G H Chen

Micromachining of SrTiO3 steps for high-Tc step edge junction dc SQUIDs

In this paper, we establish a model of the micromachined SrTiO3 substrate steps for high-T-c direct-current (dc) superconducting quantum interference devices (SQUIDs). The step angle is determined by the local ion milling rate and re-deposition rate, which is caused by back sputtering of ion milling. At dynamic balance, the maximum possible step angle is predicted to be 75degrees with an ion beam incidence angle of 45degrees, which agrees well with the measured value of 71degrees. In order to obtain a better step sidewall profile, we consider the influences of Nb metal mask micromachining. To avoid a rounded angle at the step upper corner, the minimum thickness of the Nb mask should be 440 nm when the desired step height is 300 nm. At optimized process conditions, steps with sharp, steep angles, and flawless profiles have been fabricated. Nine of the twelve dc SQUIDs thus obtained exhibited resistively shunted junction current-voltage behavior and magnetic field modulation at 77 K.

Quantum phase diffusion in a small underdamped Josephson junction.

Quantum phase diffusion in a small underdamped Nb/AlO(x)/Nb junction (∼0.4 μm(2)) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant.

A cryogen-free dilution refrigerator based Josephson qubit measurement system

We develop a small-signal measurement system on cryogen-free dilution refrigerator which is suitable for superconducting qubit studies. Cryogen-free refrigerators have several advantages such as less manpower for system operation and large sample space for experiment, but concern remains about whether the noise introduced by the coldhead can be made sufficiently low. In this work, we demonstrate some effective approaches of acoustic isolation to reduce the noise impact. The electronic circuit that includes the current, voltage, and microwave lines for qubit coherent state measurement is described. For the current and voltage lines designed to have a low pass of dc-100 kHz, we show that the measurements of Josephson junctions switching current distribution with a width down to 1 nA, and quantum coherent Rabi oscillation and Ramsey interference of the superconducting qubit can be successfully performed.

Quantum and classical resonant escapes of a strongly driven Josephson junction

The properties of phase escape in a dc superconducting quantum interference device (SQUID) at 25 mK, which is well below quantum-to-classical crossover temperature T(cr), in the presence of strong resonant ac driving have been investigated. The SQUID contains two Nb/Al-AlO(x)/Nb tunnel junctions with Josephson inductance much larger than the loop inductance so it can be viewed as a single junction having adjustable critical current. We find that with increasing microwave power W and at certain frequencies nu and nu/2, the single primary peak in the switching current distribution, which is the result of macroscopic quantum tunneling of the phase across the junction, first shifts toward lower bias current I and then a resonant peak develops. These results are explained by quantum resonant phase escape involving single and two photons with microwave-suppressed potential barrier. As W further increases, the primary peak gradually disappears and the resonant peak grows into a single one while shifting further to lower I. At certain W, a second resonant peak appears, which can locate at very low I depending on the value of nu. Analysis based on the classical equation of motion shows that such resonant peak can arise from the resonant escape of the phase particle with extremely large oscillation amplitude resulting from bifurcation of the nonlinear system. Our experimental result and theoretical analysis demonstrate that at T << T(cr), escape of the phase particle could be dominated by classical process, such as dynamical bifurcation of nonlinear systems under strong ac driving.

Absolute measurement of penetration depth in a superconducting film by the two-coil technique

A two-coil mutual-inductance apparatus that is optimized to allow for the measurement of the absolute value of penetration depth λ in superconducting films is described. Nb films with thickness d ranging from 20 to 90 nm are used to illustrate the measurement. For a 70-nm-thick Nb film at 4.2 K, with d/λ∼0.6, the uncertainty in the measured λ is about ±2.3%. From the results on the Nb film series, we show that a satisfactory determination of the absolute value of λ is possible for these films with d/λ<0.95.

A noise feedback least-mean-square algorithm of data processing for SQUID-based magnetocardiography

We present a modified normalized least-mean-square algorithm for SQUID-based magnetocardiography data processing with a new error function, in which the instantaneous signal component represented approximately by an average of near past error data has been eliminated. In this way, the rebounds of the weight vector W from its optimal value in parameter space due to the signal component can be well avoided.

Magnetic microscopy based on high-Tc SQUIDs for room temperature samples

The SQUID microscope is the most suitable instrument for imaging magnetic fields above sample surfaces if one is mainly interested in field sensitivity. In this paper, both the magnetic moment sensitivity and spatial resolution of the SQUID microscope are analysed with a simple point moment model. The result shows that the ratio of SQUID sensor size to sensor–sample distance effectively influences the sensitivity and spatial resolution. In comparison with some experimental results of magnetic images for room temperature samples from our high-Tc SQUID microscope in an unshielded environment, a brief discussion for further improvement is presented.

Macroscopic quantum tunneling and qubit design parameters of Bi2Sr2CaCu2O8+δ surface intrinsic Josephson junctions

Macroscopic quantum tunneling (MQT) has been demonstrated recently in a Bi(2)Sr(2)CaCu(2)O(8+delta) surface intrinsic Josephson junction (SIJJ) with its critical current density J(c) below 100 A cm(-2) and its size below 1 mu m. In this work, we present a study of the switching current distributions of SIJJs fabricated on the same crystal, with J(c) > 500 A cm(-2) and size of 0.8 and 1.6 mu m. MQT is clearly observed, and the crossover from MQT to thermal activation (TA) is seen. Our analysis shows that the data agree well with the theoretical predictions of MQT and TA for different-sized SIJJs when parameters that roughly scale with the SIJJ size are used. In the crossover regime, the data are found to be better fitted by considering quantum corrections to TA. We discuss the realistic design of phase- and flux-type qubits using the experimentally attainable SIJJ parameters, which shows that the SIJJs, with their controllable J(c) and size (or junction capacitance), are feasible for qubit applications in the future.

A novel quadruple excitation in high-Tc SQUID-based non-destructive evaluation

A high-Tc SQUID-based non-destructive evaluation (NDE) system has been set up in our laboratory. The SQUID was made on a 24° bicystal SrTiO3 substrate. A novel quadruple excitation coil was proposed for the first time and applied in the artificial holes in the aluminium multilayer structure in a noisy unshielded environment. The experimental data shows that it has good balance and is very effective at detecting small hole defects.

Fabrication and properties of all-refractory Nb/Al–AlOx–Ti junctions for microbolometers and microrefrigerators

Fabrication of all-refractory Nb/Al–AlOx–Ti superconducting tunnel junctions using selective titanium etching process (STEP) is described. Results including anodization properties of Ti, and junction’s I–V characteristics and subgap currents measured in the temperature range of 0.4–9.2 K are presented. The junctions show fairly high quality with respect to their stability and reproducibility. Possible utilization of these junctions as superconductor–insulator–normal metal type devices operating around 100 mK and above for ultrasensitive microbolometer and electronic microrefrigerator applications is discussed.