P. P. Nguyen

Measurements and modeling of linear and nonlinear effects in striplines

We present measurements of surface impedanceZS as a function of frequency, temperature, and rf magnetic field for high-quality epitaxial YBa2Cu3O7−x thin films using a striplineresonator technique that measures theQ of the resonator vs. input power. The results have been modeled using Ginzburg-Landau theory at moderate fields, and the Bean critical state model at high fields. Good agreement has been obtained between the model and the measurements. We also discuss the current distributions in the stripline for both low power whenZS is linear and high power whenZS is nonlinear.

Nonlinear surface impedance of YBCO thin films: Measurements, modeling, and effects in devices

High-Tc thin films continue to be of interest for passive device applications at microwave frequencies, but nonlinear effects may limit the performance. To understand these effects we have measured the nonlinear surface impedanceZs in a number of YBa2Cu3O7−x thin films as a function of frequency from 1 to 18 GHz, rf surface magnetic fieldHrf to 1500 Oe, and temperature from 4 K toTc. The results at lowHrf are shown to agree quantitatively with a modified coupled-grain model and at highHrf with hysteresis-loss calculations using the Bean critical-state model applied to a thin strip. The loss mechanisms are extrinsic properties resulting from defects in the films. We also report preliminary measurements of the nonlinear impedance of Josephson junctions, and the results are related to the models of nonlinearZS. The implications of nonlinearZS for devices are discussed using the example of a five-pole bandpass filter.

Microwave power dependence of YBa2Cu3O7 thin‐film Josephson edge junctions

Using a stripline resonator technique, we measured the dependence on microwave current of the impedance of fabricated superconductor/normal‐metal/superconductor Josephson junctions in YBa2Cu3O7 thin films. The results are compared with predictions of a resistively shunted junction model that yields good agreement at low and intermediate values of the microwave current. At high currents deviations from the model predictions are observed, which we propose are due to flux penetration into the junction. The results are consistent with previously proposed explanations of the rf power dependence of the high‐Tc materials.

ab -plane optical properties of YBa 2 Cu 3 O 7 − δ Br y single crystals

Room-temperature infrared- and optical-spectroscopic measurements are reported on brominated YBa{sub 2}Cu{sub 3}O{sub 6.2} single-crystal samples. Incorporation of bromine in the lattice was observed in both Auger electron and infrared-spectroscopy experiments but the exact lattice site for bromine dopant atoms remains unknown. Five of the six in-plane infrared-active {ital E}{sub {ital u}} phonon modes were observed in YBa{sub 2}Cu{sub 3}O{sub 6.2}Br{sub {ital y}} samples and phonon frequency softening was observed for the 591-cm{sup {minus}1} phonon (Cu(2)-O(2) in-plane stretch) mode. The frequency-dependent optical conductivity {sigma}({omega}) was obtained from the Kramers-Kronig transformation of the near-normal-incidence reflectance {ital R}({omega}) measurements, and the resulting {sigma}({omega}) showed the presence of a mid-infrared band (centered at {similar to}2500 cm{sup {minus}1}) in YBa{sub 2}Cu{sub 3}O{sub 6.2}Br{sub {ital y}}.

ab -plane optical properties of YBa sub 2 Cu sub 3 O sub 7 minus. delta. Br sub y single crystals

Room-temperature infrared- and optical-spectroscopic measurements are reported on brominated YBa{sub 2}Cu{sub 3}O{sub 6.2} single-crystal samples. Incorporation of bromine in the lattice was observed in both Auger electron and infrared-spectroscopy experiments but the exact lattice site for bromine dopant atoms remains unknown. Five of the six in-plane infrared-active {ital E}{sub {ital u}} phonon modes were observed in YBa{sub 2}Cu{sub 3}O{sub 6.2}Br{sub {ital y}} samples and phonon frequency softening was observed for the 591-cm{sup {minus}1} phonon (Cu(2)-O(2) in-plane stretch) mode. The frequency-dependent optical conductivity {sigma}({omega}) was obtained from the Kramers-Kronig transformation of the near-normal-incidence reflectance {ital R}({omega}) measurements, and the resulting {sigma}({omega}) showed the presence of a mid-infrared band (centered at {similar to}2500 cm{sup {minus}1}) in YBa{sub 2}Cu{sub 3}O{sub 6.2}Br{sub {ital y}}.

ab-plane optical properties ofYBa2Cu3O7−δBrysingle crystals

Room-temperature infrared- and optical-spectroscopic measurements are reported on brominated YBa{sub 2}Cu{sub 3}O{sub 6.2} single-crystal samples. Incorporation of bromine in the lattice was observed in both Auger electron and infrared-spectroscopy experiments but the exact lattice site for bromine dopant atoms remains unknown. Five of the six in-plane infrared-active {ital E}{sub {ital u}} phonon modes were observed in YBa{sub 2}Cu{sub 3}O{sub 6.2}Br{sub {ital y}} samples and phonon frequency softening was observed for the 591-cm{sup {minus}1} phonon (Cu(2)-O(2) in-plane stretch) mode. The frequency-dependent optical conductivity {sigma}({omega}) was obtained from the Kramers-Kronig transformation of the near-normal-incidence reflectance {ital R}({omega}) measurements, and the resulting {sigma}({omega}) showed the presence of a mid-infrared band (centered at {similar to}2500 cm{sup {minus}1}) in YBa{sub 2}Cu{sub 3}O{sub 6.2}Br{sub {ital y}}.