Claude Hilbert

Radiofrequency amplifier based on a dc superconducting quantum interference device

A low noise radiofrequency amplifier (10), using a dc SQUID (superconducting quantum interference device) as the input amplifying element. The dc SQUID (11) and an input coil (12) are maintained at superconductivity temperatures in a superconducting shield (13), with the input coil (12) inductively coupled to the superconducting ring (17) of the dc SQUID (11). A radiofrequency signal from outside the shield (13) is applied to the input coil (12), and an amplified radiofrequency signal is developed across the dc SQUID ring (17) and transmitted to exteriorly of the shield (13). A power gain of 19.5±0.5 dB has been achieved with a noise temperature of 1.0±0.4 K. at a frequency of 100 MHz.

Nuclear quadrupole resonance detected at 30 MHz with a dc superconducting quantum interference device

A dc superconducting quantum interference device is used as a tuned radio frequency amplifier at liquid helium temperatures to detect pulsed nuclear quadrupole resonance at ∼30 MHz. At a bath temperature of 4.2 K, a total system noise temperature of 6±1 K has been achieved, with a quality factor Q of 2500. A novel Q spoiler, consisting of a series array of Josephson tunnel junctions, reduces the ring‐down time of the tuned circuit after each pulse. The minimum number of Bohr magnetons observable from a free precession signal after a single pulse is ∼2×1016 in a bandwidth of 10 kHz.

Superconductor-semiconductor hybrid devices, circuits, and systems

The discovery of superconductors whose critical temperatures are above liquid nitrogen temperature has prompted considerable interest in hybrid superconducting-semiconducting electronics applications. The authors review the efforts to hybridize these technologies. Some of these efforts have already been demonstrated on a laboratory scale; others are at present just theoretical proposals. Hybridization is possible on the system, circuit, and device levels. The authors review studies of the applications of superconductors for interconnecting semiconductor systems and combining semiconductor and superconductor devices to enhance the performance of both digital and analog systems. Novel circuit combinations of superconducting and semiconducting devices are mentioned, as are proposals to combine these materials on the device level. It is noted that the use of hybrid combinations may permit some electronic functions to be performed better than either technology could perform separately. >

Hysteretic Josephson junctions from YBa2Cu3O7−x/SrTiO3/Ba1−xKxBiO3 trilayer films

We have fabricated hysteretic Josephson junctions from YBCO/SrTiO3/BKBO trilayer films. These large area rf‐sputtered junctions are reproducible, show Shapiro steps in response to microwave radiation, and their Josephson current can be modulated with a small magnetic field up to about 20 K. In addition, geometrical and magnetic field dependent resonances were observed for the first time in cuprate superconductor junctions. The devices appear to be S‐N‐S like without any gap structures discernable in the electrical characteristics. The decay length of critical current with SrTiO3 layer thickness indicates a normal‐state coherence length in the SrTiO3 of about 5 A.

YBa2Cu3O7−x‐Y2O3 system and in situ deposition of trilayer heterostructures by coevaporation

We have deposited YBa2Cu3O7−x‐Y2O3‐YBa2Cu3O7−x heterostuctures by an in situ electron‐beam coevaporation technique. Physical and chemical properties of Y2O3 were studied. The deposition conditions for Y2O3 and YBa2Cu3O7−x are completely compatible. The crystal structure of Y2O3 provides a close lattice match with YBa2Cu3O7−x and allows oxygen diffusion through Y2O3, so that heteroepitaxy and the tetragonal‐orthorhombic structural transformations of YBa2Cu3O7−x can be achieved. The heterostructures are therefore of high quality. Both the top and the bottom YBa2Cu3O7−x layers are superconducting above 85 K. Tunneling phenomena on junctions fabricated from these trilayers were observed.

Low‐leakage thin‐film superconductor‐insulator‐normal metal tunnel junctions on co‐evaporated Ba1−xRbxBiO3 and rf‐sputtered Ba1−xKxBiO3

The oxide superconductors Ba1−xKxBiO3 and Ba1−xRbxBiO3 exhibit ideal BCS‐like characteristics with Tc∼30 K. We report the deposition of thin films with zero‐resistance temperatures up to 25 K and transition widths less than 1 K by thermal co‐evaporation and rf sputtering. Thin‐film S‐I‐N tunnel junctions fabricated by evaporating Ag or Au onto a native barrier exhibit low leakage over 1 mm2 junctions and near‐ideal BCS‐like behavior with a value of 2Δ(0)/kBTco=3.5±0.2.

Input impedance of an amplifier based on a dc superconducting quantum interference device

The measured input impedance of an amplifier involving a coil tightly coupled to a dc superconducting quantum interference device (SQUID) is found to depend on the current and flux biases of the SQUID. Variations in the input inductance arise from the reflected dynamic inductance of the SQUID, and are in reasonable agreement with the predictions of an analog simulator. Variations in the input resistance, on the other hand, arise from feedback between the output of the SQUID and the input circuit, and are in approximate agreement with the predictions of a simple model.

Ba1−xKxBiO3 thin film Josephson tunnel junctions

We have fabricated sandwich‐type Josephson tunnel junction devices from Ba1−xKxBiO3 electrodes and KNbO3 insulating barriers. Near ideal superconductor‐insulator‐superconductor tunnel junction gap structure was seen in high resistance devices at 4.2 K. Lower resistance devices with similar gap structure but much higher subgap leakage were hysteretic with switching voltages up to 5 mV. Hysteresis and well‐defined gap structure persisted past 12 K. Magnetic field modulation of the zero‐bias pair current showed Fraunhofer‐like behavior with almost complete quenching at the nodes.

SUPERCONDUCTING READOUT OF SEMICONDUCTOR MEMORY AT LIQUID NITROGEN TEMPERATURE

The discovery of high‐temperature superconducting materials raises the possibility of operating semiconducting and superconducting circuits at the same temperature in intimately hybridized devices and circuits. We report the experimental readout of standard CMOS memory cells using high‐temperature superconducting flux flow devices at 77 K. Significant reduction in access times below that achievable with cryogenic CMOS alone was measured; a 4.5‐ns access time was achieved on a 1‐μm CMOS static random access memory. These results obtained using nonoptimized, separate superconductor, and semiconductor chips demonstrate the potential of hybrid superconductor‐semiconductor memories for speed and power improvements over pure semiconductors operated at cryogenic temperatures.

ALL-YBA2CU3O7 TRILAYER TUNNEL JUNCTIONS WITH SR2ALTAO6 BARRIER

A new barrier material, Sr2AlTaO6, was employed in fabricating all‐YBa2Cu3O7 trilayer tunnel junctions using in situ coevaporation and sputtering deposition. It was found that the superior material properties of Sr2AlTaO6 allow the use of a very thin barrier layer. A dramatic increase in the quasiparticle density of states at the YBa2Cu3O7 superconductive gap was observed for the first time from thin‐film all‐YBa2Cu3O7 devices. Well‐defined gap structures were observed at temperatures up to 47 K. The tunneling characteristics are consistent with the typical superconductor‐insulator‐superconductor behavior. The temperature dependence of the superconductive gap is compared with the BCS theory.