7,428 research outputs found
Second generation of vortex-antivortex states in mesoscopic superconductors: stabilization by artificial pinning
Antagonistic symmetries of superconducting polygons and their induced
multi-vortex states in a homogeneous magnetic field may lead to appearance of
antivortices in the vicinity of the superconducting/normal state boundary
(where mesoscopic confinement is particularly strong). Resulting
vortex-antivortex (V-Av) molecules match the sample symmetry, but are extremely
sensitive to defects and fluctuations and remain undetected experimentally.
Here we show that V-Av states can re-appear deep in the superconducting state
due to an array of perforations in a polygonal setting, surrounding a central
hole. Such states are no longer caused by the symmetry of the sample but rather
by pinning itself, which prevents the vortex-antivortex annihilation. As a
result, even micron-size, clearly spaced V-Av molecules can be stabilized in
large mesoscopic samples.Comment: 5 pages, 6 figure
Stabilization of vortex-antivortex configurations in mesoscopic superconductors by engineered pinning
Symmetry-induced vortex-antivortex configurations in superconducting squares
and triangles were predicted earlier; yet, they have not been resolved in
experiment up to date. Namely, with vortex-antivortex states being highly
unstable with respect to defects and temperature fluctuations, it is very
unlikely that samples can be fabricated with the needed quality. Here we show
how these drawbacks can be overcome by strategically placed nanoholes in the
sample. As a result, (i) the actual shape of the sample becomes far less
important, (ii) the stability of the vortex-antivortex configurations in
general is substantially enhanced, and (iii) states comprising novel
giant-antivortices (with higher winding numbers) become energetically favorable
in perforated disks. In the analysis, we stress the potent of strong screening
to destabilize the vortex-antivortex states. In turn, the screening-symmetry
competition favors stabilization of new asymmetric ground states, which arise
for small values of the effective Ginzburg-Landau parameter kappa.Comment: 12 pages, 20 figure
Symmetric and non-symmetric vortex-antivortex molecules in fourfold superconducting geometry
In submicron superconducting squares in a homogeneous magnetic field,
Ginzburg-Landau theory may admit solutions of the vortex-antivortex type,
conforming with the symmetry of the sample [Chibotaru et al., Nature 408, 833
(2000)]. Here we show that these fascinating, but never experimentally observed
states, can be enforced by artificial fourfold pinning, with their diagnostic
features enhanced by orders of magnitude. The second-order nucleation of
vortex-antivortex molecules can be driven either by temperature or applied
magnetic field, with stable asymmetric vortex-antivortex equilibria found on
its path.Comment: 5 pages, 5 figure
Suris tetrons: possible spectroscopic evidence for four-particle optical excitations of the 2D electron gas
The excitations of a two-dimensional electron gas in quantum wells with
intermediate carrier density (~10^{11} cm^{-2}), i.e., between the
exciton-trion- and the Fermi-Sea range, are so far poorly understood. We report
on an approach to bridge this gap by a magneto-photoluminescence study of
modulation-doped (Cd,Mn)Te quantum well structures. Employing their enhanced
spin splitting, we analyzed the characteristic magnetic-field behavior of the
individual photoluminescence features. Based on these results and earlier
findings by other authors, we present a new approach for understanding the
optical transitions at intermediate densities in terms of four-particle
excitations, the Suris tetrons, which were up to now only predicted
theoretically. All characteristic photoluminescence features are attributed to
emission from these quasi-particles when attaining different final states.Comment: 12 pages, 3 figure
Using planar laser-induced fluorescence to study the phase transformations of two-component liquid and suspension droplets
Using the planar laser-induced fluorescence (PLIF), we performed experiments to determine evaporation dynamics of homogeneous and heterogeneous droplets of liquids, conditions of their boiling, and explosive breakup. For the 1–2 mm water droplets, the distribution of highly non-homogeneous and non-steady temperature field was detected by highspeed cross-correlation video recording and the Tema Automotive software.We identified highly nonlinear dependences of evaporation rate on heating temperature and time as well as water droplet size. For the two-component liquids and water-based suspensions of graphite, we revealed unsteady temperature fields and established mechanisms and regimes of the explosive breakup of the heterogeneous droplets when heated. The regimes differ in the number and dimensions of the emerging gas–liquid fragments as well as the durations of the main stages. The three regimes of warming-up and evaporation of the heterogeneous droplets have been obtained. The explosive breakup of droplets enables provision for the secondary atomization of the liquid with the emergence of an aerosol cloud. The surface area of the liquid increases several-fold. The temperature variations at the water/solid or water/flammable component interfaces were determined corresponding to each boiling and breakup regime. Using the PLIF, we studied reasons and mechanism of the explosive breakup of water droplets with single large carbonaceous inclusions when heated
Subcortical Structures in Humans Can Be Facilitated by Transcranial Direct Current Stimulation
Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that alters cortical excitability. Interestingly, in recent animal studies facilitatory effects of tDCS have also been observed on subcortical structures. Here, we sought to provide evidence for the potential of tDCS to facilitate subcortical structures in humans as well. Subjects received anodal-tDCS and sham-tDCS on two separate testing days in a counterbalanced order. After stimulation, we assessed the effect of tDCS on two responses that arise from subcortical structures; (1) wrist and ankle responses to an imperative stimulus combined with a startling acoustic stimulus (SAS), and (2) automatic postural responses to external balance perturbations with and without a concurrent SAS. During all tasks, response onsets were significantly faster following anodal-tDCS compared to sham-tDCS, both in trials with and without a SAS. The effect of tDCS was similar for the dominant and non-dominant leg. The SAS accelerated the onsets of ankle and wrist movements and the responses to backward, but not forward perturbations. The faster onsets of SAS-induced wrist and ankle movements and automatic postural responses following stimulation provide strong evidence that, in humans, subcortical structures - in particular the reticular formation - can be facilitated by tDCS. This effect may be explained by two mechanisms that are not mutually exclusive. First, subcortical facilitation may have resulted from enhanced cortico-reticular drive. Second, the applied current may have directly stimulated the reticular formation. Strengthening reticulospinal output by tDCS may be of interest to neurorehabilitation, as there is evidence for reticulospinal compensation after corticospinal lesions
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