Junction Normal Resistance Research Articles

Large voltage modulation in superconducting quantum interference devices with submicron-scale step-edge junctions

A promising direction to improve the sensitivity of a SQUID is to increase its junction's normal resistance value, Rn, as the SQUID modulation voltage scales linearly with Rn. As a first step to develop highly sensitive single layer SQUID, submicron scale YBCO grain boundary step edge junctions and SQUIDs with large Rn were fabricated and studied. The step-edge junctions were reduced to submicron scale to increase their Rn values using focus ion beam, FIB and the measurement of transport properties were performed from 4.3 to 77 K. The FIB induced deposition layer proves to be effective to minimize the Ga ion contamination during the FIB milling process. The critical current–normal resistance value of submicron junction at 4.3 K was found to be 1–3 mV, comparable to the value of the same type of junction in micron scale. The submicron junction Rn value is in the range of 35–100 Ω, resulting a large SQUID modulation voltage in a wide temperature range. This performance promotes further investigation of cryogen-free, high field sensitivity SQUID applications at medium low temperature, e.g. at 40–60 K.

Aging- and annealing-induced variations in Nb/Al–AlOx/Nb tunnel junction properties

In this paper, we present studies of room temperature aging and annealing of Nb/Al–AlOx/Nb tunnel junctions with the size of 2–3 μm2. We observed a noticeable drop of junction normal resistance Rn unusually combined with increase in subgap resistance Rj as a result of aging. Variation in both Rn and Rj are subject to the junction size effect. An effect of aging history on the junction degradation after consequent annealing was discovered. Discussion and interpretation of the observed phenomena are presented in terms of structural ordering and reconstruction in the AlOx layer, driven by diffusion flows enhanced due to stress relaxation processes in the Al layer interfacing the AlOx layer.

Fiske steps and hysteresis in YBa2Cu3O7 grain boundary Josephson junctions: Structural information of the barrier by means of a nondestructive approach

A deep analysis of the current-voltage (I-V) characteristics of YBa2Cu3O7 grain boundary Josephson junctions (GBJJs) allows us to go much farther than the usual calculus of the transport parameters. It is possible to construct a structural image of the barrier by an exhaustive and complementary analysis of both transport and electromagnetic parameters obtained from I-V curves. For such an approach, we have chosen the following three representative bicrystalline geometries: 24° [001] asymmetric, 45° [100] asymmetric, and 24° [001] symmetric +45° [100] asymmetric. The dependence of the product ICRN on the junction normal resistance is of the ICRN−1 type pointing to a SNINS model (S denotes superconductor, I denotes insulator, and N denotes normal metal) for all our GBJJs. A satisfactory explanation of the discrepancy of the capacitance of the barrier estimated from Fiske resonance positions and hysteresis in the I-V curves needs of such a model. Moreover an estimation of the length of the normal regions adjacent to the crystallographic barrier can be made. This comparative analysis is presented in order to extract interesting information about the particular transport mechanisms involved in these GBJJs.

Reflectionless tunneling due to Andreev reflection in a gated superconductor–semiconductor junction

We have studied the differential resistance in a gated superconductor–semiconductor junction in the ballistic transport regime. The zero-bias resistance shows a minimum as a function of the gate voltage, while the junction normal resistance measured at a high bias voltage shows a monotonic increase. This is explained as follows: the conductance enhancement due to coherent Andreev reflection is controlled by the gate voltage which changes the transparency of the gate barrier. We have also fabricated a two-gated junction with a high transparency at the superconductor–semiconductor interface and measured its resistance as functions of two gate voltages. An increase in one gate voltage results in the resistance minimum as a function of the other gate voltage. The obtained results agree well with the so-called reflectionless tunneling theory.

New fabrication process elements of phase-mode logic circuits

We present the phase-mode circuits based on new fabrication process elements for high-density circuit integration. Nb/AlN/sub x//Nb overdamped junctions without external resistive shunts are applied to the phase-mode circuits. The area of a bias feed resistor can be reduced by a replacement of the metallic resistor by the junction normal resistance. As a miniaturization method of an inductance, the effective inductance of a Josephson junction is used. The phase-mode circuits based on these new integration methods have been fabricated by Nb integration technology. Various aspects which are brought about by these fabrication technologies are discussed.

Observation of the resistance minimum in a gated superconductor-semiconductor junction with variable transparency

We have studied the differential resistance in a gated superconductor-normal-metal junction The zero-bias resistance shows a minimum as a function of the gate voltage, while the junction normal resistance measured at a high bias voltage shows a monotonic increase. The results show good agreement with the reflectionless tunneling theory in the ballistic motion regime.

Superconducting transistors using InAs-inserted-channel InAlAs/InGaAs inverted HEMTs

A newly fabricated Josephson field effect transistor (JOFET) is coupled with a two-dimensional electron gas (2DEG) in a strained InAs quantum well inserted into an inverted modulation-doped structure with an HEMT-type gate. We indicate the improved characteristics of the JOFET with the HEMT-type gate, instead of the MIS-type gate. The superconducting critical current as well as the junction's normal resistance can be completely controlled via a gate voltage of about -1 V; this provides voltage gain over unity, the first time for a JOFET.

A Josephson field effect transistor using an InAs-inserted-channel In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure

A Josephson field effect transistor (JOFET) was coupled with a two-dimensional electron gas in a strained InAs quantum well inserted into an In0.52Al0.48As/In0.53Ga0.47As inverted modulation-doped structure. The characteristics of this JOFET are much improved over previous devices by using a high electron mobility transistor (HEMT)-type gate instead of the usual metal-insulator- semiconductor (MIS)-type gate. The superconducting critical current as well as the junction normal resistance are completely controlled via a gate voltage of about −1 V; this provides voltage gain over 1 for a JOFET.

Co-doped-YBCO SNS junctions for superconductive integrated circuits

We have established a high temperature superconductor SNS junction fabrication process and are reproducibly fabricating junctions whose behavior is rooted in the physics of the proximity effect. SNS edge junctions are being fabricated using YBa/sub 2/Cu/sub 2.8/Co/sub 0.2/O/sub 7-/spl delta// (Co-YBCO) as the normal barrier, and YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) as the superconductor. In the small junction regime, the junctions have RSJ characteristics with I/sub c/R/sub n//spl sim/ 50 /spl mu/V at 77 K. Junction normal resistance values are consistent with known values of the barrier bulk resistivity and junction geometry, indicating that the interface resistance between YBCO and Co-YBCO is not significant. We have demonstrated I/sub c/ and R/sub n/ 1-/spl sigma/ spreads of 33% and 17%, respectively, for 6-junction strings. Barrier thickness and/or morphology variations appear to significantly contribute to these spreads. >

Observation of the “skew supercurrent” in Josephson vacuum barrier tunnel junctions as a function of the junction normal resistance

A mechanically controllable break junction is used to obtain the smallest possible Josephson junction, formed by two superconducting electrodes, which are separated by a thin subatomic vacuum barrier. The current voltage characteristics show a “skew supercurrent”, a supercurrent-like feature with a finite resistance and a “critical skew current value” I t at a finite voltage V t. The skew supercurrent is examined over a large normal resistance range by adjustment of the vacuum barrier thickness. In the studied range of six orders of magnitude of the junction normal resistance a crossover from a clear zero voltage supercurrent to a skew supercurrent is observed, for which V t remains constant.

Electrical characteristics of HoBa 2Cu 3O 7 − x-PrBa 2Cu 3O 7 − y-HoBa 2Cu 3O 7 − x junctions with planar-type structures

All-oxide superconductor-normal conductor-superconductor (S-N-S) junctions with a planar-type structure are fabricated using HoBa 2 Cu 3 O 7 − x and PrBa 2 Cu 3 O 7 − y as the superconductor and normal conductor, respectively. Their electrical characteristics are measured. The junction's normal resistance is almost comparable to that of a single PrBa 2 Cu 3 O 7 − y film. However, it deviates from the T − 1/3 ( T is temperature) law and saturates at low temperatures for junction lengths smaller than 1.0 μm. Superconducting characteristics obtained for this type of junction may be accompanied by this saturation of normal resistance. Nonlinear current-voltage characteristics that may reflect correlation energy in the PrBa 2 Cu 3 O 7 − y layer are also detected.

Characterization of YBa2Cu3O7 step-edge Josephson junctions

The authors have fabricated and characterized thin film YBa2Cu3O7 step-edge microbridges for application in SQUIDS. Epitaxial YBCO films were pulsed-laser-deposited on SrTiO3 and had Tc's of 88-90 K. Sharp steps in SrTiO3 were obtained by photolithographic techniques and Ar ion milling with step heights (h) between 100 and 250 mm. They investigated step-edge junctions (SEJ) with different film thicknesses (d) from 100 to 250 mm but with a constant ratio d/h=1. The width of the microbridges was 2-2.5 mu m. The Tc's of the bridges were 65-85 K, depending on film thickness and the duty cycle during ion milling. The I-V curves were RSJ-like and clear Shapiro steps were observed. The junction normal resistance was independent of temperature. With applied magnetic field the critical current Ic showed a T-dependent modulation, indicating that there was self-shielding and Ic non-uniformity in the junctions. Low-frequency noise measured in RF SQUIDS was relatively low. They showed that it was generated predominantly in the junctions.

Characteristics of YBa/sub 2/Cu/sub 3/O/sub 7- delta / grain boundary junction DC-SQUIDs

The characteristics of YBa/sub 2/Cu/sub 3/O/sub 7- delta / grain boundary junction (GBJ) DC-SQUIDs with a square washer geometry design are discussed. The current-voltage characteristics of these SQUIDs are close to those predicted by the resistively shunted junction (RSJ) model. Their flux-voltage characteristics are perfectly periodic from 4.2 to 87 K. The noise characteristics have been measured as a function of frequency, temperature, bias current, and applied magnetic flux. At frequencies above about 1 kHz the magnetic flux noise energy of GBJ-SQUIDs is frequency independent and close to the theoretical value estimated from the Nyquist noise in the junction normal resistance. At lower frequencies the noise scales approximately as 1/f. Above 82-5 K the 1/f noise is dominated by flux noise, presumably caused by thermally assisted flux motion in the film. Below this temperature it is dominated by voltage noise across the junctions.

Performance limits of the Josephson junction microwave receivers

The performance limits are found for the Josephson junction microwave receivers: self- and externally-pumped mixers, as well as self-selecting and wideband quadratic detectors. The effect of the signal source impedance upon the Josephson junction properties is considered in two extreme approximations for the junc-, tion matching with the source, the approximations yielding close results. It has become possible to find the device characteristics for the arbitrary values of ratios R E /R N and F/F c , where R E is the reduced source resistance, R N is the junction normal resistance, F is the signal frequency and F C is the junction characteristic frequency. The performance factors obtained are optimized and their ultimate limits are found. The relatively poorly-developed devices without external pumping (self-pumped mixer and self-selecting detector) occur to have the best performance limits. Achievement of these limits is possible, however, only if the recently proposed specially-designed multi-junction arrays are used.

The optimization of a Josephson video-detector for millimetre wavelengths

The figure of merit of a Josephson video-detector for mm wavelengths has been evaluated accounting for the real experimental situation, i.e. for the restrictions on the junction normal resistance and the comparatively high value of the microwave coupling resistance. A simple approximation has been suggested for the spectral density of voltage fluctuations for operating points near zero voltage where the d.c. characteristic is smoothed by noise. We estimate the best realizable performance of a detector for 70 GHz radiation using resistively shunted tunnel junctions or series connections of microbridges as: responsivity ≈ 1.5.10 6 V/W; noise equivalent power per 1 Hz bandwidth ≈ 2.10 −16 W at 4 K.