Sound propagation in density wave conductors and the effect of long-range Coulomb interaction

We study theoretically the sound propagation in charge- and spin-density waves in the hydrodynamic regime. First, making use of the method of comoving frame, we construct the stress tensor appropriate for quasi-one dimensional systems within tight-binding approximation. Taking into account the screening effect of the long-range Coulomb interaction, we find that the increase of the sound velocity below the critical temperature is about two orders of magnitude less for longitudinal sound than for transverse one. It is shown that only the transverse sound wave with displacement vector parallel to the chain direction couples to the phason of the density wave, therefore we expect significant electromechanical effect only in this case.Comment: revtex, 14 pages (in preprint form), submitted to PR

Surface impedance of BaFe2-xNixAs2 in the radio frequency range

We report measurements of the temperature dependence of the surface impedance in superconducting BaFe1.93Ni0.07As2 crystals using the radiofrequency reflection technique in the 5<T<30K temperature range. An LC resonant circuit with a phase sensitive detection was used at 92MHz. A measurement assembly with point contacts was used at 30MHz. The recent discovery of iron based arsenide superconductors BaFe2-xNixAs2 has attracted much interest. For a Ni doping level of 7% the superconducting phase transition is found around 20K. The temperature dependence of the superconducting penetration depth was determined

Tb3+/Yb3+ codoped silica-hafnia glass and glass-ceramic waveguides to improve the efficiency of photovoltaic solar cells

In this paper we present the investigation of the energy transfer efficiency between Tb3+ and Yb3+ ions in silica-hafnia waveguides. Cooperative energy transfer between these two ions allows to cut one 488 nm photon in two 980 nm photons and could have important applications in improving the performance of photovoltaic solar cells. Previous works revealed that for a given concentration of donors (Tb3+, increasing the number of acceptors (Yb3+) located near to the Tb3+ ion can increase the Tb-Yb transfer probability. However, when increasing the density of active ions, some detrimental effects due to cross-relaxation mechanisms become relevant. On the basis of this observation the sample doping was chosen keeping constant the molar ratio [Yb]/[Tb] = 4 and the total rare earths contents were [Tb + Yb]/[Si + Hf] = 5%, 7%, 9%. The choice of the matrix is another crucial point to obtain an efficient down conversion processes with rare earth ions. To this respect a 70SiO(2)-30HfO(2) waveguide composition was chosen. The comparison between the glass and the glass-ceramic structures demonstrated that the latter is more efficient since it combines the good optical properties of glasses with the optimal spectroscopic properties of crystals activated by luminescent species. A maximum transfer efficiency of 55% was found for the highest rare earth doping concentration

Softening of the shear modulus C44 in (TaSe4)2I due to sliding charge density waves

Acoustic velocity of the shear mode C44 of quasi-one-dimensional (TaSe4)2I has been measured at 15 MHz frequency with the ultrasonic technique. It is found that the elastic constant C44 decreases when applied electric fields greater than the threshold ET are applied for charge-density wave (CDW) depinning. The decrease ΔC44/C44 ≈ 10-4 observed at 15 MHz for E ≈ 10 ET is two orders of magnitude smaller than the decrease reported at low frequency (kHz). Present results are compared with the effects of CDW depinning on the elastic properties predicted by several models

Optical properties of Tb3mathplusmathplus/Yb3mathplusmathplus co-doped silica-hafnia glass and glass-ceramic thin film

In this paper, optical properties of Tb3+/Yb3+ co-doped 70SiO2 - 30HfO2 glass and glass-ceramic thin films prepared by dip coating on Si O2 substrate are investigated using transmittance measurements. Such systems are usually used for down-converting the high-frequency sunlight in order to tune the solar spectrum into the bandgap of the semi-conductor based photovoltaic system. The interference modulated transmission spectra tau(lambda) of 70SiO2 - 30HfO2- x Tb3+ - y Yb3+ (with molar ratio x/y = 1.8/0, 1.8/7.2, 3/12) at normal incidence were recorded in the spectral wavelength region from 250 nm up to 2500 nm. The corresponding optical constants and thicknesses were deduced by analyzing the transmittance spectra using the straightforward method proposed by Swanepoel. The dispersion of the real part of the refractive index and the absorption were then deduced. The results show thatthe increase in the concentration of rare earth ions leads to a slight increase in the refractive index, this is also observed in thickness variation.Furthermore, the energy bandgap has been determined from the absorption coefficient values using the direct transition model proposed by Tauc. The corresponding value is around 3 eV with a slightly higher bandgap for glass films as compared to glass-ceramic films