Search NASA⌕ Search

SEARCH · Search NASA

Results for “superconducting fluctuations”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Quantum fluctuations of the superconducting cosmic string

Quantum fluctuations of the proposed superconducting string with Bose charge carriers are studied in terms of the vortices on the string world sheet. In the thermodynamical limit, it is found that they appear in the form of free vortices rather than as bound pairs. This fluctuation mode violates the topological conservation law on which superconductivity is based. However, this limit may not be reached. The critical size of the superconducting string is estimated as a function of the coupling constants involved.

Zhang, Shoucheng↗
NASA NTRS1993

Analysis of resistance data on a good ceramic sample of YBa2Cu3O(7-x)

Resistance data between 91.4 and 200 K on a ceramic sample of YBa2Cu3O(7-x) are analyzed using a two-band model for normal-state conductance plus the Lawrence-Doniach theory for effects of superconducting fluctuations. The data is fitted to a rms accuracy of 0.16 percent of the resistance in the middle of the temperature range. The steepest slope of the resistance of the sample occurs at 91.13 K, and the analysis shows that the critical region extends to less than 0.3 K above Tc. Values of parameters found indicate that defect or impurity scattering within grains is unusually small in this sample. Some difficulties in understanding Hall data using either simple two-band models or more sophisticated one-band models are discussed briefly.

Eagles, David M.↗
NASA NTRS2019

Thermal Fluctuation Noise in Mo/Au Superconducting Transition-Edge Sensor Microcalorimeters

In many superconducting transition-edge sensor (TES) microcalorimeters, the measured electrical noise exceeds theoretical estimates based on a thermal model of a single body thermally connected to a heat bath. Here, we report on noise and complex impedance measurements of a range of designs of TESs made with a Mo/Au bilayer. We have fitted the measured data using a two-body model, where the x-ray absorber and the TES are connected by an internal thermal conductance Gae. We find that the so-called excess noise measured in these devices is consistent with the noise generated from the internal thermal fluctuations between the x-ray absorber and the TES. Our fitted parameters are consistent with the origin of Gae being from the finite thermal conductance of the TES itself. These results suggest that even in these relatively low resistance Mo/Au TESs, the internal thermal conductance of the TES may add significant additional noise and could account for all the measured excess noise. Furthermore, we find that around regions of the superconducting transition with rapidly changing derivative of resistance with respect to temperature, an additional noise mechanism may dominate. These observations may lead to a greater understanding of TES devices and allow the design of TES microcalorimeters with improved performance.

Wakeham, N. A.↗
NASA NTRS2003

Metglas 2714A for Low Temperature Transformer Core and EMI Filter

We have measured the real and imaginary parts of the relative permeability of Metglas 2714A. The magnetization noise density of a toroid made of this material is also measured with a SQUID magnetometer. This noise density is found to agree very well with the fluctuation dissipation theorem, implying that superconducting transformers with predictable noise characteristic can be designed. We also find that the relative permeability is larger than 10,000 at liquid helium temperature and at frequencies from DC to 100 kHz, making it suitable to be used as EMI filter material. Its usage should be similar to that of ferrites, with the exception that it is also effective at low temperatures.

Quach, Hung↗
NASA NTRS1987

Influence of collective effects on lifetimes of condensed excited states

The possibility that collective effects may dramatically influence autoionization-limited lifetimes of condensed excited states is investigated in the context of a two-band model of an insulator in a strong magnetic field. Two different mechanisms for suppressing autoionization are discussed which may prevent the potentially catastrophic destruction of the excited state. Under appropriate circumstances, the residual low-density Auger electrons may be confined in a superconducting state and paired by excitonic fluctuations in the conduction band.

Zmuidzinas, Jonas Stasys↗
NASA NTRS1998

Frequency-Domain Analysis of Diffusion-Cooled Hot-Electron Bolometer Mixers

A new theoretical model is introduced to describe heterodyne mixer conversion efficiency and noise (from thermal fluctuation effects) in diffusion-cooled superconducting hot-electron bolometers. The model takes into account the non-uniform internal electron temperature distribution generated by Wiedemann-Franz heat conduction, and accepts for input an arbitrary (analytical or experimental) superconducting resistance-versus- temperature curve. A non-linear large-signal solution is solved iteratively to calculate the temperature distribution, and a linear frequency-domain small-signal formulation is used to calculate conversion efficiency and noise. In the small-signal solution the device is discretized into segments, and matrix algebra is used to relate the heating modulation in the segments to temperature and resistance modulations. Matrix expressions are derived that allow single-sideband mixer conversion efficiency and coupled noise power to be directly calculated. The model accounts for self-heating and electrothermal feedback from the surrounding bias circuit.

Skalare, A.↗
NASA NTRS1991

Chemical spray pyrolysis of Tl-Ba-Ca-Cu-O high-T(sub c) superconductors for high-field bitter magnets

The deposition of Tl-Ba-Ca-Cu-O thick films by spray pyrolyzing a Ba-Ca-Cu-O precursor film and diffusing thallium into the film to form the superconducting phase is examined. This approach was taken to reduce exposure to thallium and its health and safety hazards. The Tl-Ba-Ca-Cu-O system was selected because it has very attractive features which make it appealing to device and manufacturing engineering. Tl-Ba-Ca-Cu-O will accommodate a number of superconducting phases. This attribute makes it very forgiving to stoichiometric fluctuations in the bulk and film. It has excellent thermal and chemical stability, and appears to be relatively insensitive to chemical impurities. Oxygen is tightly bound into the systems, consequently there is no orthorhombic (conductor) to tetragonal (insulator) transition which would affect a component's lifetime. More significantly, the thallium based superconductors appear to have harder magnetic properties than the other high-Tc oxide ceramics. Estimates using magnetoresistance measurements indicate that at 77 K Tl2Ba2CaCu2O10 will have an upper critical field, H(sub c2) fo 26 Tesla for applied fields parallel to the c-axis and approximately 1000 Tesla for fields oriented in the a-b plane. Results to date have shown that superconducting films can be reproducibly deposited on 100 oriented MgO substrates. One film had a zero resistance temperature of 111.5 K. Furthermore, x ray diffraction analysis of the films showed preferential c-axis orientation parallel to the plane of the substrate. These results have now made it possible to consider the manufacture of a superconducting tape wire which can be configured into a topology useful for high-field magnet designs. The research which leads to the preparation of these films and plans for further development are reviewed.

Derochemont, L. Pierre↗
NASA NTRS1992

Temperature fluctuations in the canonical ensemble

We report the first quantitative measurements of spontaneous temperature fluctuations in a physical system well modeled by a canonical ensemble. Using superconducting magnetometers and a carefully controlled thermal environment, we have measured the noise spectra of paramagnetic salt thermometers that were coupled to thermal reservoirs at 2 K. The noise spectra were found to be in very good agreement with the predictions of the fluctuation-dissipation theorem. Our observations are at variance with some interpretations of fluctuations in the canonical ensemble.

Chui, T. C. P.↗
NASA NTRS1971

Fluctuation induced conductivity of superconductors above the transition temperature: Regularization of the Maki diagram

The Maki contribution to the conductivity above the superconducting transition temperature is regularized within the framework of the BCS theory. This is achieved through the renormalization of the impurity scattering vertex by inclusion of the effects of pair fluctuations. The conductivity is evaluated for a thin film. It depends only on the reduced temperature and the normal resistance per square. Fair agreement is found with Al films over a wide temperature range. Agreement is not found with experiments on Bi, Pb and Ga films, which apparently contain a strong additional pair-breaking effect. The temperature range in which interactions among fluctuations become important in the Maki conductivity is generally larger than that given by the Ginzburg criterion.

Keller, J.↗
NASA NTRS1972

Fluctuation-induced conductivity of superconductors above the transition temperature - Regularization of the Maki diagram.

The Maki contribution to the conductivity above the superconducting transition temperature is regularized within the framework of the BCS theory. This is achieved through the renormalization of the impurity-scattering vertex by inclusion of the effects of pair fluctuations. The conductivity is evaluated for a thin film. It depends only on the reduced temperature and the normal resistance per square. Fair agreement is found with Al films over a wide temperature range. Agreement is not found with experiments on Bi, Pb, and Ga films, which apparently contain a strong additional pair-breaking effect. The temperature range in which interactions among fluctuations become important in the Maki conductivity is generally larger than that given by the Ginzburg criterion.

Keller, J.↗
NASA NTRS1989

Ultra-Stable Superconducting-Maser Oscillator

Unprecedented stability of frequency in superconducting, triple-cavity ruby maser oscillator achieved by incorporation of amplitude-control subsystem. New design enhances ultra-stable measurements of time by reducing fluctuations to 2 X 10 to negative 19th power routinely, and to as little as 10 to negative 20th power in exceptional cases. Currents induced in superconducting pickup coil by changes in magnetic field in ruby. Currents from coil fed to superconducting quantum-interference device (SQUID) magnetometer, output used to generate control signal for electronically variable attenuator. Attenuator varies pump-signal amplitude in response to magnetic-field fluctuations in ruby. Very high feedback-loop gain used for sensitivity of control and adequate compensation of fluctuations.

Strayer, Donald M.↗
NASA NTRS1981

Ordered ground states of metallic hydrogen and deuterium

The physical attributes of some of the more physically distinct ordered states of metallic hydrogen and metallic deuterium at T = 0 and nearby are discussed. The likelihood of superconductivity in both is considered with respect to the usual coupling via the density fluctuations of the ions.

Ashcroft, N. W.↗
NASA NTRS1999

Precise Frequency Measurements Using a Superconducting Cavity Stabilized Oscillator

Many physics experiments call on improved resolution to better define the experimental results, thus improving tests of theories. Modern microwave technology combined with high-Q resonators can achieve frequency readout and control with resolutions up to a part in 10(exp 18). When the physical quantity in question in the experiment can be converted to a frequency or a change in frequency, a high-stability microwave oscillator can be applied to obtain state-of-the-art precision. In this work we describe the overall physical concepts and the required experimental procedures for optimizing a high-resolution frequency measurement system that employs a high-Q superconducting microwave cavity and a low-noise frequency synthesizer. The basic approach is to resolve the resonant frequencies of a high-Q (Q > 10(exp 10)) cavity to extremely high precision (one part in 10(exp 17)- 10(exp 18)). Techniques for locking the synthesizer frequency to a resonant frequency of the superconducting cavity to form an ultra-stable oscillator are described. We have recently set up an ultra-high-vacuum high-temperature annealing system to process superconducting niobium cavities, and have been able to consistently achieve Q > 10(exp 9). We have integrated high-Q superconducting cavities with a low-noise microwave synthesizer in a phase-locked-loop to verify the frequency stability of the system. Effects that disturb the cavity resonant frequency (such as the temperature fluctuations and mechanical vibrations) and methods to mitigate those effects are also considered. Applicability of these techniques to experiments will be discussed, and our latest experimental progress in achieving high-resolution frequency measurements using the superconducting-cavity-stabilized-oscillator will be presented.

Strayer, D. M.↗
NASA NTRS1986

Continuous measurement of nontidal variations of gravity

Records from seven superconducting gravimeters operated at five different locations in California and one in Boulder, CO, are examined after removal of tides and the gravitational attraction of the atmosphere. Fluctuations over periods between a few days and several months were observed at all sites with peak amplitudes of order 10 microgal. By contrast, a 640-day record obtained with one of the instruments in Germany showed peak fluctuations of only 2 microgal. In most of the records the causes of these aperiodic variations were not determined so that they serve to set limits on the vertical motion or displacement of mass at the respective locations. However, at The Geysers geothermal field, much of the gravity variation is correlated with seismic activity, reinjection rate, and rainfall. Measurements of this type were not possible prior to the development of the superconducting device. Consequently, these results provide the first evidence for the existence of gravity variations on the time scale and of the magnitude described here. Vertical crustal motion, motion related to seismic events, and hydrological phenomena can lead to variations on this scale. Unambiguous identification of causal relationships will require either special circumstances such as found at The Geysers or operation of the instruments in pairs.

Goodkind, John M.↗
NASA NTRS2010

An Investigation of the Longitudinal Proximity Effect in Superconducting and Normal Metal TES

As the TES volume and (effective) Tc become very small - for volume < 10 micrometers x 10 micrometers x 0.5 micrometers and Tc < 90 mK - we approach a regime in which the noise equivalent power is dominated by fluctuations in power dissipating from the TES electrons to its phonons. Our ultimate goal is to build a TES bolometer that operates in this regime to be used for far-infrared and sub-mm astronomy. In this study, we characterize the R vs T behavior of small TES in order to engineer a TES bolometer that has a very low Tc. Sadleir et al found that as the distance L between two superconducting leads, with the lead Tc >> the TES Tc, connected at opposite ends of TES approaches zero, superconductivity is induced parallel to the current flow, or longitudinally, and results in a much higher effective TES Te. Here we present effective Te measurements of Mo/Au TES bounded by Nb leads as a function of L which ranges between 4 and 36 micrometer. We observe that the effective Te is suppressed for current density of order 10(exp -6) A/sq micrometers. We also explore the possibility of using a normal metal TES.

Brown, Ari-David↗
NASA NTRSDate not supplied

Design of A Transition-Edge-Sensor-Based Thermal Stabilization Stage

Electrothermal feedback in a transition edge sensor suppresses thermal fluctuations in the TES and regions in close thermal contact. We designed a heavily-metallized, leg-isolated membrane with a TES which serves as a thermal stabilization stage that can be lithographically integrated with other devices. Through modeling, fabrication and test of prototype devices, we examine whether the noise reduction in this stage can reduce thermal fluctuations (e.g. 1/f noise) in the other components by providing a more stable base temperature. In one implementation, a leg-isolated TES bolometer of higher superconducting transition temperature (Tc) is encircled by a second membrane region which is metallized and thermally stabilized by a second, lower-Tc TES. We present fabrication results on the dual-Tc design and testing of the simultaneous biasing of a two-stage, two-TES device. We explore varied heat capacity of the thermally stabilized stage in the single pixel design. We also present other designs to evaluate the use of an outer thermal stabilization stage for multiple pixel arrangements.

Rachel Borrelli↗
NASA NTRS1978

High precision tide spectroscopy

Diurnal and long period earth tides were measured to high accuracy and precision with the superconducting gravimeter. The results provide new evidence on the geophysical questions which have been attacked through earth tide measurements in the past. In addition, they raise new questions of potential interest. Slow fluctuations in gravity of order 10 micron gal over periods of 3 to 5 months were observed and are discussed.

Goodkind, J. M.↗