magnetic-field-induced superconductivity is observed in
these hybrid superconductor / ferromagnet systems due to the compensation of
the applied field between the dots by the stray field of the dipole array. By
switching between different magnetic states of the nanoengineered field
compensator, the critical parameters of the superconductor can be effectively
controlled.Comment: 4 pages, 4 figure
Understanding the Physical Behavior of Plasmonic Antennas Through Computational Electromagnetics
This chapter focuses on understanding the electromagnetic response of nanoscopic metallic antennas through a classical computational electromagnetic algorithm: volumetric method of moments (V‐MoMs). Under the assumption that metals only respond to external electromagnetic disturbance locally, we rigorously formulate the light‐nanoantenna interaction in terms of a volume integral equation (VIE) and solve the equation by using the method of moments algorithm. Modes of a nanoantenna, as the excitation independent solution to the volume integral equation (VIE), are introduced to resolve the antenna’s complex optical spectrum. Group representation theory is then employed to reveal how the symmetry of a nanoantenna defines the modes’ properties and determines the antenna’s optical response. Through such a treatment, a set of tools that can systematically treat the interaction of light with a nanoantenna is developed, paving the road for future nanoantenna design
Magneto-optical study of electron occupation and hole wave functions in stacked self-assembled InP quantum dots
We have studied the magnetophotoluminescence of doubly stacked layers of self-assembled InP quantum dots in a GaInP matrix. 4.0±0.1 monolayers of InP were deposited in the lower layer of each sample, whereas in the upper layer 3.9, 3.4, and 3.0 monolayers were used. Low-temperature photoluminescence measurements in zero magnetic field are used to show that, in each case, only one layer of dots is occupied by an electron, and imply that when the amount of InP in both layers is the same, the dots in the upper layer are larger. High-field photoluminescence data reveal that the position and extent of the hole wave function are strongly dependent on the amount of InP in the stack. ©2001 American Institute of Physics
Microwave-stimulated superconductivity due to presence of vortices
The response of superconducting devices to electromagnetic radiation is a core concept implemented in diverse applications, ranging from the currently used voltage standard to single photon detectors in
astronomy. Suprisingly, a sufficiently high power subgap radiation may stimulate superconductivity itself. The possibility of stimulating type II superconductors, in which the radiation may interact also with vortex
cores, remains however unclear. Here we report on superconductivity enhanced by GHz radiation in type II superconducting Pb films in the presence of vortices. The stimulation effect is more clearly observed in the upper critical field and less pronounced in the critical temperature. The magnetic field dependence of the vortex related microwave losses in a film with periodic pinning reveals a reduced dissipation of mobile
vortices in the stimulated regime due to a reduction of the core size. Results of numerical simulations support the validy of this conclusion. Our findings may have intriguing connections with holographic
superconductors in which the possibility of stimulation is under current debateThis work has been supported in parts by Spanish MINECO (MAT2012-32743), and Comunidad de Madrid (NANOFRONTMAG-CM S2013/MIT-2850) and NANO-SC COST-Action MP-1201. A. Lara thanks UAM for FPI-UAM fellowship. The work of A.V.S. was partially supported by Mandat ‘‘d’Impulsion Scientifique’’ of the F.R.S.-FNR
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