Measurements of the temperature dependence of quasiparticle ͑QP͒ dynamics in Hg 1 Ba 2 Ca 2 Cu 3 O 8ϩ␦ with femtosecond time-resolved optical spectroscopy are reported. From the temperature dependence of the amplitude of the photoinduced reflection, the existence of two gaps is deduced, one temperature-dependent ⌬ c (T) that closes at T c , and another temperature-independent ''pseudogap'' ⌬ p . The zero-temperature magnitudes of the two gaps are ⌬ c (0)/k B T c ϭ6Ϯ0.5 and ⌬ p /k B T c ϭ6.4Ϯ0.5, respectively. The quasiparticle lifetime is found to exhibit a divergence as T→T c from below, which is attributed to the existence of a superconducting gap that closes at T c . Above T c the relaxation time is longer than expected for metallic relaxation, which is attributed to the presence of the pseudogap. The QP relaxation time is found to increase significantly at low temperatures. This behavior is explained assuming that at low temperatures the relaxation of photoexcited quasiparticles is governed by a biparticle recombination process.
The penetration resistance of a prototypical model-membrane system (HS-(CH2)11-OH self-assembled monolayer (SAM) on Au(111)) to the tip of an atomic force microscope (AFM) is investigated in the presence of different solvents. The compressibility (i.e., height vs tip load) of the HS-(CH2)11-OH SAM is studied differentially, with respect to a reference structure. The reference consists of hydrophobic alkylthiol molecules (HS-(CH2)17-CH3) embedded as nanosized patches into the hydrophilic SAM by nanografting, an AFM-assisted nanolithography technique. We find that the penetration resistance of the hydrophilic SAM depends on the nature of the solvent and is much higher in the presence of water than in 2-butanol. In contrast, no solvent-dependent effect is observed in the case of hydrophobic SAMs. We argue that the mechanical resistance of the hydroxyl-terminated SAM is a consequence of the structural order of the solvent-SAM interface, as suggested by our molecular dynamics simulations. The simulations show that in the presence of 2-butanol the polar head groups of the HS-(CH2)11-OH SAM, which bind only weakly to the solvent molecules, try to bind to each other, disrupting the local order at the interface. On the contrary, in the presence of water the polar head groups bind preferentially to the solvent that, in turn, mediates the release of the surface strain, leading to a more ordered interface. We suggest that the mechanical stabilization effect induced by water may be responsible for the stability of even more complex, real membrane systems.
We followed in real time the thermal reaction of fullerene molecules with the Si(111) surface by means of fast photoemission spectroscopy. The formation of SiC via C60 fragmentation on Si(111) is used as a key example of the capability of fast photoemission, associated with a fine temperature control, in determining the nature of thermally induced chemical reactions. By monitoring every 13 s the evolution of the C1s core level photoemission spectrum, as a function of temperature and as a function of time at fixed temperature, we were able to identify several steps in the interaction of C60 with Si(111). A model describing the thermal evolution of this interaction, in agreement with these and other experimental observations, considers the initial chemisorption of C60 in mainly metastable configurations, the evolution toward more stable configurations, allowed by molecular rotations and breaking of Si-Si bonds, the cage deformation to further increase the number of C-Si bonds, the final cage fragmentation and SiC formation only above 1050 ± 10 K.
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