YBCO Microbridge Research Articles

Vortex channel flow effect in grain boundary of YBCO thin film under inclined magnetic field

By measuring the dependence of transverse (Hall) and longitudinal magnetoresistance on temperature, the vortex easy-flow channel effect in an YBCO microbridge with an inclined GB was studied when the magnetic field was rotated in the ac plane of the film. In addition, the magnetic field angular dependence of critical current density in an YBCO microbridge with a GB perpendicular to bridge was measured when the magnetic field was rotated in the ab plane of the film. These measurement results demonstrate that there still can be vortex components moving inside and along the plane of the GB even though the vortices diagonally cross the GB.

Effects of electron-beam irradiation on the transition properties of YBa2Cu3O7−d grain boundary junctions

We have studied the effects of high-energy electron-beam irradiation on the transition properties of superconducting YBa2Cu3O7−d (YBCO) grain boundary junctions, bicrystal junctions and step-edge junctions, on SrTiO3 substrates. A uniform 1-MeV electron beam irradiated all over the samples. The irradiation doses were 0, 4.7 × 1014, 4.7 × 1015, and 4.7 × 1016 e/cm2. For each junction type, we used at least two samples for each dose level and compared the transition parameters before and after irradiation. For comparison, we also studied the same irradiation effects for YBCO microbridges. We measured the resistive transition temperature, the current-voltage characteristics, the normal-state resistance, and the critical current. The effect of irradiation was the most significant for the bicrystal grain-boundary junction and the least significant for the microbridges. The critical current data for the YBCO bicrystal grain-boundary junction showed a maximum at (0.47 ∼ 0.9) × 1015 e/cm2, and those for the microbridges showed a monotonic decrease with increasing dose. The normal-state resistance increased monotonically with increasing dose for all samples by up to ∼40% for the microbridges and ∼20% for the grain-boundary junctions at 4.7 × 1015 e/cm2. The change in the superconducting temperature (Tc) was negligible except for the bicrystal junction at 4.7 × 1016 e/cm2, which was not superconducting at 77 K. These results show that grain-boundary junctions are more susceptive to irradiation, indicating that their critical currents are controllable by using high-energy electron-beam irradiation.

Magnetic shielding performance of superconducting YBCO thin film in a multilayer device structure

Magnetic shielding performance of superconducting YBaCu2O7−x (YBCO) thin film on an YBCO microbridge was analyzed in a multilayer structure. A sandwich type multilayer structure was fabricated onto a single crystal (100) SrTiO3 (STO) substrate in the form of YBCO/STO/YBCO by depositing a thin STO interlayer in between two YBCO layers. The top YBCO was patterned as 20μm width meander-type microbridges and the bottom layer YBCO was used as magnetic shield. YBCO and STO thin films were deposited by dc and rf magnetron sputtering respectively, and the patterning was performed by using standard photolithography and wet etching. In order to enhance long-term stability of the final device, an additional STO thin film was deposited onto the device as an encapsulation layer. Electrical and magnetic characterizations of the YBCO thin film layers were carried out by means of ac magnetic susceptibility (χ–T) and resistance vs. temperature (R–T) measurements. The current–voltage (I–V) measurements were performed on the microbridges at 77K by observing the shielding performance of the bottom YBCO layer under various applied magnetic fields. The results were compared with that of a same-type single layer YBCO device without a shielding layer. The zero field critical current value of the single layer 20μm wide YBCO device was measured as 30mA and decreased down to 20mA as the field increased up to 100mT. The same measurements on the multilayer device showed that the critical current values remained almost constant around 27mA as the applied field increased.

Superconducting YBa2Cu3O7−δ Thin Film Detectors for Picosecond THz Pulses

Ultra-fast THz detectors from superconducting YBa2Cu3O7−δ (YBCO) thin films were developed to monitor picosecond THz pulses. YBCO thin films were optimized by the introduction of CeO2 and PrBaCuO buffer layers. The transition temperature of 10 nm thick films reaches 79 K. A 15 nm thick YBCO microbridge (transition temperature—83 K, critical current density at 77 K—2.4 MA/cm2) embedded in a planar log-spiral antenna was used to detect pulsed THz radiation of the ANKA storage ring. First time resolved measurements of the multi-bunch filling pattern are presented.

Investigation of Dynamic Behaviors of Low-Level Dissipation at ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7}$ Grain Boundaries Using Low-Temperature Near-Field Scanning Microwave Microscopy

Near-field scanning microwave microscopy (NSMM) provides a unique nondestructive approach for detection of local dissipation with high sensitivity and high spatial resolution. With recently improved NSMM probes of spatial resolution of up to 400 nm ( ~ 10-6 wavelength), detection of dissipation was achieved on YBCO microbridges at currents more than three orders of magnitude below the Jc(T). In this work, we report characterization of the dynamic behavior of low-level dissipation at the grain boundary of YBa2Cu3O7-δ microbridges as function of time and applied electrical current. On higher-angle grain boundary, the dissipation develops rapidly with increasing current and shows approximately linear dependence on current. On lower-angle grain boundary, nonlinear features were observed and attributed to bi-modal pattern of dissipation evolution of nucleation of isolated hot spots and their evolution. Comparison with the similar NSMM+IV measurement made on the “bulk” part of the same YBa2Cu3O7-δ microbridges on a reduced temperature scale shows higher dissipation on the grain boundary can be mostly attributed to the lower Tc values on grain boundaries.

Superconducting microlimiters based on YBCO thin-films grown on SrTiO3 substrates

It has been shown recently that superconducting microbridges implemented with YBa2Cu3O7−δ (YBCO) thin-films grown on sapphire substrates could be used as very efficient fault current limiters for microelectronic devices with some elements working at temperatures below TcI the superconducting critical temperature and, simultaneously, under very low power conditions (below 1 W). These so-called microlimiters are then well adapted to important applications of the superconducting microelectronics, as infrared detectors or SQUID based electronics. Here we will present new results obtained by using YBCO microbridges grown on SrTiO3 substrates, which have a thermal conductivity of the order of 50 times lower than the sapphire substrates used in most of our previous work. These new measurements confirm the important role played in the behaviour of microlimiters by the thermal exchanges between the microbridges with their substrates and between these last with their environment. Our present results also show that the temperature for optimal operation of the microlimiter is substrate-dependent.

On the high-resistivity contacts to YBa 2Cu 3O 7 thin films, electrical behavior in the regime of high temperatures and high-bias voltages

We prepared in-situ Au contacts on high-quality epitaxial YBa 2Cu 3O 7 (YBCO) films. Very high specific contact resistivity values up to ∼10 −2 Ω cm 2 at 4.2 K were obtained on 12×5 μm 2 contact areas. This resistivity value decreased by two orders of magnitude as the temperature was raised to room temperature. In the temperature range T<200 K, the contacts showed non-ohmic behavior suggesting the presence of a well-defined insulating native Y–Ba–Cu–O barrier between the two electrodes. The electrical transport in this barrier layer was analyzed in the limit of high temperatures and high voltages to follow Mott's variable-range hopping conduction mechanism with physically reasonable parameters describing the localized states in the barrier. The high-resistivity contacts were tested successfully in quasiparticles injection experiments where the critical current I c of the YBCO microbridge could be strongly suppressed on injection of an additional current through the contact into the superconducting channel.

Operational Characteristics of Superconducting Amplifier using Vortex Flux Flow

The operational characteristics of superconducting amplifier using vortex flux flow were analyzed from an equivalent circuit in which its current-voltage characteristics for the vortex motion in YBCO microbridge were reflected. For the analysis of operation as an amplifier, dc bias operational point for the superconducting amplifier is determined and then ac operational characteristics for the designed superconducting amplifier were investigated. The variation of transresistance, which describes the operational characteristics of superconducting amplifier, was estimated with respect to conditions of dc bias. The current and the voltage gains, which can be derived from the circuit for small signal analysis, were calculated at each operational point and compared with the results obtained from the numerical analysis for the small signal circuit. From our paper, the characteristics of amplification for superconducting flux flow transistor (SFFT) could be confirmed. The development of the superconducting amplifier applicable to various devices is expected.

Laser Scanning Microscopy Studies on Detwinned&amp;lt;tex&amp;gt;$rm YBa_2rm Cu_3rm O_7-delta$&amp;lt;/tex&amp;gt;Thin Films

YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films are heavily microtwinned due to the crystal's orthorhombic unit cell and the confining effects of underlying substrate. Single crystal YBCO can be detwinned by thermally annealing the sample while applying uniaxial pressure. A similar technique has been devised to detwin a portion of a YBCO thin film. Through the use of photolithographic techniques, a suspended YBCO microbridge structure is fabricated, but with both ends of the bridge anchored to the substrate. During a subsequent anneal, the ends of the bridge induce a uniaxial stress along its length that arises from the lattice misfit and the thermal expansion coefficient mismatch between YBCO and the underlying materials. Without the confining effects of the substrate the suspended material can either detwin or twin on a macro length scale; the extent of which is largely determined by the substrate material. The ability to produce superconducting YBCO bridges that range from heavily microtwinned, to macrotwinned, to nearly detwinned provides an ideal experimental system. To study these phenomena, an optical and laser scanning microscopy set-up was employed. The images obtained provide insight into the role that twins play in determining the transport properties of superconducting YBCO thin films.

Detection properties of slot antenna coupled YBCO microbridges based on vortex motion

High-T/sub c/ YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) microbridges coupled to a slot antenna were fabricated and the video detection properties in the resistive state were investigated under the irradiation of 94 GHz millimeter-waves. A detected voltage dominated by the nonbolometric response due to vortex motion was observed in the vicinity of the critical temperature. From measurements of the polarization dependence of the detected voltage, it was found that the detected voltage, based on the vortex motion, was not due to the photon energy but instead to a high frequency current induced in the antenna. I-V characteristics of the device with and without the external magnetic fields were also measured. Detected voltages increased with the vortex velocity and the maximum detected voltage was obtained when the vortex velocity was at its maximum.

Do superconductors have zero resistance in a magnetic field?

We show that dc voltage versus current measurements of a YBa(2)Cu(3)O(7-delta) film in a magnetic field can be collapsed onto scaling functions proposed by Fisher et al. [Phys. Rev. B 43, 130 (1991)] as is widely reported in the literature. We find, however, that good data collapse is achieved for a wide range of critical exponents and temperatures. These results strongly suggest that agreement with scaling alone does not prove the existence of a phase transition. We propose a criterion to determine if the data collapse is valid, and thus if a phase transition occurs. To our knowledge, none of the data reported in the literature meet our criterion.

Small area Y-Ba-Cu-O thin films for applications in hot-electron bolometers

We are investigating the use of very thin, small area YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) films on Si substrates for application in hot-electron bolometers. Hot-electron bolometers produced from high-T/sub c/ materials will be favored over their low-temperature counterparts in applications of radio astronomy and atmospheric physics where the higher operating temperatures provide distinct advantages. Devices on Si can help advance this technology for bolometric space applications, where a substrate is needed with good thermal conductance and excellent IR performance. Based on our experience with YBCO bolometers and YBCO film growth on Si, we have begun a study of sub-micrometer scale devices. Our typical YBCO films grown on Si by pulsed laser deposition have critical temperatures of 86 K and critical currents of 1-3/spl times/10/sup 6/ A/cm/sup 2/ at 77 K for YBCO microbridges 45 nm thick. We have made 1-2 /spl mu/m wide microbridges from YBCO films of 25 nm to 45 nm thick. These microbridges show reduced critical temperatures of 71 K to 81 K, respectively, related to the processing sequence that produces the microbridges.

Ultrafast YBCO photodetector based on the kinetic-inductive process

We make use of the nonequilibrium kinetic-inductive effect in epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) thin films to demonstrate an ultrafast, high-sensitivity, broadband photodetector. The photoresponse of a 5-/spl mu/m-wide, 10-/spl mu/m-long, 100-nm-thick YBCO microbridge embedded in a 20-/spl mu/m-wide coplanar strip transmission line was measured using a >1-THz, submillivolt-sensitivity electro-optic sampling system. We performed a comprehensive study of the optical power and wavelength dependency on the YBCO photodetector response. The quantum coherence Rothwarf-Taylor model together with the hot-electron relaxation effect were used to explain the measured data and to extract the number of generated quasiparticles for each absorbed photon, revealing the intrinsic quantum yield of our device to be /spl sim/450. The amplitude, response time, and the intrinsic gain of the YBCO photoresponses were observed to be spectrally flat over the tested range of 390 nm to 810 nm.

Fabrication of YBCO/STO/YBCO multilayer by PLD

We have measured the electrical properties of YB 2Cu 3O y/SrTiO 3/YB 2Cu 3O y (YBCO/STO/YBCO) multilayers prepared by the pulsed laser deposition method under the various deposition conditions of insulating STO layer. Superconducting properties of both YBCO layers are sensitively affected by the deposition temperature, T s , of insulating STO layer. T c of lower YBCO layer is decreased due to the oxygen loss during the deposition of STO layer, but the superconductivity of upper YBCO layer is severely deteriorated due to the poor crystalline quality for low T s . However, we have successfully fabricated YBCO/STO/YBCO multilayer with T c( zero) above 85 K for both YBCO micro-bridges of 50 μm width by depositing the insulating STO layer progressively at T s = 550°C and 780°C. The resistivity of both YBCO layers at room temperature is below 1.0 mΩ.cm.

( Mn , Sb) doped-Pb(Zr, Ti)O3 infrared detector arrays

( Mn , Sb) doped Pb(Zr, Ti)O3 (PMSZT) thin film infrared (IR) detectors were integrated with Si substrates. The epitaxial PMSZT thin films, deposited on c-axis oriented YBa2Cu3O7−y (YBCO) bottom electrodes, show good ferroelectric properties with a remnant polarization Pr of 31 μC/cm2, a spontaneous polarization Ps of 38 μC/cm2, and a coercive field Ec of 21 kV/cm under an electric field of 76 kV/cm. Doping with Mn and Sb into Pb(Zr, Ti)O3 (PZT) not only decreased the Curie temperature TC from 350 °C for PZT to 175 °C for PMSZT, but also enhanced IR responsivity significantly. PMSZT thin films show high figures of merit, Fi of 15.5×10−9 C cm/J, Fv of 1758 cm2/C and Fd of 5×10−5 Pa−1/2 at 25 °C. IR detector arrays, fabricated with PMSZT films deposited on YBCO microbridges with an air gap between them and the substrate for reduced thermal mass, show a higher IR voltage responsivity compared to those without an air gap.

Effects of sputtered SiO2 passivation layers on YBCO microbridges and step-edge junctions

High-temperature superconducting YBa2Cu3O7- microbridges and step-edge junctions (SEJs) were coated with SiO2 thin film by an rf magnetron sputtering technique. The effect of the coating on critical temperature Tc and critical current Ic was studied. It was found that Tc and Ic of microbridges increased as a result of the coating, by up to 3 K for Tc and up to 23% for Ic. We believe that this is due to the oxygen in the plasma penetrating into YBa2Cu3O7- resulting in an increase in oxygen content and restoration of the degraded microbridge edges. Ic of the SEJ, however, was found to decrease by 20-30% after deposition of SiO2 films despite no degradation in Tc. The results of ageing tests showed that SiO2 film forms a good passivation layer, protecting YBa2Cu3O7- film from reacting with water.

Mn and Sb doped-PZT thin films for uncooled infrared detector array

Mn and Sb doped-PZT (PMSZT) pyroelectric thin films have been fabricated on Si substrates for uncooled infrared detectors and thermal imaging heads. The electric properties of PMSZT thin films have been investigated. A pyroelectric coefficient p of 45 nC/cm2K, dielectric constant εr of 100, and dielectric loss tanδ of 0.01 at 25°C for the PMSZT thin films have been achieved. The PMSZT detectors exhibited high figures of merit Fv of 1768 cm2/C, and Fd of 0.048 cm3/J½ at 25°C. An IR detector array with the PMSZT thin film deposited on a YBCO microbridge structure for thermal isolation has been fabricated using micromachining techniques. The PMSZT detector with an air gap between the bottom electrode and Si substrates shows a larger voltage response than that of PMSZT detector without an air gap. At a 30 Hz frame frequency, a 3 times larger voltage response of a PMSZT detector with air gap, compared to that without an air gap, has been achieved. The PMSZT detectors with the relatively flat detectivity D*, ranging from 2.5×108 to 6.0×108 cmHz½/W in the 2.5–19.5 μm wavelength band indicate a potential for broad band IR detectors.

Picosecond response of optically driven Y-Ba-Cu-O microbridge and Josephson-junction integrated structures

We report our studies on single-picosecond electrical pulse excitation and detection in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) transmission lines containing microbridges and grain-boundary Josephson junctions. The structures were patterned in YBCO films grown by laser ablation on MgO bicrystals and consisted of 20-/spl mu/m-wide coplanar lines, separated by a 20-/spl mu/m-wide gap. Each transmission line contained a 5-/spl mu/m-wide and 10-/spl mu/m-long microbridge and a 5-/spl mu/m-wide grain-boundary weak link, and was overlaid with 50 nm of Au to improve its high frequency properties. Using a Ti:sapphire laser, we excited the microbridge with 100-fs-wide 400-nm-wavelength optical pulses and studied response of our test structures, utilizing a cryogenic electro-optic sampling system. We directly observed 2.1-ps-wide pulses generated by YBCO microbridges, as well as the single-picosecond Josephson junction response. The junction response depended directly on the current bias and its polarity with respect to the excitation electrical pulse, and exhibited single-flux-quantum-like characteristics. Our test structures can be regarded as examples of all-YBCO ultrafast optoelectronic circuits.

Characterization of the vortex dynamics in Abrikosov superconductive flux flow transistors

Abstract In order to develop superconducting thin films or multilayer structures suitable for Superconducting Flux Flow Transistors (SFFTs), we propose a model to describe the current–voltage characteristics measured on microbridges in the flux creep regime and we show how the effective superconducting thickness of the samples, as well as the pinning potential range, the maximum velocity, the pinning energy, and the depinning current of the vortices can be determined. This model is applied to YBCO microbridges in order to compare samples and to develop films which may easily be driven into the flux flow regime.

YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin-film picosecond photoresponse in the resistive state

Using a subpicosecond electro-optic sampling technique, we have characterized the photoresponse of current-biased YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) photodetectors, designed as 5-/spl mu/m-wide and 7-/spl mu/m-long microbridges patterned in 100-nm-thick, high-quality epitaxial films grown on LaAlO/sub 3/ substrates by pulsed laser deposition. The bridges were centered in a coplanar waveguide structure, allowing the photogenerated pulses to be measured 20 /spl mu/m from the detector. The experiments were conducted in the temperature range between 20 and 80 K; however, the bridges were biased in the switched (resistive) state, which corresponded to a hot-spot formation at the center of the microbridge. The photoresponse from 100-fs laser pulses (395-nm wavelength) was measured to be in the form of a single spike with the width as short as 1.3 ps. The physical origin of this ultrafast response is attributed to nonequilibrium electron heating, We extracted the intrinsic temporal parameters of the YBCO response-the electron thermalization time equal to 0.56 ps and electron-phonon energy relaxation time equal to 1.1 ps, Our measurements demonstrate that a simple YBCO microbridge can operate as a >100-GHz bandwidth photodetector, e.g., as an optical-to-electrical transducer for optoelectronic interface in YBCO digital electronics.