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The Cu02-plane optical reflectance of superconducting La2 Sr"Cu04thin films (T, =31 K) has been measured over a wide frequency and temperature range. The optical conductivity in the normal state is well described by a temperature-dependent weak-coupling (X=0.25) free-carrier term plus an overdamped, weakly temperature-dependent, midinfrared component. The free-carrier plasma frequency is nearly constant, co~a =6300 cm, whereas the relaxation rate varies linearly with temperature above T, .In the superconducting state, according to our two-component approach, most of the Drude oscillator strength condenses to a 6(co) function. A two-fluid analysis gives a rapid drop in the quasiparticle damp-0 ing rate below T, . A reasonable estimate (-2750 A) for the ab-plane London penetration depth is obtained from the superfluid density. We observe that the midinfrared strength increases below T"suggesting that some (-15%%uo) of the free carriers do not condense into superconducting pairs and may have a strong interaction with pair-breaking excitations. Two absorption edges around 80 cm (3.7 k~T, ) and 400 cm (18 kz T, ) are seen but neither is assigned to the superconducting gap. Comparisons with a one-component picture described by a frequency-dependent scattering rate and effective mass are made and discussed. The far-infrared ab-plane phonons show systematic changes with temperature, which are associated with the structural transition near 250 K.

|Ate present the results of measurements on proximitywN'ect arrays with debberate positional disorder, characterized by a parameter h, . This system provides an experimental realization of an XF magnet with random Dzyaloshinskii-Moriya interactions. In measurements of resistance versus perpendicular magnetic fieid, R(fo), we Snd strong evidence for s critical field f,~l /5 beyond which phase coherence is destroyed. Our Monte Carlo simulations show evidence for reentrant behavior in the heHcity modulus F, ' but with F always 6nite at the lowest temperatures.Such a critical Seld and reentrance vrere predicted by Granato and Kosterlitz.A single Josephson junction is isomorphic to a pair of classical XYspins, and an array of junctions is therefore a realization of an XY spin system. This equivalence has prompted the use of two-dimensional junction arrays, in zero magnetic field, as model statistical mechanical systems for studying roblems such as the Kosterlitz-Thouless transition. ' A uniform array in a perpendicular magnetic field provides a realization of the uniformly frustrated XYmagnet, with tunable frustration parametrized by f, the magnetic Sux per plaquette in units of the Sux quantum. The particular case of full frustration, where f n+ T (n an integer), has received a great deal of attention as a realization of Villain's "odd model, "3 although theory and experiment' are still far from complete in this area.

We have measured a paramagnetic Meissner effect in Nb − Al2O3 − Nb Josephson junction arrays using a scanning SQUID microscope. The arrays exhibit diamagnetism for some cooling fields and paramagnetism for other cooling fields. The measured mean magnetization is always small, Φtot −Φext < 0.3Φ0 (in terms of flux per unit cell of the array, where Φ0 is the flux quantum) for the range of cooling fields investigated (−12Φ0 to 12Φ0). We demonstrate that a new model of magnetic screening, valid for multiply-connected superconductors, reproduces all of the essential features of paramagnetism that we observe and that no exotic mechanism, such as d-wave superconductivity, is needed for paramagnetism.

We report measurements on current-voltage (1-V) characteristics of externally shunted Josephson tunnel junctions where the external shunts have nonzero inductance. At low temperatures (T< 6.6 K), we observe standard I-V curves, but as temperature is increased above 6.6 K we observe an anomaly in the I-V curves. We attribute this anomaly to the nonzero inductance in the shunt loop. Numerical simulations which properly take into account the effect of inductance show that the dynamics in the anomalous region of the I-V curve are dominated by subharmonic relaxation oscillations. We use a load-line analysis model to study the transition between the relaxation oscillations and the Josephson oscillations. The analysis is in good agreement with our data. Our analysis further predicts, and our data confirms, that in order for the relaxation oscillations to occur it is necessary to have ~c=2rrI$~CI@a<(4/~)rr)2, regardless of the value of the inductance.