A previous time-resolved optical study reported on a metastable hidden electronic state in 1T-TaS2, which is only accessible upon photoexcitation and created under non-equilibrium conditions [1]. The properties of such a state are distinct from those of any other state in the equilibrium phase diagram and it is possible to revert to the thermodynamic initial state either by illuminating with picosecond laser pulses or by applying other thermal erase procedures. In this work we show photoinduced switching to a metastable hidden state on the same material, and probe it by means of both static and time-resolved photoemission spectroscopy, thus having direct access to the electronic structure of the system. From our experimental findings and comparison with other studies, we conclude that we obtain partial switching, leading to a hidden state with persisting insulating nature but significant modifications in the electronic structure and CDW ordering.
We perform two-colour pump probe experiments on metals and superconductors using synchronized 10-fs pulses
generated by optical parametric amplifiers, tunable from the visible to the near-infrared, mapping with unprecedented
detail the energy equilibration dynamics of the free electron gas. In gold films we observe dramatic changes of the
differential reflectivity spectrum on the 100-fs timescale, corresponding to the establishment of the thermal electron
distribution, with dynamics dictated by excess energy. In high-Tc cuprate superconductors we observe fast electron
relaxation, attributed to a strong electron-phonon coupling which may play a role in the superconductivity mechanism.
KEYWORDS: Raw materials, Electron microscopy, Molybdenum, Scanning electron microscopy, Spectroscopy, Transmission electron microscopy, Carbon nanotubes, Nanotechnology, Iodine, Nanowires
The processing of most one-dimensional nano-materials such as carbon-nanotubes is hampered by the fact that they are insoluble. Here we show how a significant portion (≈12wt%) of the as-produced Mo6S4.5I4.5 nanowires is stably dispersed in isopropanol as small diameter nanowire bundles. Sedimentation studies, performed combining experiments and theory, show the presence of three phases in the raw material: impurity material, insoluble and soluble nanowire bundles. A purification procedure is also discussed. The three phases has been characterized by UV-Vix-IR spectroscopy and XPS showing their intrinsic diversity.
In this report we review recent experimental results on photoexcited carrier relaxation dynamics on high temperature superconductors (HTSC) probed by a femtosecond time-resolved optical spectroscopy, and compare the results with the data obtained on quasi two dimensional charge density waves. In these experiments, a femtosecond laser pump pulse excites electron-hole pairs via an inter-band transition in the material. These hot carriers rapidly release their energy via electron-electron and electron-phonon collisions reaching states near the Fermi energy within ~100 fs. If an energy gap is present in the low-energy density of states (DOS), it inhibits the final relaxation step and photoexcited carriers accumulate above the gap causing a transient change in reflectivity arising from excited state absorption. The relaxation and recombination processes of photoexcited quasiparticles, governed by the magnitude, anisotropy and the T-dependence of the low energy gap, are monitored by measuring the resulting photoinduced absorption as a function of time after the photoexcitation. This way, the studies of carrier relaxation dynamics give us direct information of the T-dependent changes in the low energy DOS. The technique is particularly useful to probe the systems with spatial inhomogeneities, where different local environments give rise to different relaxation rates. The data on series of HTSC-s show evidence for the coexistence of two distinct relaxation processes, whose T-dependences seem to be governed by two different energy scales: a T-independent pseudogap and a mean-field-like T-dependent gap that opens at Tc. The data suggest the origin of the two-gap behavior is in the intrinsic microscopic spatial inhomogeneity of these materials.
Temperature dependence of the relaxation of photoexcited (PE) carriers is used as a probe of the electronic structure of the high-temperature superconductor YBa2Cu3O7- (delta ) ((delta) approximately equals 0.1). The relaxation process is studied by 'counting' -- through measurement of the Raman scattering Stokes/anti-Stokes intensity ratio -- the phonons emitted in the process of carrier energy relaxation. The phonon 'shake-off' is found to be strongly temperature dependent, implying that the PE carrier relaxation proceeds via a temperature activated process, which can be understood in terms of hopping between localized states. The long PE carrier lifetime and temperature dependence of the relaxation process implies the existence of localized states within 2 eV of the Fermi energy in optimally doped high-Tc superconductor.
Ultrafast time-resolved optical absorption has been measured as a function of doping and temperature in the high temperature superconductors Bi2Sr2Ca1-yYyCu2O8 (0 less than y less than 1) and YBa2Cu3O7-(delta ) (0.06 less than 1). A fast transient response is observed, which changes sign as a function of y in Bi2Sr2Ca1-yYyCu2O8 and in YBa2Cu3O7-(delta ). Careful consideration of the possible mechanisms for the observed behavior lead us to suggest that two independent mechanisms are operating, whereby the two observed positive photoinduced absorption components are due to the electrodynamic response of photoinduced carriers, while the negative signal arises due to spectral hole burning. In the metallic samples a long-lived component is observed, which we suggest comes from photoinduced localized states intrinsic only to the superconducting species of the high-Tc materials. A very significant increase in the scattering rate (1/(tau) ) at temperatures just above Tc signifies coupling to charge and possibly spin fluctuations just above Tc. A large reduction in scattering rate is observed below Tc, similar to that observed previously in YBa2Cu3O6.9 and Tl2Ba2CaCu2O8.
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