Browsing by Subject "Josephson junction devices"

Now showing 1 - 6 of 6

Open Access
Analysis of electrical characteristics and magnetic field dependences of YBCO step edge and bicrystal grain boundary junctions for rf-SQUID applications
(Institute of Physics, 2004) Fardmanesh, M.; Schubert, J.; Akram, R.; Bick, M.; Banzet, M.; Zander, W.; Zhang, Y.; Krause, H-J.
The dc characteristics and magnetic field dependences of Y-Ba-Cu-O bicrystal grain boundary junctions (BGBJs) and step edge junctions (SEJs) were investigated for fabrication of rf-SQUIDs. Test junctions with up to 8 μm widths as well as the junctions of the two types of junction-based rf-SQUID were studied. The SEJs typically showed lower Jc and higher ρN as compared to the BGBJs, resulting in close IcRN products. All the BGBJs showed classical field dependent Ic following their junction width, resembling Fraunhofer patterns. The field sensitivity of the BGBJs' Uc led to low yield submicron BGBJ rf-SQUIDs partially impaired by the Earth's magnetic field. Two major behaviours of low and high field dependences of Ic were observed for the SEJs. Only the low field-sensitive SEJs resulted in micron size junction rf-SQUIDs not impaired by the Earth's magnetic field. The low field-sensitive SEJs led to low I/f noise magnetically stable rf-SQUIDs appropriate for applications in unshielded environments at 77 K.
Open Access
Dependence of the substrate structure and the film growth at the junction of YBCO SEJ rf-SQUIDs on the IBE process and effects on the SQUID's characteristics
(Elsevier, 2002) Fardmanesh, Mehdi; Schubert, J.; Akram, Rizwan; Banzet, M.; Zander, W.; Zhang, Y.; Schilling, M.; Krause, H-J.
Step edge junction (SEJ) rf-SQUIDs were made of 200 nm thick YBCO films on LaAlO3(100) substrates using pulsed laser deposition technique. The steps on the substrates were developed using a combination of stationary and rotating angled argon ion beams with different beam energies and intensities. While sharp clean steps with heights up to 300 nm were obtained on the substrates using the combinatorial ion beam etching (IBE) process, very shallow ramp-type surfaces were found developing on the bottom of the trench, close to the steps. The ramp-type surfaces were found to be a source of hole-type defects in the films grown at the step edges. High quality films could be obtained on the flat regions away from the steps. Higher defect densities in the films close to the SEJs resulted in devices with higher 1/f noise and wider spread of the junction parameters. The 1/f noise of such devices increased with decreasing temperature. High quality films on sharp clean steps with flat substrate surfaces, developed using optimized combinatorial IBE process, resulted in higher yield of low 1/f noise SQUIDs. The Ic of the junctions and hence the working temperature of the SQUID could also be controlled by the junction width and the step height.
Open Access
Junction characteristics and magnetic field dependencies of low noise step edge junction Rf-SQUIDs for unshielded applications
(IEEE, 2003-06) Fardmanesh, Mehdi; Schubert, J.; Akram, Rizwan; Bozbey, Ali; Bick, M.; Banzet, M.; Lomparski, D.; Zander, W.; Zhang, Y.; Krause, H-J.
Step edge grain boundary (GB) junctions and rf-SQUIDs have been made using pulsed laser deposited Y-Ba-Cu-O films on crystalline LaAlO3 substrates. The steps were developed using various ion-beam etching processes resulting in sharp and ramp type step structures. Sharp step based GB junctions showed behavior of serial junctions with resistively shunted junction (RSJ)-like I-V characteristics. The ramped type step structures resulted in relatively high critical current, Ic, junctions and noisy SQUIDs. The sharp steps resulted in low noise rf-SQUIDs with a noise level below 140 fT/Hz12/ down to few Hz at 77 K while measured with conventional tank circuits. The Ic of the junctions and hence the operating temperature range of the SQUIDs made using sharp steps was controlled by both the step height and the junction widths. The junction properties of the SQUIDs were also characterized showing RSJ-like characteristics and magnetic field sensitivities correlated to that of the SQUIDs. Two major low and high background magnetic field sensitivities have been observed for our step edge junctions and the SQUIDs made on sharp steps. High quality step edge junctions with low magnetic field sensitivities made on clean sharp steps resulted in low 1/f noise rf-SQUIDs proper for applications in unshielded environment.
Open Access
Noise, junction characteristics, and magnetic field dependencies of bicrystal grain boundary junction Rf-SQUIDs
(IEEE, 2003) Fardmanesh, Mehdi; Schubert, J.; Akram, Rizwan; Bick, M.; Banzet, M.; Zander, W.; Zhang, Y.; Krause, H-J.
Bicrystal grain boundary (GB) Josephson junctions and rf-SQUID's were made of 200 nm thick PLD YBCO films on bi-crystal SrTiO3 substrates. The junction characteristics were studied to investigate optimal parameters in the rf-SQUID) layout designs and the limits imposed by the technology. The I c of 3 to 8 μm wide test junctions scaled with the junction widths, showing clear linear RSJ-like I-V characteristics at 77 K. All the junctions showed hysteretic RCSJ-like behavior at very low temperatures. Classical Josephson flux motion type (long junction) nonlinearity in I-V curves of all the junctions was also observed at lower temperatures with systematic dependence on the junction widths. Measurements of the magnetic field dependence of the Ic of the junctions resulted in junction width dependent well-defined Fraunhofer-pattern like characteristics. The obtained characteristics of the junctions led to feasible criteria for the rf-SQUID layouts with desired device characteristics. Rf-SQUID's were made using designs for optimal performance at 77 K while avoiding large superconducting weak links across the substrate GB. Devices with low noise characteristics and junction field sensitivities proper for operation in environmental background magnetic fields were obtained. A nonsystematic spread of optimal working temperature of the SQUID's were also observed which is associated to the spread of the junction parameters caused by the defects at the GB of substrates.
Open Access
Spontaneous and persistent currents in superconductive and mesoscopic structures
(American Institute of Physics, 2004) Kulik, I. O.
We briefly review aspects of superconductive persistent currents in Josephson junctions of the S/I/S, S/O/S and S/N/S types, focusing on the origin of jumps in the current versus phase dependences, and discuss in more detail the persistent as well as «spontaneous» currents in the Aharonov-Bohm mesoscopic and nanoscopic (macromolecular) structures. A fixed-number-of- electrons mesoscopic or macromolecular conducting ring is shown to be unstable against structural transformation removing spatial symmetry (in particular, azimuthal periodicity) of its electron-lattice Hamiltonian. In case when the transformation is blocked by strong coupling to an external azimuthally symmetric environment, the system becomes bistable in its electronic configuration at certain number of electrons. At such a condition, the persistent current has a nonzero value even at the (almost) zero applied Aharonov-Bohm flux, and results in very high magnetic susceptibility dM/dH at small nonzero fields, followed by an oscillatory dependence at larger fields. We tentatively assume that previously observed oscillatory magnetization in cyclic metallo-organic molecules by Gatteschi et al. can be attributed to persistent currents. If this proves correct, it may open an opportunity (and, more generally, macromolecular cyclic structures may suggest the possibility) of engineering quantum computational tools based on the Aharonov-Bohm effect in ballistic nanostructures and macromolecular cyclic aggregates.
Open Access
Study of junction and bias parameters in readout of phase qubits
(2012) Zandi H.; Safaei, S.; Khorasani, S.; Fardmanesh, M.
The exact numerical solution of the nonlinear Ginzburg-Landau equation for Josephson junctions is obtained, from which the precise nontrivial current density and effective potential of the Josephson junctions are found. Based on the resulting potential well, the tunneling probabilities of the associated bound states are computed which are in complete agreement with the reported experimental data. The effects of junction and bias parameters such as thickness of the insulating barrier, cross sectional area, bias current, and magnetic field are fully investigated using a successive perturbation approach. We define and compute figures of merit for achieving optimal operation of phase qubits and measurements of the corresponding states. Particularly, it is found that Josephson junctions with thicker barriers yield better performance in measurements of phase qubits. The variations of characteristic parameters such as life time of the states due to the above considered parameters are also studied and discussed to obtain the appropriate configuration setup.