Spin-Waves in the Mid-Infrared Spectrum of Antiferromagnetic YBa2_2Cu3_3O6.0_{6.0}

The mid-infrared spin-wave spectrum of antiferromagnetic YBa$_2$Cu$_3$O$_{6.0}$\ was determined by infrared transmission and reflection measurements (\bbox{k} \!\! \parallel c) at $T\!=\!10\!$~K.\@ Excitation of single magnons of the optical branch was observed at $E_{\text{op}}\!=\!178.0\!$~meV.\@ Two further peaks at $346\!$~meV ($\approx\!1.94\,E_{\text{op}}$) and $470\!$~meV ($\approx\!2.6\,E_{\text{op}}$) both belong to the two-magnon spectrum. Linear spin wave theory is in good agreement with the measured two-magnon spectrum, and allows to determine the exchange constant $J$ to be about $120\!$~meV, whereas the intrabilayer coupling $J_{12}$ is approximately $0.55\,J$.Comment: 3 figures in uuencoded for

Optical conductivity of a quasi-one-dimensional system with fluctuating order

We describe a formally exact method to calculate the optical conductivity of a one-dimensional system with fluctuating order. For classical phase fluctuations we explicitly determine the optical conductivity by solving two coupled Fokker-Planck equations numerically. Our results differ considerably from perturbation theory and in contrast to Gaussian order parameter fluctuations show a strong dependence on the correlation length.Comment: 7 pages, 2 figure

Highly conducting perylene radical salts

Temperature dependent dc and microwave conductivity data together with EPR and optical reflectance measurements on the "mixed" system (pe)2(ASF6)0,75(PF6)0,35 times 0,85 CH2Cl2 are described. The data prove metallic behaviour of this organic solid down to 200 K

Optical study of orbital excitations in transition-metal oxides

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved calculation of orbital excitation energies in TiOCl, figure 16 improved, references updated, 33 pages, 20 figure

Optical properties of overdoped YBa

We report on temperature-dependent reflectance measurements on several $\rm YBa_2Cu_3O_x$ single crystals with $6.8\le x \le 7$ in the far-infrared range (5 meV $<\hbar \omega <100$ meV) with ${\bf E}\| c$. Performing a Kramers-Kronig transformation, we derive the optical conductivity $\sigma_{\|c} (\hbar \omega)$ and the real part of the dielectric constant $\epsilon_{1\|c} (\hbar \omega)$. Fitting the data with a two-fluid model, we obtained the superfluid carrier density and the scattering rate. The crystals of oxygen content x&lt; 6.93, i.e., below the maximum value of Tc, show a linear dependence of the superfluid carrier density with temperature, consistent with the $d-$wave model. The data for the overdoped crystals give evidence for superconductivity of the chain carriers