Modeling the microwave properties of superconductors

Summary form only given. We show that in a three-component plasma, e.g., electrons, ions, and negatively charged dust, such that n/sub d//spl Lt/n/sub i//spl sim/n/sub e/, the light components (electrons and ions) can rotate with a frequency, /spl Omega//sub r/=Zn/sub d//spl Omega//sub i//n/sub e/, where n/sub i,e,d/ are ion, electron, and dust densities, Z is the dust charge state, and /spl Omega/ is the ion gyro-frequency. The origin of this rotation is shown to be due to n/sub i//spl ne/n/sub e/, so that there is a current perpendicular to the magnetic field due to ion and electron E/spl times/B drift, which induces an electric field in the direction of the current. Hence the plasma is subject to electric forces simultaneously in the two orthogonal directions perpendicular to the magnetic field resulting in a rotation. A new low-frequency resonance at /spl omega/=/spl Omega//sub r/ appears in the MHD limit and affects the dispersion character of the electromagnetic waves. The magnetosonic dispersion relation [/spl omega//sup 2/=/spl Omega//sub r//sup 2/+k/sup 2/(V/sub A//sup 2/+C/sub s//sup 2/), where V/sub A/ and C/sub s/ are Alfven and ion sound speeds] is modified and becomes isomorphic to the electrostatic Langmuir wave dispersion relation. We find that the interaction of the fast rotation time-scale with the slow magnetosonic time-scale can be achieved via ponderomotive force and this could lead to a nonlinear Schrodinger equation for the magnetosonic branch. We also find that it is possible to develop nonlinear structures at very large MHD scale lengths with scale-size L/spl sim/V/sub A///spl Omega//sub r0//spl sim/(c//spl omega//sub pi/)(n/sub e//Zn/sub d/)(n/sub i//n/sub e/)/sup 1/2 / where /spl omega//sub pi/ is the ion plasma frequency, which could be comparable to astrophysical dimensions. Presently, a laboratory investigation is underway to look for signatures of the rotation in a multi-ion species plasma. The experiments focus on detection of transverse components of energetic ion flow following perturbation of the plasma. The linear stability and nonlinear structures that could arise in the frequency regime /spl omega//spl sim//spl Omega//sub r/ is discussed, along with laboratory results and applications to space plasmas.

A large amplitude upper hybrid wave has potential to be employed as a wiggler for the generation of high frequency coherent radiation via free electron laser (EEL) instability. At a density fluctuation level of a few percent, due to the upper hybrid wave, the growth time of FEL instability, with electron beam current of a few kilo ampere, turns out to be of the order of a nanosecond. The growth rate of FEL instability depends sensitively on /spl omega//sub 1/ and this dependence comes through /spl nu//sub osc/ the beam oscillatory velocity. /spl nu//sub osc/ acquires large values at k/sub 0z//spl sime/[/spl omega//sub 0/-(/spl omega//sub e///spl gamma//sub 0//sup 0/)]/spl nu//sub b//spl sime//spl omega//sub c/[1-(1//spl gamma//sub 0//sup 0/)]/spl nu//sub b/. At this value of k/sub 0z/,/spl omega//spl sime/2/spl gamma//sub 0//sup 02//spl omega//sub c/[2-(1//spl gamma//sub 0//sup 0/)]. However, this scheme of FEL operation suffers from a severe limitation due to the parametric instability of the upper hybrid wave. The process of parametric decay into lower hybrid and upper hybrid waves may have growth rate comparable to that of the FEL instability.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A special TE/sub 011+/spl delta// mode sapphire resonator and an equivalent perfect conductor calibration technique were developed to measure the surface resistance Rs of a single piece of HTS thin film non-destructively at 10 GHz and 77 K. The resonator is formed by a permanent probe and a replaceable test sample. The probe has Q-factor greater than 280000-good sample load repeatability and sensitivity about 20 /spl mu//spl Omega/ for HTS samples under test with dimension equal to or greater than /spl phi/28 mm. The calibration employs a symmetric configuration which presents an equivalent Rs of zero ohm and ensures the accuracy of the measurement. The measured Rs data of YBCO thin films made by laser ablation and magnetic sputtering are less than 250 /spl mu//spl Omega/. >

Total magnetic field measurements were obtained at ocean floor depths averaging about 100 m over a period of several days. Ambient or background fluctuations are deduced from the data by three schemes: high pass filtering in the frequency domain, detrending in the time domain, and synchronous reference subtraction. For frequencies larger than 2(32) mHz, the fluctuations have a vanishing mean, and a standard deviation of 3.3(0.1) nT. Our analysis shows that the magnitude of the background fluctuations, which set the threshold in classical target detection, varies with the high pass frequency and the detrending interval. We discuss the corresponding threshold limitations. We find that the fluctuations, when scaled by their respective standard deviations, have a common probability distribution function. This distribution is neither Gaussian nor exponential but is closely approximated by a stretched exponential (i.e. /spl sim/ e/sup -|/spl eta/|/spl gamma//) with /spl gamma/ < 1 for rms deviations 0.3 < /spl eta/ < 3. The frequency power spectra of the magnetic field measurements have f/sup /spl delta// power-law scaling with /spl delta/ /spl sim/ -2 for frequency 0.06 /spl les/ f /spl les/ 0.6 mHz and /spl delta/ /spl sim/ -11/3 for 1 < f < 10 mHz.

The nonlinear microwave surface impedances Z/sub s/ = R/sub s/ + jX/sub s/ of three epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films deposited by thermal co-evaporation on MgO substrates have been studied by the coplanar resonator technique at 8 GHz in the temperature T range 12 to 60 K in both zero and finite (/spl sim/ 10 mT) DC magnetic fields. All the films exhibited an anomalous decrease in the surface resistance R/sub s/ as a function of microwave magnetic field H/sub rf/ below 20 K. In one of the films, the application of a DC field of 10 mT has also induced an anomalous decrease in the penetration depth /spl lambda/(H/sub rf/). Analysis of the results suggests that the anomalies may be originating both in the film and in the substrate, and ways to distinguish between the two contributions are proposed. The consequences of the observed phenomena for application of YBCO films in microwave devices are discussed.

Today MOCVD, due to its inherent advantages such as scaleability and throughput, is the growth technique of choice for many compound semiconductors as a production method. However, the use of MOCVD to deposit high quality HTS material has been impeded by the lack of stable precursors. To overcome the problems of limited process reproducibility caused by these shortcomings, real time control of the gas phase composition by ultraviolet absorption sensors has been utilized. This technique allows for a very tight control of the composition of the deposited material. A surface resistance R/sub s/ lower than 250 /spl mu//spl Omega/ (77 K, scaled to 10 GHz) could be measured on 700 nm thick MOCVD YBCO films. This value correlates very well with the surface resistance extracted from measurements of the quality factor Q/sub 0/ of a simple microstrip resonator used as a process control monitor. Consistent surface resistance values below 350 /spl mu//spl Omega/ prove the reproducibility of the sensor based MOCVD approach. For cellular telecommunication devices at 845 MHz quality factors Q/sub 0/ in excess of 100,000 at 77 K are indicative of the excellent material quality.

Current-activated residual surface resistance in high temperature superconductors

We study the task of transforming a hard function f, with which any small circuit disagrees on (1 - /spl delta/)/2 fraction of the input, into a harder function f', with which any small circuit disagrees on (1 - /spl delta//sup k/)/2 fraction of the input, for /spl delta/ /spl isin/ (0,1) and k /spl isin/ /spl Nopf/. We show that this process cannot be carried out in a black-box way by a circuit of depth d and size 2/sup o(k2/d)/ or by a nondeterministic circuit of size o(k/log k) (and arbitrary depth). In particular, for k = 2/sup /spl Omega/(n)/, such hardness amplification cannot be done in ATIME(O(1), 2/sup o(n)/. Therefore, hardness amplification in general requires a high complexity. Furthermore, we show that even without any restriction on the complexity of the amplification procedure, such a black-box hardness amplification must be inherently non-uniform in the following sense. Given as an oracle any algorithm which agrees with f' on (1 - /spl delta//sup k/)/2 fraction of the input, we still need an additional advice of length /spl Omega/(k log(1//spl delta/)) in order to compute f correctly on (1 - /spl delta/)/2 fraction of the input. Therefore, to guarantee the hardness, even against uniform machines, of the function f', one has to start with a function f which is hard against non-uniform circuits. Finally, we derive similar lower bounds for any black-box construction of pseudorandom generators from hard functions.

The performance of narrowband microstrip filters with low insertion loss and high power-handling capabilities made from YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) high-temperature superconducting (HTS) thin films is presented. Results are shown for two different designs that were chosen to optimize the power-handling capability. Both filters have a 2-GHz center frequency and 5 poles that incorporate coupled resonators with 10-/spl Omega/ internal impedances. They were made on 5-cm-diameter LaAlO/sub 3/ substrates. Both designs use parallel-coupled feed lines to avoid current crowding, The first design includes backward- and forward-coupled filters, has 1% bandwidth, and has handled over 25 watts of input power at 10 K with less than 0.25 dB compression. The second design has 1.2% bandwidth and uses only forward-coupled resonators. The dissipation loss is less than 0.2 dB at 45 K and it has a third-order intercept of 62 dBm. Another similar filter handled 36 watts of power at 45 K with less than 0.15 dB compression across the passband. We have developed a technique to visualize the power dissipation of the filter by observing the bubbles created by the filter when submerged in liquid helium, showing areas with local defects or where the current distribution is at its peak value. We also discuss several planar high-power filter issues, including material selection and fabrication, device configuration trade-offs, filter structure optimization, and design approaches to maximize power-handling capacity.

Summary form only given. The feasibility of cavity-controlled spontaneous emission of a motionless two level atom is analyzed quantum mechanically and nonperturbatively. It is proposed to control the emission by putting the excited atom into the combined field of two lossless modes with their frequencies /spl omega//sub 2,1/ detuned symmetrically about the atomic one /spl omega//sub /spl alpha//, i.e. /spl omega//sub 2,1/=/spl omega//sub /spl alpha///spl plusmn//spl Delta/. The controlling /spl omega//sub 2,1/ modes are isolated from the free radiation field by an one-dimensional ideal cavity (or by two crossed ones) at zero temperature. These modes are taken to be linearly polarized in the same plane. Because the wavelength and cavity dimensions are assumed to be of the same order, the retardation effects are neglected. The spontaneous emission is treated as a radiative decay of the macromolecule-like atom+/spl omega//sub 2,1/ modes-subsystem (the H/sub s/-one) under its interaction with the free H/sub k/-field.

We present results of a systematic study of the effect of film deposition temperature on both the linear and nonlinear response of superconducting YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin films and devices at microwave frequencies. Measurements of the unpatterned films show that samples grown by pulsed laser deposition at a lower substrate temperature (740/spl deg/C) display a smaller low-temperature residual surface resistance compared to films grown at a higher substrate temperature (780/spl deg/C). However, the same films which display low residual surface resistance also show increased nonlinear effects (measured by third harmonic generation) at all temperatures. Analysis of these results suggests that the increased defects present in the films grown at the lower deposition temperature are responsible for both the lower surface resistance and the higher third harmonic generation observed in these samples. We discuss the consequences of these results for the simultaneous optimization of both linear and nonlinear microwave properties of HTS thin films and devices.

Summary form only given. Relativistic plasma waves (RPW) can accelerate electrons with gradients in excess of a few GeV/m. Theoretical studies involving RPW driven by a two-wavelength laser pulse predict that such waves are most efficiently excited when the plasma frequency /spl omega//sub p/ = /spl Delta//spl omega/, where /spl Delta//spl omega/ is the difference between the two laser frequencies. Experimentally, the amplitude of the waves driven near this resonance condition largely follows the predictions. In these experiments, either a laser pulse was used to scatter off the plasma wave at resonant densities or an injected electron beam was used to sample the plasma wave (and to accelerate electrons). In this paper a collinear Thomson scattering technique using an independent probe beam is applied to detect the RPW. In this range, very low scattering efficiencies and small frequency shifts between the probe and the scattered beam make measurements very challenging. The results reveal that, while the amount of scattered light has a peak near resonance, up to 10 times more light is scattered at plasma densities well above resonance.

We study the space complexity of refuting unsatisfiable random k-CNFs in the resolution proof system. We prove that for any large enough /spl Delta/, with high probability a random k-CNF over n variables and /spl Delta/n clauses requires resolution clause space of /spl Omega/(n/spl middot//spl Delta//sup -1+/spl epsiv//k-2-/spl epsiv//), for any 0 >/spl radic/n. This bound is nearly tight. Specifically, we show that with high probability, a random 3-CNF with /spl Delta/n clauses requires tree-like refutation size of exp(/spl Omega/(n//spl Delta//sup 1+/spl epsiv//1-/spl epsiv//)), for any 0</spl epsiv/<1/2. Our space lower bound is the consequence of three main contributions. 1. We introduce a 2-player matching game on bipartite graphs G to prove that there are no perfect matchings in G. 2. We reduce lower bounds for the clause space of a formula F in resolution to lower bounds for the complexity of the game played on the bipartite graph G(F) associated with F. 3. We prove that the complexity of the game is large whenever G is an expander graph. Finally, a simple probabilistic analysis shows that for a random formula F, with high probability G(F) is an expander. We also extend our result to the case of G-PHP, a generalization of the pigeonhole principle based on bipartite graphs G. We prove that the clause space for G-PHP can be reduced to the game complexity on G.

Study of the surface resistance of superconducting niobium films at 1.5 GHz

In this article, a novel design of non-symmetric ground /spl lambda//4 open slot antenna with microstrip-line is proposed. Choosing the proper dimension of the rectangular conducting plate which connected to the open end of the 50-/spl Omega/ microstrip-line with printed on the opposite side of the FR4 substrate and open slot, one can obtain wide impedance bandwidth and miniaturized antenna area. The proposed antenna printed on FR4 and fed by 50-/spl Omega/ microstrip-line is investigated. The frequency range of the proposed antenna is about 2.2 /spl sim/ 6.1 GHz (94%), which can be applied to mobile wireless communication products on WLAN, Bluetooth and UWB.