We measure directly both the real and imaginary parts of the sub-band-gap conductivity a in YBa2Cu3<>7 in the 15-80 cm -1 frequency range using coherent time-domain terahertz spectroscopy. We observe a peak in the real part o\ similar to the coherence peak expected in an s-wave BCS superconductor. The absence of such a peak in NMR relaxation-rate experiments suggests that its origin lies in a strongly temperature-dependent inelastic-scattering rate rather than in coherence factors.PACS numbers: 74.30. Gn, 74.70.Vy, 84.40.Cb Measurements of the dynamic response of conventional superconductors have played a crucial role in understanding the origin of superconductivity and in establishing the microscopic Bardeen-Cooper-Schrieffer x (BCS) theory of superconductivity. For instance, far-infrared transmission and reflectivity measurements provided the existence proof for the superconducting energy gap in lead 2 and indications for strong-coupling effects in NbN films. 3 In the high-temperature superconductors, reflectivity measurements on YBa2Cu3C>7 single crystals and films 4 " 6 as well as transmission measurements on Bi2Sr2CaCu20g cleaved crystals 7 have been performed, but unfortunately with widely diverging results and interpretations.In this Letter, we present measurements of both the real and imaginary parts of the conductivity a of highquality superconducting YBa2Cu3C>7 films in the frequency range from 500 GHz to 2.5 THz (15-80 cm -1 ) by coherent time-domain terahertz spectroscopy. Unlike conventional Fourier-transform far-infrared spectroscopy, owing to its phase sensitivity, this technique permits us to measure both the real and imaginary parts of the conductivity simultaneously without referring to the Kramers-Kronig relations. Furthermore, we can study thin superconducting films in transmission, which offers greatly increased accuracy compared to reflection measurements, where the reflectivity is close to unity for energies below the gap.This novel spectroscopic technique has recently been successfully applied to the measurement of the conductivity of doped semiconductors, 8 the absorption coefficient and refractive index of dielectrics, 9 the measurement of the superconducting band gap in niobium films, 10 as well as the surface impedance of YBa2Cu3C>7 superconducting films. 11 The setup of our coherent time-domain spectrometer is similar to the one described earlier, 10,11 but includes additional focusing optics for small samples. It resembles a conventional spectrometer, but here the electromagnetic far-infrared radiation is generated and detected by time-resolved optoelectronic techniques, which results in the phase sensitivity of the technique. Both source and detector consist of 50-/im dipole antennas with integrated radiation-damaged silicon-on-sapphire photoconductive switches. 12 The transmitter antenna, which is biased with a dc voltage, emits a short electromagnetic burst with a broad frequency spectrum extending from almost dc to more than 2.5 THz when the 100-fs optical pulse from a colli...

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Cd1−xZnxTe has been grown on GaAs substrates for compositions from x=0 to x=1. Binaries are shown to be of high quality, but x-ray rocking curve half-widths are extremely broad for most ternary compositions. Attempts to modify the interface yield only modest and uneven improvement in rocking curve half-widths. The poor quality appears to be due to a phase separation.

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