We report on thermal noise from the internal friction of dielectric coatings made from alternating layers of Ta2O5 and SiO2 deposited on fused silica substrates. We present calculations of the thermal noise in gravitational wave interferometers due to optical coatings, when the material properties of the coating are different from those of the substrate and the mechanical loss angle in the coating is anisotropic. The loss angle in the coatings for strains parallel to the substrate surface was determined from ringdown experiments. We measured the mechanical quality factor of three fused silica samples with coatings deposited on them. The loss angle, φ (f ), of the coating material for strains parallel to the coated surface was found to be 4.2 ± 0.3 × 10 −4 for coatings deposited on commercially polished slides and 1.0 ± 0.3 × 10 −4 for a coating deposited on a superpolished disk. Using these numbers, we estimate the effect of coatings on thermal noise in the initial LIGO and advanced LIGO interferometers. We also find that the corresponding prediction for thermal noise in the 40 m LIGO prototype at Caltech is consistent with the noise data. These results are complemented by results for a different type of coating, presented in a companion paper.

Current interferometric gravitational wave detectors use test masses with mirror coatings formed from multiple layers of dielectric materials, most commonly alternating layers of SiO2 (silica) and Ta2O5 (tantala). However, mechanical loss in the Ta2O5/SiO2 coatings may limit the design sensitivity for advanced detectors. We have investigated sources of mechanical loss in the Ta2O5/SiO2 coatings, including loss associated with the coating-substrate interface, with the coating-layer interfaces, and with the bulk material. Our results indicate that the loss is associated with the bulk coating materials and that the loss of Ta2O5 is substantially larger than that of SiO2.

We have measured the mechanical dissipation in a sample of fused silica drawn into a rod. The sample was hung from a multiple-bob suspension, which isolated it from rubbing against its support, from recoil in the support structure, and from seismic noise. The quality factor, Q, was measured for several modes with a high value of 57 million found for mode number 2 at 726 Hz. This result is about a factor 2 higher than previous room temperature measurements. The measured Q was strongly dependent on handling, with a pristine flame-polished surface yielding a Q 3-4 times higher than a surface which had been knocked several times against a copper tube.

At high flux-flow velocities in the mixed state of a type-II superconductor the nonequilibrium distribution of the quasiparticles is shifted to higher energies. As shown theoretically by Larkin and Ovchinnikov, a distinct signature of this nonequilibrium effect is an electronic instability, resulting in a sharp kink in the currentvoltage characteristic at a critical vortex velocity v * . From measurements of v * the quasiparticle-energy relaxation rate Ϫ1 can be found. We have measured this instability point for epitaxial c axis oriented films of La 1.85 Sr 0.15 CuO 4Ϫx and YBa 2 Cu 3 O 7Ϫ␦ . In both cuprates the observed temperature dependence of Ϫ1 could be well fitted with the function Ϫ1 ϭa exp͓Ϫ2⌬(T)/k B T͔, suggesting that the electron-electron recombination process, perhaps in combination with the emission of another excitation, is crucial. As an example of a low-T c superconductor we have also studied amorphous Mo 3 Si films and found that Ϫ1 can be fitted by a function similar to that we have used for the cuprates, again indicating the importance of the electron-electron interaction. ͓S0163-1829͑97͒07745-X͔

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