Pressure induced Raman and fluorescence singularities in LiYF4LiYF_4

The pressure effect on the fluoride scheelite laser host $LiYF_4$ is studied at room temperature up to 26 GPa by Raman scattering and up to 40 GPa by $P^{3+}$ fluorescence of doped sample. The Raman spectra exhibit three singularities at the vicinity of 6 GPa, 10-12 GPa and 16-17 GPa. The samples pressurized to 21 GPa or higher do not recover the original phase after being released, giving more Raman lines than original samples. The luminescence spectra of $P^{3+}$ are collected in the energy range corresponding to following transitions $^3P_{0,1}--^3H_{4,5,6}$, $^1D_2--^3H_4$ and $^3P_0--^3F_2$. Singularities are observed in the vicinity of 6 GPa, 10 GPa, 16 GPa, 23 GPa in agreement with the Raman study. Moreover, an irreversible transition occurs at 23 GPa. The samples pressurized to above 26 GPa become amorphous when released and all the sharp lines disappear. Above 31 GPa, the spectra at high pressures show only some broad bands corresponding to transitions between two multiplets of the $^4F_2$ configuration of $Pr^{3+}$. These singularities suggest possible phase transformations leading to lowering of the lattice symmetry.Comment: 12 pages, 13 figures, 2 table, LaTe

Universal scaling in sports ranking

Ranking is a ubiquitous phenomenon in the human society. By clicking the web pages of Forbes, you may find all kinds of rankings, such as world's most powerful people, world's richest people, top-paid tennis stars, and so on and so forth. Herewith, we study a specific kind, sports ranking systems in which players' scores and prize money are calculated based on their performances in attending various tournaments. A typical example is tennis. It is found that the distributions of both scores and prize money follow universal power laws, with exponents nearly identical for most sports fields. In order to understand the origin of this universal scaling we focus on the tennis ranking systems. By checking the data we find that, for any pair of players, the probability that the higher-ranked player will top the lower-ranked opponent is proportional to the rank difference between the pair. Such a dependence can be well fitted to a sigmoidal function. By using this feature, we propose a simple toy model which can simulate the competition of players in different tournaments. The simulations yield results consistent with the empirical findings. Extensive studies indicate the model is robust with respect to the modifications of the minor parts.Comment: 8 pages, 7 figure

Structure of self-organized Fe clusters grown on Au(111) analyzed by Grazing Incidence X-Ray Diffraction

We report a detailed investigation of the first stages of the growth of self-organized Fe clusters on the reconstructed Au(111) surface by grazing incidence X-ray diffraction. Below one monolayer coverage, the Fe clusters are in "local epitaxy" whereas the subsequent layers adopt first a strained fcc lattice and then a partly relaxed bcc(110) phase in a Kurdjumov-Sachs epitaxial relationship. The structural evolution is discussed in relation with the magnetic properties of the Fe clusters.Comment: 7 pages, 6 figures, submitted to Physical Review B September 200

Kondo Insulator: p-wave Bose Condensate of Excitons

In the Anderson lattice model for a mixed-valent system, the $d-f$ hybridization can possess a $p$-wave symmetry. The strongly-correlated insulating phase in the mean-field approximation is shown to be a $p$-wave Bose condensate of excitons with a spontaneous lattice deformation. We study the equilibrium and linear response properties across the insulator-metal transition. Our theory supports the empirical correlation between the lattice deformation and the magnetic susceptibility and predicts measurable ultrasonic and high-frequency phonon behavior in mixed-valent semiconductors.Comment: 5 pages, 3 encapsulated PostScript figure

Pressure effects on the Raman spectrum of CaZnF4CaZnF_4

The pressure influence on the lattice vibration of $CaZnF_4$ has been studied by Raman diffusion up to 17 GPa. Most Raman frequencies increase with increasing pressure. Three singularities in the pressure induced frequency evolution are observed around 1.5 GPa, 10 GPa and 17 GPa. The samples pressurized to 17 GPa or higher do not revert to the ambient pressure phase after being released, the new phase showing different Raman spectra from the ordinary one. It is suggested that $CaZnF_4$ undergoes probably sudden lattice deformations at about 1.5 GPa and 10 GPa, and an irreversible phase transformation above 17 GPa.Comment: LaTeX file, 3 ps figures, 8 page

Opportunities for polymer-based nanowires in optoelectronics and nanophotonics

International audienceSee document joine

Evidence for Modification of the Electronic Density-of-States by Zero-Point Lattice Motion in One-Dimension - Luminescence and Resonance Raman Studies of An Mx Solid

Luminescence spectra, both emission and excitation, and the excitation dependence of the resonance Raman spectra, have been measured for the quasi-one-dimensional charge-density-wave material [Pt(en)2][Pt(en)2Cl2](ClO4)4, en = 1,2-diaminoethane. While the luminescence experiments show the existence of tail states at low temperature in the band gap region, the Raman measurements conclusively demonstrate that this tail does not arise from ordinary static structural disorder. These results can be explained by considering the zero-point motion of the lattice

Quantum size effect on charges and phonons ultrafast dynamics in atomically controlled nanolayers of topological insulators Bi2Te3

This work was supported by the French Ministry of Education and Research, the CNRS, Region Pays de la Loire (CPER Femtosecond Spectroscopy equipment program) and the LIA-CNRS (Laboratoire International Associé) IM-LED. The partial financial support from National Science Center under project 2016/21/B/ST5/02531 is acknowledged. R. Rapacz was supported by FORSZT PhD fellowship.Heralded as one of the key elements for next generation spintronics devices, topological insulators (TIs) are now step by step envisioned as nanodevices like charge-to-spin current conversion or as Dirac fermions based nanometer Schottky diode for example. However, reduced to few nanometers, TIs layers exhibit a profound modification of the electronic structure and the consequence of this quantum size effect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. Here, thanks to a complete study of a set of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~6 nm, below which a spectacular reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2 Te3 In addition, we also evidence an A1g optical phonon mode softening together with the appearance of a thickness dependence of the photoinduced coherent acoustic phonons signals. This drastic evolution of the carriers and phonons dynamics might be due an important electron-phonon coupling evolution due to the quantum confinement. These properties have to be taken into account for future TIs-based spintronic devices.Centre National de la Recherche Scientifiqu