Y.M. Zhang

Linewidth measurements of Josephson flux‐flow oscillators in the band 280–330 GHz

An integrated receiver consisting of two Josephson flux‐flow oscillators and a superconductor‐insulator‐superconductor (SIS) mixer has been used to measure the linewidth of the oscillators. The oscillators are coupled to the SIS mixer using two centerfed interdigital capacitors, one of which with a fairly weak (−20 dB) coupling. By biasing the two oscillators (a few microvolt apart) on their velocity matching steps, the two resulting oscillations (280–330 GHz) were mixed in the SIS mixer. The difference frequency was amplified and detected on a spectrum analyzer. The combined linewidth of the two oscillators was 2.1 MHz throughout the band. The calculated available power incident on the SIS mixer was 430 nW at 320 GHz. The rf power from the oscillators, which could be tuned by changing the current bias, is high enough to pump the SIS mixer optimally.

Flux‐flow transistors based on long YBa2Cu3O7−δ bicrystal grain boundary junctions

Josephson flux‐flow transistors were made from long YBa2Cu3O7−δ bicrystal grain boundary junctions. A normal metal loop, separated from the junction by an SiO layer, was used as the current control line. The line modulated the critical current, Ic , which for temperatures ≤60 K showed the typical dependence for a long junction, while at higher temperatures the regular small junction modulation was seen. Current gain larger than unity was obtained below 70 K. The current gain increased with the junction length, and varied roughly like √Ic when decreasing the temperature. A transresistance larger than 5 Ω was obtained at 15 K. Better performance is expected from junctions with higher current densities.

Improved step edges on LaAlO3 substrates by using amorphous carbon etch masks

We report a technique for the fabrication of sharp and straight step edges on LaAlO3 (LAO) substrates by ion milling. An electron beam lithography defined amorphous carbon film was used as an etch mask. It had very low ion milling rate and was easily prepared and removed. Atomic force microscopy was used to determine the step profile. YBa2Cu3O7 step edge junctions fabricated at the LAO steps show promising results. An IcRn product of 1 mV was obtained at 30 K. A Fraunhofer‐like magnetic field dependence of Ic was obtained up to ±2 Φ0. One weak link or possibly identical weak links in series for these step edge junctions were observed from the current‐voltage (I‐V) curves as well as from the magnetic field dependence of the I‐V curves.

Intrinsic Josephson tunnel junctions fabricated on the surfaces of Bi2212 single crystals by photolithography

Abstract Stacked series arrays of intrinsic Josephson tunnel junctions were fabricated on the surfaces of single crystal Bi2Sr2CaCu2O8+x by photolithography. Both uniform Josephson coupling and a well developed gap structure are seen in the current-voltage (I–V) characteristics at temperatures up to 90 K. For sufficiently high critical current density (jc>200 A/cm2) the junctions exhibit typical non-equilibrium effects due to self-injection of quasiparticles. This is manifested in a heavy back-bending of the I–V characteristics in the sum-gap region. The phonon-trapped effective quasiparticle recombination time τeff was estimated to be about 2×10-6 s at 20 K.

Detection of mm and submm wave radiation from soliton and flux-flow modes in a long Josephson junction

The authors report on the coupling of millimeter and submillimeter waves from a long (3- mu m*350- mu m) Josephson junction to a small (2- mu m*2- mu m) superconductor-insulator-superconductor (SIS) junction by using a center-fed interdigital capacitor. Coupled power at around 100 GHz and 261 GHz was observed. Experiments confirm that the critical current density j/sub c/ of a long junction determines the fluxon motion. Coupled irradiation (>3nW) at around 100 GHz, which corresponds to resonant soliton oscillations in low j/sub c/ samples (250 A/cm/sup 2/ to 1000 A/cm/sup 2/), is detected by the SIS junction. The incident power to the SIS detector junction could be tuned with only little frequency pulling by adjusting the current bias of the oscillator. For j/sub c/>1000 A/cm/sup 2/, coupled radiation at around 100 GHz (about 3 nW) and 261 GHz (4.6 nW), from flux-flow oscillations, is also clearly seen. Flux-flow radiation at 350 GHz is also observed. It is concluded that a large j/sub c/ value is preferable for high-frequency flux-flow oscillators.<<ETX>>

Parallel plate resonators in YBa/sub 2/Cu/sub 3/O/sub 7/ bicrystal grain boundaries

Intrinsic properties, such as the London penetration depth /spl lambda//sub L/, and the surface resistance R/sub s/, can be estimated in high-T/sub c/ films by studying self-induced electromagnetic resonances (Fiske modes) in bicrystal grain boundaries. Grain boundaries, 4-30 /spl mu/m long and 0.2 /spl mu/m thick in YBa/sub 2/Cu/sub 3/O/sub 7/ made on yttria-stabilized zirconia 0-32/spl deg/ [001]-tilt bicrystals were investigated. Typical values obtained for /spl lambda//sub L/ and R/sub s/ (at approximately 200-400 GHz) are 110-170 nm, and 20-710 m /spl Omega/, respectively.<<ETX>>

Self-induced resonances in YBCO bicrystal grain boundary Josephson junctions

Abstract We have investigated YBCO grain boundary junctions formed on YSZ bicrystal (0–32°) substrates. We observed strong magnetic field dependent peaks in the Josephson current for certain voltage biases. We relate the structures to self-induced Fiske steps, where the junction electrodes along the grain boundary form a resonator of length L, and the ac Josephson current sets up a standing wave for frequencies f n = n· c /2L . For L = 20 μm, resonances occurred at f1 = 280 GHz, f2 = 490 GHz, f3 = 640 GHz. A good agreement between the calculated Fiske-modes and the experimental dependence was obtained.

Voltage dependence of the quality factor of a long Josephson junction

Abstract Velocity-matching (VM) current steps of a long Nb/NbOx/PbBi Josephson junction could be tuned continuously over a wide voltage range, from about 0.34mV to 1.37mV, by an applied magnetic field. Knowing that the steps steepness is proportional to the inverse of the junction quality factor Q J , we determined the voltage dependence of Q J in this voltage range. For voltages below 0.4mV, Q J is higher than 15 because of the resonant mode. For voltages above 0.4mV, Q J is between 4 to 8 for the flux flow mode. We also found the dynamic resistance of the VM step increases with bias voltage in this voltage range.

Resonant steps due to Josephson flux-flow in long YBa2Cu3O7 bicrystal junctions

Abstract Resonant steps due to viscous Josephson flux-flow were observed in the current-voltage characteristics of long YBa2Cu3O7−δ bicrystal junctions. With an applied dc magnetic field of 0.20 to 0.55 Gauss, the step voltage could be linearly tuned from about 0.4 to 2 mV for a 30 μm long, 0–32° [001]-tilt junction at 16K. From the linear dependence of the step voltage with the applied field, we extract 1) the ratio between the barrier thickness and the relative dielectric constant t/e r =2.9 A and 2) the quasiparticle loss α=1.3, at 16K.