LaNiO3: a promising material for contact with YBa2Cu3O7-x thin films

Epitaxial LaNiO₃ metallic oxide thin films have been grown on c-axis oriented YBa₂Cu₃O_7-δ thin films on LaAlO₃ substrates by pulsed laser deposition technique and the interface formed between the two films has been examined by measuring the contact conductance of the same. The specific contact conductance of the interface measured using a modified four probe method was found to be 1.4 to 6×10⁴ ohm^-1 cm^-2 at 77 K. There are indications that contact conductance can be brought closer to that obtained for noble metal-YBCO interface     

Noise sources across a normal-to-superconducting boundary in bulkYBa2Cu3O7-x

Experiments in which a normal-to-superconducting (NS) boundary is created in an originally superconducting bar of bulk YBa₂Cu ₃O_7-x is described. The first noise measurements associated with such a boundary are presented. A high-density (5.20 g-cm ^-3) sample (sample A) and a low density (4.23 g-cm^-3 ) sample (sample B) have been investigated. Common to both samples is 1/f² noise observed in the frequency region between about 15 and 50 Hz that can be associated with the NS boundary itself. Sample A also exhibits conventional 1/f noise as expected for the normal part of the boundary, below about 15 Hz. In contrast, sample B shows 1/f^1.5 noise in this region; since the sample contains considerably fewer small grains than sample A, any 1/f noise is presumably masked in B. It is suggested that the 1/f² noise might be associated with flux-flow noise     

Stripline resonator measurements of Zs versus Hrf in YBa2Cu3O7-x thin films

A report is presented on measurements of the surface impedance, Z_S, of YBa₂Cu₃O_7-x thin films using a stripline resonator. The films were deposited on LaAlO₃ substrates by off-axis magnetron sputtering. The authors obtained Z_S as a function of frequency from 1.5 to 20 GHz, as a function of temperature from 4 K to the transition temperature (~90 K), and as a function of the RF magnetic field from zero to 300 Oe. At low temperatures the surface resistance, R _S, of the films shows a very weak dependence on the magnetic field up to 225 to 250 Oe. At 77 K, R_S is proportional to the square of the field. The penetration depth shows a much weaker dependence on the field than does R_S. The origins of the magnetic field dependence of Z_S are also discussed     

Critical current characteristics of YBa2Cu3O7 thin films on (110) SrTiO3

The material and electrical characteristics of YBa₂Cu ₃O₇ (YBCO) thin films deposited by inverted cylindrical magnetron sputtering on (110) SrTiO₃ (STO) were investigated. X-ray diffractometry shows the grain orientations to be predominantly the YBCO (110) and (103) with no evidence of c-axis grains, Electron micrographs show the film surface to consist of coupled elongated grains parallel to the (110) STO edge. The films were patterned into small 2.5 mm squares parallel to the substrate edges for electrical characterization. Transport currents parallel and perpendicular to the (110) substrate edge showed a 945:1 anisotropy in film resistance and a factor of two in critical current density for temperatures below 60% of the transition temperature (T_c). The temperature dependence of the critical current near T_c was quadratic-like and strongly dependent on the value of T_c used in the analysis. For the two orientations, there was nearly a 6 K difference in T_c as determined by the point at which the critical current became zero. The response of the critical current to small magnetic fields was greater for transport current along the c-axis direction and was observable over a temperature interval nearly four times greater than for current along the basal plain. These YBCO thin films have good response to small magnetic fields and are suitable for vortex flow device development     

Possibility ofNd1.9Ba1.1Cu3O7+δ andPr1.14Ba1.86Cu3O7-δsingle crystals for insulator in high-speed superconductingcircuits

Dielectric properties of both Nd_1.9Ba_1.1Cu ₃O_7+δ (NBCO-213) and Pr_1.14Ba_1.86Cu₃O_7-δ (Pr-rich PBCO) single crystals have been examined at low temperature. These materials have good lattice matching to high-T_c superconductors (HTS), but they are conductive at room temperature. Below 80 K, they are insulators with low dielectric constants, ε _τ below 25, and low dielectric loss tan δ below 0.1 at 100 kHz. The value of ε_τ is suitable for insulators in integrated circuits using strip line widths of 10 μm order, providing short delay time, no excitation of surface wave, and low radiation loss. The value of tan δ is comparable to loss of superconducting surface resistance above 100 GHz. These results indicate the applicability for the insulator layers in multilayer superconducting electronic devices such as Single Flux Quantum (SFQ) circuits operated at high speed     

Template patterning of YBa2Cu3O7

We demonstrate the use of thin chromium layers to define structures in YBa₂Cu₃O₇ deposited by laser ablation. The chromium reacts with copper to form an insulating crystalline compound containing copper and chromium in equal amounts. Although the reaction will proceed up to approximately 1 μm into the defined pattern, the formation of a crystalline compound means that the extent of the reaction is limited by the availability of chromium. We have defined large-area, regular grids of tiny chromium pads, yielding a grid of insulating islands of approximately 600 nm in size after YBa₂Cu₃O₇ deposition. Electrical measurements show the influence of such a pattern on the R-T characteristics of the YBa₂Cu₃O₇.     

Temporal stability of circulating current in thin film Tl2Ba2CaCu2O8 and YBa2Cu3O7

The temporal stability of trapped transport current in annular thin film Tl₂Ba₂CaCu₂O₈ (TBCCO) and YBa₂Cu₃O₇ (YBCO) wafers has been accurately measured and has been found to be of suitable quality for the stringent requirements of nuclear magnetic resonance (NMR) magnets. No detectable decay, to the limit of the experimental apparatus (2*10^-14 Ω), was detected in those wafers with transport current at or below the critical current density J_c. The critical current density, as previously determined from 12 μm meander lines, was confirmed in a wafer with a width of 1.9 cm. The profile of trapped magnetic field resulting from induced current was modeled in order to assess its effect on the uniformity of an NMR magnet     

YBa2Cu3O7 superconductors forhigh-speed interconnects

The surface resistance of high-quality YBa₂Cu₃ O₇ superconducting films measured over a frequency range from 10 to 500 GHz using high-speed optoelectronic techniques is discussed. A direct comparison is made with the surface resistance of gold and superconducting niobium conductors. Using the measured surface resistance, the propagation characteristic of interconnects based on YBa ₂Cu₃O₇ superconductors at 77 K is simulated and compared to that of gold transmission lines at 77 K and superconducting niobium lines at 7.7 K     

Optical detector using granularYBa2Cu3O7-δ weak link-typejunctions

Experimental photoresponse study of granular YBa₂Cu₃O_7-x (YBCO) weak link is reported. The Josephson critical current I_c is suppressed with light illumination of the junction, produced mainly from optical generation of quasiparticles. At lower temperature (T=64.5 K), the hysteresis in I-V characteristic of the weak link appears, which may be found useful for optical detection purpose. This was supported from preliminary experiments     

YBa2Cu3-xZnxO7薄膜红外探测器的研究

在对红外探测器进行理论分析的基础上,设计并研制了液氮温度下的Yba2Cu3-xZnxO7薄膜红外探测器,系统地测试了器件的特征参数.最好的结果为:对于波长为10μm,调制频率为f=500Hz,带宽为Δf=1Hz的红外输入辐射Rv(500,10,1)=3587V/W,NEP(500,10,1)=6.5×10-12W/Hz1/2,D*(500,10,1)=7.2×1012cmHz1/2/W,τ(500,10,1)=1.2ms.     

Switched YBa2Cu3O7 lumped elementbandstop filter

The switching capability of a simple high temperature superconducting (HTS) lumped element bandstop filter is demonstrated. A bias current of the order of the critical current of the HTS material is necessary to switch the filter response from high to low attenuation at the stop-band frequency. A simple theoretical model to describe the ON and OFF states is presented. First measurements of the response time to a pulsed bias current show that the switching time is <2 μs     

On the design and performance of electrically small printed thickfilm YBa2Cu3O7-x antennas

Strip material and substrate losses, that occur at the radiating element and the feeding-matching network of electrically small printed YBa₂Cu₃O_7-x (HTS) antenna systems, can substantially degrade their performance and, therefore, have to be considered in optimum efficiency and SNR designs. Identification and numerical estimation of such loss mechanisms, and assessment of the overall efficiency of the antenna system, which is of particular importance in design, is facilitated by fundamental microwave network analysis and the method of moments (MM). Four efficient designs, incorporating different feeding-matching configurations, are experimentally tested, and their performance is compared with the theoretical predictions. Superior gain can be demonstrated over similar supercooled Cu and Ag antennas. The applicability of low frequency efficiency measurement methods for highly efficient HTS antenna systems is outlined and extended for improved accuracy     

The propagation characteristics of wave-guiding structures withvery thin superconductors; application to coplanar waveguide YBa2Cu3O7-x resonators

The propagation characteristics of waveguiding structures with superconductors which are thin compared to the magnetic penetration depth are analyzed. The complex propagation constant is evaluated within the framework of the modified spectral domain method without the need for numerical calculations in the complex plane. Good agreement is found with the results of other methods. The numerical analysis is instrumental in deducing results for the penetration depth and the surface resistance of YBa₂Cu₃O_7-x thin films on sapphire with a PrBa₂Cu₃O_7-x buffer layer. Recent observations of a non-single-gap BCS temperature dependence are confirmed     

Optimized technology for freely positionable Josephson junctions inYBa2Cu3O7-x fabricated by local iondamaging

Freely positionable Josephson junctions in high-temperature superconductor thin films are fabricated by a combination of photo- and electron-beam lithography. In selected regions of YBCO thin films point defects are induced by oxygen implantation. These weak links exhibit resistively shunted junction (RSJ)-like voltage-current characteristics close to the junction transition-temperature. Below this temperature, a transition from RSJ to flux-flow behavior is observed. Fraunhofer patterns appear in magnetic modulation measurements at different temperatures. The temperature dependence of the critical current reveals superconductor/normal conductor/superconductor (SNS) features. The dependence of Shapiro steps on microwave power demonstrates Josephson contributions over the entire temperature range     

DC characteristics of patternedYBa2Cu3O7-x superconducting thin-filmbolometers: artifacts related to Joule heating, ambient pressure, andmicrostructure

Joule heating due to the bias current and resistance of the material in patterned YBa₂Cu₃O_7-x, superconducting films on 250-500-μm-thick MgO, LaAlO₃, and SrTiO₃ crystalline substrates, results in a number of effects: (1) a temperature rise in the film with respect to the measured temperature at the bottom of the substrate; (2) a possible thermal runaway, which may be local or uniformly distributed in the film, depending upon the dimensions of the superconducting pattern relative to that of the substrate; (3) an apparently sharper normal-to-superconducting transition in the measure R versus T curve; and (4) decrease of T_c to 60 K (ΔT_x>20 K) after being subjected to high-bias currents j~10⁵ A/cm² under vacuum, with recovery of T_c after exposure to room atmosphere. The magnitude of R at T_c*onset is found to be dependent on bias current in granular samples, with a lower R at currents higher than some on-set value. The slope of R versus T in the transition region in our granular samples is found to be lower at higher bias currents, since the widening of the transition overcomes the shift caused by the Joule heating. These various phenomena impact the responsivity of bolometers made from these films, as well as the predictions of possible attainable responsivity and speculations of mechanisms occurring in the films. In particular, misinterpretation of the Joule heating sharpening of the R versus T curve has led to predictions of responsivities over one order of magnitude higher than are justified, and shifts in properties of the films due to heating have been misinterpreted as nonequilibrium responses of the films     

氧缺陷的YBa2Cu3O7-x高温超导体光辐照研究

利用Janis CCS-350制冷机(最低可达10 K)提供低温环境, 从室温到200 K, 利用真空泵持续抽取真空, 使氧缺陷YBa2Cu3O7-x(YBCO)超导体中的氧重新分布。实验表明, 在缺氧环境下, 样品氧缺陷现象加剧, 超导临界温度降低, 直至到正常态, 利用原位光辐照氧缺陷YBCO样品产生持续光电导效应或光诱导超导电性效应, 可以部分地抵消氧缺陷对超导电性所引起的破坏, 而对YBCO,x<0.1时, 其转变温度Tc高于90 K的样品, 在缺氧环境下, 样品氧缺陷现象变化和光辐照效应不明显。讨论了YBCO超导体在能隙比一般半导体光电材料至少要窄约二个数量级, 又具有较高的光学吸收系数的情况下, 光子作用下高温超导体内各种元激发之间的相互作用过程, 对实际应用YBCO超导体光学性能, 理解机制设计仪器有现实意义。     

Coaxial line configuration for microwave power transmission studyof YBa2Cu3O7-δ thin films

Microwave transmission measurements through YBa₂Cu₃O_7-δ (YBCO) high-transition-temperature superconducting thin films on lanthanum aluminate (LaAlO₃) have been performed in a coaxial line at 10 GHz. LaAlO₃ substrates were ultrasonically machined into washer-shaped discs, polished, and coated with laser-ablated YBCO. These samples were mounted in a 50-Ω coaxial air line to form a short circuit. The power transmitted through the films as a function of temperature was used to calculate the normal state conductivity and the magnetic penetration depth for the films     

Inductance measurements in YBa2Cu3O7 DC SQUID's with a submicrometer line width

We measured the inductances of submicrometerwide strip lines by using DC superconducting quantum interference devices (SQUID's) fabricated from a single layer of YBa₂Cu₃O₇ thin film, The measured inductances coincided well with some numerically calculated inductances consisting of a magnetic and kinetic component. The proportion of the magnetic component in the total inductance for the line width of 2 μm was estimated from calculation to be 56%, The kinetic component increased with the decrease of the line width and was dominant for line widths less than 1 μm. The inductance per square at 4.2 K was measured to be 1.3 pH/□ for line widths from 0.5 to 2 μm     

Improved electrical properties using SrTiO3/Y2O3 bilayer dielectrics for MIM capacitor applications

In this paper, we show that the capacitance–voltage linearity of MIM structures can be enhanced using SrTiO₃ (STO)/Y₂O₃ dielectric bilayers. The C(V) linearity is significantly improved by combining two dielectric materials with opposite permittivity-voltage responses. Three STO/Y₂O₃ stacks with different thicknesses were realized and compared to a 20 nm STO single layer structure. We observed that an increase in the Y₂O₃ thickness leads to an improvement in the voltage linearity, while maintaining an overall capacitance density greater than 10 fF/μm².     

Anomalous microwave response ofYBa2Cu3O7-δ granular thin filmsunder magnetic fields

The microwave response of YBa₂Cu₃O_7-δ (YBCO) granular films has been studied at a microwave frequency of 30.5 GHz. In absence of a magnetic field, the dependencies of a normal microwave response on the bias current was observed at a temperature close to T_c. When a magnetic field ranging from 5.0 to 33.0 mT is applied, the responses broaden and shift toward a lower temperature. In the superconducting state, the responses were found to be highly dependent on the magnetic field. For a current of 5.0 mA and a magnetic field above 17.0 mT, the responses increased and did not vanish, even at a very low temperature, which is believed to be correlative with the anisotropic character of the structure