angle ϳ 90°causes the magnetization to orient perpendicular to the applied field, while maintaining the normal incidence of the pump beam. In this sample geometry, a linearly polarized pump pulse is used in place of the circularly polarized pump pulse in the TRFR measurement so that the optical excitation imparts no angular momentum to the sample. However, a time scan with B app ϭ 1000 G exhibits precession of coherent electron spins (Fig. 5C) (27), indicating that these electrons acquire a component of spin-polarization perpendicular to the field (i.e., along the magnetization). Because this angular momentum does not originate from the pump pulse, it demonstrates that the ferromagnet polarizes some fraction of the photoexcited electrons along the magnetization. Currently, it is unclear how the ferromagnet polarizes the photoexcited electrons; some possibilities include polarization during the absorption process, spin-dependent recombination with spin-polarized holes, and spin-dependent scattering at the ferromagnetic interface. Nonetheless, this result implies that photoexcited electrons mediate the transfer of angular momentum from the ferromagnet to the nuclear spins in GaAs.Finally, we show that the nuclear polarization depends primarily on the component of magnetization parallel to the applied field. The angle between the magnetization M and B app is varied systematically by rotating the sample in-plane (Fig. 5A). Returning to TRFR measurement with a circularly polarized pump pulse, L is measured for several angles (Fig. 5D), and the nuclear polarization corresponds to L in excess of the Zeeman contribution (dashed line). As the parallel component of magnetization vanishes at ϭ 90°, the nuclear polarization also approaches zero. This behavior is explained by considering the components of electron spin involved in DNP. The circularly polarized pump induces electron spin S x normal to the sample (axes defined in Fig. 5A). The ferromagnet induces in-plane spin-polarization both parallel to the field S z ϳ M z ϳ cos and perpendicular to the field S y ϳ M y ϳ sin . DNP depends primarily on the projection of the total electron spin on B app (i.e., S z ) (9), explaining why M z is the most important component for generating nuclear polarization. for ferromagnetic layers:T growth ϭ 240°C (MnAs) or 260°C [(Ga,Mn)As, DFH, single layer]; As 4 /Ga flux ratio ϳ25; GaAs rate ϳ5 nm/min; MnAs rate ϳ0.3 nm/min. 14. H. Tanobe, F. Koyama, K. Iga, Jpn. J. Appl. Phys. 31, 1597. 15. The n-GaAs control sample has g ϭ -0.50. 16. Supplemental figure is available at www.sciencemag. org/cgi/content/full/294/5540/131/DC1. 17. This evolution of L includes a zero-crossing at ϳ2 min after reversal, which indicates that L changes sign (i.e., spin precession changes direction) in response to a magnetization reversal. 18. Therefore, the exact shape of the L curves in Fig. 2 depends on the field step and acquisition rate. 19. As L approaches zero, its value cannot be resolved below 0.1 GHz due to the limited range of ⌬t (0 to 2600 ps) access...
Recent findings on the connection between the dielectric breakdown strength and the contact angle saturation in electrowetting triggered further investigation of the underlying mechanisms towards reporting the consequences of the proposed relation. High sensitivity current measurements are conducted to monitor the dielectric leakage current during a standard electrowetting experiment by testing thin (15–500 nm) dielectric films of materials widely used in microelectronics industry (SiO2, tetra-ethoxy-silane, Si3N4). The measurements confirmed that the current is negligible as long as the applied, direct current, voltage is kept below a critical value at saturation onset. This current, however, exhibits a sharp increase at higher voltages. By exploiting the increased breakdown strength of stacked oxide-nitride-oxide dielectrics, the appearance of the contact angle saturation is inhibited, suggesting the use of such composites for the design of efficient electrowetting devices.
Instability of electrowetting on a dielectric substrate J. Appl. Phys. 109, 034309 (2011); 10.1063/1.3544460Illuminating the connection between contact angle saturation and dielectric breakdown in electrowetting through leakage current measurementsa)
Electrowetting on dielectric (EWOD) is simulated by solving the equations of capillary electrohydrostatics, by the Galerkin/finite element method. Aiming to provide reliable predictions of the voltage dependence of the apparent contact angle, close to or beyond the saturation limit, special attention is given in the treatment of the dielectric properties of the solid dielectric where the liquid sits. It is proposed that in regions where the electric field strength locally exceeds the material breakdown strength, the dielectric locally switches to a conductor. Without using any fitting parameter, the implementation of the proposed phenomenological idea realized a surprising matching of published experimental data concerning materials ranging from SiO(2) to Parylene N and Teflon. Charge trapping is naturally connected to the field-induced transition, and its distribution as well as its dependence on the applied voltage is calculated.
Using a recently realized ''addressable catalyst surface'' [Science 294, 134 (2001)] we study the interaction of chemical reaction waves with prescribed spatiotemporal fields. In particular, we study how a traveling chemical pulse is ''dragged'' by a localized, moving temperature heterogeneity as a function of its intensity and speed. The acceleration and eventual ''detachment'' of the wave from the heterogeneity is also explored through simulation and stability analysis. DOI: 10.1103/PhysRevLett.90.018302 PACS numbers: 82.40.Bj, 05.45.-a, 82.40.Np, 82.45.Jn As the elements of spontaneous pattern formation are progressively understood through theory, experimentation, and scientific computation [1], ways to purposefully interact with the coherent structures (pulses and fronts) that constitute the building blocks of spatiotemporal patterns are becoming the focus of extensive research [2,3]. In particular, the stabilization and control of various patterns [3-9] through local, nonlocal, or global feedback, and pattern formation in media with designed heterogeneities [10 -12] are the source of novel insights for spatiotemporal dynamics.To explore such phenomena, we have recently constructed an ''addressable catalyst'': A focused laser beam, manipulated through computer-controlled mirrors and capable of ''writing'' spatiotemporal temperature heterogeneity patterns on a metal single crystal catalyst. The loop between this actuation and sensing (both resolved in space and time) through nonintrusive microscopies is then closed through the computer or the experimentalist herself/himself in real time. Our model system is the low-pressure catalytic oxidation of CO on Pt(110), a reaction exhibiting well-documented spatiotemporal patterns [13][14][15][16], and whose macroscopic modeling has reached an advanced level [17][18][19][20].In this Letter we study the interaction of reactive pulses with a single, spatially coherent but temporally mobile heterogeneity. In particular, we use a temperature heterogeneity, localized in space and steadily moving in time, to ''drag'' spontaneously isothermally forming reactive pulses and fronts with speeds differing from their natural speed. We examine the shapes acquired by these dragged waves and their limits of stability (that is, the range of dragging speeds for which they can exist). Through computer-aided analysis we examine the nature of the detachment instability, marking the loss of the ability of the heterogeneity to drag a pulse in 1D or 2D. Two possible paths to instability (depending on the linearized spectrum crossing) are detected. We explore the postdetachment transient dynamics and the interactions of a pulse with successive elements of a 1D periodic, constant speed array of identical heterogeneities. The theme of pulse dragging is currently also being explored in a variety of physically relevant Hamiltonian systems, or weakly perturbed dissipative variations thereof (targeted transfer of pulses in optical media or the motion/displacement of harmonic traps or optical l...
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