Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in YSr x Ba 2Ϫx Cu 4 O 8 as a function of temperature and polarization. The data show a two-component relaxation similar to YBa 2 Cu 3 O 7Ϫ␦ , one component being associated with the superconducting transition, and the other with the pseudogap below T*. The relaxation time p associated with the pseudogap is found to be T independent, while the relaxation time G of the component observed only below T c exhibits a clear divergence near T c . A strong polarization anisotropy of the picosecond transient is observed below T c which is attributed to the anisotropy of the probe transition matrix elements.Femtosecond time-domain spectroscopy is capable of giving important information on the quasiparticle excitations and a low-energy structure of correlated electron systems, and particularly high-T c superconducting cuprates ͑HTSC͒. Measurements on YBa 2 Cu 3 O 7Ϫ␦ ͑Y123͒ at different doping levels have shown two-component relaxation dynamics on the picosecond time scale, that was attributed to the simultaneous existence of two gaps in optimally doped and overdoped regions. 1 Moreover, similar two-component relaxation dynamics has been thus far observed in several HTSC, 2-8 suggesting this being a general feature in these materials. The magnitude and sign of the two components at temperatures below T c were found to depend on the material, probe wavelength, 9 and also the effects of probe polarization dependence were investigated on untwinned Y123 single crystals, 10 where a response parallel and perpendicular to the Cu-O chains was separately probed and found to be different. In some cases ͑Tl2201 2 , Bi2212 3 ͒ the signs of the different components observed in the relaxation were opposite. However, the anisotropy of the photoinduced signal with respect to the probe pulse polarization has not been discussed in detail thus far.The purpose of this paper is twofold. First, we show that the two-component relaxation behavior observed in Y123 is similar also in YBa 2 Cu 4 O 8 ͑Y124͒, and second, utilizing the fact that Y124 has a fixed oxygen content and a well-defined untwinned orthorhombic structure, to probe the polarization dependence of the photoinduced signal.We report on a systematic investigation of Y124 and YBa 1.6 Sr 0.4 Cu 4 O 8 ͑Y124:Sr͒ using the usual femtosecond time-resolved pump-probe technique. As discussed in detail previously, 11 a short ͑ϳ80 fs͒ pump laser pulse excites the carriers in the sample. Photoexcited electrons and holes with energies on the order of photon energy quickly thermalize via electron-electron and electron-phonon thermalization, reaching states just above the gap in a time short compared to the pulse duration. The gap in the density of states presents a relaxation bottleneck, and the relaxation of photoexcited carrier density near E F is measured through measurement of small changes in the optical reflectivity ⌬R/R or transmittance ⌬T/T of the sample as a function of a time delay between the pu...

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We demonstrate the application of Mo(6)S(3)I(6) molecular wire bundles for electrically controllable two-terminal on-off switches. We investigate how changes in the contact electrode material and geometry influence the device characteristics, hysteretic switching behavior and device stability. We also determine the device operating parameters, particularly the Young's moduli (40-270 GPa), operating current densities (3.2 x 10(5)-7 x 10(6) A m(-2)) and force constants. Although qualitatively, the properties of Mo(6)S(3)I(6) nanowires in nanoelectromechanical (NEM) switches are similar to those of carbon nanotubes (CNTs), their lower friction coefficient, higher mechanical stability and higher operation voltages give specific advantages in terms of smaller differences in on-off operating potentials, higher switching speeds and lower energy consumption than CNTs, which are critical for applications in NEM devices.

An ultrafast paralell data processor is described in which amplitude mode excitations of a charge density wave (CDW) are used to encode data on the surface of a 1-T TaS2 crystal. The data are written, manipulated and read using parallel femtosecond laser pulse beams, and the operation of a database search algorithm is demonstrated on a 2-element array.

Longitudinal resistivity measurements on single Mo6S9−xIx (x = 4.5, 6 and 7) molecular nanowire bundles ranging in diameter from d = 7 nm to 1 µm are performed to investigate the longitudinal transport properties of individual bundles. Different contacting methods are used to study diverse nanocircuit manufacturing technologies that can be used for interconnects based on Mo6S9−xIx. The measurements show ubiquitously linear I–V characteristics with Pd, Au, Ag and Ti contact metals. The highest room-temperature conductivity achieved is σ0∼10 S m−1 using Ag contacts. The critical current densities typically achieved are Jc∼104 A cm−2. The observed metallic behaviour at room temperature is consistent with the band structure calculated using density functional theory (DFT). At low temperatures, the conductivity is found to decrease, following variable range hopping (VRH) behaviour of the form σ = σ0exp−(T0/T)β reasonably well, but the exponent β changes upon annealing. From fits to the temperature dependence of the conductivity, a change from β∼1/4 to β∼1/2 is observed, which may be explained by a change in dimensionality from 3D-like VRH to 1D-like VRH following the removal of intra-bundle interstitial iodine.