M. Neuhaus

Measured performance parameters of gradiometers with digital output

A planar first-order gradiometer and a DC SQUID (superconducting quantum interference device) comparator have been integrated on the same 6*6 mm/sup 2/ chip in Nb-Nb/sub 2/O/sub 5/-PbInAu technology and operated in a flux-locked loop. Owing to the digital gradiometer output, a very low flux noise of 7*10/sup -7/ phi /sub 0// square root Hz, a coupled noise energy of 34 times Plancks constant h, and a gradient noise of 3.8 pT/m square root Hz have been achieved in the white noise region above about 60 Hz. In spite of the very low flux noise, a maximum slew rate of 1.5*10/sup 5/ phi /sub 0//s at a signal frequency of 4 kHz has been measured. A simple modulation scheme has been developed to cancel Josephson junction critical-current fluctuations, yielding 4.5*10/sup -6/ phi /sub 0// square root Hz at 0.1 Hz. >

Anisotropy on the a, b-plane of c-axis orientated almost twin-free YBCO films on NdGaO3

Abstract Thin YBCO films have been grown epitaxially on twin-free (001) NdGaO 3 substrates with off-axis DC magnetron sputtering at right angle from a stoichiometric target. The films are 200 nm thick and have c -axis orientation. A very small amount of a -axis orientated grains has been achieved. According to diffractometric measurements the a -axis of the major part of the YBCO film has only one common direction in the plane of the substrate. The critical temperature for zero DC resistance is T co = 88 K. The expected anisotropy of the specific resistances in the a , b -plane has been demonstrated with DC measurements. The ratio of the specific resistances in a - and b -direction is almost temperature independent between 110 K and room temperature. Ratios up to ϱ a ϱ b = 2.1 have been achieved. At 100 K the specific resistance in the direction of the b -axis is ϱ b ≈ 50 μΩ cm for a ratio ϱ a ϱ b = 2 .

Experimental SFQ interferometer shift register prototype with Josephson junctions

A shift register prototype has been implemented with lead-alloy technology. Single-flux quanta have been shifted in both directions by local magnetic fields with a maximum repetition frequency of 250 MHz. The absence or presence of a flux quantum at a given position is read out nondestructively. The speed and the miniaturisation potential of the proposed shift register is discussed.

Measured temperature dependence of the intermodulation product of coplanar waveguides with s- and d-wave superconductors

Abstract The measured intermodulation product is compared with the calculation that follows the interpretation of the BCS-theory according to Dahm and Scalapino in postulating a specific current density and temperature dependence of the London penetration depth: λ(T,j)≈λ LT (T=0) 1−(T/T c ) 4 1−b Θ (T)(j/j c0 ) 2 −0.5 where j c0 is the current density at zero temperature and b Θ ( T ) the nonlinearity factor. A good agreement between calculated and measured values b Θ ( T ) is obtained if the theoretical d-wave temperature dependence is assumed for YBCO and s-wave behavior for niobium.

A NRZ-output amplifier for RSFQ circuits

Interfaces between RSFQ circuits in the Return-to-Zero (RZ) and semiconductor circuits in the Non-Return-to-Zero (NRZ) mode are developed for data transmission at about 10 GHz clock frequencies reducing the errors owing to a finite sampling jitter of conventional room temperature equipment. A prototype of a NRZ output interface with the Nb-Al/sub 2/O/sub 3/-Nb technology has been simulated, implemented and successfully tested at 100 MHz. The interface can easily be combined with a superconducting pulse amplifier for NRZ output signals in the range of several mV. Corresponding simulations and a layout of the amplifier are presented.

Off-axis sputtered Y1Ba2Cu3O7-δ films on NdGaO3

Abstract Y 1 Ba 2 Cu 3 O 7-δ films have been deposited in situ on twin-free NdGaO 3 (100) substrates at ≈ 750°C by off-axis sputtering from a stoichiometric target. Critical temperatures of 89 K, specific resistances at 100 K of ρ n (100) = 80 μ cm and ρ n (300)/ ρ n (100) ratios of ≈ 3.1 have been achieved. The extrapolated zero temperature specific resistance, ρ n (0) = 16 μΩ cm . The full width at half maximum of the (005) rocking curve, δψ = 0.14°. Only a small amount of 45° grain boundaries could be found ( κ 45 −3 ). The 10 GHz surface resistances (not yet optimized) are below 50 μΩ at 4.2 K and 375 μΩ at 77 K.

High slew rate gradiometer prototype with digital feedback loop of variable step size

Abstract A gradiometer with two 31 mm 2 pick-up coil areas, a 12 mm baseline and a digital output signal has been built into a NbPb In Au system on a single 6 × 18 mm 2 chip, the measured white magnetic flux and field noise are below 1 μΦ 0 Hz − 1 2 and 0.4 pT m −1 Hz − 1 2 , respectively, at 1 kHz for a clock frequency of 18 MHz in a digital flux locked loop operation with a 12 bit counter of constant step size. With a digital feedback of variable step size and a 50 MHz clock frequency a maximum slew rate of 2 × 10 6 Φ 0 s −1 has been achieved with a signal frequency of 7.5 kHz.

Tunnel barrier growth dynamics of Nb/AlOxAl/Nb and Nb/A1NxAl/Nb Josephson junctions

The growth of very thin aluminum oxide and aluminum nitride tunnel barriers on top of an about 7 nm aluminum layer is deduced by measurements of the reflectivity change of a laser beam due to the decrease of the aluminum laser thickness. Within the first seconds of the process thermal aluminum oxide grows much faster than aluminum nitride in a nitrogen plasma. For both barrier types the reflectivity change can be correlated with the Josephson current density of the finished junctions. In a semi-logarithmic scale the current density versus reflectivity change can be approximated by a straight line up to 20 kA/cm2. High current densities with AINx seem to be more easily controllable than with AlOx.

Laser reflection measurements for end point detection and analysis in Nb/Al2O3Al/Nb Josephson circuit fabrication

Abstract A simple optical reflection measurement arrangement is described which allows one to monitor the reflectivity of etched layers. The application of this method for end point detection in fabricating Josephson circuits based on Nb/Al 2 O 3 -Al/Nb trilayers is presented. Moreover, the thin intermediate Al layer can be examined by reflection analysis during etching of the complete trilayer. The application of reflectivity measurements to the fabrication of vertically stacked Josephson devices is discussed.

An experimental Josephson interferometer memory cell for nondestructive read out

Abstract An interferometer memory cell with two unequal Josephson junctions for nondestructive read out of single flux quantum states has been implemented and tested successfully. The agreement between the measurements and computer simulations is very satisfactory.