Stimuli-responsive (active) materials undergo large-scale shape or property changes in response to an external stimulus such as stress, temperature, light or pH. Technological uses range from durable, shape-recovery eye-glass frames, to temperature-sensitive switches, to the generation of stress to induce mechanical motion. Here, we demonstrate that the uniform dispersion of 1-5 vol.% of carbon nanotubes in a thermoplastic elastomer yields nanocomposites that can store and subsequently release, through remote means, up to 50% more recovery stress than the pristine resin. The anisotropic nanotubes increase the rubbery modulus by a factor of 2 to 5 (for 1-5 vol.%) and improve shape fixity by enhancing strain-induced crystallization. Non-radiative decay of infrared photons absorbed by the nanotubes raises the internal temperature, melting strain-induced polymer crystallites (which act as physical crosslinks that secure the deformed shape) and remotely trigger the release of the stored strain energy. Comparable effects occur for electrically induced actuation associated with Joule heating of the matrix when a current is passed through the conductive percolative network of the nanotubes within the resin. This unique combination of properties, directly arising from the nanocomposite morphology, demonstrates new opportunities for the design and fabrication of stimuli-responsive polymers, which are otherwise not available in one material system.

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We report the study of optically induced terahertz (THz) electromagnetic radiation from (110) oriented zinc-blende crystals. This work extends our previous studies of (100) and (111) GaAs. Excellent agreement between calculated results and experimental data indicates that, under conditions of moderate optical fluence and normal incidence on the unbiased sample, second-order optical rectification is the major nonlinear process that generates THz radiation.

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The electronic structure of La2-Sr Cu04+~has been studied by measuring 0 1s absorption edges using high-energy electron-energy-loss spectroscopy in transmission.Upon doping the insulating compound the conduction-band states are reduced and states in the gap are formed. When going from insulating to conducting compounds there is a continuous increase of states at the Fermi level, which is at the bottom of the gap. The insulator-metal transition is probably driven by delocalization of these states.Among the cuprates showing high-T, superconductivity, the system La2 "M"Cu04 (M Ba,Sr), first discovered by Bednorz and Miiller, ' is one of the most suitable to study the electronic structure of high-T, superconductors. The undoped parent compound La2Cu04 is an antiferromagnetic insulator. This is due to strong electronic correlation efl'ects on the Cu sites which force the otherwise metallic compound with a half-filled band to become a charge-transfer insulator.A charge-transfer gap appears between the valence band (ligand band) with mainly 0 2p character and the conduction band (upper Hubbard band) with mainly Cu 3d character. Upon p-type doping with Sr, antiferromagnetism disappears and an insulatormetal transition occurs near x=0.06. For 0.06~x (0.30 superconductivity is observed with a maximum T, near x =0.15. Though numerous experimental and theoretical studies have been carried out on this system, the electronic structure as a function of dopant concentration, the mechanism of high-T, superconductivity, and even the nature of the normal state for x-0.15 still remain unclear.In this Rapid Communication we report on changes of the electronic structure of La2Cu04 upon doping with Sr, which are probably essential to understand the normalstate properties of high-T, superconductors.With increasing dopant concentration there is a continuous increase of the density of states at the Fermi level, which is located at the bottom of the gap between the ligand band and the upper Hubbard band. No discontinuity is observed close to the insulator-metal transition. This points out that this transition is driven by delocalization. The present results probably can be generalized to other cuprate systems showing high-T, superconductivity. 0 ls absorption edges of La2 "Sr"Cu04+s have been measured in the concentration range 0(x (0.30 by electron-energy-loss spectroscopy (EELS) in transmission. These edges probe the local unoccupied density of states with p symmetry at the 0 sites provided the interaction of the excited electron with the core hole is small. The latter assumption is supported, at least for metallic cuprates, by the strong similarity of 0 ls absorption edges and resonant bremsstrahlung isochromat spectroscopy (BIS) spectra on Bi2SrqCaCuqQs, and by the rather good agreement between experimental C and N ls core-level absorption spectra with the calculated total density of states in transition-metal carbides and nitrides. Ceramic samples were prepared as described previously.From these samples about 1000-A-thick films were c...