A fundamental parameter of sample quality in epitaxial films of high Tc oxides is the effective penetration depth λ∥ of the superconducting sheet. A contactless audio-frequency method is described, in which an epitaxial YBa2Cu3O7 film is sandwiched between sets of stacked coils, and the procedure is given for computing the complex sheet impedance and hence λ∥ from the change in mutual inductance produced by screening currents in the film. Temperature dependence of the complex impedance is presented.

We report on a new process to make films of Y1Ba2Cu3O7 using coevaporation of Y, Cu, and BaF2 on SrTiO3 substrates. The films have high transition temperatures (up to 91 K for a full resistive transition), high critical current densities (106 A/cm2 at 81 K), and a reduced sensitivity to fabrication and environmental conditions. Because of the lower reactivity of the films, we have been able to pattern them in both the pre-annealed and post-annealed states using conventional positive photoresist technology.

We have measured the distribution in current for the onset of voltage in small-area Josephson junctions for temperatures down to ~1.6 K. The lifetime of the (V) = 0 state for our highest current density junctions becomes temperature independent for k B T %H(j)i/20 t SLT much less than the WKB prediction (co 0 is the junction resonant frequency). This is consistent with the Caldeira-Leggett theory, which includes damping effects on the quantum tunneling rate. PACS numbers: 74.50.+r, 05.04,+j, 72.70.+m Many experiments have studied the ramifications of the classical Langevin equation, but few have probed the low-temperature, high-frequency regime where quantum corrections to the picture must be expected. High-current-density Josephson tunnel junctions 1 at temperatures of a few kelvins make possible experimental studies 2 in this regime. Biased in the {V) = 0 state at currents near the critical current, the junctions are sensitive to fluctuations in the terahertz range, centered about their natural frequency, a> 0 . Fluctuation effects may be observed by sweeping the junction current through the critical current and noting the distribution, P{i), of current values at which voltage sets in. 3 P(i) should depend on whether k B T»Hu 0 /2 (classical regime) or k B T «h(x) Q /2 (quantum regime). 4 " 6 We have measured the width, a, of P(i) for junctions in the quantum regime and found that quantum effects do not appear until k B T)) = 2yMk B T.Without fluctuations, the phase can be trapped in a local minimum at cp^sin^x if x/[(1 -x 2 ) l/2 -x cos~lx]. No dc voltage develops across the junctions since {d

l the potential wells vanish and the phase increases at a speed determined by the damping and the overall steepness of the potential. This corresponds to the finite voltage state of the junction. We are interested in x slightly less than 1, in the presence of fluctuations. The phase is now restrained by a low barrier (inset, Fig. 1) which, in the absence of fluctuations, would hold ( V) = 0 until i exceeds i c . Fluctuations, however, cause p...

We measure the conductance variations of submicrometer inversion-layer segments in silicon devices, systematically changing the length, width, inelastic diffusion length, gate voltage, magnetic field, and temperature. Results agree with the theory of universal conductance fluctuations, demonstrating that random quantum interference causes rms conductance changes AG^^^e^/h in each phase-coherent subunit of each segment. The random quantum interference is extremely sensitive to change of a single scatterer.PACS numbers: 72.20. My, 72.20.Dp, 73.40.Qv It is now well understood that low-temperature conductance changes due to weak localization (coherent backscattering) are an example of electron quantum interference in disordered metals.^ In recent theoretical investigations,^"^ a new type of sample-specific random quantum interference of a surprisingly universal character is asserted to affect to varying degrees all conductance measurements on disordered metals. In this Letter, we experimentally confirm key predictions of this theory of ''universal conductance fluctuations."' Random quantum interference arises from the scattering of electron waves by the particular disordered configuration of scatterers ("impurities") in each specimen. A single specimen should exhibit different conductances G corresponding to changes of magnetic field (phase) and electron energy (wavelength) sufficient to rerandomize the interference. Macroscopically similar specimens-having the same dimensions, electron density, and average density of scatterers-should show a similar range of specimento-specimen variation due to microscopic differences of configuration. In both cases, for phase-coherent specimens, the theory predicts an rms variation 8G with a universal magnitude of approximately e^/h = (25.8 kfi)"^ For specimens consisting of A^ phase-coherent subunits, it predicts that the fractional fluctuations are smaller by A^^^^ because of selfaveraging.Two manifestations of this random interference are the aperiodic magnetoresistance fluctuations observed in small metal wires^*^^ and narrow Si inversion layers,^^'^^ and the periodic A/^ Aharonov-Bohm oscillations observed in metal^^"^^ and quantum-well^^ rings. Prior comparisons between theory and the aperiodic phenomena have been based on one or two devices with very small fractional effects. In the present Letter, we systematically confirm a broad range of key predictions of the universal conductance fluctuations theory, including systems in which the fractional effect is of order unity.We have fabricated dozens of Si inversion-layer segments of various lengths and widths in the range 0.04-1.0 fxm and have measured the conductance of each at a variety of gate voltages, magnetic fields, and temperatures. Our devices are /^channel metaloxide-silicon field-effect transistors (MOSFETs) with multiple contacts and narrow channel segments. The device structure and fabrication is described in Mankiewich.^^ Each device contains a narrow channel of width W with several side branches that are used ...

The a-b-plane penetration depth X(T) in YBa2Cu3C>7 films, determined from kinetic inductance measurements, follows the temperature dependence of weak-coupling mean-field BCS theory near T c to within ~0.5 K of T c , resolution limited either by homogeneity or for the thinnest films by phase fluctuations. The Kosterlitz-Thouless transition in a thin film is the first observation in a clean type-II superconductor, noted by the universal superfluid jump and the algebraic-decay law with a square-root cusp exponent.A kinetic inductance method in which supercurrent flow is induced in the a-b basal plane of oaxis-oriented epitaxial YBa2Cu3C>7 films grown on SrTiChOOO) was used to measure the weak-field in-plane component of the penetration depth X. [ Properties of the high-r c cuprates such as the short coherence length £o, comparatively long mean free path /, large a-b vs c anisotropy, superconducting energy gap A(T), together with thermodynamic fluctuations near T c will influence the detailed temperature dependence of X(T), which is a measure of the superelectron density and of the observed dissipation, which is associated with vortex motion. For temperatures not too close to T c , the weak-coupling mean-field BCS theory with A/&r c = 1.76 and X(0)=0.15 /im provides a good fit to X(T) in films with the sharpest transitions. Broadening by film inhomogeneity correlates with a systematic increase in the magnitude of X(T). Phase fluctuations driven by the presence of quantized vortices are clearly observed in 500-A films, where the entire film fluctuates as a two-dimensional (2D) superconductor. The data presented here are the first to show the Kosterlitz-Thouless (KT) transition 2 in an intrinsic (/ > £o) type-II superconductor, as distinct from the earlier restriction to high-resistivity dirty-limit oxides. 3 The intriguing possibility of renormalization by 2D fluctuations within the CuC>2 planes is consistent with the data.Various films from 500 to 2000 A thick were prepared by coevaporation from Cu, Y, and BaF2 sources onto polished SrTiO 3 (100) and by reaction at 800 °C in 0 2 with water vapor used to catalyze oxidation. 4 The method yields high-quality c-axis perpendicular films with 10-//m grains, critical current density ^:2xl0 6 A cm ~2 at 77 K, and a-b -plane resistivity of 400 /iftcmat 295 K. We found the penetration-depth measurement itself to be a sensitive and critical test of film quality.The penetration depth X was calculated from a measurement of the complex ac sheet impedance of the film, Z~R + icoL. 1 Under conditions of low dissipation, the sheet kinetic inductance of the superconducting state dominates, and is given by L =4KC ~2d ~lX 2 , where d is the film thickness. The London equation may be inadequate very close to 7 C , because of fluctuations, the finite transition width, and the appearance of the dissipative component R. Applied perpendicular magnetic fields enhance L (and X) and contribute to R.The apparatus contained astatically wound 1-mmdiam drive and receive coils positioned with a 1.5-m...