Applied Superconductivity最新文献

Microwave properties of YBa₂Cu₃O_7-δ (YBCO) films grown on (100) LaAlO₃ (LAO), (110) NdGaO₃ (NGO) and (001) SrLaAlO₄ (SLAO) substrates were studied in the form of a microstrip ring resonator at temperatures above 20 K. The YBCO resonator on a SLAO substrate showed microwave properties better than or comparable to other YBCO resonators on LAO substrates. For the YBCO resonators on LAO and SLAO substrates, both Q_U and f₀ appeared to decrease as the temperature was raised. Meanwhile the resonator on a NGO substrate showed different behaviors with Q_U showing a peak at ∼70 K, which are attributed to the unique temperature dependence of the loss tangent of the NGO substrate. An X-band oscillator with a YBCO ring resonator coupled to the circuit was prepared and its properties were investigated at low temperatures. The frequency of the oscillator signal appeared to change from 7.925 GHz at 30 K to 7.878 GHz at 77 K, which was mostly attributed to the change in f₀ of the YBCO ring resonator. The signal power appeared to be more than 4.5 mW at 30 K and 2.1 mW at 77 K, respectively. At 55 K, the frequency of the oscillator signal was 7.917 GHz with the 3 dB-linewidth of 450 Hz.

A process has been developed to fabricate NbN tunnel junctions and 1.5 THz SIS mixers with Al electrodes and Al/SiO₂/Al microstrip tuning circuits on thin Si membranes patterned on silicon on insulator wafers (SIMOX). High Josephson current density (J_c up to 2×10⁴ A/cm²) NbN/AlN/NbN and NbN/MgO/NbN SIS junctions have been fabricated with a reasonably good V_m quality factor and energy gap values close to 5 meV at 4.2 K on (100) oriented 3 inches SIMOX wafers covered by a thin (∼8 nm) MgO buffer layer. The sputtering conditions critically influence the dielectric quality of both AlN and MgO tunnel barriers as well as the surface losses of NbN electrodes. 0.6-μm Si/SiO₂ membranes are obtained after processing of a whole wafer and etching the individual chips in EDP. Such a technology is applied to the development of a waveguide/membrane SIS mixer for use around 1.5 THz.

We have developed an on-chip signal-pattern generator (SPG) for high-speed testing of latching-type Josephson logic circuits. The basis of the SPG is using a feedback shift register, in which the complement output of the last-stage LATCH gate (a D flip-flop) is fed back to the first-stage LATCH gate. Since the SPG consists of only LATCH gates and requires no external input signal, the design and high-speed operation are greatly simplified. We performed a high-speed measurement of the 1-bit SPG and found that the SPG has the potential to operate at a speed of more than 4.6 GHz. We also demonstrated a high-speed testing of a 2-bit logic circuit with the 2-bit SPG up to a clock frequency of 1 GHz.

For many SQUID applications robust, compact magnetometers are required with low flux noise and high effective area at 77 K. Low flux noise is achieved by using a low inductance SQUID. A low inductance SQUID will also have a high transfer function, allowing simplified direct readout schemes to be used. For maximum field sensitivity the SQUID needs to be coupled to a pickup loop of large area and therefore large inductance. We have modelled, designed and fabricated a novel HTS magnetometer consisting of a low inductance (<16 pH) dc SQUID coupled to a 28 nH pickup loop flux transformer fabricated on the same 1 cm² substrate. Efficient coupling was achieved by using a flip-chipped intermediate flux transformer. This configuration produces a magnetometer with a high sensitivity per unit area. The magnetometers incorporate PrBa₂Cu₃O₇ isolation layers and two 2 μm diameter CAM variant junctions. Thick YBa₂Cu₃O₇ layers were used to improve coupling and decrease the demagnetisation factor wherever possible. Our device had a transfer function of 913 μV/Φ₀ which allows direct readout without any matching or additional positive feedback arrangements. The measured effective area at 77 K was 0.58 mm² (3.6 nT/Φ₀). The magnetometer white noise was 18 fT/$\text{Hz}$ and at 1 Hz was 380 fT/$\text{Hz}$. This was reduced at 1 Hz to 120 fT/$\text{Hz}$ using bias reversal. The measured white noise was higher than the design value and is largely due to Johnson noise from the PrBa₂Cu₃O₇ isolation layer used in our devices.

We have constructed two pulsed NMR spectrometers in which the signal is coupled to the input coil of a low T_c DC SQUID using a superconducting flux transformer, yielding broadband response, with bandwidth determined by the SQUID electronics. A 50 kHz bandwidth commercial system has been used to observe free induction decay signals from platinum powder, bulk platinum, ³He gas and surface monolayers of ³He in the temperature range from 1.4 to 4.2 K and at frequencies from 5 to 40 kHz. The observed signal-to-noise ratio is as calculated with the noise dominated by flux noise in the SQUID in all samples but the bulk metal. A second system, which operates in flux-locked loop mode with bandwidth of 3.4 MHz using a SQUID with additional positive feedback, has been used to observe NMR signals from platinum powder at frequencies from 38 to 513 kHz and at a temperature of 4.2 K. The advantage of this technique in the study of systems with short T₂ at frequencies below 1 MHz is discussed. In addition we discuss the benefits of both broadband and tuned input circuits for NMR detection and we describe the performance of a spectrometer with a tuned input circuit which has been used to obtain signals at 1 MHz from platinum powder at 4.2 K and from ∼2 layers of ³He absorbed on a surface area of 0.11 m² at 1.7 K. The amplifier noise temperature is predicted to be 60 mK in the ³He experiment. This demonstrates the potential of the tuned set-up for measurements at low millikelvin temperatures on systems with low spin density and with T₂ greater than several hundred microseconds.

We systematically investigated the characteristics of Nb/Al/AlO_x/Al/AlO_x/Nb junctions having a structure in which Al/AlO_x films are placed in a Nb/Al/AlO_x/Nb trilayer junction. The junction characteristics reflect superconductivity in the second Al film due to the proximity effect. As a result, the critical current density J_c and the hysteresis on its current–voltage (I–V) curve strongly depend on the transparencies of the AlO_x barriers. The hystereses of the junctions are smaller than those of Nb/Al/AlO_x/Nb junctions at the same J_c. Intrinsically overdamped junctions are also realized. The intrinsically overdamped junctions can considerably reduce an area occupied by a single gate. Furthermore, the spread of critical current I_c for the junctions, which have hystereses, connected in series, is ±1.2%. This shows that the junctions with additional Al/AlO_x have uniform enough characteristics to be used in integrated circuits.

The response of the circulating screening current to applied magnetic flux in a variety of DC SQUIDs has been studied in a regime in which thermal noise induces rapid switching between the internal flux states of the SQUID. We observe an unexpected jump of 10 dB to 25 dB in the amplitude and signal to noise ratio (SNR) at the output of the SQUID in response to input signals of frequency below the knee of the switching spectrum. The magnitude of the gain in SNR has been measured as a function of both barrier height and energy difference between local minima of the SQUID energy potential revealing new features of SQUID behavior. A new analysis is put forth for the DC SQUID which is able to reproduce the key features of these observations.

Circular shift registers (CSRs) can be used in the implementation of superconducting digital signal processing blocks requiring the storage of data that needs to be accessed periodically with short access times and high throughput rate. The clock distribution networks of these shift registers has the unique constraint that the overall clock skew must be zero. Centered around this requirement, a design methodology for the design of these circuits has previously been developed and presented, resulting in three different designs for 64-bit versions of CSRs. We now present experimental results of the functional testing of two of these designs. These results show correct operation up to 13 GHz and set an important step for the complete validation of the design methodology presented earlier.

We have measured the low frequency (5 mHz<f<30 Hz) noise in current biased aluminium single electron tunneling (SET) transistors. A refined high frequency (HF) shielding allows us to maintain and study a given background charge configuration for many hours at T<100 mK. At frequencies below 10 Hz the noise is mainly due to charge traps, and the noise pattern superimposed on the V(V_g)-curve strongly depends on the particular background charge configuration resulting from the cooling sequence and the applied RF irradiation, including thermal radiation from the 4.2 K environment. The noise spectra, which show both 1/f and 1/f^1/2 dependencies and saturate at f<100 mHz can be fitted by two-level fluctuators (TLF) with Debye–Lorentz spectra and relaxation times on the order of seconds.

The noninvasive measurement of magnetic heart activity based on high-T_C DC SQUIDs, could be a tool for investigating cardiac electrophysiological properties, if such a system is able to work in unshielded environments. For the realization of such a system we use thin film planar SQUID gradiometers with bicrystal or step-edge Josephson junctions. Even for a small baseline of approximately 4 mm (limited by substrate dimensions), the field gradent resolution is in the range required for clinical analysis. We use a portable system which consists of a glass fiber cryostat with a measuring unit where we can place up to four sensors, different read-out electronics, and signal filtering methods. Firstly, measurements inside a magnetically shielded room were used to show the capability of our magnetocardiography (MCG) system in medical applcations. The system works in an unshielded environment also, without additional field compensation. We demonstrate a measurement of a magnetocardiogram in a real clinical environment, and discuss the possibilties as well as the limitations of this system in magnetocardiography.