Browsing by Author "Fardmanesh, Mehdi"

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Open Access
Analytic modeling of patterned high-Tc superconductive bolometers: film and substrate interface effects
(SPIE, 1998) Fardmanesh, Mehdi; Rothwarf, A.
Superconducting film and substrate interface effects on the response of superconductive edge-transition bolometers are modeled with a one dimensional thermal model in closed form, for samples with large area patterns compared to the substrate thickness. The results from the model agree with experimental results on samples made of meander line patterned granular YBCO films on crystalline substrates, in both the magnitude and phase of the response versus modulation frequency up to about 100 KHz, the limit of the characterization setup. Using the fit of the calculated frequency response curves obtained from the model to the measured ones, values of the film-substrate and substrate-holder thermal boundary resistance, heat capacity of the superconducting film, and the thermal parameters of the substrate materials could be investigated. While the calculated magnitude and phase of the response of the SrTiO3 substrate samples obtained from the model is in a very good agreement with the measured values, the calculated response of the LaAlO3 and MgO substrate samples deviate slightly from the measured values at very low frequencies, increasing with an increase in the thermal conductivity of the substrate material. Using the fit of the calculated response to the measured values, film-substrate thermal boundary resistances in the range of 4.4* 10-3 to 4.4* 10-2 K-cm2-w-1 are obtained for different substrate materials. The effect of substrate optical absorption in the response of the samples is also investigated.
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
Effect of rf pumping frequency and rf input power on the flux to voltage transfer function of rf-SQUIDs
(IEEE, 2007) Akram, Rizwan; Eker, Taylan; Bozbey, Ali; Fardmanesh, Mehdi; Schubert, J.; Banzet, M.
We present the results on the correlation between the flux to voltage transfer function, Vspp, of the rf-SQUID and the rf-bias frequency as well as rf-bias power. Measurements were performed for different SQUID gradiometer samples chosen from the same batch or different batches. In order to have full control on the electronics parameters, an experimental rf-SQUID circuit was designed and implemented with an operation frequency of 600 MHz to 900 MHz. According to our findings, It has been observed that at any particular rf-bias power, Vspp vs. rf-bias frequency shows Sine-like behavior. We observed that the main lobe maxima exist close to the resonance frequency of the LC tank circuit and by changing only the power, amplitude of the main lobe and side lobes can be controlled. The Vspp vs. rf-bias power analysis shows that maximum of Vspp, strongly depends on the bias frequency. This can be correlated with the S11 parameter of LC tank circuit. We also observed that the devices from the same batch show main lobe maxima at different frequencies and/or power. Our SQUIDs with high working frequency gave their maxima at lower rf-bias powers leading to the need of having high frequency electronics with low bias power handling capabilities. It has also been observed that the SQUIDs from the same chip show similar characteristics regarding Vspp vs. frequency and power while the SQUIDs from different batches show completely different behavior for a fixed LC tank circuit configuration.
Open Access
Effect of the superconductivity transition on the response of YBCO edge transition bolometers
(2003) Bozbey, Ali; Fardmanesh, Mehdi; Askerzade, I. N.; Banzet, M.; Schubert, J.
Dependence of the phase and magnitude of the response of Y-Ba-Cu-O edge transition bolometers on the superconducting transition is studied. The responses of both large and small area devices were investigated and several anomalies are observed. The response of small area LaAlO3 devices considerably differed from that expected based on the dR/dT curve. This discrepancy is observed to be strongly dependent on the superconducting transition. Both the phase and magnitude/(dR/dT) of the response of the devices showed abrupt changes for below the Tc-onset when measured versus temperature, while the phase variation also showed strong dependence on the modulation frequency. We present the analysis and propose mechanisms responsible for the modulation frequency dependence of the response characteristics versus temperature, within the superconductivity transition region of the devices.
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
Response analysis and modeling of high temperature superconductor edge transition bolometers
(Springer, 2004) Fardmanesh, Mehdi; Narlikar, A. V.
One of the promising devices made of high temperature superconducting (HTSC) materials are edge transition bolometers. Since the discovery of high-temperature superconductors, many works have been focused on the application of these materials in different types of bolometers for the near to far infrared wavelength regime [l]-[9]. They can be used to detect electromagnetic radiation over the whole spectrum from x-ray to the far-infrared [1], [9]-[13]. The superconductor bolometers typically consist of patterned thin or thick superconducting films deposited on crystalline substrates such as MgO, SrTiO3, and LaA103. Their operation is based upon their steep drop in the resistance, R, at their transition temperature, Tc. The detector is typically held at a temperature close to the middle of the superconducting transition, where the dR/dT is maximum. When the detector is illuminated its temperature rises by an amount proportional to the input radiation power resulting in a ΔR. The response obtained by the above mechanism is the so called the bolometric, or equilibrium response, as opposed to typically faster non-bolometric or intrinsic response caused by other mechanisms such as direct depairing. A typical response of an YBCO sample versus temperature at low frequencies is shown in Figure 1.1.