Optical Sum Rule anomalies in the High-Tc Cuprates

We provide a brief summary of the observed sum rule anomalies in the high-T$_c$ cuprate materials. A recent issue has been the impact of a non-infinite frequency cutoff in the experiment. In the normal state, the observed anomalously high temperature dependence can be explained as a `cutoff effect'. The anomalous rise in the optical spectral weight below the superconducting transition, however, remains as a solid experimental observation, even with the use of a cutoff frequency.Comment: 4 pages, 2 figures, very brief review of optical sum rule anomal

Superconductivity from Undressing. II. Single Particle Green's Function and Photoemission in Cuprates

Experimental evidence indicates that the superconducting transition in high $T_c$ cuprates is an 'undressing' transition. Microscopic mechanisms giving rise to this physics were discussed in the first paper of this series. Here we discuss the calculation of the single particle Green's function and spectral function for Hamiltonians describing undressing transitions in the normal and superconducting states. A single parameter, $\Upsilon$, describes the strength of the undressing process and drives the transition to superconductivity. In the normal state, the spectral function evolves from predominantly incoherent to partly coherent as the hole concentration increases. In the superconducting state, the 'normal' Green's function acquires a contribution from the anomalous Green's function when $\Upsilon$ is non-zero; the resulting contribution to the spectral function is $positive$ for hole extraction and $negative$ for hole injection. It is proposed that these results explain the observation of sharp quasiparticle states in the superconducting state of cuprates along the $(\pi,0)$ direction and their absence along the $(\pi,\pi)$ direction.Comment: figures have been condensed in fewer pages for easier readin

Interplane Transport and Superfluid Density in Layered Superconductors

We report on generic trends in the behavior of the interlayer penetration depth $\lambda_c$ of several different classes of quasi two-dimensional superconductors including cuprates, Sr$_2$RuO$_4$, transition metal dichalcogenides and organic materials of the $(BEDT-TTF)_2X$-series. Analysis of these trends reveals two distinct patterns in the scaling between the values of $\lambda_c$ and the magnitude of the DC conductivity: one realized in the systems with a Fermi liquid (FL) ground state and the other seen in systems with a marked deviation from the FL response. The latter pattern is found primarily in under-doped cuprates and indicates a dramatic enhancement (factor $\simeq 10^2$) of the energy scale $\Omega_C$ associated with the formation of the condensate compared to the data for the FL materials. We discuss implications of these results for the understanding of pairing in high-$T_c$ cuprates.Comment: 4 pages, 2 figure

Superconducting Fluctuation investigated by THz Conductivity of La2−x_{2-x}Srx_xCuO4_4 Thin Films

Frequency-dependent terahertz conductivities of La$_{2-x}$Sr$_x$CuO$_4$ thin films with various carrier concentrations were investigated. The imaginary part of the complex conductivity considerably increased from far above a zero-resistance superconducting transition temperature, $T_\text{c}^\text{zero}$, because of the existence of the fluctuating superfluid density with a short lifetime. The onset temperature of the superconducting fluctuation is at most $\sim 2T_\text{c}^\text{zero}$ for underdoped samples, which is consistent with the previously reported analysis of microwave conductivity. The superconducting fluctuation was not enhanced under a 0.5 T magnetic field. We also found that the temperature dependence of the superconducting fluctuation was sensitive to the carrier concentration of La$_{2-x}$Sr$_x$CuO$_4$, which reflects the difference in the nature of the critical dynamics near the superconducting transition temperature. Our results suggest that the onset temperature of the Nernst signal is not related to the superconducting fluctuation we argued in this paper.Comment: J. Phys. Soc. Jpn. in pres

Laser annealing study of PECVD deposited hydrogenated amorphous silicon carbon alloys films

The influence of carbon content on the crystallization process has been investigated for the excimer laser annealed hydrogenated amorphous silicon carbon alloy films deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) technique, using silane methane gas mixture diluted in helium, as well as for the hydrogenated microcrystalline silicon carbon alloy films prepared by PECVD from silane methane gas mixture highly diluted in hydrogen, for comparison. The study demonstrates clearly that the increase in the carbon content prevents the crystallization process in the hydrogen diluted samples while the crystallization process is enhanced in the laser annealing of amorphous samples because of the increase in the absorbed laser energy density that occurs for the amorphous films with the higher carbon content. This, in turn, facilitates the crystallization for the laser annealed samples with higher carbon content, resulting in the formation of SiC crystallites along with Si crystallites

Porous inorganic thin films from bridged silsesquioxane sol-gel precursors

International audienceA sol-gel process was exploited to produce porous inorganic thin films from phenyl-bridged silsesquioxanes. The evolution of both structural and optical properties of the starting hybrid so-gel films were monitored, during synthesis and successive thermal curing steps, by Fourier transform infrared (FT-IR) spectroscopy, differential thermal analysis (DTA), thermogravimetric analysis (TGA) and spectroscopic ellipsometry (SE). Involved chemical species, structural and chemical modifications were identified when thermal treatments at increasing temperatures in the range of 60-800 degrees C were applied to the hybrid films. The progressive formation of a crosslinked silica network, template elimination and film densification were observed, resulting in completely inorganic porous thin films of low refractive index. The distinctiveness of this system directly comes from the extremely controlled and uniform dispersion of the porogen at a molecular level, which is intrinsic to the bridged silsesquioxane precursor choice. A quantitative porosity analysis was performed by environmental ellipsometric porosimetry (EEP), studying inorganic film optical and mechanical properties under different relative humidity conditions. A transmission electron microscopy (TEM) in-situ characterization of porosity size and distribution confirms the presence of a spatially structured organization of pores of a few nanometers in diameter

Interplay of Order and Disorder in the High-Energy Optical Response of Three-Dimensional Photonic Crystals

21 páginasOpal-like structures, consisting of lattices of dielectrics spheres, are the most commonly studied example of three-dimensional (3D) photonic crystals (PCs). Since they were proposed as new materials to mold the flow of light, they have become an important area of research because of theit technological potential and fundamental interest. Among all fabrication techniques developed up to date to prepare opaline PCs, those based on evaporation-induced self-assembly (EISA) are some of the most frequently used and thoroughly analyzed. The advent and subsequent improvement of fabrication techniques that take advantage of self-organizing properties of dielectric spheres in the micrometer scale have permitted to obtain solid colloidal crystals that exhibit PC properties, whose optical response has been studies in depth in the low-energy range, where the lattice parameter is smaller than the incident wavelength.Peer reviewe