Josephson Vortex Flow Transistors Research Articles

Parallel array of YBa2Cu3O7−δ superconducting Josephson vortex-flow transistors with high current gains

We have developed a Josephson vortex-flow transistor based on a parallel array of 440 YBa2Cu3O7−δ bicrystal grain boundary Josephson junctions. The array's critical current Ic was measured as a function of the control current Ictrl through a control line that is inductively coupled to the array. The device has a highly asymmetric Ic(Ictrl) curve with several regions where a switching behaviour is observed characterized by a maximum current gain gmax = ∂Ic/∂Ictrl of 19 and a significant dynamic range of 20 μA at 77 K. In the range 4.7–92 K gmax versus temperature is non-monotonic with a maximum recorded at 77 K.

High-sensitive scanning laser magneto-optical imaging system

A high-sensitive scanning laser magneto-optical (MO) imaging system has been developed. The system is mainly composed of a laser source, galvano meters, and a high-sensitive differential optical-detector. Preliminary evaluation of system performance by using a Faraday indicator with a Faraday rotation coefficient of 3.47 x 10(-5) rad/microm Oe shows a magnetic sensitivity of about 5 microT, without any need for accumulation or averaging processing. Using the developed MO system we have succeeded in the fast and quantitative imaging of a rotationally symmetric magnetic field distribution around an YBa(2)Cu(3)O(7-delta) (YBCO) strip line applied with dc-biased current, and also succeeded in the detection of quantized fine signals corresponding to magnetic flux quantum generation in a superconducting loop of an YBCO Josephson vortex flow transistor. Thus, the developed system enables us not only to do fast imaging and local signal detection but also to directly evaluate both the strength and direction of a magnetic signal.

Optical Responses of Josephson Vortex Flow Transistors under Irradiation of Femtosecond Laser Pulses

We have fabricated Josephson vortex flow transistors (JVFTs) that consist of a parallel array of Josephson junctions and a control current line to apply the external magnetic field at the junctions. We observed modulations of the flow voltage with a DC current input. From these results, we estimated the mutual inductance and the self-inductance, and calculated the operation frequency of the JVFT. Next, femtosecond laser pulses modulated by an optical chopper were irradiated to the Josephson junction array. As a result, we observed an increase in the output voltage in the current-voltage characteristics, and the clear modulation of the output voltage with the same period as the optical chopping rate. These results suggest that a JFVT can operate as an optical input interface for superconducting circuits.

Study on Sub-THz Signal Input for Superconducting Electronic Devices

Development of ultrafast optical interfaces that can operate in sub-terahertz region is important to apply superconducting electronic devices to the high-end systems. We have performed several fundamental researches to realize the ultrafast optical input interface for superconducting electronic devices. Firstly, we observed optical response of amorphous Ge thin films, and the results indicated that an amorphous Ge photoconductive switch could stably operate in a terahertz frequency range as an optical-to-electrical signal converter in the low-temperature region below T c of YBCO. Next, we have fabricated optical-to-electrical signal conversion system with photomixing technique, and we have demonstrated the generation and the detection of high frequency signals over 50 GHz. Finally, we have observed optical responses of a Josephson vortex flow transistor under irradiation of femtosecond laser pulses, and the results suggeste that the device has high potential as an optical interface.

DC and AC Responses of Josephson Vortex Flow Transistors with High Tc Superconducting Thin Films

We fabricated Josephson vortex flow transistors (JVFTs) with a parallel array of Josephson junctions that were prepared using c-axis-oriented 100-nm-thick YBa 2 Cu 3 O 7-δ (YBCO) thin films grown on 24° bicrystal MgO (100) substrates. We observed clear modulations of the critical current and the flow voltage with DC current input to the control line that was inductively coupled to the array of junctions. From the results, we estimated the parameters of the device, e.g., the mutual inductance and the self-inductance, and calculated the operation frequency at which the device potentially exhibited these parameters. Moreover, the current gain and the transresistance were evaluated and found to be 0.5 and 0.15 Ω, respectively. In addition, we observed the high-frequency responses of the JVFT to the input AC current of the sine wave or the square pulse wave. A clear oscillation of the output voltage could be observed with a 1 MHz sine wave and 250 kHz square pulse wave. We also discussed the feasibility of higher frequency operation by using it as an input interface for a single flux quantum (SFQ) logic circuit.

Flux transistor made from a single-layer niobium thin-film superconducting quantuminterference device

A micrometre-size Josephson vortex flow transistor is demonstrated experimentally. Thedrain–source of this device is the output of a single-layer Nb superconducting quantuminterference device (SQUID) with a nanometre-size hole, forming a nanoSQUID. The deviceinput is a Nb strip, which is closely coupled to the nanoSQUID. The change of the inputcurrent induces a change in the magnetic flux coupling to the nanoSQUID, and hencemodulates the current–voltage characteristics of the nanoSQUID. The current gain andtransresistance of the device were investigated by evaluating the mutual coupling and thedynamic resistance of the nanoSQUID. A current gain of 0.3 and a transresistance>1 Ω were achieved.

Comparison of Josephson vortex flow transistors with different gate line configurations

We performed numerical simulations and experiments on Josephson vortex flow transistors based on parallel arrays of YBa2Cu3O7–δ grain boundary junctions with a cross gate line allowing us to operate the same devices in two different modes named the Josephson fluxon transistor (JFT) and Josephson fluxon–antifluxon transistor (JFAT). The simulations yield a general expression for the current gain versus number of junctions and normalized loop inductance and predict higher current gain for the JFAT. The experiments are in good agreement with simulations and show improved coupling between gate line and junctions for the JFAT as compared to the JFT.

Behaviour of HTS Josephson vortex flow transistors

We have studied both theoretically and experimentally the low-frequency behaviour of discrete HTS Josephson vortex flow transistors. The devices were fabricated from YBa 2Cu 3O 7 thin films grown on 24° SrTiO 3 bicrystal substrates. They consist of an asymmetric array of 10 or 20 Josephson junctions, with a control current fed through an independent line that is inductively coupled to the array. We have measured current gains up to 40 at 77 K and substantially higher at lower temperatures. We discuss the results of simulations of the device behaviour and consider the effects of junction critical current, number of junctions and array loop inductance. We compare the measured device behaviour at 77 K with simulations which include the effects of thermal noise.

High current gain HTS Josephson vortex flow transistors

We have fabricated discrete Josephson vortex flow transistors from yttrium barium copper oxide thin-films on 24 degree strontium titanate bicrystals. The devices have an asymmetric design with the control current fed through an independent control line. We have measured high current gains, in excess of 20 at 77 K for a range of several devices, and substantially higher at lower temperatures. The performance at 77 K has been studied and compared with theoretical simulations, which have included the effects of noise.

High frequency operation of Josephson vortex flow transistor with a controlled damping

Both HTS and LTS junctions, respectively YBaCuO grain boundary junctions made on MgO 24/spl deg/ [001] tilted bicrystals and shunted Nb/AlOx-Al/Nb SNOP tunnel junctions deposited on sapphire have been used in asymmetrically biased Josephson vortex flow transistors (JVFT). As shown previously by Gross et al. (1995), we verify that the self field effect induced by current bias in asymmetrically designed JVFT with large screening parameter /spl beta//sub L/ can be used for increasing the current gain. Gain of about 10 up to 77 K are obtained with practical YBaCuO devices. High frequency operation of the devices has been experimentally verified up to the MHz range and extrapolated to GHz by using the control input gate coupled in a coplanar configuration to the vortex flow SQUID array, while reasonable values of transresistance, bandwidth and dynamical range are observed. Influence of the damping on the JVFT performances has been also studies experimentally by varying the subgap resistance of the Nb tunnel junctions. Amplifiers made of asymmetrical HTS JVFT may be competitive with SQUID or FET amplifiers for NMR, Electromagnetic Compatibility or Infrared imaging applications.

High-temperature superconducting Josephson Vortex Flow Transistors: numerical simulations and experimental results

Josephson Vortex Flow Transistors (JVFTs) based on high transition temperature superconductors (HTS) are promising candidates for three-terminal devices, which may be used e.g. at the interface between superconducting and semiconducting electronics. We have investigated the performance of JVFTs based on parallel arrays and on long HTS Josephson junctions, both theoretically and experimentally. Our numerical simulation results reveal the dependence of the current gain on various device parameters, such as number of junctions, loop size, and screening parameter /spl beta/=L/L/sub j/, where L is the loop inductance and L/sub j/ the Josephson inductance of a single junction. We have fabricated various devices using symmetrically and asymmetrically injected bias currents. The experimental results are in good agreement with our simulation results and it is shown that for asymmetric devices high values of the current gain above 20 can be obtained for temperatures below 60 K.

New vortex flow transistors made of YBa/sub 2/Cu/sub 3/O/sub y/ thin films

We have investigated the performances of several types of vortex flow transistors including nanobridge vortex flow transistors (NBVFTs) based on a parallel array of nanobridges, planar Josephson vortex flow transistors (planar JVFTs) based on a parallel array of grain boundary Josephson junctions, and new JVFTs with a stacked structure (stacked JVFTs). Considering the integration and the reduction of the L/R time constant, the areas of the transistors were restricted to less than 350 /spl mu/m/sup 2/. A NBVFT showed a flux-to-voltage transfer function of 2.6 mV//spl Phi//sub 0/, which was one order of magnitude larger than that of the other transistors. In contrast, the NBVFTs showed a very small current gain due to a large kinetic inductance of a nanobridge, while the NBVFT had the smallest area among the three. A planar JVFT with asymmetric geometry was easy to fabricate and showed a current gain of 2.2 at 4.2 K. However, the planar JVFT requires a large area, leading to a long response time other than the internal delay time. A stacked JVFT also showed a current gain of 2.5 at 4.2 K. A layered structure yielded a strong coupling between the body of the JVFT and the control line. Due to this strong coupling, the response time of the stacked JVFT was considerably improved compared to that of the planar JVFT.

High-temperature superconducting Josephson fluxon–antifluxon transistors

We have realized the Josephson fluxon–antifluxon transistor (JFAT) in high temperature superconductivity using an asymmetric control line on top of either bicrystal junctions or Co-doped YBa2Cu3Ox superconductor–normal–superconductor (SNS) junctions. We have measured current gains as high as 6 for 30 μm-wide bicrystal JFATs (30 K) and as high as 3 for Co-doped SNS JFATs (50 K). An improvement in gain over the Josephson vortex flow transistor, due to improved coupling efficiency, is demonstrated. There is also a reduction of control line inductance that should lead to an improvement in gate speed.

Theory of operation of high temperature Josephson fluxon-antifluxon transistor

We provide two qualitative models for the operation of Josephson fluxon–antifluxon transistor based on the pendulum model and image currents induced in the junction. This device, which is a variant of a Josephson vortex flow transistor, utilizes high temperature superconductor long Josephson junctions with a control line on top of the junction. Our models are consistent with the experimental observation that the coupling efficiency increases with this geometry. We also provide a quantitative model using numerical simulations to confirm the static and dynamic characteristics predicted by the intuitive models.

Asymmetric high temperature superconducting Josephson vortex-flow transistors with high current gain

We have fabricated Josephson vortex-flow transistors (JVFTs) based on asymmetric parallel arrays of YBa2Cu3O7−δ bicrystal grain boundary junctions. The critical current Ic and the voltage V at fixed bias current Ib were measured as a function of the control current Ictrl through a control line inductively coupled to the array. For JVFTs with an asymmetric in-line geometry a high current gain g=∂Ic/∂Ictrll ranging between about 20 at 40 K and 14 at 70 K was achieved. This current gain is much higher than that obtained for symmetric devices and results from the self-field effect of the electrode currents. In contrast to the current gain the transresistance rm=∂V/∂Ictrl of the JVFTs could not be enhanced by an asymmetric device structure. The current-voltage characteristics of the asymmetric JVFTs show pronounced step-like structures caused by the self-field of the electrode currents.

Physics and performance of high temperature superconducting vortex flow transistors

Abstract The physics and performance of high temperature superconducting vortex flow transistors is analyzed both theoretically and experimentally. Vortex flow transistors based on Josephson vortices are found to be superior to those based on Abrikosov vortices with respect to sensitivity, speed and impedance level. For Josephson vortex flow transistors based on single long junctions and parallel arrays of a large number of short junctions, expressions for the current gain, the transresistance and the response time are derived as a function of the various device parameters. Our analysis shows that Josephson junctions with high products of their normal resistance and critical current are required in order to obtain large current gain, high transresistance and small response time. Both symmetric and asymmetric Josephson vortex flow transistors based on YBa 2 Cu 3 O 7 − δ bicrystal grain boundary junctions have been fabricated. Our experimental results agree well with the model predictions. Transresistance values of several Ω have been achieved for devices based on YBa 2 Cu 3 O 7 − δ grain boundary Josephson junctions. The current gain could be increased by using an asymmetric bias current injection. For asymmetric devices a current gain above 20 has been obtained at temperatures below 60 K.

Josephson vortex-flow transistors based on parallel arrays of YBa/sub 2/Cu/sub 3/O/sub 7-δ/ bicrystal grain boundary junctions

Three-terminal electronic devices based on flux-flow phenomena have attracted considerable interest due to their potential application in the interface between superconducting and semiconducting electronics. We have fabricated Josephson vortex flow transistors based on parallel arrays of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) bicrystal Grain Boundary Junctions (GBJs). The array critical current I/sub c/ has been measured as a function of an external magnetic field B or the magnetic field of the current I/sub ctrl/ that is fed through the control line coupled inductively to the array. By varying the device geometry we have studied the influence of the coupling of the magnetic control and the array parameters such as the number N of discrete Josephson elements and the screening parameter /spl beta//sub L/ on the I/sub c/(B) dependences of the arrays. Going from /spl beta//sub L//spl Lt/1 to /spl beta//sub L//spl Gt/1 the measured I/sub c/(B) curves changed from a dependence similar to the diffraction pattern of a grid to a dependence dominated by the parameters of a single array cell. The impact on the available current gain is discussed. >