Research and Some Applications of High-T c SQUIDs

In the work, we studied the noise characteristics of electronic gradiometers in unshielded environments. In the off-axis electronic gradiometers for biomagnetic measurements, we optimized low frequency noise and performed two-dimensional magnetic mapping of magnetic signal from human heart. The measured magnetocardiography (MCG) signal was averaged according to the simultaneously recorded electrocardiography signal to further enhance the signal-to-noise ratio. The off-axis configuration in our gradiometer system offers the flexibility for multichannel SQUID-based MCG applications. In the study of the SQUID microscope for circuit detection in unshielded environment, we fabricated SQUIDS, considered the design of cryostat, and used the lock-in technique to examine the circuit board. We also examined the magnetic field pattern from the magnetized magnetic thin film.     

Integrated niobium DC SQUID gradiometers for non-destructive evaluation and biomagnetism

We describe the design and fabrication of thin-film Nb gradiometers with integrated DC SQUIDs for use in non-destructive evaluation (NDE) and biomagnetism. Issues of sensitivity, imbalance and field response are considered. Results are presented from eddy-current NDE in an unshielded environment of aluminium plates with sub-surface flaws, and from biomagnetic measurements of spinal and peripheral nerve evoked fields.     

Radiation amplification with help of the intrinsic Josephson effect

The use of the intrinsic Josephson effect in layered high-T_c superconductors for amplification of a signal in the presence of a strong pump radiation is investigated. Calculations are done both for thin films and massive material with a surface parallel to the c-direction, assuming normal incidence of the radiation with electric field parallel to the c-direction for parameter values relevant for high-T_c materials. In both cases high signal gain is obtained in a limited range of frequencies and pump power.     

High-Tc dc-SQUID magnetometer and its application in eddy current non-destructive evaluation

We have fabricated and tested a high-T_c dc-SQUID magnetometer patterned on a YBCO thin film deposited on a SrTiO₃ bicrystal substrate. The measurement of noise spectrum at 77 K showed that the magnetometer has a white noise of 333 fT/Hz^1/2 without superconducting shield, corresponding to a flux sensitivity of 14.5 μΦ₀/Hz^1/2. We constructed a test system for eddy current non-destructive evaluation of conducting aluminum plates by using the magnetometer. Flaws over 10 mm below the surface can be clearly identified. By mapping the magnetic field distribution, we can locate the flaw position. The signals caused by flaw were expanded due to the use of double-D type driving coil with a wider central slit. The experimental results confirmed that this test system could operate in a magnetic unshielded environment.     

Steps Toward Application of Photoinduced Phenomena in Superconducting Devices

The interaction of electromagnetic radiation with thin film devices consisting of superconducting, normal conducting, semiconducting, insulating, and magnetic materials can be used for variety of different applications. Some examples will be discussed, including laser modification for patterning and adjustment of thin film high-T_c superconducting devices, ultrafast response, and the photoinduced superconductivity in high-T_c materials which offers interesting possibilities for optoelectronical conversation and controllable weak links in superconducting electronics.     

A symmetric third order gradiometer without external balancing or magnetocardiography

A feasibility study is being carried out on the operation of a SQUID gradiometer without any balancing techniques in a non-shielded environment. This paper compares empirically the performances of second and third order gradiometers; it also presents a general procedure for the design of third order gradiometers, taking into account the measured spatial dependence of the magnetic field of the human heart.The performance of a third order gradiometer is very promising for a noisy environment; it has a better low frequency noise rejection than the second order gradiometers tested, and needs no external balancing.     

Theoretical and experimental investigation of phase coherence in shorted 2D arrays of high-T c Josephson junctions

Phase coherence in shorted 2D Josephson junction arrays shunted by resistive load in presence of disorder and magnetic field are investigated theoretically and experimentally. Using computer simulation it is shown that magnetic field negatively influences radiation properties of such arrays. Similarly to XY-type of 2D arrays maximal spread in junction parameters which allows stable phase-locking state is ±11%. 2D arrays with topology of those investigated theoretically were fabricated using high-T_c superconductor junction technology. The phase-coherence (voltage locking) between rows was observed experimentally.     

Synthesis and properties of Bi1.6Pb2-xSr2-xCa2Cu3Oy

Bi_1.6Pb_0.4Sr_2-xCa₂Cu₃O_y (x = 0, 0.2, 0.4) high-T_c materials were prepared from oxide-carbonate mixtures and presynthesized mixed oxides. The 2223 superconducting phase was found to be formed most rapidly in thex = 0.4 sample if the Pb-and Ca-containing starting reagents were separated in the initial stage of synthesis. The highest superconducting transition temperature, T_c (R = 0) = 104.3 K, was attained in the stoichiometric 2223 material. The materials were characterized by electrical resistivity, magnetic susceptibility, and Hall effect measurements. The Hall data were used to evaluate carrier concentration and mobility. The 77-K resistivity of the materials was measured as a function of magnetic field.     

Investigation of the interface problem between high temperature superconductor and ionic conductors

The charge carriers transfer process across the interface between a superconductor and an ionic conductor, around T _c is investigated. Low temperature electrochemical measurements are carried out on the interfaces between different polycrystalline high-T_c superconductors (HTSC) and RbAg₄l₅ and Ag⁺ ion-conducting glass. The experiments cover a temperature range down to 10 K in the de-frequency range. Atransient technique in the time domain and electrochemical impedance spectroscopy (EIS) in the frequency domain are used to study the silver (Ag⁺) deposition as the Faradaic charge transfer process at that interface. The results show significant enhancement of the charge transfer observed around the critical temperature. This was indicated by either an admittance peak in the transient measurements or a corresponding decrease of the polarization resistance (R _p) in EIS measurements. This enhancement of the charge transfer is correlated to the formation of Cooper pairs at T T _c and interpreted on the basis of a band structure model as a quantumelectrochemical phenomenon with the tunnelling of Cooper pairs through the electrochemical double layer.     

超导梯度计及其对环境噪声的抑制能力

研究了超导磁传感器的交流标定方法,并在此基础上研究了不同软件梯度计的实现方案及其对环境干扰磁场的抑制能力。采用多个频率、多个强度的参考磁场对传感器进行标定得到的结果一致性良好。合成的一阶软件梯度计经过实验验证具有很好的微弱磁信号检测能力,在城市实验室噪声环境下成功实现了约50pT微弱磁场信号的探测。还对标定精度以及实验中观察到的不同软件梯度计方案结果之间的差别进行了讨论。     

On the principles of vortex localization and motion in superconductor thin films with artificially patterned cavities

In this paper we summarize the results of our recent studies of vortex matter in high-T_c superconductors with antidots, i.e. lithographically patterned holes in superconductor thin films. We analyze theoretically and experimentally the distribution of magnetic flux and effective pinning energy in perforated samples: the equilibrium occupation number (number of flux quanta accomodated by the hole) is evaluated numerically in a range of temperatures and fields for antidot sizes typically used in experiments; magnetic field profiles and activation energy of thermally activated flux creep is obtained from magnetic relaxation measurements by the means of Hall probes array. Possible mechanisms of vortex penetration and propulsion in such structures are discussed.     

Development of High-T c SQUID System for Nondestructive Evaluation

Nondestructive evaluation (NDE) system with high spatial resolution power based on high-T _c superconducting quantum interference device (SQUID) has been developed. The SQUID is fabricated by YBCO thin film and mounted in vacuum at the end of a cold finger fixed at the inner wall bottom of the dewar, which is filled with liquid nitrogen (77 K) to cool the SQUID sensor. The sensor faces a thin flat sapphire window on the outer wall of the dewar and is thermally isolated from it by a vacuum space at a distance of less than 1 mm. The magnetic field sensitivity of SQUID is about 40 pT/ Hz in magnetically unshielded environment. This NDE system was used to detect the flaws buried into materials. The remnant magnetic field inducted by the local damages in a stainless steel sample was measured. Incorporated a gradiometric double-D excitation coil, the eddy current NDE was carried out. Small cracks in the aluminum below the surface can be detected effectively in unshielded environment.      

Using acoustic resonators to study superfluid-filled silica aerogel,highT c superconductors,and quasicrystals

Investigations of many quantum and solid-state systems of interest require measurements on samples that are either very fragile, as in the aerogel- ⁴ He system, very small, such as samples of high-T_c materials, or both small and fragile, such as a single grain quasicrystal. Many relevant properties of these systems can be deduced from sound-velocity measurements; we present here some techniques for making such measurements, using acoustic resonators and novel transduction mechanisms. The use of acoustic resonance, as opposed to pulses, for measuring sound velocity permits relatively low frequencies to be used; in addition, frequency becomes the primary quantity of interest, which can be measured with ease and precision.     

Formation and properties of copper film on Tl-(Pb,Bi)-Sr-Ca-Cu-O superconductor by electrodeposition from aqueous solution

A new process of electrodeposition in saturated cupric sulphate aqueous solution was successfully developed for the formation of copper film on a high-T_c, three Cu-O layered Tl-(Bi, Pb)-Sr-Ca-Cu-O superconductor substrate surface for the first time. Scanning electron microscopy and electron micro-probe analysis were used to investigate the morphology of the substrate surface and the composition of the copper superconductor interface. After the electrodeposition process, no evident changes in the temperature dependence of electrical resistivity were found by four-point probe measurement. The difference of magnetic properties before and after electrodeposition was investigated from magnetization measurements. Almost no degradation of the bulk properties was observed from the susceptibility data. The copper-superconductor contact was confirmed to show Ohmic behaviour by two-pointI–V characterization at liquid nitrogen boiling temperature.     

Effect of rare-earth (Eu, Yb and Ag) substitutions on superconducting properties of the Bi1.7Pb0.3 Sr2Ca2−xRx(R = Eu, Yb, and Ag)Cu3OY system

The superconducting properties of Bi^1.7Pb_0.3Sr₂Ca^2–xR^x(R = Eu, Yb and Ag)Cu₃O_Y have been investigated by X-ray diffraction, electrical resistance and the peritectic transition of these superconductors was studied kinetically under different atmospheres and temperature gradients. X-ray diffraction results show that the volume fraction of the high-T_c (2 2 2 3) phase decreases and that of the low-T_c (2 2 1 2) phase increases as Eu, Yb and Ag concentrations increase. The resistivity measurements reveal that the T_c onset decreases down to 100 K for Eu, 85 K for Yb and 106 K for Ag concentrations. These results are explained on the basis of possible variations of hole concentration with trivalent rare earth ion substitutions. Activation energies and frequency factors for crystallisation were determined by non-isothermal differential thermal analysis (DTA), employing different models. It was found that both peritectic transition and reaction rate were dependent on the ambient atmosphere. Kinetic studies under different atmospheres revealed that the thermal stability of Bi-2 2 1 2 phase was greatly enhanced under oxygen atmosphere.     

Integrated Planar HTS SQUID Gradiometer for NDE Application

The performance of a SQUID NDE probe with integrated planar SQUID gradiometers was compared to that with an electronic axial SQUID gradiometer. A circular excitation coil was used for the SQUID NDE probe with the planar gradiometer, whereas a differential excitation coil was adopted for the one with the axial gradiometer. These NDE systems are used to detect a buried flaw in aluminum plates, and the phase-depth relations of the buried flaw were found to show similar linear dependences. Overall, the SQUID NDE system with the planar gradiometer is relatively simple and practical for unshielded operations.     

Review of Thermal Boundary Resistance of High-Temperature Superconductors

Solid-solid thermal boundary resistance plays an important role in the thermal stability of many electronic circuits, microdevices, and superconducting devices. The thermal boundary resistance (R _b ) at any interface causes a temperature discontinuity, which can result in heat accumulation on one side of the boundary and raise the temperature much above the stable region, causing device failure. With the advent of high-critical-temperature (high-T _c ) superconductors, it is possible to make superconducting devices at practically achievable temperatures. As the current trend goes toward the development of more and more high-T_c superconducting devices, the need for a better understanding of the thermal boundary resistance of high-T_c superconductors becomes mandatory. This paper compiles all the theoretical and experimental work to date onR _b in high-T_c superconductors, both in thin-film and bulk forms, and provides a critical review of the cited works. This paper also describes the possible effect of the superconducting state onR _b for high-T _c superconductors, based on the experiments for both high-T_c and low-T_c bulk superconductors, and a possible explanation for these data based on the existing theory for low-T _c superconductors.     

On the Principles of Vortex Localization and Motion in Superconductor Thin Films with Artificially Patterned Cavities

No Heading In this paper we summarize the results of our recent studies of vortex matter in high-T_c superconductors with antidots, i.e. lithographically patterned holes in superconductor thin films. We analyze theoretically and experimentally the distribution of magnetic flux and effective pinning energy in perforated samples: the equilibrium occupation number (number of flux quanta accomodated by the hole) is evaluated numerically in a range of temperatures and fields for antidot sizes typically used in experiments; magnetic field profiles and activation energy of thermally activated flux creep is obtained from magnetic relaxation measurements by the means of Hall probes array. Possible mechanisms of vortex penetration and propulsion in such structures are discussed.PACS numbers: 74.78.–w, 74.25.Qt     

Vortex Loop Distribution Near a Superfluid Phase Transition

The distribution of thermally excited vortex loops near a superfluid phase transition is calculated. The number density of loops with a given perimeter is found to change from an exponential decay with increasing perimeter to algebraic decay as T _c is approached from below, in agreement with recent simulations of cosmic strings and of the high-T _c superconducting transition in zero field. The total line density is calculated and compared with the simulations. A different viewpoint of the Kibble-Zurek scenario for rapidly quenched transitions is proposed, and the role of vortices above the transition temperature is briefly discussed.     

Large ultrasonic attenuation at Tc in YBa2Cu3O7?x

We investigated elastic anomalies of sintered YBa₂Cu₃O_7–x (x=0.35) superconductor around the critical temperature (T_c=60 K). We observed the non-symmetric and large attenuation peak (2 dB/cm at 9.7 MHz) at T_c, which was due to the lattice softening caused by the strong correlation between carriers and local lattice distortion. The rapid decrease of ultrasonic attenuation just below T_c suggested a condensation mechanism. These supported the polaronic condensation in high-T_c superconductivity.