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When viewed at frequencies below about 8 THz (250 cm-1; 30 meV) the ab-plane optical conductivity of the cuprate superconductors (in their normal state) is well described by a Drude model. Examples include optimally-doped YBa2Cu3O7-δ and Bi2Sr2CaCu2O8; even the underdoped phases have a Drude character to their optical conductivity. A residual Drude-like normal fluid is seen in the superconducting state in most cases; the scattering rate of this quasiparticle contribution collapses at Tc.

We report the far-infrared properties of superconducting ab plane-oriented YBa 2Cu3O7-d films in magnetic fields of up to 27 T. The applications of magnetic field (withH perpendicular to the ab plane and with unpolarized light) at low temperature produces no discernible field dependence. This observation differs from other previous far-infrared measurements in this temperature range. Only at fields and temperatures where the dc resistance is not zero ‐ on account of dissipative flux motion ‐ is a field-induced effect observed.q 1998 Elsevier Science Ltd. All rights reserved.

Superconducting hot-electron bolometers (HEB) represent a promising candidate for heterodyne mixing at frequencies exceeding 1 THz. Nb HEB mixers offer performance competitive with tunnel junctions without the frequency limit imposed by the superconducting energy gap. Although the performance of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// HEB mixers is not projected to be superior to that of Nb devices, which operate at low temperatures, they introduce the possibility of sensitive, low power heterodyne detectors operating at temperatures approaching 90 K for applications requiring portability and closed-cycle refrigeration. We report on the fabrication and characterization, both DC and RF, of high-T/sub c/ mixers based on ultra-thin (/spl les/20 nm) YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films patterned to micrometer dimensions and incorporated into 2.5 THz planar mixer circuits.

The discovery of superconductors with high critical temperatures has led to a considerable effort to fabricate Josephson junctions operating at temperatures approaching, or even exceeding, 77 K for both scientific investigations and potential applications. Superconductor - normal - superconductor (SNS) devices, with noble or oxide metals as normal interlayers, are perhaps the most widely explored high- junction type at present. Although demonstrations of individual high- SNS devices exhibiting excellent current - voltage characteristics, high critical current - resistance products, and low noise behaviour have been made, reproducible devices suitable for electronic applications are elusive. It is therefore important to ask how well these nominally SNS high- junctions are understood. We review the available data, with emphasis on junction critical currents, and conclude that there is little evidence supporting a conventional proximity effect interpretation in the majority of reported high- devices. The strongest candidates for SNS behaviour are junctions in which N is a superconductor above its transition temperature. We discuss the present experimental and theoretical understanding of SNS junctions with emphasis on the implications for future research and development of these devices.

We have performed measurements on one-dimensional (1D) arrays of underdamped Josephson junctions connected in parallel by superconducting wires which in a magnetic field show resonances in the current-voltage characteristics. From analytical calculations, we find that the position of the steps is determined by the dispersion relation \ensuremath{\omega}(k) of a 1D discrete, linear transmission line, where the Josephson junction has been replaced by the capacitor and the resistance of the junction. When nearest-neighbor inductances are accounted for, the model agrees well with the measured positions of the steps. Numerical calculations of the full dynamics with inclusion of inductances between all cells support our model. As an example of a system with a more complicated dispersion relation, we have measured 1D arrays with two different junctions per primitive cell. The measured dispersion relation shows an acoustic and an optical branch in agreement with numerical simulations of the full dynamics.

We have fabricated an overdamped superconducting discrete vortex flow transistor (DVFT) which consists of an array of Josephson junctions coupled together in parallel. Measurements on this DVFT indicate a current gain of 1.2 when the control current is injected parallel to the array of junctions. We have developed a model that successfully predicts the performance of our low-T/sub c/ DVFT, including its current gain. When the current is injected parallel to the array, the gain is linear in the number of junctions and can be made much greater than 1. Except for the output voltage, the results of the DVFT are comparable to those of the present high-T/sub c/ devices. >

The dynamics of discrete-one-dimensional parallel arrays of Josephson junctions has been studied experimentally and numerically. The energy barriers for vortex motion from cell to cell have been calculated as a function of the discreteness parameter, the ratio of the Josephson inductance to the self-inductance of the cell. The experimental data show resonances in the current-voltage characteristic, analogous to Fiske steps in a single long Josephson junction. These steps provide a novel experimental method for measuring the self-inductance of 1D parallel arrays. >

Several scaling relations are inferred from magnetization data taken in fields up to 20 T on single crystal YBa 2 Cu 3-x Ni x O 7 where 0≤x≤0.03. A Bean-like critical state exists in the samples at high fields (≤20 T) and low temperatures (4.2 K) regardless of x. Strong systematics are also displayed in the temperature dependent data: it is found that beyond an applied field breakpoint, which itself scales with temperature as (1-T/T c ) 3/2 , all the scaled hysteresis loops decrease in a universal manner

The voltage generated by moving ballistic vortices with a mass ${\mathit{m}}_{\ensuremath{\nu}}$ in a two-dimensional superconducting ring is quantized, and this quantization depends on the amount of charge enclosed by the ring. The quantization of the voltage is the dual to flux quantization in a superconductor, and is a manifestation of the Aharonov-Casher effect. The quantization is obtained by applying the Bohr-Sommerfeld criterion to the canonical momentum of the ballistic vortices. The results of this quantization condition can also be used to understand the persistent voltage predicted by van Wees for an array of Josephson junctions.

Transport and magnetization measurements are presented for YBa/sub 2/Cu/sub 3-x/Ni/sub x/O/sub 7/ single crystals, where x ranges from 0.0 to 0.03. The presence of nickel systematically sharpens the resistive transition of the material in a high magnetic field, but it does not correlate with the critical current as determined from magnetization measurements. The breadth of the transition for both pure and doped samples, although different, scales according to the model proposed by M. Tinkham Phys. Rev. Lett., (vol.61, p.1658, 1988).

We demonstrate that recent data on the length and temperature dependence of the critical current in Josephson junctions with YBa2Cu3O7−x electrodes coupled by Ca‐ and Co‐substituted YBa2Cu3O7−x interlayers are consistent with the conventional theory of the proximity effect.

We describe the fabrication and properties of Nb‐AlOx‐Nb Josephson tunnel junctions having critical current densities as high as 400 kA/cm2, roughly an order of magnitude larger than any previously reported for this materials system. The quality of the junction characteristics, as indicated by the current level at subgap voltages, is considerably lower in high‐Jc than in low‐Jc junctions. However, over the entire high‐Jc range of 20–400 kA/cm2, the quality of our junctions remains the same (i.e., subgap current scales with critical current).

A Sensor Web is a system of intra-communicating spatially distributed sensor pods that can be deployed to monitor and explore new environments. By its very nature, the Sensor Web provides spatio-temporal data in a form consistent with that needed for environment modeling. Sensor Web-style instruments represent a new paradigm fey space and planetary exploration and will significantly impact the way that NASA designs missions and spacecraft. For example, a wireless web of scattered sensor pods on the Martian surface is an ideal way to pursue gaseous biosignature searches. Webs can be deployed in environments as diverse as the aqueous environment expected under Europa’s ice crust or in the rocky terrain around IO’S volcanoes. In addition, the opportunities on Earth for climate and agricultural monitoring (particularly when coupled with remote measurements) represent ways that this technology can be spun off into the commercial sector. These ideas are currently being developed and tested with a modest Sensor Web deployed in Baja California to examine

We investigate both experimentally and numerically the dynamics of discrete one-dimensional parallel arrays of underdamped Josephson junctions. In a magnetic field, measurements show steps in the current-voltage characteristics which are the discrete analogs of Fiske steps in a long Josephson junction. From the position of the steps, one can construct a plot of the dispersion relation Ω(k) . We observe a sine-dependence in the dispersion relation due to the discrete nature of our arrays. We also observe an additional, smaller gap at a k-value determined by the periodicity of the vortex lattice. Our measurements are supported by numerical simulations of the full dynamics. The Fiske steps provide an experimental method to measure the self-inductance of 1D parallel arrays.

The upper critical fields ${B}_{c2}(T)$ were measured up to 28 T for ${R}_{1}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$, where $R=\mathrm{N}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{u},\phantom{\rule{0ex}{0ex}}\mathrm{G}\mathrm{d},\phantom{\rule{0ex}{0ex}}\mathrm{D}\mathrm{y},\phantom{\rule{0ex}{0ex}}\mathrm{H}\mathrm{o},\phantom{\rule{0ex}{0ex}}\mathrm{E}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{a}\mathrm{n}\mathrm{d}\phantom{\rule{0ex}{0ex}}\mathrm{T}\mathrm{m}$, and for $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$. The midpoints ${(\frac{d{B}_{c2}}{\mathrm{dT}})}_{T={T}_{c}}$ cluster about (2.8\ifmmode\pm\else\textpm\fi{}0.2) T/K, the ${T}_{c}'\mathrm{s}=(93\ifmmode\pm\else\textpm\fi{}2)$ K show a slight systematic decrease from Nd to Tm, and the extrapolated values of ${B}_{c2}(0)$ cluster about (160\ifmmode\pm\else\textpm\fi{}20) T. The wide range of normal-state resistivities indicates that they are not intrinsic to the superconductor. The anisotropy $A=\frac{{B}_{c2\ensuremath{\parallel}}}{{B}_{c2}\ensuremath{\perp}}$ is estimated to be less than 12 based on the data for the resistive transition. Criteria for the midpoint resistive transition are also reexamined.

The upper critical fields B/sub c//sub 2/(T) of La/sub 2-//sub x/Sr/sub x/CuO/sub 4/(x = 0.15 and x = 0.20) were measured near T/sub c/ in dc fields to 23 T and at 4.2 K with pulsed fields to 45 T. Resistive midpoint transitions for the x = 0.15 sample yield (dB/sub c//sub 2//dT)/sub T //sub = //sub =/ = 2.2 T/K, T/sub c/ = 38.1 K, and B/sub c//sub 2/(0) = 58 T assuming a dirty type-II superconductor with no Pauli paramagnetic limiting. Data for the onset yield 4.5 +- 0.5 K/sup -1/, 40.1 K, and 125 +- 15 T, respectively. Pulsed-field data show that fractions of both materials are superconducting above 45 T at 4.2 K. Calculations of limits of B/sub c//sub 2/(T) are presented and experimental limitations of measurements in these high-field superconductors are indicated.

We have measured the upper critical field ${B}_{c2}$(T) of ${\mathrm{Y}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathrm{y}}$ with x=0.4 near ${T}_{c}$ in dc fields up to 20 T and at 77 K with pulsed fields to 45 T. The data yield (${\mathrm{dB}}_{c2}$/dT${)}_{T={T}_{c}}$=1.3 T/K and ${T}_{c}$=89.5 K for the midpoint of the resistive transitions, and (${\mathrm{dB}}_{c2}$/dT${)}_{T={T}_{c}}$=5\ifmmode\pm\else\textpm\fi{}1 T/K and ${T}_{c}$=95.1 K for the onset. The estimates for ${B}_{c2}$(0) range from 80 to 320 T. Pulsed-field data suggest that a fraction of the material is still superconducting above 45 T at 77 K.