Engineering topics

Stan, M. A.

Publications and source records attributed to Stan, M. A..

11 records

NASA NTRS1995

Conductor-Backed Superconductive Coplanar-Waveguide Resonators

Report describes comparative tests of conductor-backed coplanar-waveguide resonators with design resonant frequency of 10.8 GHz. Dielectric layers in resonators made of LaAIO3 and conductive ground planes on back surfaces of dielectric layers made of gold. TI/Ba/Ca/Cu/O and Y/Ba/Cu/O units performed better than all-gold-conductor unit did, at all but highest temperatures. Results suggest TI/Ba/Ca/Cu/O may be material of choice for cryogenic microwave applications.

Miranda, F. A.↗

NASA NTRS1993

Microwave properties and characterization of co-evaporated BSCCO thin films

An extensive characterization of Bi-Sr-Ca-Cu-O (BSCCO) thin films deposited by co-evaporation on LaAlO3 and SrTiO3 substrates was performed. The films had a T(sub c) (R = O) of approximately 78 K, and were predominantly c-axis oriented, with critical current densities (J(sub c)) at 4.5 K of 1.6 x 10(exp 6) and 1.1 x 10(exp 6) A cm(sup -2), for the samples on SrTiO3 and LaAlO3, respectively. The microwave properties of the films were examined by three techniques. The complex conductivity sigma(sub *) = sigma(sub 1) - j(sigma(sub 2)) and the magnetic penetration depth (A) were measured by power transmission at 30.6 GHz; the surface resistance (R(sub s)) was measured using a cavity resonator at 58.9 GHz, and the transmission line losses were determined by measuring the quality factor (Q) of a linear microstrip resonator at 10.4 and 20.2 GHz. The complex conductivity for the film on LaAlO3 was determined to be (2.0-j10) x 10(exp 5) S/m at 77 K. It was observed that in the superconducting state sigma(sub 1) deviates from both the Bardeen-Cooper-Schrieffer (BCS) theory and the two-fluid model. Values of lambda were found to be approximately 2.0 and 1.1 microns at 77 K and 20 K respectively, and were obtained for the film on LaAlO3. The value of lambda at 20 K was approximately three times larger than that of BSCCO single crystals. R(sub s) values of 865 and 1391 mOmega were obtained for the films on SrTiO3 and LaAlO3, respectively, at 77 K and 58.9 GHz. Unloaded Q factors at 20 K of approximately 1100 and 800 at 10.4 and 20.2 GHz respectively, were measured for the BSCCO resonator. Unloaded Q values of 290 and 405 at 20 K were obtained at 10.4 GHz and 20.2 GHz respectively, for an all gold (Au) resonator.

Miranda, F. A.↗

NASA NTRS1993

The effect of fluctuations on the electrical transport behavior in YBa2Cu3O(7-x)

The excess conductivity behavior of highly oriented YBa2Cu3O(7-x) thin films prepared by both coevaporation and laser ablation was studied in detail in the reduced-temperature range 9 x 10(exp -4) is less than t is less than 1. The excess conductivity in all the films studied was found to diverge sharply near T(sub c), in agreement with the conventional mean-field theory. However, the detailed temperature dependence could not be fitted to either the power-law or the logarithmic functional forms as predicted by the theory. The excess conductivity of all the films was found to be exponentially dependent on the temperature over nearly three decades for 9 x 10(exp -4) is less than t is less than 10(exp -1), in contradiction to the mean-field theory.

Vitta, Satish↗

NASA NTRS1992

Conductor-backed coplanar waveguide resonators of Y-Ba-Cu-O and Tl-Ba-Ca-Cu-O on LaAlO3

Conductor-backed coplanar waveguide (CBCPW) resonators operating at 10.8 GHz have been fabricated from Tl-Ba-Ca-O (TBCCO) and Y-Ba-Cu-O (YBCO) thin films on LaAlO3. The resonators consist of a coplanar waveguide (CPW) patterned on the superconducting film side of the LaAlO3 substrate with a gold ground plane coated on the opposite side. These resonators were tested in the temperature range from 14 to 106 K. At 77 K, the best of our TBCCO and YBCO resonators have an unloaded quality factor (Qo) 7 and 4 times, respectively, larger than that of a similar all-gold resonator. In this study, the Qo's of the TBCCO resonators were larger than those of their YBCO counterparts throughout the aforementioned temperature range.

Miranda, F. A.↗

NASA NTRS1992

Magnetic flux relaxation in YBa2Cu3O(7-x) thin film: Thermal or athermal

The magnetic flux relaxation behavior of YBa2Cu3O(7-x) thin film on LaAlO3 for H parallel c was studied in the range of 4.2-40 k and 0.2-1.0 T. Both the normalized flux relaxation rate (S) and the net flux pinning energy (U) increase continuously from 1.3 x 10 exp -2 to 3.0 x 10 exp -2 and from 70-240 meV respectively, as the temperature (T) increases from 10 to 40 K. This behavior is consistent with the thermally activated flux motion model. At low temperatures, however, S is found to decrease much more slowly as compared with kT, in contradiction to the thermal activation model. This behavior is discussed in terms of the athermal quantum tunneling of flux lines. The magnetic field dependence of U, however, is not completely understood.

Vitta, Satish↗

NASA NTRS1991

Temperature dependence of the anisotropy in magnetic relaxation in YBa2Cu3O(7-x) thin films

The relaxation of diamagnetic magnetization in the c-axis-aligned YBa2Cu3O(7-x) thin film is studied as a function of orientation and temperature in the range of 5-50 K at H = 0.2 T. The magnetization M(T,H) in the orientations H parallel to c and H perpendicular to c and at all the temperatures is found to decrease logarithmically with time t. The activation energy for the movement of flux lines U is found to be 30-110 meV in the range 5-50 K. For H parallel c, U increases continuously with T, whereas for H perpendicular c, U has two apparent maxima: at T = 10 K and T greater than 50 K. These results are discussed in terms of the thermally activated flux motion model.

Vitta, Satish↗

NASA NTRS1991

Electromigration failure in YBa2Cu3O(7-x) thin films

Electromigration failure in highly oriented YBa2Cu3O(7-x) thin films below the superconducting transition temperature is reported here for the first time. The film on SrTiO3 failed at 86 K, 230,000 A/sq cm, while that on LaAlO3 failed at 84 K, 930,000 A/sq cm. Scanning electron microscopy and energy dispersive X-ray analysis of the films after failure shows that Cu migrates preferentially away from the failure region toward the electrode.

Vitta, Satish↗

NASA NTRS1991

Magnetic flux relaxation in YBa2Cu3)(7-x) thin film: Thermal or athermal

The magnetic flux relaxation behavior of YBa2Cu3O(7-x) thin film on LaAlO3 for H is parallel to c was studied in the range 4.2 - 40 K and 0.2 - 1.0 T. Both the normalized flux relaxation rate S and the net flux pinning energy U increase continuously from 1.3 x 10(exp -2) to 3.0 x 10(exp -2) and from 70 to 240 meV respectively, as the temperature T increases from 10 to 40 K. This behavior is consistent with the thermally activated flux motion model. At low temperatures, however, S is found to decrease much more slowly as compared with kT, in contradiction to the thermal activation model. This behavior is discussed in terms of the athermal quantum tunneling of flux lines. The magnetic field dependence of U, however, is not completely understood.

Vitta, Satish↗

NASA NTRS1990

Ellipsometric study of YBa2Cu3O(7-x) laser ablated and co-evaporated films

High temperature superconducting films of YBa2Cu3O(7-x) (YBCO) were grown on SrTiO3, LaAl03, and YSZ substrates using two techniques: excimer laser ablation with in situ annealing and co-evaporation of Y, Cu, and BaF2 with ex-situ annealing. Film thicknesses were typically 5000 A, with predominant c-axis alignment perpendicular to the substrate. Critical temperatures up to Tc(R = 0) = 90 K were achieved by both techniques. Ellipsometric measurements were taken in the range 1.6 to 4.3 eV using a variable angle spectroscopic ellipsometer. The complex dielectric function of the laser ablated films was reproducible from run to run, and was found to be within 10 percent of that previously reported for (001) oriented single crystals. A dielectric overlayer was observed in these films, with an index of refraction of approximately 1.55 and nearly zero absorption. For the laser ablated films the optical properties were essentially independent of substrate material. The magnitude of the dielectric function obtained for the co-evaporated films was much lower than the value reported for single crystals, and was sample dependent.

Alterovitz, S. A.↗

NASA NTRS1990

Ellipsometric study of YBa2Cu3O(7-x) laser ablated and co-evaporated films

High temperature superconducting films of YBa2Cu3O(7-x) (YBCO were grown on SrTiO3, LaA1O3, and YSZ substrates using two techniques: excimer laser ablation with in situ annealing and co-evaporation of Y, Cu, and BaF2 with ex-situ annealing. Film thicknesses were typically 5000 A, with predominant c-axis alignment perpendicular to the substrate. Critical temperatures up to Tc(R=O)=90 K were achieved by both techniques. Ellipsometric measurements were taken in the range 1.6 to 4.3 eV using a variable angle spectroscopic ellipsometer. The complex dielectric function of the laser ablated films was reproducible from run to run, and was found to be within 10 percent of that previously reported for (001) oriented single crystals. A dielectric overlayer was observed in these films, with an index of refraction of approximately 1.55 and nearly zero absorption. For the laser ablated films the optical properties were essentially independent of substrate material. The magnitude of the dielectric function obtained for the co-evaported films was much lower than the value reported for single crystals, and was sample dependent.

Alterovitz, S. A.↗

NASA NTRS1990

Electrical transport measurements on polycrystalline superconducting Y-Ba-Cu-O films

The current-voltage, I-V, characteristics of polycrystalline Y-Ba-Cu-O films have been measured as a function of temperature. The I-V characteristics are interpreted using a model based upon an array of weak links with a statistical distribution of critical currents. In addition, evidence is found that the supercurrents flow in nearly independent filaments near Tc. Various criteria are discussed with respect to the definition of the transport critical current, Ic, in these films. A temperature dependence for Ic has also been deduced from the I-V data by appealing to an empirical scaling law. It is proposed that this temperature dependence is representative of the weaker links within the critical current distribution.

Stan, M. A.↗